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• Published: 29 February 2024

Understanding energy performance in drinking water treatment plants using the efficiency analysis tree approach

• Alexandros Maziotis 1 &
• Maria Molinos-Senante   ORCID: orcid.org/0000-0002-6689-6861 2 , 3  

npj Clean Water volume  7 , Article number:  13 ( 2024 ) Cite this article

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• Environmental economics
• Water resources

Water treatment processes are known to consume substantial amounts of energy, making it crucial to understand their efficiency, drivers, and potential energy savings. In this study, we apply Efficiency Analysis Tree (EAT), which combines machine learning and linear programming techniques to assess the energy performance of 146 Chilean drinking water treatment plants (DWTPs) for 2020. Additionally, we utilize bootstrap regression techniques to examine the influence of operating characteristics on energy efficiency. The results indicate that the evaluated DWTPs exhibited poor energy performance, with an average energy efficiency score of 0.197. The estimated potential energy savings were found to be 0.005 kWh/m 3 . Several factors, such as the age of the facility, source of raw water, and treatment technology, were identified as significant drivers of energy efficiency in DWTPs. The insights gained from our study can be valuable for policymakers in making informed decisions regarding the adoption of practices that promote efficient and sustainable energy use within the water cycle.

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Introduction

Energy plays a crucial role in various aspects of managing water resources in urban settings, including abstraction, treatment, and distribution 1 . In particular, the production of drinking water is an energy intensive activity because raw water needs to be cleaned by removing high levels of pollutants 2 . The Sustainable Development Goals established by the United Nations, specifically Goals 6 and 7, impose an obligation on governments to ensure universal access to clean water and enhance energy efficiency 3 . Consequently, it is crucial for water resources to meet rigorous quality standards before being deemed suitable for drinking purposes. Simultaneously, the challenges posed by climate change and population growth necessitate the sustainable and efficient utilization of water resources 4 , 5 . Moreover, drinking water needs to be provided to people at an affordable price 6 .

To address the aforementioned challenges, it is crucial to gain a deeper understanding of the energy efficiency of the water treatment process, including the factors that influence energy performance and the potential for energy savings. By examining the energy efficiency of the water treatment process and exploring opportunities for energy conservation, we can work towards achieving sustainable and efficient use of energy in the water sector. Previous studies, such as those conducted by Loubet et al. 7 , Chini et al. 8 , and Lam et al. 9 , have examined the relationship between energy intensity and the water treatment process. These studies have highlighted the growing energy demands in water treatment resulting from factors like climate change, population growth, and urbanization. However, it is worth noting that these studies did not specifically explore the link between energy efficiency and the water treatment process.

Energy intensity and energy efficiency are different concepts 10 . Energy intensity is the energy consumed [kWh] per unit volume [m 3 ] of drinking water produced and therefore, it does not consider how the quality of raw water and drinking water affected the energy consumed by drinking water treatment plants 11 , 12 . On the other hand, energy efficiency is a synthetic indicator which in addition to the volume of drinking water produce integrates the quantity of pollutants removed from raw water 13 .

Only a limited number of previous studies have specifically focused on evaluating the energy efficiency of drinking water treatment plants (DWTPs). These studies, conducted by Molinos-Senante and Guzman 14 , Molinos-Senante and Sala-Garrido 10 , 15 , Ananda 16 , Sala-Garrido and Molinos-Senante 17 , and Maziotis et al. 13 , utilized the data envelopment analysis (DEA) method. DEA is a non-parametric technique that employs linear programming to measure energy efficiency, enabling the integration of multiple inputs and outputs for each DWTP 18 . One advantage of DEA is that it does not require a priori definition of the functional form of the production frontier, which represents the relationship between inputs and outputs 19 . While DEA has its positive characteristics in evaluating energy efficiency of DWTPs, it is important to acknowledge its limitations. One such limitation is its deterministic nature, which makes it sensitive to outliers in the data 20 . Consequently, the presence of extreme values can significantly impact the efficiency scores derived from DEA analysis.

DEA method needs implicitly to determine the boundary of the underlying technology, which constitutes the reference benchmark 21 . Its estimation allows the calculation of the corresponding inefficiency score for each unit (DWTP in our case study) as the deviation of each activity or production plan from the frontier of the production possibility set. For this reason, by definition, DEA approach suffers from an overfitting problem 21 , 22 , 23 . Overfitting occurs when the model becomes too closely tailored to the specific dataset used for analysis, potentially resulting in less robust efficiency scores. Therefore, caution should be exercised when interpreting and relying solely on efficiency scores derived from DEA analysis.

In order to address the limitations of DEA and enhance the accuracy and robustness of efficiency scores, Esteve et al. 21 introduced a novel method called Efficiency Analysis Trees (EAT). The EAT method combines machine learning and linear programming techniques to measure efficiency. Specifically, it utilizes regression (or decision) trees to estimate the value of the response variable based on thresholds of predictor variables. The EAT approach assumes free disposability to estimate a step function production frontier and calculate efficiency scores. Esteve et al. 21 demonstrated from a mathematical point of view how the EAT method overcomes overfitting improving the accuracy of the efficiency results. In particular, they demonstrated that the EAT method outperforms other non-parametric techniques such as DEA because the estimated values are not overfitted, ensuring more reliable efficiency measurements 24 .

In light of the aforementioned context, the main objective of this study is to assess the energy efficiency of the drinking water treatment process using the newly developed method EAT. By employing the EAT approach, this study aims to quantify the optimal level of energy consumption at various thresholds of volume pollutants removed. Additionally, the EAT method allows for the estimation of potential energy savings that could be achieved with efficient water treatment practices. Moreover, in order to gain a deeper understanding of the factors influencing energy efficiency in the water treatment process, this study utilizes bootstrap regression techniques. Specifically, it examines the impact of operational characteristics such as the age of the facility and the type of treatment technology on energy performance. Through these analyses, this study seeks to provide valuable insights into improving energy efficiency in drinking water treatment operations.

Our study makes several significant contributions to the existing literature. Firstly, it stands as the pioneering research that applies machine learning and linear programming techniques to assess the energy performance of the drinking water treatment process. By employing the EAT approach, we are able to generate robust efficiency scores that are not overfitted, in contrast to other non-parametric methods like DEA. Furthermore, our study estimates the potential energy savings achievable in the drinking water treatment process. This allows us to gain insights into the optimal energy utilization required for different pollutant removal volumes. This information is invaluable for managers and decision-makers as it sheds light on the factors influencing energy intensity and aids in the decision-making process. Importantly, our innovative research was implemented and applied to multiple DWTPs in Chile, enhancing its applicability and relevance to real-world scenarios. This empirical application further strengthens the validity and reliability of our findings.

Results and discussion

Optimal energy use in drinking water treatment.

By applying the EAT algorithm, we can determine the optimal level of energy utilization in DWTPs based on the volume of drinking water produced and efficiency in pollutants removal. The findings from our analysis, as depicted in Fig. 1 , highlight the significant impact that removing arsenic and sulfates from raw water has on the energy consumption of DWTPs. By contrast, the removal of the other pollutants considered in this study, i.e., turbidity and total dissolved solids, does not significantly explain energy use in DWTPs. This finding aligns with Molinos-Senante and Sala-Garrido 25 conclusions, which assessed how various pollutants and the volume of treated water affect the energy intensity in a range of water treatment facilities. They evidenced that total dissolved solids only affect the energy usage in DWTPs employing coagulation-flocculation and pressure filtration techniques. Conversely, the reduction of turbidity was found to influence energy consumption only in DWTPs that utilize pressure filtration. However, for facilities relying on rapid gravity filtration, the energy usage is not affected by the removal of turbidity and total dissolved solids.For those facilities producing more than 2,111,834 m^−3 per year of drinking water adjusted by arsenic removal efficiency the maximum energy use is 1,054,754 kWh per year, i.e., 0.499 kWh m^−3. In the case of DWTPs that produce less than 2,111,834 m^−3 per year of drinking water adjusted by arsenic removal and of more than 428,440 m^−3 per year adjusted by sulfates removal, the maximum use of energy utilization could reach the level of 539,412 kWh per year. Hence, the optimal energy usage ranges from 0.255 kWh m^−3 to 1.259 kWh m^−3 depending on whether the assessment takes into account the efficiency of arsenic or sulfates removal, respectively. Finally, DWTPs producing less than 428,440 m^−3 per year adjusted by sulfates efficiency removal, the maximum energy use required could be lower than 136,201 kWh/year, i.e., 0.318 kWh m^−3. It is illustrated the large range of optimal energy use depending on the volume of drinking water produced and the quantity of arsenic and sulfates removed. Considering that the average volume of water produced adjusted by arsenic removal is 1,998,544 m^−3 per year, results on Fig. 1 , illustrate that smaller facilities can have an energy intensity as low (or lower) than larger ones as even the quantity of sulfates to be removed is large (>428,440 m^−3 per year).

The volume of drinking water produced and the quantities of arsenic and sulphates removed from raw water influences of the energy use of DWTPs.

Indeed, the results of our study indicate that optimal energy utilization in DWTPs can vary based on different thresholds of water treated to remove specific pollutants such as sulfates and arsenic. This finding underscores the importance of tailoring energy consumption to the specific requirements of pollutant removal in order to achieve optimal energy efficiency. By considering different thresholds or levels of pollutant removal, DWTPs can determine the appropriate energy usage for their particular circumstances. The results suggest that the energy required to remove pollutants like sulfates and arsenic can have a significant impact on overall energy consumption in DWTPs. Therefore, by optimizing energy usage based on these thresholds, water treatment facilities can enhance their energy efficiency and reduce unnecessary energy consumption.

Energy efficiency of drinking water treatment plants

The findings of our study indicate that the water treatment processes examined exhibit high levels of energy inefficiency, with an average energy efficiency score of 0.197 (Fig. 2 ). This implies that, on average, there is significant room for improvement in terms of energy consumption, with a potential reduction of almost 80% in energy usage. The distribution of energy efficiency scores among DWTPs is shown in Supplementary Fig. 1 .

Relevant differences on the energy efficiency among drinking water treatment plants are reported.

Moreover, it is observed that a small proportion of the evaluated facilities demonstrated full energy efficiency. Specifically, out of the 146 DWTPs analyzed, only four facilities, representing ~2.7% of the total, achieved a full efficiency score of 1.00. This indicates that these particular facilities have effectively optimized their energy usage and are operating at the highest level of energy efficiency within the context of our study. The four energy-efficient facilities utilize pressure filters (PF) for drinking water production. However, they differ in their primary raw water sources, with two of them using groundwater and the other two relying on surface water.

Findings from this study slightly differ from those of Molinos-Senante and Sala-Garrido 15 , who reported an average energy efficiency score of 0.28 and classified 6 out of 146 DWTPs as energy-efficient. Notably, two facilities deemed efficient in their study were considered inefficient in ours. Furthermore, there are more pronounced discrepancies when compared to Molinos-Senante and Maziotis 26 , who reported an average energy efficiency score of 0.462, with none of the evaluated facilities being fully energy-efficient. These variations can be attributed to the different methodological approaches used in assessing energy efficiency. Molinos-Senante and Sala-Garrido 15 utilized a double-bootstrap DEA method, which reduces data uncertainty but does not address overfitting issues. In contrast, Molinos-Senante and Maziotis 26 used stochastic non-parametric envelopment of data (StoNED), a technique that incorporates both inefficiency and noise in the assessment but still has overfitting limitations. Our study, however, employed the EAT approach, a method that is not prone to overfitting issues. This methodological improvement strengthens the validity of our findings and underscores the need for targeted efforts to improve energy efficiency in water treatment plants.

Figure 3 provides valuable insights into the distribution of energy efficiency scores across the evaluated DWTPs. The majority of the facilities reported energy efficiency scores below 0.21, indicating a significant level of energy inefficiency. On average, these plants would need to reduce their energy consumption by nearly 80% to achieve optimal energy efficiency. On the other hand, there are 18 treatment plants that reported relatively higher energy efficiency scores. However, their scores still fall within the range of 0.21 and 0.61, indicating room for improvement. These facilities have the potential to achieve substantial energy savings, ranging from 40% to 80% on average, and bridge the gap with the most energy-efficient plants in the sector. No common characteristics were observed in terms of source of raw water, treatment train, and ownership for this group of DWTPs. The treatment trains employed are coagulation-flocculation with rapid gravity filters (CF-RGF) by 8 facilities, coagulation-flocculation with pressure filters (CF-PF) by 5, and PF alone by another 5. Similarly, the distribution of the main source of raw water varies, with 8 DWTPs treating mixed raw water, 5 treating groundwater, and 5 treating surface water. Ownership-wise, 14 of the 18 DWTPs are fully privately owned, while the remaining 4 are operated by concessioned companies.

Most of the facilities evaluated present a very poor energy efficiency with a score lower than 0.21.

Furthermore, the analysis reveals a group of 11 DWTPs that can be considered as best performers. These facilities attained energy efficiency scores ranging between 0.81 and 1.00, indicating a high level of energy efficiency. These best performers serve as examples of successful energy management practices and provide insights into the potential for achieving optimal energy efficiency in the water treatment sector. The shared characteristic among this group of DWTPs is their use of PF as the primary treatment method for producing drinking water. Additional information of the influence of treatment train on the energy efficiency of DWTPs is shown in Fig. 5 . As with the previous group, diverse features are observed in terms of ownership and the main source of raw water. Of these DWTPs, 8 out of 11 are owned by full private water companies, while the remaining three are operated by concessioned water companies. Regarding the source of water, the distribution is as follows: 5 out of 11 DWTPs treat mixed water, 4 out of 11 treat groundwater, and 2 out of 11 treat surface water.

In our study, we investigated the energy-saving potential of the energy-inefficient DWTPs. By applying Eq. ( 5 ) and considering the current energy use of the 146 assessed DWTPs, we estimated the potential energy savings for these facilities, as depicted in Fig. ( 4 ). The results revealed that the assessed DWTPs have a combined potential energy-saving of 13,344,093 kWh/year. This indicates the substantial opportunity for reducing energy consumption in these facilities while maintaining the same volume of drinking water production and pollutant removal from raw water. The mean potential energy savings for the 146 assessed DWTPs were estimated to be 0.005 kWh m^−3. Additionally, we analyzed the distribution of potential energy savings across the assessed DWTPs. The 25th percentile indicates that 25% of the facilities could achieve energy savings of 0.008 kWh per cubic meter water, while the 75th percentile suggests that 25% of the facilities could achieve energy savings of 0.146 kWh m^−3. Current energy intensity of DWTPs ranges between 0.002 kWh m^−3 and 0.215 kWh m^−3 with an average value of 0.007 kWh m^−3. The distribution of potential energy savings among DWTPs is shown in Supplementary Fig. 2 .

DWTPs with the lowest energy performance can save up to 2.45 kWh per cubic meter of water.

Factors influencing energy efficiency of DWTPs

To gain a deeper understanding of the factors impacting the energy performance DWTPs, it is essential to assess how their operational characteristics influence the previously estimated energy efficiency scores. The findings of this analysis are presented in Table 1 . We check the existence of multicollinearity among the explanatory variables in the regression using the variance inflation factor (VIF) test. The estimated value of VIF was 2.31 indicating that there is no multicollinearity in the regression model. The results indicate that the age of the treatment plant, the source of raw water, and the type of treatment technology have a negative effect on energy efficiency. By contrast, the ownership of the facility does not statistically influence on its energy performance which is consistent with past research 15 , 26 , 27 .

Specifically, the study reveals that older DWTPs tend to have lower energy performance. This can be attributed to the lack of updates and improvements in energy-efficient equipment within these aging plants. Molinos-Senante and Sala-Garrido 15 found that newer facilities tend to exhibit better energy performance, suggesting a positive correlation between a facility’s age and its energy efficiency. Conversely, the study by Molinos-Senante and Maziotis 26 did not provide a definitive conclusion on how the age of a facility influences its energy efficiency, indicating that the relationship between these factors remains unclear and further research is needed. Furthermore, the analysis demonstrates that DWTPs relying on mixed water resources, such as both surface and groundwater, experience a decrease in energy efficiency. This implies that treating water from multiple sources necessitates extensive treatment processes, potentially leading to higher energy consumption. Consequently, this combination of surface and groundwater treatment may have a detrimental impact on overall energy performance. This finding is consistent with those reported by Molinos-Senante and Maziotis 26 whereas Molinos-Senante and Sala-Garrido 15 did not identify the source of raw water as an explanatory factor of energy efficiency of DWTPs.

When focusing on the primary technology used for treating raw water, there are statistically significant differences in energy efficiency scores. Figure 5 provides an illustration of these differences, highlighting that the most energy-efficient technology is PF. On the other hand, treatment plants utilize rapid gravity filters (RGF) technology to remove pollutants from raw water are the least energy-efficient. Treatment plants that utilize a combination of coagulation and flocculation (CF) and PF or RGF to purify water demonstrated slightly higher energy efficiency scores. However, there is still considerable room for improvement in their energy performance to catch up with plants utilizing more energy-efficient technologies. These findings present a partial divergence from the conclusions of Molinos-Senante and Sala-Garrido 10 , who conducted a metafrontier DEA assessment on DWTPs. Their study concluded that DWTPs utilizing a combination of CF and rapid gravity filtering (CF + RGF) were the most energy-efficient. They also found evidence that facilities employing RGF as their treatment process were the least energy-efficient. It is important to note that differences in methodologies, data sources, and specific contexts may contribute to variations in the results between studies.

DWTPs using PF are those with the best energetic performance.

Results from this study provide evidence that the evaluated DWTPs exhibit inadequate energy performance, highlighting significant opportunities to reduce energy consumption. Such reductions can lead to cost savings and help mitigate greenhouse gas emissions, particularly if the energy sources are non-renewable. Water managers and regulators can implement various actions and policies to enhance energy efficiency in water treatment processes (Fig. 6 ). Potential strategies could be categorized as follows:

Energy efficiency improvement can be achieved by applying a diverse range of approaches.

DWTPs can improve energy efficiency by both reducing energy use and by removing more pollutants from raw water. Focusing on the first alternative, as it has reported by Sowby et al. 28 , some practices for managing energy in DWTPs are: (i) Implementing optimized operational procedures can help minimize energy waste. This includes strategies such as optimizing flow rates, ensuring appropriate maintenance of equipment, and adopting efficient operating schedules; (ii) Energy recovery systems, such as energy-efficient pumps or turbines, can be installed to capture and utilize energy that would otherwise be wasted during the treatment process and (iii) Deploying advanced monitoring and control systems can enable real-time monitoring of energy consumption and process optimization. This allows operators to identify areas of energy inefficiency promptly and take corrective actions. Results from this study evidenced that older DWTPs tend to have lower energy performance. Thus, water managers can invest in modernizing treatment plant equipment to utilize more energy-efficient technologies. This may involve replacing outdated machinery with newer models that offer improved energy performance. Generation and transfer of specific knowledge is also a valuable tool for improving energy efficiency. Thus, training programs and awareness campaigns can educate DWTP staff about the importance of energy efficiency and provide them with the necessary knowledge and skills to identify energy-saving opportunities in their daily operations. Moreover, establishing platforms for collaboration and knowledge sharing among water managers, researchers, and industry experts can facilitate the exchange of best practices and innovative approaches to energy efficiency in water treatment processes. Finally, given the large room of Chilean DWTPs to improve energy efficiency, the Chilean water regulators can introduce incentives and policies to encourage water companies to prioritize energy efficiency. These can include offering financial incentives for adopting energy-efficient technologies or setting energy efficiency targets that must be met to ensure tariff adjustments or other benefits.

The policy implications of your study’s findings are indeed significant and can provide valuable guidance to stakeholders involved in water treatment processes. By employing a novel approach that combines machine learning and linear programming techniques, this study offers a visually intuitive way for water regulators to understand the maximum energy requirements for different pollutant removal scenarios. This can aid decision-making processes by providing clear insights into the energy implications of water treatment operations. The new method used overcomes overfitting issues often encountered in other efficiency techniques. As a result, the energy efficiency scores derived from this approach are more robust and reliable. This increased reliability can contribute to more informed decision-making, as water regulators can have greater confidence in the efficiency assessments provided. The analysis conducted identifies key factors influencing energy performance in water treatment processes. This knowledge enables water regulators to gain insights into the specific aspects that impact efficiency. For example, recognizing that newer treatment plants tend to be more energy-efficient can inform decisions regarding facility upgrades or replacements. Similarly, understanding the energy intensity associated with different water sources and treatment technologies can guide choices in resource allocation and process optimization. In this context, Sowby 29 empirically proved that those water utilities with energy management policies or plans use less energy. This correlation was attributed to the organization´s culture and operation and also to the identification of energy use as a relevant topic within the organization.

Energy performance assessment based on efficiency analysis tree

According to Esteve et al. 21 , let’s make the assumption that there is a vector of predictors variables, i.e., factors influencing energy use in DWTPs, defined as \({x}_{1},\ldots ,{x}_{m}\) with \({{\boldsymbol{x}}}_{{\boldsymbol{i}}}{{\in }}{{\boldsymbol{R}}}^{{\boldsymbol{m}}}\) . Let’s also assume that this set of variables are employed to predict a vector of response variables, i.e., energy used, defined as \(y,\ldots ,{y}_{n}\) with \({{\boldsymbol{y}}}_{{\boldsymbol{i}}}{{\in }}{{\boldsymbol{R}}}^{{\boldsymbol{n}}}\) . The EAT approach selects a predictor variable \(j\) and a threshold \({{\boldsymbol{s}}}_{{\boldsymbol{j}}}{{\in }}{{\boldsymbol{S}}}_{{\boldsymbol{j}}}\) where \({{\boldsymbol{S}}}_{{\boldsymbol{j}}}\) consists of the vector of potential thresholds for the variable \(j\) to split the dataset into the right and left node, \({t}_{R}\) and \({t}_{L}\) , respectively 22 . The mean squared error (MSE) is used to define the threshold and consequently, the right and left node. This is shown mathematically as follows:

where \(t\) shows the node of the regression tree, \(R(t)\) presents the MSE of each node \(t\) , \(n\) is the size of the sample, and \(y\left({t}_{L}\right)\) and \(y\left({t}_{R}\right)\) are the estimated values of the response variable \(y\) . Note that the nodes \({t}_{L}\) and \({t}_{R}\) present the left and right nodes of the tree, respectively. The value of the response variable is calculated using the data that goes to nodes, \({t}_{L}\) and \({t}_{R}\) .

The estimated values of the response variable for each node of the regression tree are calculated as follows 23 :

where \(T\) is the sub-tree that is derived using the EAT method, \(k\) is the number of splits, \(y({I}_{T\left({k|}{t}^{* }\to {t}_{L},{t}_{R}\right)}\left({t}_{L}\right))\) and \(y({I}_{T\left({k|}{t}^{* }\to {t}_{L},{t}_{R}\right)}\left({t}_{R}\right))\) is the set of leaf nodes of the regression tree generated after performing the \(k\) -th break that Pareto dominates node \({t}_{L}\) and \({t}_{R}\) 21 , 23 .

To avoid any overfitting problems, the EAT approach employs cross validation techniques to select the best regression tree 21 . Therefore, the production technology that is estimated takes the following form:

where \({d}_{{T}_{k}}\left(x\right)\) is the predictor estimator regarding the sub-tree \({T}_{k}.\)

The energy efficiency score, which is a synthetic index embracing energy use and pollutants removed from raw water, for each analyzed DWTP is derived after solving the following linear programming model:

where \(\theta\) is the energy efficiency score which is ranged between 0 and 1. We note that when energy efficiency score equals to one, then the unit is energy-efficient. A value lower than one indicates energy inefficiency. \({a}^{t},{d}_{{T}^{* }}({a}^{t}\) ) are locations in the input-output space for all \(t\in {T}^{* }\) where * presents the final sub-tree, and \(\lambda\) are intensity variables that are part of the process to construct the efficient frontier 30 .

Based on the energy efficiency scores estimated by using Eq. ( 4 ), potential energy savings if a DWTP was efficient can be estimated using the following equation:

where \({{Energy}}_{s}\) is the potential saving in energy and \({{Energy}}_{c}\) is the actual level of energy consumption of the evaluated DWTP.

In the second step of our analysis, we examine the relationship between energy efficiency scores of DWTPs and their operating characteristics. To accomplish this, we utilize bootstrap truncated regression techniques, as proposed by Simar and Wilson 31 . The choice of employing a truncated regression approach is motivated by the fact that energy efficiency scores are bounded between zero and one. This approach allows us to account for this bounded nature and ensure that the estimated relationship is valid within this range.

By using bootstrap regression techniques, we can mitigate any potential issues related to serial correlation among efficiency scores, error terms, and the operating characteristics. This is an improvement over the standard Tobit regression approach, which may encounter difficulties when dealing with such correlations 31 . The bootstrap method provides a robust framework for analyzing the relationship between energy efficiency scores and the various operating characteristics of DWTPs in our study.

Mathematically, the regression model takes the following form:

where \({\theta }_{i}\) is the EAT energy efficiency score, \({\mu }_{0}\) is the constant term, \({{\boldsymbol{\xi }}}_{{\boldsymbol{i}}}^{{\boldsymbol{{\prime} }}}\) is the set of operating characteristics of any DWTP \(i\) , and \({\mu }_{i}\) are parameters that are estimated. Finally, \({v}_{i}\) is the error term which is distributed following the standard normal distribution 24 .

Case study and variables used

The case study conducted in Chile focuses on assessing the energy performance of 146 DWTPs in the country. The study focused on assessing the energetic performance of water treatment facilities excluding energy use for raw water abstraction. It is important to note that the water industry in Chile operates under a system of private ownership, which was established during the privatization process between 1998 and 2004. Two types of water companies emerged from this process: full private water companies, responsible for the long-term operation and maintenance of the water network, and concessionary water companies, tasked with supplying water for a specific period, typically around 30 years 32 . Due to the monopolistic nature of the water sector, a national regulator called the Superintendencia de Servicios Sanitarios (SISS) was established. This regulatory body is responsible for setting water tariffs for customers, using an efficient company standard as a benchmark 33 . Additionally, the national regulator, SISS, monitors the environmental performance of the water sector. The Ministry of Health establishes quality standards that must be met by treated water before it is distributed to end-users for consumption. These quality standards are based on guidelines set by the World Health Organization 10 .

The selection of predictor and response variables is based on past literature on this topic and data availability 34 , 35 , 36 , 37 , 38 , 39 . The response variable is captured by the energy consumption and is measured in kWh per year. In order to account for the removal of pollutants during the water treatment process and consider its impact on energy efficiency, our study incorporates four quality adjusted predictor variables. Following past practice 10 , 40 , 41 , they are estimated as follows:

where \({{Volume}}_{w}\) denotes the volume of drinking water produced and is measured in m 3 per year; \({{Pollutant}}_{\sin }\) is the concentration of the pollutant \(s\) in the influent and \({{Pollutant}}_{{sef}}\) is the concentration of the pollutant \(s\) in the effluent. Pollutant concentrations are measured in g/m 3 . This study employs four quality adjusted predictors because four pollutants are removed during the water treatment process. The four pollutants considered are sulfates, turbidity, arsenic and total dissolved solids. The four pollutants considered are sulfates, turbidity, arsenic and total dissolved solids. This selection is based on their significant impact on the energy consumption of Chilean DWTPs 10 , 25 .

Regarding operational characteristics influencing the energetic performance of DWTPs, the following variables are considered: (i) age of the DWTP measured in years; (ii) source of the raw water treated (surface water; groundwater or mixed water resources, which involves groundwater and surface water blending before its treatment); (iii) ownership of the DWTP which is captured through the use of a dummy variable, i.e., whether the treatment plant owned by a full private or concessionary water company and; (iv) the type of treatment technology used in the DWTPs, i.e., PF ( n  = 66), RGF ( n  = 36), CF-PF ( n  = 18) and CF-RGF ( n  = 26). The pretreatment of all facilities assessed is a simple screening process and all use chlorine for water disinfection. The descriptive statistics of the variables used in our analysis are reported in Table 2 . Data of the variables (predictor, response variables, and operational characteristics of DWTPs) was provided by the Chilean Urban Water Regulator (SISS) requested under the right to public information in Chile and correspond to 2020.

Data availability

The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Code availability

The codes generated and/or used during the current study are available from the corresponding author upon reasonable request.

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Acknowledgements

This work has been supported by project CL-EI-2021-07 funded by the Regional Government of Castilla y León and the EU-FEDER and projects TED-130807A-100 and CNS2022-135573 funded by MCIN/AEI/10.13039/501100011033 and by the “European Union NextGenerationEU/PRTR”.

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Alexandros Maziotis

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Maria Molinos-Senante

Centro de Desarrollo Urbano Sustentable ANID/FONDAP/15110020, Av. Vicuña Mackenna, 4860, Santiago, Chile

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Alexandros Maziotis: Methodology; Sofware; Writing—Original Draft. María Molinos-Senante: Conceptualization; Visualization; Writing—Review & Editing.

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Maziotis, A., Molinos-Senante, M. Understanding energy performance in drinking water treatment plants using the efficiency analysis tree approach. npj Clean Water 7 , 13 (2024). https://doi.org/10.1038/s41545-024-00307-8

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• Remediation Evaluation Model for Fuel Hydrocarbons (REMFuel) REMfuel is n analytical solution for simulating the transient effects of groundwater source and plume remediation for fuel hydrocarbons. REMFuel can also simulate zero order or Monod's kinetics for decay of fuel components in the plume. The decay rates and other reaction coefficients are variable functions of time and distance in the plume. This approach allows for flexible simulation of enhanced plume remediation that may be temporary in time, limited in space, and which may have different effects on different contaminant species in the plume.
• Regulatory and Investigative Treatment Zone (RITZ) Model RITZ is a screening model for simulation of unsaturated zone flow and transport of oily wastes during land treatment. RITZ was developed to help decision makers systematically estimate the movement and fate of hazardous chemicals during land treatment of oily wastes. The model considers the downward movement of the pollutant with the soil solution, volatilization, and loss to the atmosphere and degradation. The model incorporates the influence of oil upon the transport and fate of the pollutant.
• Vadose Zone Leaching (VLEACH) Model VLEACH  is a one-dimensional, finite difference model for making preliminary assessments of the effects on groundwater from the leaching of volatile, sorbed contaminants through the vadose zone.  In an individual run, it can simulate leaching in a number of distinct polygons, which may differ in terms of soil properties, recharge rates, depth of water, or initial conditions. Modeling results in an overall, area-weighted assessment of ground-water impact.

Water Quality: Pathogens and Nutrient Loading

• Virtual Beach (VB) VB is a software package designed for developing site-specific statistical models for the prediction of pathogen indicator levels at recreational beaches. It is primarily designed for beach managers responsible for making decisions regarding beach closures due to pathogen contamination. However, others interested in studying relationships between water quality indicators and ambient environmental conditions will find VB useful.
• Water Quality Analysis Simulation Program (WASP) WASP is a spatially and temporally dynamic, mechanistic modeling framework that can assist states by simulating solids and contaminants in the surface water and the underlying sediment layers, with flexibility to handle different complexities of such systems as ponds, lakes, streams, rivers and estuaries. WASP has been widely applied in the development of TMDLs. EPA’s Office of Wastewater Management routinely uses this model to address nitrogen (N) and phosphorus (P) loadings.

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• Toxicity Forecaster (ToxCast) ToxCast generates data and predictive models on thousands of chemicals of interest to the EPA, and uses high-throughput screening methods and computational toxicology approaches to rank and prioritize chemicals. It has data on over 1,800 chemicals from a broad range of sources including industrial and consumer products, food additives, and potentially "green" chemicals that could be safer alternatives to existing chemicals. ToxCast screens chemicals in over 700 high-throughput assays that cover a range of high-level cell responses and approximately 300 signaling pathways.

Drinking Water Treatment and Security

• Breakpoint Chlorination Simulator for Drinking Water Systems   EXIT Although a reasonable reaction scheme that describes inorganic chloramine formation and decay over a range of conditions applicable to drinking water exists, a widely and freely accessible one does not. This web-based application relevant to drinking water practice was developed to assist water utilities in generating chlorine breakpoint curves. The simulator generates two side-by-side breakpoint curves for comparison purposes with user defined conditions. 
• CANARY   CANARY software evaluates standard water quality data (e.g., free chlorine, pH, total organic carbon) over time and uses mathematical and statistical techniques to identify the onset of anomalous water quality incidents. Before using CANARY for the first time, historical utility data must be used to determine the natural variation of these water quality parameters. This allows the water utility to adapt CANARY to work accurately at multiple locations within the water distribution system and helps utility operators to understand the expected false alarm rates associated with CANARY and contamination incident detection. 
• Chloramine Formation and Decay Simulator for Drinking Water Systems   EXIT Although a reasonable reaction scheme that describes inorganic chloramine formation and decay over a range of conditions applicable to drinking water exists, a widely and freely accessible one does not. This web-based application relevant to drinking water practice was developed to simulate inorganic chloramine formation and subsequent stability, including a simple inorganic chloramine demand reaction for organic matter. It provides two side-by-side simulations and associated graphs to allow comparison of input choices on chloramine formation and decay. 
• Threat Ensemble Vulnerability Assessment – Sensor Placement Optimization Tool (TEVASPOT) Graphical User Interface TEVA-SPOT is used by water utilities to optimize the number and location of contamination detection sensors so that economic and/or public health consequences are minimized. It is interactive, allowing a user to specify the minimization objective and specify constraints. Installation and maintenance costs for sensor placement can also be factored into the analysis.

Ecosystem Services and Restoration

• Causal Analysis/Diagnosis Decision Information System (CADDIS) CADDIS is a web-based technical support system for implementing the stressor identification process for determining environmental causes. Biological indices are the principal monitoring tool for evaluating the biological condition of water bodies in all 50 states, many territories and tribal lands. Yet when a biological assessment indicates a problem, it may not be readily apparent what caused the problem. CADDIS provides states a causal assessment framework by which data and other information are organized and evaluated, using quantitative and logical techniques, to determine the likely cause of an observed condition needed to identify appropriate remediation or restoration actions. 
• EPA H2O EPA H2O is a desktop GIS based decision support tool for assessing the provision of ecosystem services under different land use scenarios. Users can explore the spatial arrangement of ecosystem goods and services at regional to local scales, complete spatial queries along hydrological networks, and generate customized reports for scenario comparisons, all to gain a better understanding of where ecosystem services are produced and how land use change might affect future production. This tool can be used by any community as long as they can develop their local database. States can generate pdf summary reports of what is in both an area of interest and areas upstream or connected via transportation network, compare different alternative future land use scenarios for an area, and generate custom designed scenarios including changing the placement and shape of land use parcels as well as modifying the monetary value benefit functions.
• Rapid Benefit Indicators (RBI) Approach The RBI approach is an easy-to-use process for assessing restoration sites using non-monetary benefit indicators. It uses readily-available data to estimate and quantify benefits to people around an ecological restoration site. Whether you are a federal, state, or local manager, or a member of an interest group or funding organization, this simple yet powerful site analysis will allow you and your stakeholders to systematically and equitably incorporate social benefits in restoration decisions.

​Geostatistical Analysis

• Geostatistical Environmental Assessment (GEOEAS)  GEOEAS is a collection of interactive software tools for performing two-dimensional geostatistical analyses of spatially distributed data. The principal function of the package is the production of grids and contour maps of interpolated (kriged) estimates from sample data. GEOEAS can produce data maps, univariate statistics, scatter plots/linear regression, and variogram computation and model fitting.
• Geostatistical Software Package (GEOPACK) GEOPACK is a comprehensive geostatistical software package that allows both novice and advanced users to undertake geostatistical analyses of spatially correlated data. It allows users to incorporate additional programs at a later date without having to alter previous programs or recompile the entire system.

Green Infrastructure and Stormwater Management 

• Green Infrastructure Modeling Toolkit This toolkit contains five EPA green infrastructure models and tools, along with manuals, a summary video, facts sheets, and a green infrastructure brochure. It can be used as a teaching tool and as a quick reference resource for use by planners and developers when making green infrastructure implementation decisions, and can also be used for low impact development design competitions.  
• Green Infrastructure Wizard (GIWiz) GIWiz is an interactive web application that provides users with customized reports containing the EPA tools and resources they select, direct links, and overview information about each.
• Retention Curve (RETC) Computer Program RETC  is a program for analyzing the hydraulic conductivity properties of unsaturated soils. The parametric models of Brooks-Corey and van Genuchten are used to represent the soil water retention curve. The theoretical pore-size distribution models of Mualem and Burdine predict the unsaturated hydraulic conductivity function. The simulation can be generated from observed soil water retention data, assuming that one observed conductivity value (not necessarily at saturation) is available. The program also permits users to fit analytical functions simultaneously to observed water retention and hydraulic conductivity data.
• Sanitary Sewer Overflow Analysis and Planning (SSOAP) Toolbox SSOAP is a suite of computer software tools used for the quantification of rainfall-derived infiltration and inflow and capacity analysis and condition assessment of sanitary sewer systems. This toolbox includes EPA’s  Storm Water Management Model Version 5 (SWMM5)  for performing dynamic routing of flows through the sanitary sewer systems.
• Watershed Management Optimization Support Tool (WMOST) WMOST is a software application designed to facilitate integrated water resources management across wet and dry climate regions. It allows water resources managers and planners to screen a wide range of practices across their watershed or jurisdiction for cost-effectiveness and environmental and economic sustainability. WMOST allows users to select up to fifteen stormwater management practices, including traditional grey infrastructure, green infrastructure, and other low impact development practices.

Watershed Management

• Automated Geospatial Watershed Assessment Tool (AGWA) AGWA is a geographic information systems (GIS) interface designed to help manage and analyze watershed water quantity and quality. Developed by EPA, the U.S. Department of Agriculture, and the University of Arizona, the tool is designed to provide qualitative estimates of runoff and erosion relative to landscape change. 
• Estuary Data Mapper (EDM) EDM is a downloadable application that can help states view and access data for estuary-scale geographical regions of interest. Data types include nitrogen sources and loads for coastal watersheds and estuaries, including atmospheric deposition, point source loads and nonpoint source loads as well as response endpoints, such as seagrass and chlorophyll a.   
• Watershed Deposition Tool (WDT) WDT is a software tool for mapping deposition estimates from the Community Multi-scale Air Quality (CMAQ) model to watersheds. It provides users with the linkage of air and water needed for the total maximum daily load (TMDL) and related nonpoint-source watershed analyses. WDT takes gridded atmospheric deposition estimates from CMAQ and allocates them to user-defined hydrologic cataloging units of rivers and streams within a watershed, state or region and provides output as text or shapefiles.
• Watershed Health Assessment Tools Investigating Fisheries (WHATIF) WHATIF is software that integrates a number of calculators, tools, and models for assessing the health of watersheds and streams with an emphasis on fish communities in the Mid-Atlantic Highland region.
• Chemical and Product Categories (CPCat) Database CPCat is a database containing information mapping more than 43,000 chemicals to a set of terms categorizing their usage or function. The comprehensive list of chemicals with associated categories of chemical and product use was compiled from publically available sources. Unique use category taxonomies from each source are mapped onto a single common set of approximately 800 terms. Users can search for chemicals by chemical name, Chemical Abstracts Registry Number, or by CPCat terms associated with chemicals.

Drinking Water Treatment

• Drinking Water Treatability Database (TDB) The TDB presents referenced information on the control of contaminants in drinking water. It allows drinking water utilities, first responders to spills or emergencies, treatment process designers, research organizations, academicians, regulators and others to access referenced information gathered from thousands of literature sources and assembled on one site. Over time, the TDB will expand to include over 200 regulated and unregulated contaminants and their contaminant properties. It includes more than 25 treatment processes used by drinking water utilities. 

Drinking Water and Wastewater Infrastructure

• National Database Structure for Life Cycle Performance Assessment of Water and Wastewater Rehabilitation Technologies (Retrospective Evaluation)  (Registration required)  Exit This database houses performance evaluation data for rehabilitation technologies used in the water and wastewater sectors on a national basis, including additional cured-in-place pipe liner testing. The database will improve the capability of utilities to sustainably manage their aging and deteriorating water distribution, stormwater and wastewater collection systems, and will help increase acceptance of new and innovative technologies by decision makers who adopt, regulate, and design infrastructure technologies. The databases can also assist utilities to more effectively implement comprehensive asset management, provide reliable service to their customers, and meet their Clean Water Act and Safe Drinking Water Act requirements.
• Water Infrastructure Database (WATERiD)   Exit WATERiD is a database used for helping utilities choose the best pipe rehabilitation, condition assessment, and pipe-location determining technologies for both wastewater conveyance systems and drinking water distribution systems. It includes primary information about individual renewal technologies' cost and performance, case studies for their real world applications, and the list of vendors, consultants, and contractors available for a particular technology on a regional basis. The database allows utilities to input their experiences in these areas for the benefit of other utilities.

Ecosystems and Watersheds

• Freshwater Biological Traits Database (Traits) This database contains traits data for 3,857 North American macroinvertebrate taxa, and includes habitat, life history, mobility, morphology and ecological trait data. These data were compiled for a project on climate change effects on river and stream ecosystems.
• Stream-Catchment (StreamCat) Dataset StreamCat is an extensive collection of landscape metrics for 2.6 million streams and associated catchments within the conterminous U.S. It includes both natural and human-related landscape features. The data are summarized both for individual stream catchments and for cumulative upstream watersheds. StreamCat data are being utilized to develop national maps of aquatic condition and watershed integrity, and can be used to model and predict reference condition for the National Rivers and Streams Assessment. The data will also be useful to states that are conducting similar assessments .
• GeoPlatform Best Management Practice Performance Database This database allows users to see how well certain BMPs and low impact development (LID) approaches control stormwater runoff in different parts of the U.S. This web map displays sites where the performance of BMPs and LID approaches for controlling stormwater runoff have been monitored and reported on. Each site also contains a link back to the database where more detailed information on the site and its performance can be found.

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Overview of Drinking Water Treatment Technologies

On this page:, granular activated carbon, packed tower aeration, multi-stage bubble aeration, anion exchange, cation exchange, biological treatment, reverse osmosis/nanofiltration, adsorptive media.

• Ultraviolet Photolysis and Advanced Oxidation Processes

Caustic Feed

Phosphate feed, nontreatment options, what is granular activated carbon.

Granular activated carbon (GAC) is a porous adsorption media with extremely high internal surface area. GACs are manufactured from a variety of raw materials with porous structures including:

• bituminous coal
• lignite coal
• coconut shells

Physical and/or chemical manufacturing processes are applied to these raw materials to create and/or enlarge pores.  This results in a porous structure with a large surface area per unit mass.

Why is it useful?

GAC is useful for the removal of taste- and odor-producing compounds, natural organic matter, volatile organic compounds (VOCs), synthetic organic compounds and disinfection byproduct precursors. Organic compounds with high molecular weights are readily adsorbable.

Treatment capacities for different contaminants vary depending on the properties of the different GACs, which in turn vary widely depending on the raw materials and manufacturing processes used.

What are the advantages of using GAC?

GAC is a proven technology with high removal efficiencies (up to 99.9%) for many VOCs, including trichloroethylene (TCE) and tetrachloroethylene (PCE). In most cases, GAC can remove target contaminants to concentrations below 1 µg/l. Another advantage is that regenerative carbon beds allow for easy recovery of the adsorption media.

What are the disadvantages of using GAC?

The media has to be removed and replaced or regenerated when GAC capacity is exhausted. In some cases, disposal of the media may require a special hazardous waste handling permit. Other adsorbable contaminants in the water can reduce GAC capacity for a target contaminant.

How can the WBS model for GAC be used?

The work breakdown structure (WBS) model can estimate costs for two types of GAC systems where:

• the GAC bed is contained in pressure vessels in a treatment configuration similar to that used for other adsorption media (for example, activated alumina), referred to as pressure GAC
• the GAC bed is contained in open concrete basins in a treatment configuration similar to that used in the filtration step of conventional or direct filtration, referred to as gravity GAC

The WBS model for GAC includes standard designs to estimate costs for treatment of a number of different contaminants, including atrazine and various VOCs. The WBS model can also be used to estimate the cost of GAC treatment for removal of other contaminants.

To simulate the use of GAC for treatment of other contaminants, users will need to adjust default inputs (for example, bed volumes before breakthrough, bed depth) and, potentially, critical design assumptions (for example, minimum and maximum loading rates).

Where can I find more information on GAC?

The technical report Work Breakdown Structure-Based Cost Model for Granular Activated Carbon Drinking Water Treatment Technologies discusses GAC technology in detail.

What is packed tower aeration?

Aeration processes, in general, transfer contaminants from water to air. Packed tower aeration (PTA) uses towers filled with a packing media designed to mechanically increase the area of water exposed to non-contaminated air. Water falls from the top of the tower through the packing media while a blower forces air upwards through the tower. In the process, volatile contaminants pass from the water into the air.

PTA is useful for removing volatile contaminants including:

• Volatile organic compounds (VOCs)
• Disinfection byproducts
• Hydrogen sulfide
• Carbon dioxide
• Other taste- and odor-producing compounds

The more volatile the contaminant, the more easily PTA will remove it. PTA readily removes the most volatile contaminants, such as vinyl chloride. With sufficient tower height and air flow, PTA can even remove somewhat less volatile contaminants, such as 1,2-dichloroethane.

What are the advantages of using PTA?

PTA is a proven technology and can achieve high removal efficiencies (99 percent or greater) for most VOCs. PTA removal efficiency is independent of starting concentration. Therefore, it can remove most volatile contaminants to concentrations below 1 µg/L. PTA generates no liquid or solid waste residuals for disposal.

What are the disadvantages of using PTA?

Depending on the location and conditions, air quality regulations might require the use of air pollution control devices with PTA, increasing the technology cost. PTA uses tall towers that could be considered unsightly in some communities. Under certain water quality conditions, scaling or fouling of the packing media can occur if precautions are not taken.

How can the WBS model for PTA be used?

The work breakdown structure (WBS) model for PTA includes standard designs to estimate costs for treatment of a number of different contaminants, including methyl tertiary-butyl ether (MTBE) and various VOCs. However, the WBS model can be used to estimate the cost of PTA treatment for removal of other contaminants as well.

To simulate the use of PTA for treatment of other contaminants, users will need to adjust default inputs (for example, Henry’s coefficient, molecular weight) and, potentially, critical design assumptions (for example, minimum and maximum packing height).

Where can I find more information on PTA?

The technical report Work Breakdown Structure-Based Cost Model for Packed Tower Aeration Drinking Water Treatment Technologies discusses PTA technology in detail.

What is multi-stage bubble aeration?

Aeration processes, in general, transfer contaminants from water to air. Multi-stage bubble aeration (MSBA) uses shallow basins that are divided into smaller compartments, or stages, using baffles.

Inside each stage, diffusers (consisting of perforated pipes or porous plates) release small air bubbles that rise through the water. The bubbles and their resulting turbulence cause volatile contaminants to pass from the water into the air.

MSBA is useful for removing volatile contaminants including:

The more volatile the contaminant, the more easily MSBA will remove it. Vendors supply MSBA in skid-mounted, pre-packaged systems that can be particularly suitable for small systems.

What are the advantages of using MSBA?

MSBA is a proven technology. In recent EPA pilot tests, MSBA achieved high removal efficiencies (98 percent to greater than 99 percent) for most VOCs, removing them to concentrations below 1 µg/L. MSBA is a low-profile aeration technology that does not require tall, potentially unsightly towers. MSBA generates no liquid or solid waste residuals for disposal.

What are the disadvantages of using MSBA?

Depending on the location and conditions, air quality regulations might require the use of air pollution control devices with MSBA, increasing the technology cost.

MSBA is less efficient at removing contaminants than packed tower aeration, requiring high air flow rates to remove the most recalcitrant VOCs. Treating large water flows with MSBA can require a large number of basins. This might not be practical for large systems.

How can the WBS model for MSBA be used?

The work breakdown structure (WBS) model for MSBA includes standard designs for the treatment of a number of contaminants, including various VOCs. However, the WBS model can be used to estimate the cost of MSBA treatment for removal of other volatile contaminants as well.

To simulate the use of MSBA for treatment of other contaminants, users will need to adjust default inputs (for example, air-to-water ratio, number of stages) and, potentially, critical design assumptions (for example, maximum air surface intensity).

Where can I find more information on MSBA?

The technical report Work Breakdown Structure-Based Cost Model for Multi-stage Bubble Aeration Drinking Water Treatment Technologies discusses MSBA technology in detail.

What is anion exchange?

In an anion exchange treatment process, water passes through a bed of synthetic resin. Negatively charged contaminants in the water are exchanged with more innocuous negatively charged ions, typically chloride, on the resin’s surface.

Anion exchange is useful for the removal of negatively charged contaminants including arsenic, chromium-6, cyanide, nitrate, perchlorate, per- and polyfluoroalkyl substances (PFAS), sulfate, and uranium.

Treatment capacities for different contaminants vary depending on the properties of the resin used and characteristics of the influent water. Several of vendors manufacture different resins, including those designed to selectively remove specific contaminant ions.​

What are the advantages of using anion exchange?

Anion exchange is a proven technology that can achieve high removal efficiencies (greater than 99 percent) for negatively charged contaminants. When the capacity of the resin is exhausted, it can be regenerated to restore it to its initial condition. The regeneration process uses a saturated solution, usually of sodium chloride (also known as brine). An alternative to regeneration is to dispose of the exhausted resin and replace it with fresh resin. This alternative is often employed when selective resins are used to remove perchlorate or PFAS.

What are the disadvantages of using anion exchange?

The spent regenerant brine is a concentrated solution of the removed contaminants and will be high in dissolved solids and excess regenerant ions (e.g., sodium, chloride). This waste stream will require disposal or discharge. Anion exchange treatment also can lower the pH of the treated water and, therefore, may require post-treatment corrosion control. When replacement with fresh resin is used as an alternative to regeneration, the spent resin, loaded with removed contaminants, will require disposal. In some cases, disposal of the resin may require a special hazardous waste handling permit.

How can the WBS model for anion exchange be used?

The primary work breakdown structure (WBS) model for anion exchange includes standard designs to estimate costs for treatment of arsenic and nitrate. EPA has developed separate WBS models, also available on this page, to estimate costs for treatment of perchlorate and PFAS. In addition, the WBS anion exchange models can be used to estimate the cost of anion exchange treatment for removal of other contaminants.

To simulate the use of anion exchange for treatment of other contaminants, users will need to adjust default inputs (for example, bed volumes before regeneration, bed depth) and, potentially, critical design assumptions (for example, minimum and maximum loading rates).

Where can I find more information on anion exchange?

The technical report Work Breakdown Structure-Based Cost Model for Anion Exchange Drinking Water Treatment discusses anion exchange technology in detail.

What is cation exchange?

In a cation exchange treatment process, water passes through a bed of synthetic resin. Positively charged contaminants in the water are exchanged with more innocuous positively charged ions, typically sodium, on the resin’s surface.

Cation exchange is useful for water softening by removing hardness ions such as calcium and magnesium. It can also remove other positively charged contaminants including barium, radium and strontium.

Treatment capacities for different contaminants vary depending on the properties of the resin used and characteristics of the influent water. A number of vendors manufacture different resins, including those designed to selectively remove specific contaminant ions.​

What are the advantages of using cation exchange?

Cation exchange is a proven technology for water softening and removal of positively charged contaminants. It can achieve high removal efficiencies (greater than 99 percent) for positively charged contaminants. When the capacity of the resin is exhausted, it can be regenerated to restore it to its initial condition. The regeneration process uses a saturated solution, usually of sodium chloride (also known as brine).

What are the disadvantages of using cation exchange?

The spent regenerant brine is a concentrated solution of the removed contaminants and also will be high in dissolved solids and excess regenerant ions (e.g., sodium, chloride). This waste stream will require disposal or discharge.

How can the WBS model for cation exchange be used?

The work breakdown structure (WBS) model for cation exchange includes standard designs for water softening. The same designs may also be appropriate for radium removal. The WBS model can also be used to estimate the cost of cation exchange treatment for removal of other contaminants.

To simulate the use of cation exchange for treatment of other contaminants, users will need to adjust default inputs (for example, bed volumes before regeneration, bed depth) and, potentially, critical design assumptions (for example, minimum and maximum loading rates).

Where can I find more information on cation exchange?

The technical report Work Breakdown Structure-Based Cost Model for Cation Exchange Drinking Water Treatment discusses cation exchange technology in detail.

What is biological treatment?

Biological treatment of drinking water uses indigenous bacteria to remove contaminants. The process has a vessel or basin called a bioreactor that contains the bacteria in a media bed. As contaminated water flows through the bed, the bacteria, in combination with an electron donor and nutrients, react with contaminants to produce biomass and other non-toxic by-products. In this way, the biological treatment chemically “reduces” the contaminant in the water.

Biological treatment is useful for the removal of contaminants including nitrate and perchlorate. Following a startup period, the bacterial population in the water will adapt to consume the target contaminants as long as favorable conditions, such as water temperature and electron donor and nutrient concentrations, are maintained.

What are the advantages of using biological treatment?

Biological treatment can achieve high removals (greater than 90 percent) of nitrate and perchlorate. The process destroys contaminants, as opposed to removing them, and, therefore, does not produce contaminant-laden waste streams. Biological treatment remains effective even in the presence of certain co-occurring contaminants.

What are the disadvantages of using biological treatment?

An active bioreactor will have a continuous growth of biomass that needs to be periodically removed. Although the excess biomass will not be contaminant-laden, it still requires disposal. Also, biological treatment adds soluble microbial organic products and can deplete the oxygen in treated water. Post-treatment processes are needed to control these effects.

How can the WBS model for biological treatment be used?

The work breakdown structure (WBS) model can estimate costs for anoxic biological treatment using three types of bioreactors:

• pressure vessels with a fixed media bed
• open concrete basins with a fixed media bed
• pressure vessels with a fluidized media bed.

The WBS model for biological treatment includes standard designs for perchlorate and nitrate treatment. However, the model can also be used to estimate the cost of biological treatment for the removal of other contaminants.

To simulate the use of biological treatment for other contaminants, users will need to adjust default inputs (e.g., electron donor and nutrient doses) and critical design assumptions (e.g., minimum and maximum loading rates).

Where can I find more information on biological treatment?

The technical report Work Breakdown Structure-Based Cost Model for Biological Drinking Water Treatment discusses the technology in detail.

What are reverse osmosis and nanofiltration?

Reverse osmosis (RO) and nanofiltration (NF) are membrane separation processes that physically remove contaminants from water. These processes force water at high pressure through semi-permeable membranes that prevent the passage of various substances depending on their molecular weight. Treated water, also known as permeate or product water, is the portion of flow that passes through the membrane along with lower molecular weight substances. Water that does not pass through the membrane is known as concentrate or reject and retains the higher molecular weight substances, including many undesirable contaminants.

Why are they useful?

RO and NF are useful for the removal a wide range of contaminants. RO can remove contaminants including many   inorganics, dissolved solids, radionuclides and synthetic organic chemicals. RO can also be used for removing salts from brackish water or sea water. NF is useful for removal of hardness, color and odor compounds, synthetic organic chemicals and some disinfection byproduct precursors.

What are the advantages of using RO and NF?

RO and NF are proven technologies that can achieve high removals of a broad range contaminants at once. They do not selectively target individual contaminants and remain effective for water that contains mixtures of contaminants. The processes do not usually require adjustment based on the specific trace contaminants present.

What are the disadvantages of using RO and NF?

RO and NF reject part of the feed water (15 to 30 percent) that enters the process. This “loss” of water as concentrate can present a problem when water is scarce. Furthermore, this large volume concentrate stream is laden with removed contaminants, salts and dissolved solids and will require discharge or disposal. Also, the high pressures used in these treatment processes can result in significant energy consumption. Pre-treatment processes are frequently required to prevent membrane fouling or plugging. Finally, RO can lower the pH of treated water and, therefore, may require post-treatment corrosion control.

How can the WBS model for RO and NF be used?

The work breakdown structure (WBS) model can estimate costs for either RO or NF. It includes standard designs for feed waters of various quality in terms of gross chemical composition (e.g., salt concentrations). The design parameters typically do not require adjustment to target a specific trace contaminant, other than selecting the appropriate type of membrane (e.g., RO or NF) given the contaminant’s molecular weight and other characteristics.

Where can I find more information on RO and NF?

The technical report Work Breakdown Structure-Based Cost Model for Reverse Osmosis/Nanofiltration Drinking Water Treatment discusses these technologies in detail.

What is adsorptive media?

Adsorptive water treatment technologies involve passing contaminated water through a media bed. The contaminants in the water adsorb to empty pore spaces on the surface of the adsorptive media as the water passes through. Granular activated carbon (GAC), described above, is one type of adsorptive media, but other types exist, including aluminum-based, iron-based, titanium-based, zirconium-based and other types of media.

Adsorptive media treatment is useful for removal of inorganic contaminants including antimony, arsenic, beryllium, fluoride, selenium, thallium, and uranium. The capacity of the media to adsorb different contaminants depends on the specific type of media used, the water chemistry (e.g., pH), and contaminant valence.

What are the advantages of using adsorptive media?

Adsorptive media is a proven technology with high removal efficiencies for certain inorganic contaminants (e.g., up to greater than 99% for arsenic, up to 99% or more for fluoride). When the appropriate media is used in combination with the appropriate water quality conditions (e.g., pH), the process can remove selected target contaminants to concentrations below relevant regulatory limits. Another advantage is that some types of adsorptive media can be regenerated in place after their capacity is exhausted. The regeneration process typically uses an acid wash, followed by a caustic wash.

What are the disadvantages of using adsorptive media?

The media has to be removed and replaced or regenerated when its adsorptive capacity is exhausted. When regeneration is employed, the spent regenerant is a concentrated solution of the removed contaminants and will require disposal or discharge. When replacement with fresh media is used as an alternative to regeneration, the spent media, loaded with removed contaminants, will require disposal. In some cases, disposal of the media may require a special hazardous waste handling permit.

How can the WBS model for adsorptive media be used?

The work breakdown structure (WBS) model can estimate costs for the following combinations of media and target contaminant:

• Conventional activated alumina for removal of arsenic
• Conventional activated alumina for removal of fluoride
• Iron-modified activated alumina (also known as AAFS-50) for removal of arsenic
• Granular ferric oxide (GFO) for removal of arsenic
• Granulated ferric hydroxide (GFH) for removal of arsenic.

The WBS model can also estimate costs for treatment using alternative media and/or other contaminants, if the user provides appropriate assumptions about the media and adjusts default inputs (e.g., bed volumes before breakthrough, bed depth).

The technical report Work Breakdown Structure-Based Cost Model for Adsorptive Media Drinking Water Treatment discusses the technology in detail.

Ultraviolet Photolysis and Advanced Oxidation Processes (UVAOP)

What is ultraviolet photolysis and advanced oxidation.

Ultraviolet (UV) light can be used on its own (in photolysis), or in combination with chemical addition (in UV advanced oxidation), to reduce the concentration of organic contaminants. In UVAOP drinking water treatment, water passes through a reactor vessel equipped with lamps that emit UV light. In photolysis, the contaminants are degraded by the photons emitted by the UV lamps. Advanced oxidation adds chemicals such as hydrogen peroxide (H 2 O 2 ) or chlorine. These chemicals react with the UV light to generate radicals (such as hydroxyl) that in turn oxidize the contaminants.

UVAOP is useful to reduce the concentration of organic micropollutants that may be difficult to address with other technologies including 1,4-dioxane, N-nitrosodimethylamine (NDMA), and methyl tert-butyl ether (MTBE). The process can also be useful for treatment of taste and odor issues. The effectiveness of the process depends on the UV dose, chemical dose (in advanced oxidation), contact time, concentration of the target contaminants, and other water quality parameters (e.g., UV transmittance, presence of radical scavengers).

What are the advantages of using UVAOP?

UVAOP can achieve high removal efficiencies for 1,4-dioxane (up to greater than 99%) and MTBE (greater than 90%). The process destroys contaminants, as opposed to removing them, and therefore, does not produce contaminant-laden waste streams.

What are the disadvantages of using UVAOP?

UVAOP is non-selective and can oxidize non-target organic compounds present in the water. In some cases, this oxidation can increase the potential for formation of disinfection byproducts in the drinking water distribution system. Also, in advanced oxidation, the process will not completely consume the entire dosage of the chemical added. The presence of the excess chemical in the treated water may be of concern. Both of these disadvantages may require post-treatment using a process such as GAC. Finally, operating the UV lamps can consume significant electrical energy and the lamps themselves usually require periodic cleaning and replacement.

How can the WBS model for UVAOP be used?

The work breakdown structure (WBS) model can estimate costs for the following combinations of treatment processes and target contaminant:

• Treatment of 1,4-dioxane using UV and H 2 O 2 (UV/H 2 O 2 )
• Treatment of 1,4-dioxane using UV and chlorine (UV/Cl)
• Treatment of NDMA using direct photolysis.

The WBS model can also estimate costs for treatment of other contaminants by UV/ H 2 O 2 , UV/Cl, or direct photolysis, if the user adjusts default inputs (e.g., UV energy input, oxidant dose).

Where can I find more information on UVAOP?

The technical report Work Breakdown Structure-Based Cost Model for Drinking Water Treatment by Ultraviolet Photolysis and Advanced Oxidation Processes discusses the technology in detail.

What is caustic feed?

Caustic soda, also known as sodium hydroxide (NaOH), is sometimes added to drinking water to raise the water’s pH, making the water less acidic.

Caustic feed can be useful on its own to attain and maintain a desired pH and prevent downstream corrosion in a drinking water distribution system. It can also be useful following treatment processes that lower the pH of water to return the water to its original or more neutral pH. It may also be useful prior to certain treatment processes to optimize the pH of the water feeding those processes.

What are the advantages of using caustic feed?

Caustic soda is a liquid chemical that can rapidly change the pH of water without requiring extensive equipment for feeding and mixing.

What are the disadvantages of using caustic feed?

Concentrated caustic soda is harmful to human skin and therefore requires handling precautions and secondary containment. At higher concentrations, caustic soda will freeze at moderate temperatures (i.e., 50 percent solution freezes at 58 degrees Fahrenheit), so storage tanks may need to be indoors and/or equipped with special heating equipment. This disadvantage can be mitigated by using lower concentration caustic soda.

How can the WBS model for caustic feed be used?

The work breakdown structure (WBS) model can estimate costs for a process to add caustic soda into a water pipeline at an existing drinking water treatment plant. It includes several pre-defined scenarios of starting pH, target pH, and other water quality parameters. It can easily estimate costs for other scenarios if the user adjusts default inputs.

Where can I find more information on caustic feed?

The technical report Work Breakdown Structure-Based Cost Model for Caustic Feed Drinking Water Treatment discusses the technology in detail.

What is phosphate feed?

Phosphate-based chemicals, such as phosphoric acid, zinc orthophosphate, or others, are sometimes added to drinking water to control corrosion in a distribution system.

Phosphate addition is among the treatment strategies for compliance with the federal Lead and Copper Rule.

What are the advantages and disadvantages of using phosphate feed?

Phosphate corrosion control chemicals containing orthophosphate are believed to combine with lead and copper in plumbing materials to form insoluble compounds, thus reducing lead and copper release at the tap. The effectiveness of this process depends on chemical dosage and pH. However, the addition of these chemicals does not permanently eliminate sources of lead and copper release (e.g., service lines). Changes in influent water quality can require re-optimization of corrosion control practices. In addition, different phosphate chemical formulations have different advantages and disadvantages. For example, phosphoric acid is potentially cheaper than zinc orthophosphate, but is a strong acid that can require safety precautions.

How can the WBS model for phosphate feed be used?

The work breakdown structure (WBS) model can estimate costs for a process to add phosphoric acid or zinc orthophosphate into a water pipeline at an existing drinking water treatment plant. It can estimate the cost of phosphate feed using different chemical formulations, if the user provides appropriate inputs for the alternative chemical (e.g., solution strength, density, price).

The technical report Work Breakdown Structure-Based Cost Model for Phosphate Feed Drinking Water Treatment discusses the technology in detail.

What are nontreatment options?

Instead of treating a contaminated water source, nontreatment options replace the source with water that meets applicable drinking water standards. Examples include interconnection with another system and drilling a new well to replace a contaminated one.

Nontreatment can provide a route to compliance with drinking water standards for various contaminants, as long as an alternate water source is available.

What are the advantages of using nontreatment options?

Small water utilities, particularly those that lack financial and/or technical capacity, might be able to use nontreatment approaches to avoid the cost and labor associated with installing and operating new treatment processes.

What are the disadvantages of using nontreatment options?

Interconnection requires a neighboring utility with excess capacity that is willing to sell water to the affected utility. Installation of a new well requires the existence and accessibility of an uncontaminated aquifer.

How can the WBS model for nontreatment options be used?

The work breakdown structure (WBS) model can estimate costs for either of two nontreatment options:

• interconnection with another system
• drilling a new well to replace a contaminated one

Where can I find more information on nontreatment options?

The technical report Work Breakdown Structure-Based Cost Model for Nontreatment Options for Drinking Water Compliance discusses these options in detail.

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International Journal of Phytomedicine and Phytotherapy

• Open access
• Published: 24 March 2021

Plant active products and emerging interventions in water potabilisation: disinfection and multi-drug resistant pathogen treatment

• Adeyemi O. Adeeyo   ORCID: orcid.org/0000-0001-6940-6955 1 ,
• Joshua N. Edokpayi 2 ,
• Mercy A. Alabi 3 ,
• Titus A. M. Msagati 4 &
• John O. Odiyo 2 , 5  

Clinical Phytoscience volume  7 , Article number:  31 ( 2021 ) Cite this article

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6 Citations

Metrics details

This review aims at establishing the emerging applications of phytobiotics in water treatment and disinfection.

Statistical analysis of data obtained revealed that the use of plant product in water treatment needs more research attention. A major observation is that plants possess multifaceted components and can be sustainably developed into products for water treatment. The seed (24.53%), flower (20.75), leaf (16.98%) and fruit (11.32%) biomasses are preferred against bulb (3.77%), resin (1.89%), bark (1.89%) and tuber (1.89%). The observation suggests that novel applications of plant in water treatment need further exploration since vast and broader antimicrobial activities (63.63%) is reported than water treatment application (36.37%).

Conclusions

This review has revealed the existing knowledge gaps in exploration of plant resources for water treatment and product development. Chemical complexity of some plant extracts, lack of standardisation, slow working rate, poor water solubility, extraction and purification complexities are limitations that need to be overcome for industrial adoption of phytochemicals in water treatment. The field of phytobiotics should engage modern methodologies such as proteomics, genomics, and metabolomics to minimise challenges confronting phytobiotic standardisation. The knowledge disseminated awaits novel application for plant product development in water treatment.

Introduction

Although plant products are available [ 1 , 2 , 3 ]and exhibit different mechanisms of action from conventional antimicrobials [ 4 , 5 ], there are critical gaps in the exploration of plant resources [ 6 ] for development of useful products [ 7 ]. The variation and complexity in chemical compositions of plants potentiate their activity [ 5 ]. Different phytochemicals present in plants such as phenols, quinones, flavonols, tannins, coumarins and alkaloids are responsible for plant activities. Flavonols and phenolics have been reported for their antioxidant activity exerted by scavenging the ‘free-oxygen radicals’ thereby giving rise to a fairly ‘stable radical. Cinnamaldehyde and other polyphenols have been known for their anti-diabetic activity by enhancing the amount of insulin-like TTP (Thrombotic Thrombocytopenic Purpura), IR (Insulin Resistance), and GLUT4 (Glucose Transporter-4) in 3 T3-L1 Adipocytes. Phenolic acids are known for their antimicrobial activity by reduction of adherence of organisms to cells and essential oils for their anti-inflammatory effect by suppressing nitric oxide production [ 8 , 9 ].

Current interventions of plants dealing with microbial resistance, immunomodulation, as antitumour agents, maintenance of illnesses affecting immune systems and in microbial virulence attenuation have been described in various researches [ 10 , 11 , 12 , 13 ]. In the early days of use, plants were used in their crude state [ 14 ] and contribute greatly to the health care system of local communities [ 15 ]. Ethnomedicinally, plants have found application in the treatment of several diseases and ailments. Examples are Astragalus membranaceus , roots of Trichosanthes kirilowii , roots and rhizomes of Panax quinoquefolusin and pulps of Cornus officinalis reported for treatment of diabetes mellitus.

Terminalia chebula and Adenocarpus mannii are known for their immunomodulation properties [ 16 , 17 ], Thea assamica in the treatment of impetigo [ 18 ], Ocimum gratissimum in the treatment of acne [ 19 ], Drechslera rostata and Polygala molluginifolia for their antitumor activities [ 20 , 21 ]. Medicinal plants including Ageratum conyzoides, Celosia trigyna, Centella asiatica, Brassica nigra, Racunculus oreophytus, Azadiracha indica, Ficus exasperate, Senna hirsuta, Morus alba, Artocarpus heterophyllus have all been reported for their ethnomedicinal use in various ethnic groups [ 22 , 23 , 24 ].

Microbial resistance has increased with drug discovery resulting in serious health concerns globally [ 25 , 26 ] and indiscriminate use of antimicrobial remains the main cause [ 27 ]. Therefore, the emergence of multidrug resistant microbes in water and undesirable effects of conventional antimicrobials call for alternative means of water treatment of plant origin [ 28 ]. Various microbial contamination of human origin in water may be controlled by extracts of plants [ 29 ]. In Kirui et al. [ 30 ]. Aqueous extract of Acacia nilotica, Acacia seyal, Acacia tortilis, Acacia etbaica, Albizia anthelmintica, Euclea divinorum and Plumbago zeylanica were investigated for their water treatment capacity and report indicates a notable effect. Extracts of Moringa oleifera , Jatropha curcas and Guar gum have also been investigated for their water treatment potential and reduction in turbidity of the treated water was observed [ 31 ]. These observations show the potential of plant product for water treatment. This attempts made to finding alternative way to combat resistant microorganisms and water disinfection is noble. It is of low cost and mostly available for safe water especially, in Africa. This review discusses the prospects of plant and plant products in less reported areas of water and antimicrobial resistance interventions.

Methodology

A desktop structured study of scholarly published articles was employed in the study of over 200 relevant literatures. The searched databases included Science Direct, Google Scholar and Web of Science. The searched terms and keywords included history, types, mechanisms of action, standardisation and application of phytobiotics in various fields. The search was restricted to articles written in English language and covered the period between 1993 and 2020. A review of studies reporting the use of plants as alternative against resistant microorganism especially in water treatment was attempted. Studies reporting the mechanisms of action of phytobiotics, methods employed in standardisation of herbal drug and current existing challenges in this field were examined. Raw information obtained were computed in MS-excel 2016 to convert data into processed statistics for the interpretation of the data. Tables, figures charts and simple percentages were used to present and interpret the results of data.

Results and discussion

Classification of synthetic and plant-based antimicrobials.

The treatment of microbial infections and contamination has mainly involved the use of antimicrobial agents like antiseptics, sanitizers, disinfectants, as well as antibiotics [ 32 ]. Antibiotic could specifically denote a substance with the capacity to inhibit, that is, cause static or cidal effect to microbes at low concentrations [ 33 ]. Pharmaceutical agents such as antibacterial, antifungal, antiviral, and antiparasitic drugs are broadly referred to as antibiotics [ 34 ]. Among the several classification schemes for antibiotics, those based on the molecular structures, spectrum of activity and modes of action [ 35 ] are more preferred as indicated in Table  1 . Antibiotics can also be classified as injectable, oral or topical based on route of administration. Antibiotics with similar structure will usually exhibit similar trends of actions and effects.

The antimicrobial activity of plants has been credited to the existence of phytochemicals in specific parts of plants [ 38 ] where they contribute to enhanced plant’s survival by warding off pathogenic microorganisms [ 39 , 40 ]. Some major groups of antimicrobials derived from plants include saponins, polyphenols, alkaloids, lectins, tannins, flavonoids, and terpenoids [ 41 ]. Synthetic pathways of some phytochemicals and related enzymes are indicated in Fig.  1 .

Biosynthetic pathways of plant phytochemicals and their related enzymes. Source: [ 42 ]

Basic phenolic acids and phenols

These are made of mono-substituted ring of phenol [ 43 ]. It is thought that the site(s) as well as numbers of hydroxyl components in this group influence the level of toxicity against microorganisms as it is evident that higher hydroxylation correlates with improved toxicity [ 44 ]. When phenolics possess a lower level of oxidation and a C 3 side chain, it is referred to as an essential oil [ 45 ]. This group includes cinnamic acids, caffeic acids, and pyrogallol with proven toxicity against microbes. The defensive functions of phenolic compounds in plants include antimicrobial activities as well as cell wall repair and strength [ 46 ].

These are classes of cyclic organic compounds with two carbonyl groups characterised with high reactivity and ubiquity. They possess aromatic rings and 2 ketone substitutes viewed as an important phytochemical group and possess excellent antimicrobial activities [ 5 ]. Quinones are responsible for the natural activity of browning reaction on plant. In microbial cell, quinones target surface-exposed adhesins, membrane bound enzymes and cell wall polypeptides [ 45 ]. Quinones may as well render substrate unavailable to microorganisms. An example is anthraquinone with a wide spectrum of antimicrobial actions [ 5 ].

Flavonols and flavones

These classes of flavonoids possess a double bond between position 2 and 3, and oxygen (a ketone group) in position 4 of the C ring (Fig.  2 ). Flavones have demonstrated excellent antibiosis against broad groups of microbes [ 47 ]. Reported antiviral and other bioactive effects of these groups of phytochemicals include the action of herperetin, galangin and alpinumisoflavone against Human Immunodeficiency Virus, poliovirus type 1, gram positive bacteria, fungi, and schistosomal infections [ 46 ].

Structure of flavonoids [ 48 ]

Tannins are poly-phenolics with wide distribution in various plant parts and are involved in many physiological activities of plant such as stimulation of phagocytic cells and anti-infective activities [ 47 ]. The antibacterial activities of tannins are attributed to their capacity to disrupt bacterial enzymes, cell envelope, adhesins and transport proteins. They are toxic to fungi, bacteria and yeasts cells [ 5 ]. Their strong affinity for iron on cell membrane results in inactivation of membrane-bound protein, which is responsible for wide antibacterial activities of gallotannin containing plants [ 49 ].

These are phytochemicals with bonded alpha pyrone and benzene. Coumarins may exhibit selective antiviral effects. Warfarin is a commonly reported coumarin which produces diverse biological activities and has been proved in-vitro to inhibit the growth of Candida albicans [ 47 ]. They can stimulate macrophages and reduce tenacity of microbial infection. Warfarin has also been a prescribed drug therapy for prevention of thromboembolic conditions for decades [ 50 ]. Esculetin, 6-nitro-7-hydroxycoumarin, scopoletin, 7,8 –dihydroxy-4-methylcoumarin have also been reported for cytotoxicity activity against cancer cell lines [ 51 ].

Alkaloids are nitrogenous heterocyclic compounds. The first medically engaged alkaloid- morphine, was obtained from Papaver somniferum. This group of phytochemicals proved to be microbiocidal (against Entamoeba spp. and Giardia) and are antidiarrheal. Examples of alkaloids include diterpene alkaloids, berberine (isoquinoline alkaloid) and solamargine (glycoalkaloid) used against a wide range of fungi, protozoa, bacteria, viruses and in maintenance of HIV [ 5 , 52 ]. They penetrate cells, intercalate DNA and target several nucleic acid enzymes, resulting in severe damages to microbial cells [ 53 ].

Emerging interventions of phytobiotics

Intervention in water treatment.

The use of plant derivatives as microbial inhibitors has been greatly reported [ 54 , 55 , 56 , 57 ]. However, limited literatures exist on the application of plant as disinfectants in water treatment. Winward et al. [ 58 ] reported the antimicrobial activity of 8 mixtures of different plant extracts which were studied for disinfection of coliform in grey water. Another study using thyme oils recorded higher inactivation of E. coli when compared to chlorinedioxide and ozonation for disinfection of water [ 59 ]. Extracts of plants such as M. oleifera , J. curcas , Guar gum [ 60 ], Terminalia glaucescens, Zanthoxylum zanthoxyloides, Gongronema latifolium [ 61 , 62 ], Azadirachta indica oil extracts [ 63 ] and Luffa cylindrica fruit extracts [ 64 ] have been reported for use in water treatment. It must be noted that high concentrations of crude plant extracts are not desired in water treatment since they result into undesired amount of suspended solids and contribute to taste and colour development. Hence, purified active plant-derived compounds rather than crude extracts or powders are preferred for water treatment.

Interventions in microbial resistance

Arctostaphylos uvaursi , Vaccinium macrocarpon, Hydrastis canadensis as well as oil extracts of Melaleuca alternifolia and Echinacea species have been used for the treatment of microbes of urinary tract, skin and lung origin [ 56 , 65 ]. Curative potential of plant extracts has been investigated and developed as novel drugs to control microbial infections and those with minimum inhibitory concentration of 100–1000 mg/ml are accepted and classed as antimicrobials [ 56 ]. Reports by various investigators had confirmed the antimicrobial potency of different plant materials [ 66 ]. Plants like Holarrhenea antidyssentrica [ 67 ], Tapinthus senssilifolius [ 68 ], Psidium guajava, Mangifera indica [ 69 ], Rauelfia tetraphylla, Physalis minima [ 70 ], Salvia spp. [ 71 ] and Salicornia brachiata [ 72 ] have demonstrated antimicrobial effects.

Plant products have been considered as alternatives to synthetic counterparts with significant results, including commercial antiseptics [ 73 ], sanitisers [ 74 ] and antibiotics [ 75 ]. Several other plant materials and formulations have been tested against different bacterial and fungal isolates with satisfactory results in literature [ 76 , 77 , 78 , 79 ]. Furthermore, plants active compounds have been considered useful in cases of multidrug resistance [ 80 ] and inhibition of biofilm formation [ 81 ]. Selected plants bio-activities against multi-drug resistant microbes are presented in Table  2 . They have been considered for their effects in efflux pump inhibition [ 82 ]. Fungi as well as bacteria have all been treated by several plant compounds, reducing their virulence and pathogenicity through modulation of gene transcription, expression of proteins and quorum sensing [ 83 , 84 , 85 , 86 , 87 , 88 , 89 ].

Plant products are also considered in adjuvant application. Since phytochemicals possess varied minimum inhibitory concentrations (MIC) from synthetic antibiotics, phytochemicals may be a good adjuvant for potentiating the activities of conventional biocides to improve efficacy and reduce the dosage of synthetic disinfectants [ 93 , 94 ]. Many reviews have dealt with reports on system of actions of plant materials and extensive list of herbs with antimicrobial activity exists [ 95 , 96 , 97 , 98 , 98 , 99 , 100 , 101 , 102 , 103 , 104 , 105 ]. Some examples of interventions of phytobiotics as antimicrobial are presented in Table  3 .

Mechanisms of action of antibiotics and phytobiotics

Diverse mechanisms exist to define the actions of phytochemicals in different bioactivities. They may prevent the growth of microorganisms, interfere with some biological metabolic processes or may modulate signal transduction and gene expression pathways [ 110 , 111 , 112 ]. Multiple molecular targets of phytochemicals have been identified to include cell cycle proteins, cell adhesion molecules, protein kinases, transcription factors and cell growth pathways [ 113 , 114 , 115 ]. Multi-molecular targets of plant phytochemicals account for multi-mechanisms of action in plant product [ 116 ]. Phytochemicals may modulate transcription factors [ 117 ], redox-sensitive transcription factors [ 118 ], redox signalling, and inflammation.

The general antimicrobial activities of conventional antimicrobials are hinged on inhibition of several cellular functions and structure, including cell membrane function, cell wall synthesis, nucleic acid and protein synthesis, as well as blockage of key metabolic pathways. Phytochemicals act majorly by collapsing cell walls and membranes, resulting in leakage of the cell component, interruption of proton motive force, dysfunction of efflux pump and enzymes, all leading to cytosis [ 119 ]. Some phytochemicals inhibit or minimise quorum sensing and this signifies a feasible method of countering antibiotic resistance in microorganisms since quorum sensing is partly involved in the mechanism of antibiotic-resistance in microbes [ 120 ].

A group of cell membrane disruptors include amphotericin B, polyenes, imidazole, triazole and polymyxins [ 36 ]. This group disrupts the structure of the membrane in cytoplasm of microorganisms resulting in the escape of macromolecules and ion from the cell, which results in lethal effects [ 121 ]. Antibiotics inhibiting cell wall synthesis are vancomycin, bacitracin, penicillin and cephalosporin. These antibiotics manipulate specific steps in homeostatic cell wall biosynthesis, in inhibition of peptide bond formation reaction catalysed by transglycosylase and transpeptidases and then activation of autolytic enzymes [ 122 ]. Antibiotics can block nucleic acid replication and halt transcription by inhibition of DNA polymerase, helicase or RNA polymerase. Examples are rifampin, trimethoprim, quinolones and sulphonamides [ 37 , 123 ].

Antibiotics inhibiting protein synthesis may either block the initiation of protein translation or peptidyl tRNAs translocation, which inhibit peptidyltransferase reaction involved in elongating the nascent peptide chain [ 124 ]. Examples of protein inhibiting antibiotics are chloramphenicol, tetracycline, erythromycin, lincomycin, and aminoglycosides. Some antibiotics like sulphonamides and trimethoprim mimic important substrate needed for cellular metabolism in microbes. This deception results in microbial enzyme attachment to antibiotic rather than the needed substrate [ 125 ] resulting in blockages of key metabolic pathways of survival. An example of metabolic pathway blocking antibiotics is sulfonamides, which are structurally identical to p -aminobenzoic acid needed in the synthesis of folic acid, thus disrupting the nucleic acid synthesis and amino acid production, since they imitate materials needed for folic acid metabolism [ 125 ].

Conventional chemically synthesised antibiotics and phytobiotics significantly differ with respect to frequency in spatial arrangement and radical composition [ 126 ]. The latter is with less nitrogen, phosphorus, sulphur, halogens and exhibit diverse and enhanced scaffold formation, stereo-chemical conformation, molecular complexity, varied ring system and carbohydrate compositions [ 127 ]. Furthermore, phytochemicals can disrupt protein-protein reactions and act as immune modulators and modulators of mitosis with less resistance from microbes due to the aforementioned complexity of plant phytochemicals [ 128 ]. Plant phytochemicals therefore exert activities via highly complex and diverse mechanisms, including disruption in cell quorum sensing, membranes, structures, nucleic acid synthesis, cytoplasmic material and cell metabolism [ 129 , 130 , 131 , 132 , 133 ]. A common phenomenon is that several compounds in crude plant extract act at different target sites in pathogens and contribute to optimum efficacy of plant extracts. Phytochemicals may exhibit antimicrobial effect in microbes not only through direct lethal activity, but also by altering key events in pathogenesis [ 134 ].

Standardisation of phytobiotics

Herbs comprise of crude plant materials such as fruits, flowers, stems, wood, leaves, seeds or other parts of plants in whole or parts. Herbal products are prepared through different carefully selected processes of solvent extraction and purification, and more recently by novel advanced instrumentation techniques by physical, chemical and biological processes alone or in combination with conventional extraction process. Products which have been modified with synthetic compounds or other chemically defined, active substances as well as isolated constituent from herbal materials may not wholly be accepted as herbal [ 135 ].

Standardisation in phytomedicine refers to the procedure for ensuring quality, standard characteristics, persistent nature and absolute quantifiable values with a guarantee of effectiveness, non-toxicity, excellence and reproducibility [ 136 ]. Validation of herbal drugs and recognition of counterfeits from quality herbal products are necessary for public health and quality reproducibility in herbal medicine. Standardisation reduces batch differences, guarantee effectiveness, originality, safety and acceptability of herbal products [ 137 ]. Some recent techniques of herbal standard verification include Thin Layer Chromatography, High Performance Thin Layer Chromatography, Gas Chromatography, Super Critical Fluid Chromatography, Chromatographic Fingerprinting and DNA Fingerprinting. Brief details on herbal drug standardisation are given in Table  4 .

Studies on commercial disinfectant for water treatment

When choosing a disinfectant for water processing, there is a need to consider if it follows all regulatory approvals [ 146 ]. Through the use of disinfectants, pathogenic (resistant) bacteria present in water can be destroyed to make water safe for drinking [ 147 ]. Plant disinfectants have also been produced as alternative to the chemical disinfectant counterparts. Tannins, plant gums and celluloses are examples of plant products that have been reported as effective natural disinfectants [ 148 , 149 ]. Tannins are produced from polyphenolic metabolites from bark, fruits and leaves of plants [ 150 ]. Mimosa bark tannin, quebracho wood tannin, pine bark tannin and eucalyptus species bark tannin are common tannins used in for water treatment. The coagulation effect of tannins have been tested for the treatment of raw water in the removal of suspended and colloidal materials, removal of dyes, pigments as well as inks from ink-containing wastewater [ 151 , 152 ].

Flocculants have also been derived from several plants gums and mucilages. These are obtained after aqueous extraction, precipitation with alcohol and drying. It has been used in the treatment of landfill leachate, textile wastewater, tannery effluent and sewage effluent [ 153 , 154 ]. From the report from Agarwal et al. [ 155 ], result showed 85% removal of suspended solids and 90% colour removal using these plant-based products. Cellulose is another alternative to synthetic disinfectant in water purification. Its water purification effect is due to the abundant free –OH groups on the chain that enables the removal of metal ions and organic matter from water [ 156 ]. However, the use of cellulose is limited because of its poor solubility and low chemical reactivity. This disadvantage can be taken care of by carboxymethylation [ 157 ].

Commercially, Tanfloc have been produced by a Brazilian company, and TANAC from the bark of Acacia tree [ 158 ]. Tanfloc allows for the removal of biological oxygen demand and chemical oxygen demand and generates a sludge volume that is biodegradable. Tanfloc has been tested to remove heavy metals from polluted surface water and municipal wastewater [ 150 ]. Another company in Italy, Silvateam also produced a commercially available plant based disinfectant called SilvaFLOC from the bark of S. balansae . Silvafloc has been tested on surface river water and has been reported safe for use in drinking water treatment [ 159 ]. It is found to be more efficient than aluminium sulphate for water clarification.

Data analyses

Reported use and chemical components of some plants are presented in Tables  5 and 6 . A total of 44 plants are presented in Table  6 with members of the family, Compositae mostly reported. Compositae (asteraceae) is the most diverse family of angiosperms and has a worldwide distribution. The family has been reported for its enormous importance in popular medicine and is the major plant studied for use in many ethno-medicinal researches [ 160 , 161 , 162 ]. The family Compositae is nested high in the Angiosperm phyleny. The family contains the largest number of described, accepted species of any plant family [ 163 ]. The diverse application of Compositae has been attributed to the wide array of bioactive component they contain as well as the higher likeliness of the people to experiment with members of this family. Conversely, the survey by Lawal et al. [ 164 ] reveals the family Leguminosae as the mostly used family and that compositae was barely used. However, Ageratum conyzoides and Vernonia amygdalina, both of which belong to the family Compositae were reported as commonly used species ethno-medicinally.

During the survey, it was apparent that whole plants and seeds (24.53%) are mostly used. Since individual plant parts have been reported for effective activity, the whole plant biomass is assumed to possess better activity and may account for the high value of whole plant material use as compared to other plant parts. The use of whole plants is usually not preferred since the removal of whole plant threatens conservation of plant species. The seeds (24.53%), flowers (20.75), leaves (16.98%) and fruit (11.32%) are therefore preferable as observed in Fig.  3 as against bulb (3.77%), resin (1.89%), bark (1.89%) and tuber (1.89%). This result does not align with the findings of Ozioma et al. [ 205 ] who reported leaves to possess more effective properties than other parts. As established by Ullah et al. [ 206 ], leaves are more reportedly used and followed by fruit (15%) among plant parts used during an indigenous study. Leaves, roots and bulbs are the most desirable parts because they contain a high concentration of bioactive compounds. Compared with the whole plant and roots, the use of leaves or arial part of plant is much better for sustainability of natural plant products and biotechnology [ 207 ]. The use of plant products in water treatment needs to gain more research attention as they can be effective alternatives for conventional agents of water disinfection. The observation in Fig.  4 suggests that novel applications of plant products need to be explored further since more antimicrobial activities (63.63%) exist for plant materials than water treatment application (36.37%). Moringa seeds have been greatly studied for water treatment due to the presence of cationic proteins (dimeric) responsible for their anticoagulant potential [ 208 , 209 ]. Moringa oleifera extracts as well as other natural coagulants are presently in demand because they are less toxic and ecofriendly [ 183 ]. Reports have also shown that combined treatment can present better coagulation effect as seen in Alam et al. [ 183 ] report. There is always a need to carry out test to ascertain the toxicity of plants extracts to be used in water treatment and ensure its effect falls within the WHO guideline values, to be proved effective [ 60 ].

Percentage use of plant based on the parts

Frequency (%) of reported antimicrobial activity of plants

Current challenges

There are current issues which call for caution during herbal and plant products usage. Safe plants and those with positive health effects must be identified prior to use and product formulation in water disinfection. Regulations in herbal remedies and isolation of pure and safe compounds rather than crude usage may be necessary during the considerations of plant products. Herbal remedies can be risky to human health [ 210 ] when inappropriately used. Inappropriate combination with synthetic biocides may act to reduce the potency of conventional products. Risks may exist and be triggered by age, genetics and concurrent use of other drugs [ 211 ] for products involving plant materials. Alkaloids and cardiac glycosides have been reported for adverse effects. Some herbs with adverse effects are described by Reid et al. [ 212 ], Allard et al. [ 213 ], Maffe et al. [ 214 ], and Fatima and Nayeem [ 215 ]. Some plants previously reported in literature for adverse effects include Allium sativum, Panax ginseng, Silybam marianum, Vitis vinifera, Aloe barbadensis, Valeriana officinalis and Salix daphnoides , [ 210 , 216 , 217 , 218 ]. Leaves of Ginkgo biloba have been reported for allergic skin reaction and seizures [ 210 ]. Chemical complexity of some plant extracts, lack of standardisation, slow working rate, poor water solubility, extraction and purification complexities are limitations that need to be overcome for industrial adoption of phytochemicals in water treatment.

Conclusion and key report findings

This review aimed at establishing emerging applications of phyto-biotics in water treatment and associated challenges in comabating multidrug resistant organism in water disinfection. It has been established that plant-derived compounds are environmentally friendly, usually less toxic and have a broad medicinal application. These plant products are generally widespread, affordable, and have significant antimicrobial efficacy. Secondary metabolites from plants have found great usefulness against resistant microorgamisms and extracts of plants have been used in water treatment as natural coagulants and in reduction of microbial count of water borne pathogens. A major observation is that plant materials possess multifaceted components with manifold actions and capabilities in different fields; a characteristic not commonly found in synthetic counterpart. Challenges impeding progress and development of plants as useful biotechnological products, therefore beckon for attention to aid wide applications of phytobiotics. The use of phytochemicals in combination with synthetic antimicrobials as adjuvant needs a boost as this is a current problematic area. Novel investigations in the field of phytobiotics should engage modern methodologies such as proteomics, genomics, and metabolomics to screen safe herbs and isolate pure compounds in order to minimise challenges confronting phytobiotic safety and standardisation.

Availability of data and materials

The content of this work is original and all ethical issues were well considered during the manuscript development.

Abbreviations

• Microbial resistance

Human immunodeficiency virus 1

Deoxyribonucleic acid

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• Phytochemicals
• Water treatment
• Standardisation

Wastewater Treatment and Reuse: a Review of its Applications and Health Implications

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• Published: 10 May 2021
• Volume 232 , article number  208 , ( 2021 )

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• Kavindra Kumar Kesari   ORCID: orcid.org/0000-0003-3622-9555 1   na1 ,
• Ramendra Soni 2   na1 ,
• Qazi Mohammad Sajid Jamal 3 ,
• Pooja Tripathi 4 ,
• Jonathan A. Lal 2 ,
• Niraj Kumar Jha 5 ,
• Mohammed Haris Siddiqui 6 ,
• Pradeep Kumar 7 ,
• Vijay Tripathi 2 &
• Janne Ruokolainen 1  

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Water scarcity is one of the major problems in the world and millions of people have no access to freshwater. Untreated wastewater is widely used for agriculture in many countries. This is one of the world-leading serious environmental and public health concerns. Instead of using untreated wastewater, treated wastewater has been found more applicable and ecofriendly option. Moreover, environmental toxicity due to solid waste exposures is also one of the leading health concerns. Therefore, intending to combat the problems associated with the use of untreated wastewater, we propose in this review a multidisciplinary approach to handle wastewater as a potential resource for use in agriculture. We propose a model showing the efficient methods for wastewater treatment and the utilization of solid wastes in fertilizers. The study also points out the associated health concern for farmers, who are working in wastewater-irrigated fields along with the harmful effects of untreated wastewater. The consumption of crop irrigated by wastewater has leading health implications also discussed in this review paper. This review further reveals that our current understanding of the wastewater treatment and use in agriculture with addressing advancements in treatment methods has great future possibilities.

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1 Introduction

Rapidly depleting and elevating the level of freshwater demand, though wastewater reclamation or reuse is one of the most important necessities of the current scenario. Total water consumption worldwide for agriculture accounts 92% (Clemmens et al., 2008 ; Hoekstra & Mekonnen, 2012 ; Tanji & Kielen, 2002 ). Out of which about 70% of freshwater is used for irrigation (WRI, 2020 ), which comes from the rivers and underground water sources (Pedrero et al., 2010 ). The statistics shows serious concern for the countries facing water crisis. Shen et al. ( 2014 ) reported that 40% of the global population is situated in heavy water–stressed basins, which represents the water crisis for irrigation. Therefore, wastewater reuse in agriculture is an ideal resource to replace freshwater use in agriculture (Contreras et al., 2017 ). Treated wastewater is generally applied for non-potable purposes, like agriculture, land, irrigation, groundwater recharge, golf course irrigation, vehicle washing, toilet flushes, firefighting, and building construction activities. It can also be used for cooling purposes in thermal power plants (Katsoyiannis et al., 2017 ; Mohsen, 2004 ; Smith, 1995 ; Yang et al., 2017 ). At global level, treated wastewater irrigation supports agricultural yield and the livelihoods of millions of smallholder farmers (Sato et al., 2013 ). Global reuse of treated wastewater for agricultural purposes shows wide variability ranging from 1.5 to 6.6% (Sato et al., 2013 ; Ungureanu et al., 2018 ). More than 10% of the global population consumes agriculture-based products, which are cultivated by wastewater irrigation (WHO, 2006 ). Treated wastewater reuse has experienced very rapid growth and the volumes have been increased ~10 to 29% per year in Europe, the USA, China, and up to 41% in Australia (Aziz & Farissi, 2014 ). China stands out as the leading country in Asia for the reuse of wastewater with an estimated 1.3 M ha area including Vietnam, India, and Pakistan (Zhang & Shen, 2017 ). Presently, it has been estimated that, only 37.6% of the urban wastewater in India is getting treated (Singh et al., 2019 ). By utilizing 90% of reclaimed water, Israel is the largest user of treated wastewater for agriculture land irrigation (Angelakis & Snyder, 2015 ). The detail information related to the utilization of freshwater and treated wastewater is compiled in Table 1 .

Many low-income countries in Africa, Asia, and Latin America use untreated wastewater as a source of irrigation (Jiménez & Asano, 2008 ). On the other hand, middle-income countries, such as Tunisia, Jordan, and Saudi Arabia, use treated wastewater for irrigation (Al-Nakshabandi et al., 1997 ; Balkhair, 2016a ; Balkhair, 2016b ; Qadir et al., 2010 ; Sato et al., 2013 ).

Domestic water and treated wastewater contains various type of nutrients such as phosphorus, nitrogen, potassium, and sulfur, but the major amount of nitrogen and phosphorous available in wastewater can be easily accumulated by the plants, that’s why it is widely used for the irrigation (Drechsel et al., 2010 ; Duncan, 2009 ; Poustie et al., 2020 ; Sengupta et al., 2015 ). The rich availability of nutrients in reclaimed wastewater reduces the use of fertilizers, increases crop productivity, improves soil fertility, and at the same time, it may also decrease the cost of crop production (Chen et al., 2013 a; Jeong et al., 2016 ). The data of high nutritional values in treated wastewater is shown in Fig. 1 .

Nutrient concentrations (mg/L) of freshwater/wastewater (Yadav et al., 2002 )

Wastewater reuse for crop irrigation showed several health concerns (Ungureanu et al., 2020 ). Irrigation with the industrial wastewater either directly or mixing with domestic water showed higher risk (Chen et al., 2013). Risk factors are higher due to heavy metal and pathogens contamination because heavy metals are non-biodegradable and have a long biological half-life (Chaoua et al., 2019 ; WHO, 2006 ). It contains several toxic elements, i.e., Cu, Cr, Mn, Fe, Pb, Zn, and Ni (Mahfooz et al., 2020 ). These heavy metals accumulate in topsoil (at a depth of 20 cm) and sourcing through plant roots; they enter the human and animal body through leafy vegetables consumption and inhalation of contaminated soils (Mahmood et al., 2014 ). Therefore, health risk assessment of such wastewater irrigation is important especially in adults (Mehmood et al., 2019 ; Njuguna et al., 2019 ; Xiao et al., 2017 ). For this, an advanced wastewater treatment method should be applied before release of wastewater in the river, agriculture land, and soils. Therefore, this review also proposed an advance wastewater treatment model, which has been tasted partially at laboratory scale by Kesari and Behari ( 2008 ), Kesari et al. ( 2011a , b ), and Kumar et al. ( 2010 ).

For a decade, reuse of wastewater has also become one of the global health concerns linking to public health and the environment (Dang et al., 2019 ; Narain et al., 2020 ). The World Health Organization (WHO) drafted guidelines in 1973 to protect the public health by facilitating the conditions for the use of wastewater and excreta in agriculture and aquaculture (WHO, 1973 ). Later in 2005, the initial guidelines were drafted in the absence of epidemiological studies with minimal risk approach (Carr, 2005 ). Although, Adegoke et al. ( 2018 ) reviewed the epidemiological shreds of evidence and health risks associated with reuse of wastewater for irrigation. Wastewater or graywater reuse has adverse health risks associated with microbial hazards (i.e., infectious pathogens) and chemicals or pharmaceuticals exposures (Adegoke et al., 2016 ; Adegoke et al., 2017 ; Busgang et al., 2018 ; Marcussen et al., 2007 ; Panthi et al., 2019 ). Researchers have reported that the exposure to wastewater may cause infectious (helminth infection) diseases, which are linked to anemia and impaired physical and cognitive development (Amoah et al., 2018 ; Bos et al., 2010 ; Pham-Duc et al., 2014 ; WHO, 2006 ).

Owing to an increasing population and a growing imbalance in the demand and supply of water, the use of wastewater has been expected to increase in the coming years (World Bank, 2010 ). The use of treated wastewater in developed nations follows strict rules and regulations. However, the direct use of untreated wastewater without any sound regulatory policies is evident in developing nations, which leads to serious environmental and public health concerns (Dickin et al., 2016 ). Because of these issues, we present in this review, a brief discussion on the risk associated with the untreated wastewater exposures and advanced methods for its treatment, reuse possibilities of the treated wastewater in agriculture.

2 Environmental Toxicity of Untreated Wastewater

Treated wastewater carries larger applicability such as irrigation, groundwater recharge, toilet flushing, and firefighting. Municipal wastewater treatment plants (WWTPs) are the major collection point for the different toxic elements, pathogenic microorganisms, and heavy metals. It collects wastewater from divergent sources like household sewage, industrial, clinical or hospital wastewater, and urban runoff (Soni et al., 2020 ). Alghobar et al. ( 2014 ) reported that grass and crops irrigated with sewage and treated wastewater are rich in heavy metals in comparison with groundwater (GW) irrigation. Although, heavy metals classified as toxic elements and listed as cadmium, lead, mercury, copper, and iron. An exceeding dose or exposures of these heavy metals could be hazardous for health (Duan et al., 2017 ) and ecological risks (Tytła, 2019 ). The major sources of these heavy metals come from drinking water. This might be due to the release of wastewater into river or through soil contamination reaches to ground water. Table 2 presenting the permissible limits of heavy metals presented in drinking water and its impact on human health after an exceeding the amount in drinking water, along with the route of exposure of heavy metals to human body.

Direct release in river or reuse of wastewater for irrigation purposes may create short-term implications like heavy metal and microbial contamination and pathogenic interaction in soil and crops. It has also long-term influence like soil salinity, which grows with regular use of untreated wastewater (Smith, 1995 ). Improper use of wastewater for irrigation makes it unsafe and environment threatening. Irrigation with several different types of wastewater, i.e., industrial effluents, municipal and agricultural wastewaters, and sewage liquid sludge transfers the heavy metals to the soil, which leads to accumulation in crops due to improper practices. This has been identified as a significant route of heavy metals into aquatic resources (Agoro et al., 2020 ). Hussain et al. ( 2019 ) investigated the concentration of heavy metals (except for Cd) was higher in the soil irrigated with treated wastewater (large-scale sewage treatment plant) than the normal ground water, also reported by Khaskhoussy et al. ( 2015 ).

In other words, irrigation with wastewater mitigates the quality of crops and enhances health risks. Excess amount of copper causes anemia, liver and kidney damage, vomiting, headache, and nausea in children (Bent & Bohm, 1995 ; Madsen et al., 1990 ; Salem et al., 2000 ). A higher concentration of arsenic may lead to bone and kidney cancer (Jarup, 2003 ) and results in osteopenia or osteoporosis (Puzas et al., 2004 ). Cadmium gives rise to musculoskeletal diseases (Fukushima et al., 1970 ), whereas mercury directly affects the nervous system (Azevedo et al., 2014 ).

3 Spread of Antibiotic Resistance

Currently, antibiotics are highly used for human disease treatment; however, uses in poultries, animal husbandries, biochemical industries, and agriculture are common practices these days. Extensive use and/or misuse of antibiotics have given rise to multi-resistant bacteria, which carry multiple resistance genes (Icgen & Yilmaz, 2014 ; Lv et al., 2015 ; Tripathi & Tripathi, 2017 ; Xu et al., 2017 ). These multidrug-resistant bacteria discharged through the sewage network and get collected into the wastewater treatment plants. Therefore, it can be inferred that the WWTPs serve as the hotspot of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs). Though, these antibiotic-resistant bacteria can be disseminated to the different bacterial species through the mobile genetic elements and horizontal gene transfer (Gupta et al., 2018 ). Previous studies indicated that certain pathogens might survive in wastewater, even during and after the treatment processes, including methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE) (Börjesson et al., 2009 ; Caplin et al., 2008 ). The use of treated wastewater in irrigation provides favorable conditions for the growth and persistence of total coliforms and fecal coliforms (Akponikpe et al., 2011 ; Sacks & Bernstein, 2011 ). Furthermore, few studies have also reported the presence of various bacterial pathogens, such as Clostridium , Salmonella , Streptococci , Viruses, Protozoa, and Helminths in crops irrigated with treated wastewater (Carey et al., 2004 ; Mañas et al., 2009 ; Samie et al., 2009 ). Goldstein ( 2013 ) investigated the survival of ARB in secondary treated wastewater and proved that it causes serious health risks to the individuals, who are exposed to reclaimed water. The U.S. Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) have already declared the ARBs as the imminent hazard to human health. According to the list published by WHO, regarding the development of new antimicrobial agents, the ESKAPE ( Enterococcus faecium , S. aureus , Klebsiella pneumoniae , Acinetobacter baumannii , Pseudomonas aeruginosa , and Enterobacter species) pathogens were designated to be “priority status” as their occurrence in the food chain is considered as the potential and major threat for the human health (Tacconelli et al., 2018 ).

These ESKAPE pathogens have acquired the multi drug resistance mechanisms against oxazolidinones, lipopeptides, macrolides, fluoroquinolones, tetracyclines, β-lactams, β-lactam–β-lactamase inhibitor combinations, and even those antibiotics that are considered as the last line of defense, including carbapenems and glycopeptides (Giddins et al., 2017 ; Herc et al., 2017 ; Iguchi et al., 2016 ; Naylor et al., 2018 ; Zaman et al., 2017 ), by the means of genetic mutation and mobile genetic elements. These cluster of ESKAPE pathogens are mainly responsible for lethal nosocomial infections (Founou et al., 2017 ; Santajit & Indrawattana, 2016 ).

Due to the wide application of antibiotics in animal husbandry and inefficient capability of wastewater treatment plants, the multidrug-resistant bacteria such as tetracyclines, sulfonamides, β-lactam, aminoglycoside, colistin, and vancomycin in major are disseminated in the receiving water bodies, which ultimately results in the accumulation of ARGs in the irrigated crops (He et al., 2020 ).

4 Toxic Contaminations in Wastewater Impacting Human Health

The release of untreated wastewater into the river may pose serious health implications (König et al., 2017 ; Odigie, 2014 ; Westcot, 1997 ). It has been already discussed about the household and municipal sewage which contains a major amount of organic materials and pathogenic microorganisms and these infectious microorganisms are capable of spreading various diseases like typhoid, dysentery, diarrhea, vomiting, and malabsorption (Jia & Zhang, 2020 ; Numberger et al., 2019 ; Soni et al., 2020 ). Additionally, pharmaceutical industries also play a key role in the regulation and discharge of biologically toxic agents. The untreated wastewater also contains a group of contaminants, which are toxic to humans. These toxic contaminations have been classified into two major groups: (i) chemical contamination and (ii) microbial contamination.

4.1 Chemical Contamination

Mostly, various types of chemical compounds released from industries, tanneries, workshops, irrigated lands, and household wastewaters are responsible for several diseases. These contaminants can be organic materials, hydrocarbons, volatile compounds, pesticides, and heavy metals. Exposure to such contaminants may cause infectious diseases like chronic dermatoses and skin cancer, lung infection, and eye irritation. Most of them are non-biodegradable and intractable. Therefore, they can persist in the water bodies for a very long period and could be easily accumulated in our food chain system. Several pharmaceutical personal care products (PPCPs) and surfactants are available that may contain toxic compounds like nonylphenol, estrone, estradiol, and ethinylestradiol. These compounds are endocrine-disrupting chemicals (Bolong et al., 2009 ), and the existence of these compounds in the human body even in the trace amounts can be highly hazardous. Also, the occurrence of perfluorinated compounds (PFCs) in wastewater, which is toxic in nature, has been significantly reported worldwide (Templeton et al., 2009 ). Furthermore, PFCs cause severe health menaces like pre-eclampsia, birth defects, reduced human fertility (Webster, 2010 ), immunotoxicity (Dewitt et al., 2012 ), neurotoxicity (Lee & Viberg, 2013 ), and carcinogenesis (Bonefeld-Jorgensen et al., 2011 ).

4.2 Microbial Contamination

Researchers have reported serious health risks associated with the microbial contaminants in untreated wastewater. The diverse group of microorganisms causes severe health implications like campylobacteriosis, diarrhea, encephalitis, typhoid, giardiasis, hepatitis A, poliomyelitis, salmonellosis, and gastroenteritis (ISDH, 2009 ; Okoh et al., 2010 ). Few bacterial species like P. aeruginosa , Salmonella typhimurium , Vibrio cholerae , G. intestinales , Legionella spp., E. coli , Shigella sonnei have been reported for the spreading of waterborne diseases, and acute illness in human being (Craun et al., 2006 ; Craun et al., 2010 ). These aforementioned microorganisms may release in the environment from municipal sewage water network, animal husbandries, or hospitals and enter the food chain via public water supply systems.

5 Wastewater Impact on Agriculture

The agriculture sector is well known for the largest user of water, accounting for nearly 70% of global water usage (Winpenny et al., 2010 ). The fact that an estimated 20 million hectares worldwide are irrigated with wastewater suggests a major source for irrigation (Ecosse, 2001 ). However, maximum wastewater that is used for irrigation is untreated (Jiménez & Asano, 2008 ; Scott et al., 2004 ). Mostly in developing countries, partially treated or untreated wastewater is used for irrigation purpose (Scott et al., 2009 ). Untreated wastewater often contains a large range of chemical contaminants from waste sites, chemical wastes from industrial discharges, heavy metals, fertilizers, textile, leather, paper, sewage waste, food processing waste, and pesticides. World Health Organization (WHO) has warned significant health implications due to the direct use of wastewater for irrigation purposes (WHO, 2006 ). These contaminants pose health risks to communities (farmers, agricultural workers, their families, and the consumers of wastewater-irrigated crops) living in the proximity of wastewater sources and areas irrigated with untreated wastewater (Qadir et al., 2010 ). Wastewater also contains a wide variety of organic compounds. Some of them are toxic or cancer-causing and have harmful effects on an embryo (Jarup, 2003 ; Shakir et al., 2016 ). The pathway of untreated wastewater used in irrigation and associated health effects are shown in Fig. 2 .

Exposure pathway representing serious health concerns from wastewater-irrigated crops

Alternatively, in developing countries, due to the limited availability of treatment facilities, untreated wastewater is discharged into the existing waterbodies (Qadir et al., 2010 ). The direct use of wastewater in agriculture or irrigation obstructs the growth of natural plants and grasses, which in turn causes the loss of biodiversity. Shuval et al. ( 1985 ) reported one of the earliest evidences connecting to agricultural wastewater reuse with the occurrence of diseases. Application of untreated wastewater in irrigation increases soil salinity, land sealing followed by sodium accumulation, which results in soil erosion. Increased soil salinity and sodium accumulation deteriorates the soil and decreases the soil permeability, which inhibits the nutrients intake of crops from the soil. These causes have been considered the long-term impact of wastewater reuse in agriculture (Halliwell et al., 2001 ). Moreover, wastewater contaminated soils are a major source of intestinal parasites (helminths—nematodes and tapeworms) that are transmitted through the fecal–oral route (Toze, 1997 ). Already known, the helminth infections are linked to blood deficiency and behavioral or cognitive development (Bos et al., 2010 ). One of the major sources of helminth infections around the world is the use of raw or partially treated sewage effluent and sludge for the irrigation of food crops (WHO, 1989 ). Wastewater-irrigated crops contain heavy metal contamination, which originates from mining, foundries, and metal-based industries (Fazeli et al., 1998 ). Exposure to heavy metals including arsenic, cadmium, lead, and mercury in wastewater-irrigated crops is a cause for various health problems. For example, the consumption of high amounts of cadmium causes osteoporosis in humans (Dickin et al., 2016 ). The uptake of heavy metals by the rice crop irrigated with untreated effluent from a paper mill has been reported to cause serious health concerns (Fazeli et al., 1998 ). Irrigating rice paddies with highly contaminated water containing heavy metals leads to the outbreak of Itai-itai disease in Japan (Jarup, 2003 ).

Owing to these widespread health risks, the WHO published the third edition of its guidelines for the safe use of wastewater in irrigating crops (WHO, 2006 ) and made recommendations for threshold contaminant levels in wastewater. The quality of wastewater for agricultural reuse have been classified based on the availability of nutrients, trace elements, microorganisms, and chemicals contamination levels. The level of contamination differs widely depending on the type of source, household sewage, pharmaceutical, chemical, paper, or textile industries effluents. The standard measures of water quality for irrigation are internationally reported (CCREM, 1987 ; FAO, 1985 ; FEPA, 1991 ; US EPA, 2004 , 2012 ; WHO, 2006 ), where the recommended levels of trace elements, metals, COD, BOD, nitrogen, and phosphorus are set at certain limits. Researchers reviewed the status of wastewater reuse for agriculture, based on its standards and guidelines for water quality (Angelakis et al., 1999 ; Brissaud, 2008 ; Kalavrouziotis et al., 2015 ). Based on these recommendations and guidelines, it is evident that greater awareness is required for the treatment of wastewater safely.

6 Wastewater Treatment Techniques

6.1 primary treatment.

This initial step is designed to remove gross, suspended and floating solids from raw wastewater. It includes screening to trap solid objects and sedimentation by gravity to remove suspended solids. This physical solid/liquid separation is a mechanical process, although chemicals can be used sometimes to accelerate the sedimentation process. This phase of the treatment reduces the BOD of the incoming wastewater by 20–30% and the total suspended solids by nearly 50–60%.

6.2 Secondary (Biological) Treatment

This stage helps eliminate the dissolved organic matter that escapes primary treatment. Microbes consume the organic matter as food, and converting it to carbondioxide, water, and energy for their own growth. Additional settling to remove more of the suspended solids then follows the biological process. Nearly 85% of the suspended solids and biological oxygen demand (BOD) can be removed with secondary treatment. This process also removes carbonaceous pollutants that settle down in the secondary settling tank, thus separating the biological sludge from the clear water. This sludge can be fed as a co-substrate with other wastes in a biogas plant to obtain biogas, a mixture of CH 4 and CO 2 . It generates heat and electricity for further energy distribution. The leftover, clear water is then processed for nitrification or denitrification for the removal of carbon and nitrogen. Furthermore, the water is passed through a sedimentation basin for treatment with chlorine. At this stage, the water may still contain several types of microbial, chemical, and metal contaminations. Therefore, to make the water reusable, e.g., for irrigation, it further needs to pass through filtration and then into a disinfection tank. Here, sodium hypochlorite is used to disinfect the wastewater. After this process, the treated water is considered safe to use for irrigation purposes. Solid wastes generated during primary and secondary treatment processes are processed further in the gravity-thickening tank under a continuous supply of air. The solid waste is then passed into a centrifuge dewatering tank and finally to a lime stabilization tank. Treated solid waste is obtained at this stage and it can be processed further for several uses such as landfilling, fertilizers and as a building.

Other than the activated sludge process of wastewater treatment, there are several other methods developed and being used in full-scale reactors such as ponds (aerobic, anaerobic, facultative, and maturation), trickling filters, anaerobic treatments like up-flow anaerobic sludge blanket (UASB) reactors, artificial wetlands, microbial fuel cells, and methanogenic reactors.

UASB reactors are being applied for wastewater treatment from a very long period. Behling et al. ( 1996 ) examined the performance of the UASB reactor without any external heat supply. In their study, the COD loading rate was maintained at 1.21 kg COD/m 3 /day, after 200 days of trial. They achieved an average of 85% of COD removal. Von-Sperling and Chernicharo ( 2005 ) presented a combined model consisted of an Up-flow Anaerobic Sludge Blanket-Activated Sludge reactor (UASB–AS system), using the low strength domestic wastewater with a BOD 5 amounting to 340 mg/l. Outcomes of their experiment have shown a 60% reduction in sludge construction and a 40% reduction in aeration energy consumption. In another experiment, Rizvi et al. ( 2015 ) seeded UASB reactor with cow manure dung to treat domestic wastewater; they observed 81%, 75%, and 76% reduction in COD, TSS, and total sulfate removal, respectively, in their results.

6.3 Tertiary or Advanced Treatment Processes

The tertiary treatment process is employed when specific constituents, substances, or contaminants cannot be completely removed after the secondary treatment process. The tertiary treatment processes, therefore, ensure that nearly 99% of all impurities are removed from wastewater. To make the treated water safe for drinking purposes, water is treated individually or in combination with advanced methods like the US (ultrasonication), UV (ultraviolet light treatment), and O 3 (exposure to ozone). This process helps to remove bacteria and heavy metal contaminations remaining in the treated water. For the purpose, the secondarily treated water is first made to undergo ultrasonication and it is subsequently exposed to UV light and passed through an ozone chamber for the complete removal of contaminations. The possible mechanisms by which cells are rendered inviable during the US include free-radical attack and physical disruption of cell membranes (Phull et al., 1997 ; Scherba et al., 1991 ). The combined treatment of US + UV + O 3 produces free radicals, which are attached to cell membranes of the biological contaminants. Once the cell membrane is sheared, chemical oxidants can enter the cell and attack internal structures. Thus, the US alone or in combination facilitates the deagglomeration of microorganisms and increases the efficiency of other chemical disinfectants (Hua & Thompson, 2000 ; Kesari et al., 2011a , b ; Petrier et al., 1992 ; Phull et al., 1997 ; Scherba et al., 1991 ). A combined treatment method was also considered by Pesoutova et al. ( 2011 ) and reported a very effective method for textile wastewater treatment. The effectiveness of ultrasound application as a pre-treatment step in combination with ultraviolet rays (Blume & Neis, 2004 ; Naddeo et al., 2009 ), or also compared it with various other combinations of both ultrasound and UV radiation with TiO 2 photocatalysis (Paleologou et al., 2007 ), and ozone (Jyoti & Pandit, 2004 ) to optimize wastewater disinfection process.

An important aspect of our wastewater treatment model (Fig. 3 ) is that at each step of the treatment process, we recommend the measurement of the quality of treated water. After ensuring that the proper purification standards are met, the treated water can be made available for irrigation, drinking or other domestic uses.

A wastewater treatment schematic highlighting the various methods that result in a progressively improved quality of the wastewater from the source to the intended use of the treated wastewater for irrigation purposes

6.4 Nanotechnology as Tertiary Treatment of Wastewater Converting Drinking Water Alike

Considering the emerging trends of nanotechnology, nanofillers can be used as a viable method for the tertiary treatment of wastewater. Due to the very small pore size, 1–5-nm nanofillers may eliminate the organic–inorganic pollutants, heavy metals, as well as pathogenic microorganisms and pharmaceutically active compounds (PhACs) (Mohammad et al., 2015 ; Vergili, 2013 ). Over the recent years, nanofillers have been largely accepted in the textile industry for the treatment of pulp bleaching pharmaceutical industry, dairy industry, microbial elimination, and removal of heavy metals from wastewater (Abdel-Fatah, 2018 ). Srivastava et al. ( 2004 ) synthesized very efficient and reusable water filters from carbon nanotubes, which exhibited effective elimination of bacterial pathogens ( E. coli and S. aureus ), and Poliovirus sabin-1 from wastewater.

Nanofiltration requires lower operating pressure and lesser energy consumption in comparison of RO and higher rejection of organic compounds compared to UF. Therefore, it can be applied as the tertiary treatment of wastewater (Abdel-Fatah, 2018 ). Apart from nanofilters, there are various kinds of nanoparticles like metal nanoparticles, metal oxide nanoparticles, carbon nanotubes, graphene nanosheets, and polymer-based nanosorbents, which may play a different role in wastewater treatment based on their properties. Kocabas et al. ( 2012 ) analyzed the potential of different metal oxide nanoparticles and observed that nanopowders of TiO 2 , FeO 3 , ZnO 2 , and NiO can exhibit the exceeding amount of removal of arsenate from wastewater. Cadmium contamination in wastewater, which poses a serious health risk, can be overcome by using ZnO nanoparticles (Kumar & Chawla, 2014 ). Latterly, Vélez et al. ( 2016 ) investigated that the 70% removal of mercury from wastewater through iron oxide nanoparticles successfully performed. Sheet et al. ( 2014 ) used graphite oxide nanoparticles for the removal of nickel from wastewater. An exceeding amount of copper causes liver cirrhosis, anemia, liver, and kidney damage, which can be removed by carbon nanotubes, pyromellitic acid dianhydride (PMDA) and phenyl aminomethyl trimethoxysilane (PAMTMS) (Liu et al., 2010 ).

Nanomaterials are efficiently being used for microbial purification from wastewater. Carbon nanotubes (CNTs) are broadly applied for the treatment of wastewater contaminated with E. coli , Salmonella , and a wide range of microorganisms (Akasaka & Watari, 2009 ). In addition, silver nanoparticles reveal very effective results against the microorganisms present in wastewater. Hence, it is extensively being used for microbial elimination from wastewater (Inoue et al., 2002 ). Moreover, CNTs exhibit high binding affinity to bacterial cells and possess magnetic properties (Pan & Xing, 2008 ). Melanta ( 2008 ) confirmed and recommended the applicability of CNTs for the removal of E. coli contamination from wastewater. Mostafaii et al. ( 2017 ) suggested that the ZnO nanoparticles could be the potential antibacterial agent for the removal of total coliform bacteria from municipal wastewater. Apart from the previously mentioned, applicability of the nanotechnology, the related drawbacks and challenges cannot be neglected. Most of the nanoengineered techniques are currently either in research scale or pilot scale performing well (Gehrke et al., 2015 ). Nevertheless, as discussed above, nanotechnology and nanomaterials exhibit exceptional properties for the removal of contaminants and purification of water. Therefore, it can be adapted as the prominent solution for the wastewater treatment (Zekić et al., 2018 ) and further use for drinking purposes.

6.5 Wastewater Treatment by Using Plant Species

Some of the naturally growing plants can be a potential source for wastewater treatment as they remove pollutants and contaminants by utilizing them as a nutrient source (Zimmels et al., 2004 ). Application of plant species in wastewater treatment may be cost-effective, energy-saving, and provides ease of operation. At the same time, it can be used as in situ, where the wastewater is being produced (Vogelmann et al., 2016 ). Nizam et al. ( 2020 ) analyzed the phytoremediation efficiency of five plant species ( Centella asiatica , Ipomoea aquatica , Salvinia molesta , Eichhornia crassipes , and Pistia stratiotes ) and achieved the drastic decrease in the amount of three pollutants viz. total suspended solids (TSS), ammoniacal nitrogen (NH 3 -N), and phosphate levels . All the five species found to be efficient removal of the level of 63.9-98% of NH 3 -N, TSS, and phosphate. Coleman et al. ( 2001 ) examined the physiological effects of domestic wastewater treatment by three common Appalachian plant species: common rush or soft rush ( Juncus effuses L.), gray club-rush ( Scirpus Validus L.), and broadleaf cattail or bulrush ( Typha latifolia L.). They observed in their experiments about 70% of reduction in total suspended solids (TSS) and biochemical oxygen demand (BOD), 50% to 60% of reduction in nitrogen, ammonia, and phosphate levels, and a significant reduction in feacal coliform populations. Whereas, Zamora et al. ( 2019 ) found the removal efficiency of chemical oxygen demand (COD), total solids suspended (TSS), nitrogen as ammonium (N-NH 4 ) and nitrate (N-NO 3 ), and phosphate (P-PO 4 ) up to 20–60% higher using the three ornamental species of plants viz. Canna indica , Cyperus papyrus , and Hedychium coronarium . The list of various plant species applied for the wastewater treatment is shown in Table 3 .

6.6 Wastewater Treatment by Using Microorganisms

There is a diverse group of bacteria like Pseudomonas fluorescens , Pseudomonas putida , and different Bacillus strains, which are capable to use in biological wastewater systems. These bacteria work in the cluster forms as a floc, biofilm, or granule during the wastewater treatment. Furthermore, after the recognition of bacterial exopolysaccharides (EPS) as an efficient adsorption material, it may be applied in a revolutionary manner for the heavy metal elimination (Gupta & Diwan, 2017 ). There are few examples of EPS, which are commercially available, i.e., alginate ( P. aeruginosa , Azotobacter vinelandii ), gellan (Sphingomonas paucimobilis ), hyaluronan ( . aeruginosa , Pasteurella multocida , Streptococci attenuated strains ), xanthan (Xanthomonas campestris ), and galactopol ( Pseudomonas oleovorans ) (Freitas et al., 2009 ; Freitas, Alves, & Reis, 2011a ; Freitas, Alves, Torres, et al., 2011b ). Similarly, Hesnawi et al. ( 2014 ) experimented biodegradation of municipal wastewater using local and commercial bacteria (Sludge Hammer), where they achieved a significant decrease in synthetic wastewater, i.e., 70%, 54%, 52%, 42% for the Sludge Hammer, B. subtilis , B. laterosponus , and P. aeruginosa , respectively. Therefore, based on the above studies, it can be concluded that bioaugmentation of wastewater treatment reactor with selective and mixed strains can ameliorate the treatment. During recent years, microalgae have attracted the attention of researchers as an alternative system, due to their applicability in wastewater treatment. Algae are the unicellular or multicellular photosynthetic microorganism that grows on water surfaces, salt water, or moist soil. They utilize the exceeding amount of nutrients like nitrogen, phosphorus, and carbon for their growth and metabolism process through their anaerobic system. This property of algae also inhibits eutrophication; that is to avoid over-deposit of nutrients in water bodies. During the nutrient digestion process, algae produce oxygen that is constructive for the heterotrophic aerobic bacteria, which may further be utilized to degrade the organic and inorganic pollutants. Kim et al. ( 2014 ) observed a total decrease in the levels of COD (86%), total nitrogen (93%), and total phosphorus (83%) after using algae in the municipal wastewater consortium. Nmaya et al. ( 2017 ) reported the heavy metal removal efficiency of microalga Scenedesmus sp. from contaminated river water in the Melaka River, Malaysia. They observed the effective removal of Zn (97-99%) on the 3 rd and 7 th day of the experiment. The categorized list of microorganisms used for wastewater treatment is presented in Table 4 .

7 The Computational Approach in Wastewater Treatment

7.1 bioinformatics and genome sequencing.

A computational approach is accessible in wastewater treatment. Several tools and techniques are in use such as, sequencing platforms (Hall, 2007 ; Marsh, 2007 ), metagenome sequencing strategies (Schloss & Handelsman, 2005 ; Schmeisser et al., 2007 ; Tringe et al., 2005 ), bioinformatics tools and techniques (Chen & Pachter, 2005 ; Foerstner et al., 2006 ; Raes et al., 2007 ), and the genome analysis of complex microbial communities (Fig. 4 ). Most of the biological database contains microorganisms and taxonomical information. Thus, these can provide extensive details and supports for further utilization in wastewater treatment–related research and development (Siezen & Galardini, 2008 ). Balcom et al. ( 2016 ) explored that the microbial population residing in the plant roots immersed in the wastewater of an ecological WWTP and showed the evidence of the capacity for micro-pollutant biodegradation using whole metagenome sequencing (WMS). Similarly, Kumar et al. ( 2016 ) revealed that bioremediation of highly polluted wastewater from textile dyes by two novel strains were found to highly decolorize Joyfix Red. They were identified as Lysinibacillus sphaericus (KF032717) and Aeromonas hydrophila (KF032718) through 16S rDNA analysis. More recently, Leddy et al. ( 2018 ) reported that research scientists are making strides to advance the safety and application of potable water reuse with metagenomics for water quality analysis. The application of the bio-computational approach has also been implemented in the advancements of wastewater treatment and disease detection.

A schematic showing the overall conceptual framework on which depicting the computational approach in wastewater treatment

7.2 Computational Fluid Dynamics in Wastewater Treatment

In recent years, computational fluid dynamics (CFD), a broadly used method, has been applied to biological wastewater treatment. It has exposed the inner flow state that is the hydraulic condition of a biological reactor (Peng et al., 2014 ). CFD is the application of powerful predictive modeling and simulation tools. It may calculate the multiple interactions between all the water quality and process design parameters. CFD modeling tools have already been widely used in other industries, but their application in the water industry is quite recent. CFD modeling has great applications in water and wastewater treatment, where it mechanically works by using hydrodynamic and mass transfer performance of single or two-phase flow reactors (Do-Quang et al., 1998 ). The level of CFD’s capability varies between different process units. It has a high frequency of application in the areas of final sedimentation, activated sludge basin modeling, disinfection, and greater needs in primary sedimentation and anaerobic digestion (Samstag et al., 2016 ). Now, researchers are enhancing the CFD modeling with a developed 3D model of the anoxic zone to evaluate further hydrodynamic performance (Elshaw et al., 2016 ). The overall conceptual framework and the applications of the computational approach in wastewater treatment are presented in Fig. 4 .

7.3 Computational Artificial Intelligence Approach in Wastewater Treatment

Several studies were obtained by researchers to implement computer-based artificial techniques, which provide fast and rapid automated monitoring of water quality tests such as BOD and COD. Recently, Nourani et al. ( 2018 ) explores the possibility of wastewater treatment plant by using three different kinds of artificial intelligence methods, i.e., feedforward neural network (FFNN), adaptive neuro-fuzzy inference system (ANFIS), and support vector machine (SVM). Several measurements were done in terms of effluent to tests BOD, COD, and total nitrogen in the Nicosia wastewater treatment plant (NWWTP) and reported high-performance efficiency of artificial intelligence (Nourani et al., 2018 ).

7.4 Remote sensing and Geographical Information System

Since the implementation of satellite technology, the initiation of new methods and tools became popular nowadays. The futuristic approach of remote sensing and GIS technology plays a crucial role in the identification and locating of the water polluted area through satellite imaginary and spatial data. GIS analysis may provide a quick and reasonable solution to develop atmospheric correction methods. Moreover, it provides a user-friendly environment, which may support complex spatial operations to get the best quality information on water quality parameters through remote sensing (Ramadas & Samantaray, 2018 ).

8 Applications of Treated Wastewater

8.1 scope in crop irrigation.

Several studies have assessed the impact of the reuse of recycled/treated wastewater in major sectors. These are agriculture, landscapes, public parks, golf course irrigation, cooling water for power plants and oil refineries, processing water for mills, plants, toilet flushing, dust control, construction activities, concrete mixing, and artificial lakes (Table 5 ). Although the treated wastewater after secondary treatment is adequate for reuse since the level of heavy metals in the effluent is similar to that in nature (Ayers & Westcot, 1985 ), experimental evidences have been found and evaluated the effects of irrigation with treated wastewater on soil fertility and chemical characteristics, where it has been concluded that secondary treated wastewater can improve soil fertility parameters (Mohammad & Mazahreh, 2003 ). The proposed model (Fig. 3 ) is tested partially previously at a laboratory scale by treating the wastewater (from sewage, sugar, and paper industry) in an ultrasonic bath (Kesari et al., 2011a , b ; Kesari & Behari, 2008 ; Kumar et al., 2010 ). Advancing it with ultraviolet and ozone treatment has modified this in the proposed model. A recent study shows that the treated water passed quality measures suited for crop irrigation (Bhatnagar et al., 2016 ). In Fig. 3 , a model is proposed including all three (UV, US, nanoparticle, and ozone) techniques, which have been tested individually as well as in combination (US and nanoparticle) (Kesari et al., 2011a , b ) to obtain the highest water quality standards acceptable for irrigation and even drinking purposes.

A wastewater-irrigated field is a major source of essential and non-essential metals contaminants such as lead, copper, zinc, boron, cobalt, chromium, arsenic, molybdenum, and manganese. While crops need some of these, the others are non-essential metals, toxic to plants, animals, and humans. Kanwar and Sandha ( 2000 ) reported that heavy metal concentrations in plants grown in wastewater-irrigated soils were significantly higher than in plants grown in the reference soil in their study. Yaqub et al. ( 2012 ) suggest that the use of US is very effective in removing heavy or toxic metals and organic pollutants from industrial wastewater. However, it has been also observed that the metals were removed efficiently, when UV light was combined with ozone (Samarghandi et al., 2007 ). Ozone exposure is a potent method for the removal of metal or toxic compounds from wastewater as also reported earlier (Park et al., 2008 ). Application of US, UV, and O 3 in combination lead to the formation of reactive oxygen species (ROS) that oxidize certain organics, metal ions and kill pathogens. In the process of advanced oxidizing process (AOP) primarily oxidants, electricity, light, catalysts etc. are implied to produce extremely reactive free radicals (such as OH) for the breakdown of organic matters (Oturan & Aaron, 2014 ). Among the other AOPs, ozone oxidization process is more promising and effective for the decomposition of complex organic contaminants (Xu et al., 2020 ). Ozone oxidizes the heavy metal to their higher oxidation state to form metallic oxides or hydroxides in which they generally form limited soluble oxides and gets precipitated, which are easy to be filtered by filtration process. Ozone oxidization found to be efficient for the removal of heavy metals like cadmium, chromium, cobalt, copper, lead, manganese, nickel, and zinc from the water source (Upadhyay & Srivastava, 2005 ). Ultrasonic-treated sludge leads to the disintegration of biological cells and kills bacteria in treated wastewater (Kesari, Kumar, et al., 2011a ; Kesari, Verma, & Behari, 2011b ). This has been found that combined treatment with ultrasound and nanoparticles is more effective (Kesari, Kumar, et al., 2011a ). Ultrasonication has the physical effects of cavitation inactivate and lyse bacteria (Broekman et al., 2010 ). The induced effect of US, US, or ozone may destroy the pathogens and especially during ultrasound irradiation including free-radical attack, hydroxyl radical attack, and physical disruption of cell membranes (Kesari, Kumar, et al., 2011a ; Phull et al., 1997 ; Scherba et al., 1991 ).

8.2 Energy and Economy Management

Municipal wastewater treatment plants play a major role in wastewater sanitation and public health protection. However, domestic wastewater has been considered as a resource or valuable products instead of waste, because it has been playing a significant role in the recovery of energy and resource for the plant-fertilizing nutrients like phosphorus and nitrogen. Use of domestic wastewater is widely accepted for the crop irrigation in agriculture and industrial consumption to avoid the water crisis. It has also been found as a source of energy through the anaerobic conversion of the organic content of wastewater into methane gas. However, most of the wastewater treatment plants are using traditional technology, as anaerobic sludge digestion to treat wastewater, which results in more consumption of energy. Therefore, through these conventional technologies, only a fraction of the energy of wastewater has been captured. In order to solve these issues, the next generation of municipal wastewater treatment plants is approaching total retrieval of the energy potential of water and nutrients, mostly nitrogen and phosphorus. These plants also play an important role in the removal and recovery of emerging pollutants and valuable products of different nature like heavy and radioactive metals, fertilizers hormones, and pharma compounds. Moreover, there are still few possibilities of improvement in wastewater treatment plants to retrieve and reuse of these compounds. There are several methods under development to convert the organic matter into bioenergy such as biohydrogen, biodiesel, bioethanol, and microbial fuel cell. These methods are capable to produce electricity from wastewater but still need an appropriate development. Energy development through wastewater is a great driver to regulate the wastewater energy because it produces 10 times more energy than chemical, thermal, and hydraulic forms. Vermicomposting can be utilized for stabilization of sludge from the wastewater treatment plant. Kesari and Jamal ( 2017 ) have reported the significant, economical, and ecofriendly role of the vermicomposting method for the conversion of solid waste materials into organic fertilizers as presented in Fig. 5 . Solid waste may come from several sources of municipal and industrial sludge, for example, textile industry, paper mill, sugarcane, pulp industry, dairy, and intensively housed livestock. These solid wastes or sewage sludges have been treated successfully by composting and/or vermicomposting (Contreras-Ramos et al., 2005 ; Elvira et al., 1998 ; Fraser-Quick, 2002 ; Ndegwa & Thompson, 2001 ; Sinha et al., 2010 ) Although collection of solid wastes materials from sewage or wastewater and further drying is one of the important concerns, processing of dried municipal sewage sludge (Contreras-Ramos et al., 2005 ) and management (Ayilara et al., 2020 ) for vermicomposting could be possible way of generating organic fertilizers for future research. Vermicomposting of household solid wastes, agriculture wastes, or pulp and sugarcane industry wastes shows greater potential as fertilizer for higher crop yielding (Bhatnagar et al., 2016 ; Kesari & Jamal, 2017 ). The higher amount of solid waste comes from agricultural land and instead of utilizing it, this biomass is processed by burning, which causes severe diseases (Kesari & Jamal, 2017 ). Figure 3 shows the proper utilization of solid waste after removal from wastewater; however, Fig. 5 showing greater possibility in fertilizer conversion which has also been discussed in detail elsewhere (Bhatnagar et al., 2016 ; Nagavallemma et al., 2006 )

Energy production through wastewater (reproduced from Bhatnagar et al., 2016 ; Kesari & Jamal, 2017 )

9 Conclusions and future perspectives

In this paper, we have reviewed environmental and public health issues associated with the use of untreated wastewater in agriculture. We have focused on the current state of affairs concerning the wastewater treatment model and computational approach. Given the dire need for holistic approaches for cultivation, we proposed the ideas to tackle the issues related to wastewater treatment and the reuse potential of the treated water. Water resources are under threat because of the growing population. Increasing generation of wastewater (municipal, industrial, and agricultural) in developing countries especially in India and other Asian countries has the potential to serve as an alternative of freshwater resources for reuse in rice agriculture, provide appropriate treatment, and distribution measures are adopted. Wastewater treatment is one of the big challenges for many countries because increasing levels of undesired or unknown pollutants are very harmful to health as well as environment. Therefore, this review explores the ideas based on current and future research. Wastewater treatment includes very traditional methods by following primary, secondary, and tertiary treatment procedures, but the implementation of advanced techniques is always giving us a big possibility of good water quality. In this paper, we have proposed combined methods for the wastewater treatment, where the concept of the proposed model works on the various types of wastewater effluents. The proposed model not only useful for wastewater treatment but also for the utilization of solid wastes as fertilizer. An appropriate method for the treatment of wastewater and further utilization for drinking water is the main futuristic outcome. It is also highly recommendable to follow the standard methods and available guidelines provided WHO. In this paper, the proposed role of the computational model, i.e., artificial intelligence, fluid dynamics, and GIS, in wastewater treatment could be useful in future studies. In this review, health concerns associated with wastewater irrigation for farmers and irrigated crops consumers have been discussed.

The crisis of freshwater is one of the growing concerns in the twenty-first century. Globaly, about 330 km 3 of municipal wastewater is generated annually (Hernández-Sancho et al., 2015 ). This data provides a better understanding of why the reuse of treated wastewater is important to solve the issues of the water crisis. The use of treated wastewater (industrial or municipal wastewater or Seawater) for irrigation has a better future for the fulfillment of water demand. Currently, in developing countries, farmers are using wastewater directly for irrigation, which may cause several health issues for both farmers and consumers (crops or vegetables). Therefore, it is very imperative to implement standard and advanced methods for wastewater treatment. A local assessment of the environmental and health impacts of wastewater irrigation is required because most of the developed and developing countries are not using the proper guidelines. Therefore, it is highly required to establish concrete policies and practices to encourage safe water reuse to take advantage of all its potential benefits in agriculture and for farmers.

Abdel-Fatah, M. A. (2018). Nanofiltration systems and applications in wastewater treatment: Review article. Ain Shams Engineering Journal, 9 , 3077–3092.

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Acknowledgements

All the authors are highly grateful to the authority of the respective departments and institutions for their support in doing this research. The author VT would like to thank Science & Engineering Research Board, New Delhi, India (Grant #ECR/2017/001809). The Author RS is thankful to the University Grants Commission for the National Fellowship (201819-NFO-2018-19-OBC-UTT-78476).

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Kavindra Kumar Kesari and Ramendra Soni contributed equally to this work.

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Department of Applied Physics, Aalto University, Espoo, Finland

Kavindra Kumar Kesari & Janne Ruokolainen

Department of Molecular and Cellular Engineering, Sam Higginbottom University of Agriculture, Technology and Sciences, Naini, Allahabad, India

Ramendra Soni, Jonathan A. Lal & Vijay Tripathi

Department of Health Informatics, College of Public Health and Health Informatics, Qassim University, Al Bukayriyah, Saudi Arabia

Qazi Mohammad Sajid Jamal

Department of Computational Biology and Bioinformatics, Sam Higginbottom University of Agriculture, Technology and Sciences, Naini, Allahabad, India

Pooja Tripathi

Department of Biotechnology, School of Engineering & Technology, Sharda University, Greater Noida, UP, India

Niraj Kumar Jha

Department of Bioengineering, Faculty of Engineering, Integral University, Lucknow, India

Mohammed Haris Siddiqui

Department of Forestry, NERIST, Nirjuli, Arunachal Pradesh, India

Pradeep Kumar

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Kesari, K.K., Soni, R., Jamal, Q.M.S. et al. Wastewater Treatment and Reuse: a Review of its Applications and Health Implications. Water Air Soil Pollut 232 , 208 (2021). https://doi.org/10.1007/s11270-021-05154-8

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DOI : https://doi.org/10.1007/s11270-021-05154-8

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• Bioengineered
• v.14(1); 2023
• PMC10376923

Recent advances in microbial engineering approaches for wastewater treatment: a review

Monika sharma.

a Department of Zoology, University of Jammu, Jammu and Kashmir, India

Sangita Agarwal

b Department of Applied Science, RCC Institute of Information Technology Kolkata, West Bengal, India

Richa Agarwal Malik

c Department of Environmental Studies, PGDAV College, University of Delhi, New Delhi, India

Gaurav Kumar

Dan bahadur pal.

d Department of Chemical Engineering, Harcourt Butler Technical University, Kanpur, Uttar Pradesh, India

Mamun Mandal

e Laboratory of Applied Stress Biology, Department of Botany, University of Gour Banga, Malda, West Bengal, India

Abhijit Sarkar

Farkad bantun.

f Department of Microbiology, Faculty of Medicine, Umm Al-Qura University, Makkah, Saudi Arabia

Shafiul Haque

g Research and Scientific Studies Unit, College of Nursing and Allied Health Sciences, Jazan University, Jazan, Saudi Arabia

j Gilbert and Rose-Marie Chagoury School of Medicine, Lebanese American University, Beirut, Lebanon

k Centre of Medical and Bio-Allied Health Sciences Research, Ajman University, Ajman, United Arab Emirates

Pardeep Singh

Neha srivastava.

h Department of Chemical Engineering and Technology, Indian Institute of Technology (BHU), Varanasi, India

Vijai Kumar Gupta

i Biorefining and Advanced Materials Research Center, SRUC, UK

In the present era of global climate change, the scarcity of potable water is increasing both due to natural and anthropogenic causes. Water is the elixir of life, and its usage has risen significantly due to escalating economic activities, widespread urbanization, and industrialization. The increasing water scarcity and rising contamination have compelled, scientists and researchers, to adopt feasible and sustainable wastewater treatment methods in meeting the growing demand for freshwater. Presently, various waste treatment technologies are adopted across the globe, such as physical, chemical, and biological treatment processes. There is a need to replace these technologies with sustainable and green technology that encourages the use of microorganisms since they have proven to be more effective in water treatment processes. The present review article is focused on demonstrating how effectively various microbes can be used in wastewater treatment to achieve environmental sustainability and economic feasibility. The microbial consortium used for water treatment offers many advantages over pure culture. There is an urgent need to develop hybrid treatment technology for the effective remediation of various organic and inorganic pollutants from wastewater.

• Microbial engineering approaches for wastewater treatment.
• Current and emerging sources of water pollution are discussed.
• Various treatment technologies for wastewater treatment.
• Biological methods and microbes are used for degradation.
• Parameters responsible for the degradations processes of wastewater.

GRAPHICAL ABSTRACT

1. Introduction

World water consumption has almost doubled in a few decades [ 1 ]. The growing concerns over water contamination have led to extensive research and development in water treatment techniques. They are expanding to promote the reuse of water and improve the quality of water for human consumption. Water pollution is a worldwide issue that presents a severe threat to the survival of all life forms. Aquatic pollution can be caused by organic and inorganic impurities and microbiological contaminants. Population growth, industrial and mining activities, sewage and wastewater, radioactive waste, chemical fertilizers, pesticides, urban development, and other anthropogenic sources are all responsible for rising levels of aquatic pollution. Water quality is determined by the concentrations of particles and chemicals in water, such as heavy metals, nutrients, microorganisms, polycyclic aromatic hydrocarbon (PAH), and other pollutants. Many organic contaminants are endocrine-disrupting chemicals associated with testicular, prostate, and breast cancers. They can also cause serious complications in human and animal reproductive health, such as sperm count reduction in males and the production of fragile eggs in females, among other things [ 2 ].

Wastewater treatment has evolved as a feasible technique for tackling water scarcity and protecting the ecosystem from the harmful impacts of polluted/wastewater in the contemporary environment [ 3 ]. Many countries have passed stricter laws for treating sewage water before dumping it into water bodies. From sustainability, improved water management and wastewater recycling have begun to get active attention [ 4 ]. Many physical and chemical processes (flotation, precipitation, oxidation, adsorption, etc.) used for wastewater treatment are expensive, demand high maintenance, and require a complicated functional setup. As a result, there is an urgent necessity to move to green and sustainable technologies such as microbial wastewater treatment to serve as a long-term alternative to traditional wastewater treatment methods [ 3 ]. Wastewater treatment using microbes such as fungi, bacteria, microalgae, and others has caught the researcher’s attention in recent years. The presence of a significant amount of nutrients such as nitrogen, phosphorus, and carbon in wastewater emanating from different sources can aid in the growth and survival of these microbes [ 5 ].

Phytoremediation is frequently used to remediate wastewater. On the other hand, excessive salts in wastewater can induce plant toxicity. To solve the problem, Sarawaneeyaruk et al. [ 6 ] isolated multifarious plant growth-promoting bacteria (PGPB) Bacillus spp from the municipal wastewater and by using this PGPB they successfully enhanced plant growth under municipal wastewater irrigation. Hence, such green technology would be sustainable and help maintain a balance between socio-economic and environmental perspectives [ 3 ].

Food, water, and energy are interconnected and wastewater is crucial in this nexus. Although various wastewater treatment methods are available the aim of this review is to draw attention on the importance of microbes in wastewater. The goal of this review is to help in protecting water resources using effective treatment method. This review illustrates the wastewater treatment process by utilizing microorganisms (bacteria, fungi, microalgae, and yeast), highlighting the advantages and applications of microbes over other conventional approaches. This review further aims to provide useful information to researchers working in relevant fields worldwide to pique their interest in using microbes to improve and cost-effectively treat wastewater ( Table 1 ).

Categories of water pollutants and their probable effects are tabulated as.

2. Emerging sources of water pollution

A variety of organic contaminants can be found in water, including insecticides, herbicides and organohalides [ 7 ]. Industrial effluents contain inorganic pollutants such as silt from stormwater runoff and heavy metals from acid mine drainage. Different sources of domestic waste enter water bodies. Pesticides used in gardens and lawns may also enter the water bodies [ 8 ]. Cleaning products, detergents, and skincare products contain significant amounts of pollutants that can pollute water bodies and make them unfit for human consumption [ 9 ]. Chemicals and acids from industries like steel and paper are discharged into rivers [ 10 ]. Water bodies receive over 70% of industrial waste, containing many toxins [ 8 ]. Major agricultural wastes are fertilizers, pesticides (herbicides, insecticides), and other agrochemicals. Fertilizer production continues to rise year after year to increase productivity, resulting in increased waste generation. Irrigation contributes significantly to surface water pollution in China and is also a cause of nitrogen groundwater pollution in the United States [ 9 ]. Toxic chemicals can accumulate in the body and eventually reach toxic levels, causing food chain disruption. Another major source of water pollution is nutrient enrichment.

3. Emerging technologies for wastewater treatment

Physical treatment of wastewater entails removing contaminants from the water without affecting the biochemical properties of contaminants. Physical treatments usually follow chemical and biological treatments. Screening, Flotation, Flow equalization, Membrane-based technology, Thermal treatment, and other physical treatment techniques are commonly used. Some of the common chemical unit processes used in wastewater treatment technology include precipitation, adsorption, disinfection, chlorination, neutralization, chemical exchange, etc. [ 14 ] to bring changes in the quality of water.

3.1. Biological treatment for the wastewater

Biological treatment involves removing contaminants from wastewater using biological organisms or processes. Microbes are critical to wastewater treatment and reclamation, making them a promising green technology tool. The biological treatment uses bacteria, fungi, microalgae, yeast, and other microbial groups. Biological treatments are less expensive than physical and chemical treatments Table 2 [ 15 ]. Among the most widely used biological wastewater treatment methods are:

Showing advantages of biological methods in wastewater treatment [ 17 ].

3.1.1. Activated sludge method

One of the most commonly used biological processes in wastewater treatment to date is the activated sludge. It has been effectively used to treat industrial and municipal wastewater. This process uses a biological floc that consists of bacteria and protozoa under aerobic conditions [ 16 ]. The basic principle behind all activated sludge processes is that microbes grow within metabolizing organic material where they form clumps. The oxidizable matter is used as food for the microorganisms forming a suspended floc in the wastewater. The aeration/agitation provides the continued oxygen supply. The mixed liquor, which is the mix of wastewater and activated sludge, is allowed to settle down to segregate the activated sludge solids from treated wastewater while a part of the settled activated sludge is returned to the aeration site. The entire activated sludge process comprises interlinked elements such as an aeration tank, source of aeration, clarifier, and a collection system. The biological reaction occurs in an aeration tank fitted with a stirrer for mixing and source of oxygen, which is connected to a tank/clarifier where the settled solids are segregated from treated water along with a collection loop which either returns the activated sludge back to the aeration tank or is removed from the process. The process has high efficiency and can also be used for nutrient removal [ 18 ].

The activated sludge process is used in the treatment of industrial wastewater as well as domestic. In spite of having advantages like low operational cost with added treatment efficiency, the major drawback is the generation of excess quantity of waste activated sludge. The organic matter generated by the process needs to be properly treated and managed to reduce the ecological and financial burden [ 19 ].

3.1.2. Bioreactors and biofilters

Using physical retention and microbial biodegradation, membrane bioreactors eliminate pollutants from wastewater [ 20 ]. Biofilters use biological processes to filter wastewater [ 21 ]. It grows on top of the media, which is composed of gravel, sand, and ceramic. For example, a biofilm can contain a microbial (bacterial) community that helps to decompose organic content in water [ 37 ]. This process has been extensively used to remove H 2 S from municipal wastewater, according to Zhang et al. [ 38 ].

3.1.3. Biosorption

Certain biological molecules naturally can accumulate metals like copper, zinc, nickel, chromium, palladium from wastewater [ 39 ]. The process of biosorption is complex and involves various interactive mechanisms like ion-exchange, absorption, precipitation, and complexation through the participation of functional groups like hydroxyl, carbonyl, etc. [ 40 ]. It is a reversible process that involves interactions rather than oxidation to bind the biosorbent in an aqueous solution.[ 41 ]

The biosorbent is suspended in a contaminant solution (e.g. metal ions). After a while, contaminant-rich biosorbent can be separated. Microbes immobilize on adsorbants to form a biosorbent that captures contaminants [ 42 ].

Agricultural waste, microbial biomasses, industrial-by-products offer advantages over chemical methods in terms of efficiency, large abundance and low cost [ 43 ]. Factors like pH, concentration of the metals, ionic strength, other pollutants present in wastewater, temperature, etc., effect the process of biosorption [ 44 ]

4. Different microbial groups in wastewater degradation

Microbial treatment can be used instead of traditional wastewater treatment methods because it is cheaper, more efficient, and more competent [ 45 ]. Bioremediation involves bacteria, fungi, microalgae, yeast, etc. [ 46 ]. These microbes are responsible for degrading or converting contaminants into lesser harmful products [ 47 ]. They have become ideal bioagents of remediation owing to their high surface area-to-volume ratio, small size, and substantial surface area [ 48 ]. These microbes use biosorption and bioaccumulation to bioremediate. Adsorption occurs when pollutants (metals) interact with functional groups on the cell surface [ 49 ]. Biosorption can use both live and dead biomass. Bioaccumulation involves intracellular and extracellular processes. Toxins bioaccumulate when they are absorbed from the environment. Bioaccumulation uses only living biomass, limiting its reuse, and costing more than biosorption [ 50 ].

4.1. Bacterial removal of organic and inorganic pollutants

The treatment of wastewater effluents is based on the capability of bacterial cells to concentrate pollutants (metals). The microbial population and xenobiotic content determine the rate of biodegradation. Plants feed rhizosphere microbes’ organic carbon, which helps degrade pollutants. Aquatic plants’ biofilms can degrade organics like phenols, amines, and aliphatic aldehydes [ 51 ]. Methanotrophs use methane to obtain carbon and energy and break down various harmful organic compounds [ 52 ]. Eichhornia crassipes can help clean up eutrophic water by influencing nitrogen production [ 53 ]. Tolypothrixceytonica and Anabaena oryzae have also been shown to be effective in treating industrial wastewater [ 54 ]. Aphanocapsu sp . and Plectonema sp . have the ability to degrade crude oil [ 55 ]. Anaerobic bacteria in sewage treatments include sulfate-reducing bacteria like Desulfovibrio, Desulfotomaculum, Desulfobacter, and Desulfococcus genera [ 56 ].

The factors like abundance, size, growth under controlled conditions and resistance to environmental changes have marked bacteria as important biosorbents [ 57 ]. Metal ion biosorption into the cell wall can be active or passive. Passive biosorption occurs in both living and dead/inactive bacterial cells [ 58 ]. Active biosorption includes metal ion uptake within living bacterial cells. Metal ion binding involves ion exchange, chelation, complexation, and micro precipitation [ 59 ].

4.1.1. Dye degradations

Synthetic dyes have many advantages over natural dyes in terms of color variety, speed of coloration, absorption and water solubility [ 60 ], which explains the global dye production of 800,000 tons per year [ 61 ]. The impact of textile effluents on the overall health of the aquatic ecosystem is growing in concern as dye demand and production rise. Textile wastewater contains inorganic and organic additives and chemicals [ 62 ] as well as dyes [ 63 ] in concentrations ranging from 10 to 200 mg/L. In textile industries, the azo dyes (70%) are commonly used because of their low cost and ease of use. Since all dyes do not fix to fabrics during dyeing, unfixed dyes are washed out and found in high concentrations in effluents [ 64 ]. Bacterial-assisted dye degradation is nontoxic and can decolorize colored complex dyes. Table 3 lists some studies on bacterial dye degradation.

Shows the bacterial degradation of dyes and the mechanism involved.

Aeromonas hydrophila, Bacillus subtilis, Bacillus cereus have been studied effectively and have the potential for bioremediation of azo dyes [ 65 ]. Under anoxic conditions, Pseudomonas sp., Pseudomonas luteola, Proteus mirabilis have the ability to degrade azo dyes [ 66 ]. These bacteria utilize oxidoreductive enzymes for dye degradation. Aerobic bacteria use oxygen-catalyzed azoreductase to break the azo bonds [ 67 ]. Some bacterial strains degrade dyes in aerobic conditions and use mono and dioxygenase for oxidizing the aromatic ring of organic compounds [ 68 ]. Anaerobic bacteria use the enzyme azoreductase to degrade azo dyes. And generally, anaerobic conditions favor decolorization (Chang et al., 2001b). Mostly first-order kinetics is followed with respect to the concentration of the dye in the decolorization reaction and in some; zero-order kinetics is also seen [ 69 ]. The oxidoreductive enzymes also are involved in hydroxylation, desulfonation, and deamination. Pseudomonas aeruginosa could decolorize various azo dyes [ 70 ] and Navitan Fast Blue S5R under aerobic conditions.

4.1.2. Petrochemicals degradation

Petroleum hydrocarbons are divided into resins, asphaltenes, aromatics, and saturates [ 79 ]. Their degradation by microbes ( Figure 1 ) is complex, dependent on the nature and number of hydrocarbons available. The biodegradation of hydrocarbons is determined by agents such as temperature and concentration of inorganic nutrients such as phosphorus, nitrogen, and iron in some instances [ 80 ]. The vulnerability of hydrocarbons to attack by microbes is different, with linear alkanes being most susceptible and cyclic alkanes the least [ 81 , 82 ]. Polycyclic hydrocarbons having higher molecular weight might not be degraded [ 83 ]. Acinetobacter sp. degrades n-alkane having chain length C 10 -C 40, utilizing carbon as the sole source [ 84 ]. Mycobacterium , Burkholderia, Gordonia , Brevibacterium , Dietzia, Aeromicrobium, Pseudomonas, Aeromonas, Flavobacteria, Nocardia, Modococci, Chrobacteria, Moraxella, Cyanobacteria, Streptomyces, Bacilli, Arthrobacter , and other bacteria can degrade petroleum products [ 85 ]. The poly-aromatic hydrocarbons could be degraded by Sphingomonas [ 86 ]. The biodegradation efficiency of soil bacteria [ 87 ] and marine bacteria [ 88 ] are not the same. The microbes utilize specific enzymes systems (oxygenase, peroxidase, and hydroxylase) in degrading the petroleum hydrocarbons in aerobic conditions, and it starts with the attachment of microbial cells onto the substrates and is followed by the production of biosurfactants [ 89 , 90 ]. Biosurfactants are synthesized by various microorganisms ( Table 4 ) and are heterogeneous surface-active compounds. Biosurfactants are involved in enhancing the solubility and, finally, the removal of the contaminant [ 91 ]. They augment the surface area and the amount of oil available for the bacteria to utilize [ 92 ] and decrease surface tension to help form micelles.

Microorganism mediated degradation of hydrocarbons.

Biosurfactants produced by the various microbes for degradation of petroleum hydrocarbons.

4.1.3. Pharmaceutical and personal care products

Pharmaceuticals and personal care products (PPCPs) are emerging persistent pollutants [ 93 ]. Pharmaceuticals have increased steadily globally [ 94 ], especially since the COVID-19 pandemic. Individuals contribute PPCPs to the aquatic ecosystem ( Figure 2 ) by using sanitizers, shampoos, household cleaners, detergents, and medicines. PPCPs are complex and persistent molecules that reenter the hydrologic cycle, increasing antibacterial resistance, reproductive abnormalities, and tumor growth. These unregulated pollutants persist in water bodies, and many metabolites are converted back to their parent form [ 95 ]. They precipitate specific pollutants into complex and toxic forms that easily spread in aquatic phases. Figure 2 shows the circulation of PPCP in the surrounding [ 96 ]. The breakdown of PPCPs by microorganisms is difficult because pharmaceuticals were designed to be toxic to bacteria [ 97 ]. Nonetheless, some native bacterial species can help degrade pharmaceutical pollutants [ 98 ]. Microbes reduce or degrade the complex structure to a nontoxic or less toxic form.

Representative diagram of pharmaceuticals and personal care products circulating in the environment adopted from [ 73 ].

4.1.4. Pesticides

Pesticides are chemicals utilized to kill pests and are classified based on their functions (herbicides, algicides, fungicides, bactericides, nematicides, rodenticides, and insecticides) [ 99 ]. Chemical classes of organic pesticides include organophosphorus, organochlorine, carbamates, acetamides, neonicotinoids, pyrethroids, triazoles, and triazines. Inorganic pesticides include lead arsenate, and boric acid complexes, etc. Organochlorine compounds like chlordane, DDT, toxaphene, and heptachlor have been included in the list of persistent organic pollutants [ 100 , 101 ].

It has been reported by many researchers that effectively, less than 5% of total used pesticides are involved in targeting the pests while the rest of them are precipitated in the surrounding water and soil [ 102 ]. The pesticides left in the ecosystem have a detrimental effect on the ecosystem [ 103 ] and need to be removed. The chemical and physical methods of pesticide removal are unsustainable [ 104 ], hence bacteria could function as bio-weapon to fight toxic agricultural chemicals [ 105 ]. Various studies have elaborated on the role of bacteria in bioremediating pesticides, viz. Endosulfan removal by Bacillus and Staphylococcus [ 106 ]; Malathion removal by Arthrobacter sp ., Pseudomonas putida [ 107 ]; Ridomil and Fitoraz removal by Pseudomonas putida and Acinetobacter sp . [ 108 ]; Napthelene removal by Cyanobacteria [ 109 ]; Endosulfan removal by Staphylococcus aureus , Achromobacter sp ., Rhodococcus sp . [ 110 ]; Malathion, Ridomil and Fitoraz removal by Pseudomonas putida , Rhodococcus and Arhtrobacter sp . [ 111 ].

4.1.5. Heavy metals degradation

Heavy metals such as lead (Pb), cadmium (Cd), chromium (Cr), arsenic (As), and mercury (Hg) are ubiquitous environmental pollutants, having high toxicity and density. Natural and anthropogenic sources of heavy metal pollution cause detrimental effects on all living beings [ 112 ]. The microbial cells require cations for numerous cellular activities, but increasing concentration may retard growth by forming internal complexes [ 113 ]. Bacteria have the ability to immobilize and also mobilize, transform and uptake heavy metals [ 114 ]. Many studies have been published on the role of endophytic bacteria in heavy metal bioaccumulation and detoxification [ 115 , 116 ]. These studies show that bacteria secrete organic acids to help with the bioremediation process. Bacteria also produce biosurfactants released as root exudates and increase metal bioavailability in aquatic environments [ 117 ]. It was found that glutathione was involved in the intracellular sequestration of cadmium ions in the cells of Rhizobium leguminosarum [ 118 ]. Heavy metals can be reduced to less or nontoxic metals by iron-reducing bacteria like Geobacter sp. and sulfur-reducing bacteria like Desufuromonas sp. Sulfate-reducing bacteria and metal-reducing bacteria, for example, can convert chromium from the highly toxic Cr (VI) to the less toxic Cr (III) [ 119 ]. Sulfate-reducing bacteria produce a lot of hydrogen sulfide, which causes metal cations to precipitate [ 120 ]. Vibrio harveyi strain could precipitate the divalent lead as a salt of lead phosphate [ 121 ]. Many ionizable cell wall groups can help bacteria absorb metal ions (amino, carboxyl, phosphate, and hydroxyl gp). In metal remediation, microbial methylation is also important. For example, Bacillus sp., Clostridium sp., Pseudomonas sp., and Escherichia sp., can biomethylate Hg (II) [ 122 ]. Various heavy metals respond to the microorganism differently depending on the conditions. Some bacterial cells produce siderophores, and they form metal complexes, limiting their bioavailability and removing their toxicity [ 123 ]. Some of the bacterial species involved in the bioremediation of heavy metals ( Table 5 ) have been tabulated.

Some Bacterial species used for the removal of heavy metals.

4.2. Fungi and yeast

Fungi can help in the removal of pollutants (heavy metals) by increasing their bioavailability and converting them to lesser toxic forms [ 124 ]. Fungi are simple to grow and produce a significant amount of biomass. Several fungal strains have shown the ability to digest a variety of environmental contaminants, including dyes, pharmaceutical drugs, aromatic hydrocarbons, and heavy metals [ 125 , 126 ]. The two important characteristics of fungi that make them an ideal candidate for wastewater treatment are the secretion of many extracellular enzymes [ 127 ] and the hyphal mesh of fungi that protects the internal sensitive organelles from the ill effects of contaminants. Fungi are drawn to the rhizosphere by root exudates. Many factors influence plant-fungi interactions in the rhizosphere, including soil characteristics, plant species, water type, climate, and other microorganisms [ 128 ]. Plant-fungi interactions perform a variety of important functions, including metal-chelating siderophores emission, denitrification, and detoxification ( Figure 3 ). The organic wastes are transformed into industrially important biochemicals and other valuable compounds by fungi, which is an advantage of using fungal culture in wastewater treatment over bacterial culture (proteins, organic acids). Animal feed can also be made from fungal biomass [ 129 ]. Pleurotus pulmonarius, Stachybotrys sp., Cephalosporium aphidicola, Aspergillus parasitica, Verticillum terrestre, Candida sp., Acremonium sp., Glomus sp., Minimedusa sp., Talaromyces, Hydnobolites, Peziza , and other fungal species can be used in wastewater treatment [ 130 ]. Table 6 shows the effective fungal degradation of wastewater.

A schematic diagram of wastewater treatment by microorganisms with their different bioremediation mechanisms.

Some common fungal species used in pollutant remediation in wastewater.

Over the last years, many research studies have suggested the effective role of ligninolytic fungi in degrading synthetic dyes [ 131 ]. Interestingly, fungi possess ligninolytic enzymes that degrade complex dyes, including laccase, manganese peroxidase, and lignin peroxidase. Some research studies reflecting fungi’s role in degrading dyes have been elucidated in Table 7 .

Shows the degradation of some dyes by fungi along with the mechanism involved.

Many studies have shown that yeast can be used to eliminate pollutants (heavy metals) from the environment [ 132 ]. Yeast can also help reduce COD levels and remove mono and polyphenols [ 133 ] because yeast can absorb, accumulate, and degrade toxic compounds into nontoxic forms. It can be used to treat textile wastewater. Saccharomyces cerevisiae, Galactomyces geotrichum , Trichosporon beigelii , and Candida krusei can degrade dyes in wastewater [ 134 ].

4.3. Microalgae

These include the use of eukaryotic algae and cyanobacteria for biological wastewater treatment [ 135 ]. The term ‘phycoremediation’ refers to the use of algal species for bioremediation. Chlorella sp., Picochlorum sp., Tetraselmis sp., Scenedesmus sp., and other algal and cyanobacterial strains like Anabaena sp., Oscillatoria sp., Spirulina sp., Chroococcus sp., Pseudospongiococcus sp., Scytonema sp., Dolichospermum [ 136 ] are used in wastewater treatment. Microalgae have the following characteristics that make them an ideal candidate for wastewater treatment:

• Capability to utilize both inorganic and organic carbon, nitrogen, and phosphorus present in wastewater for growth [ 137 ].
• The life cycle is short and requires less nutrients [ 164 ].
• Scope of re-using algal biomass through adsorption/desorption mechanism [ 165 ].
• The growth of algal biomass is independent of environmental conditions, hence can be produced throughout the year [ 166 ].
• The efficiency of algal biomass is better than membranes to remove heavy metals [ 167 ].
• Source of Oxygen and helps in degradation process by heterotrophic bacteria [ 168 ].
• Useful in both anaerobic and aerobic effluent treatment plants [ 169 ].

Depending on the nutrient source, capital investment, and culture conditions (biofuels, CO 2 capture), microalgae culture-based wastewater treatments can be open or closed.

4.3.1. Open type

Algae are grown in open systems in places like ponds, lagoons, and deep channels. Natural (ponds, lagoons) or artificial (man-made ponds, tanks, containers) sites can be used. For domestic and industrial wastewater treatment, stabilization ponds containing bacteria and microalgae culture are most commonly used in temperate and tropical climates. Many studies have demonstrated the effective use of open microalgal cultured treatment plants in treating wastewater [ 170 ].

4.3.2. Closed systems

Microalgae is grown in closed environments in such systems. Photobioreactors are one example of such a system. Reduced water evaporation, higher biomass yield, and contamination elimination are all advantages of closed-type treatment over open type [ 171 ]. A pilot-scale tubular bioreactors are used to grow a diverse range of microalgae, including Arthrospira sp., Chlorella sp., Haematococcus sp., Spirulina and Phaeodactylum sp [ 172 ].

Algal biosorbents have a high sorption capacity [ 173 ]. Using algae-based biosorption, heavy metal ion extraction from wastewater could be an environmentally friendly, cost-effective, and efficient method [ 174 ]. Textile wastewater contains algae cultivation nutrients (phosphates, nitrates, micronutrients, etc.) as well as organic dyes [ 175 ]. Many studies ( Table 8 ) have shown that microalgae can remove pollutants from wastewater; for example, C. vulgaris and S. quadricauda can remove nitrate [ 190 ]; Chlorella sp., Scenedesmus sp., Cosmarium sp. for wastewater treatment [ 191 ]; Chlorella sp., Scenedesmus sp., Cosmarium sp. for treatment of wastewater (aquaculture wastewater and textile wastewater [ 192 ]. According to Ojha et al. [ 207 ], C. vulgaris and S. quadricauda cultures can be used for wastewater remediation.

Some microalgae species used in wastewater treatment.

Organic dyes are major pollutants in water. They can be found in many manufacturing industries, including textiles, plastics, and medicines. These dyes, when accumulated in aquatic systems, results into eutrophication and limited reoxygenation capacity. The production of poisonous amines during the decomposition of dyes is one of the most serious concerns [ 208 ]. Microalgae and Cyanobacteria represent a possible option for the bioremediation of wastewater. Microalgae decolorize dyes by adsorption or degradation. Microalgae can make use of wastewater dyes and nutrients. During the bioconversion process, microalgae can consume the dyes as a source of carbon and convert them to metabolites. The degradation of dyes by microalgae has been elucidated through some research studies in ( Table 9 ).

Shows the degradation of some dyes by microalgae along with the mechanism involved.

5. Factors affecting microbial biodegradation

Microbes can degrade various physical and chemical wastes through removal, alteration, immobilization, or detoxification. Microbes play a role because of their enzymatic pathways. Many factors influence bioremediation efficiency, including soil type, temperature, pH, oxygen, and other electron acceptors, nutrients, biological factors, and so on.

5.1. Environmental determinants

Temperature is the most important of all the physical factors that influence microorganism survival [ 209 ]. Microbial enzymes involved in biodegradation require the right temperature to metabolize substances. The rate of microbial activity increases as the temperature rises and peaks at the optimum temperature. The temperature of water influences various processes such as mineralization, diffusion, and chemical reactions [ 210 ]). Temperature extremes can kill bacteria and other microbes, affecting their growth [ 211 ]. Increases in temperature within the optimum range raise the reaction temperature, thereby increasing the solubility of contaminants, improving diffusion, and so on. The bacterial consortium of Bacillus pumilus HKG212 and Zobellella taiwanensis AT was used by [ 212 ] to degrade reactive green 19, and their findings revealed that the highest degradation occurs at 32.04°C.

The measurement of pH indicates microbial growth potential [ 224 ]. The pH range determines the survival of bacterial species, and thus bioremediation. Acidophilic, neutrophilic, and alkaliphilic biodegrading bacteria require acidic, neutral, and basic media for optimal activity [ 225 ]. According to [ 226 ], the pH of the affected site can be changed to achieve the desired biodegradation results. At pH 4.5, they were able to degrade malachite green by 98% using RuO 2 -TiO 2, and Pt coated Ti mesh electrodes.

Moisture has an impact on the rate of biodegradation because moisture affects the content and concentration of soluble materials available, as well as the osmotic pressure and pH of aquatic systems.

Different microbes require different oxygen levels, such as aerobic, anaerobic, and semi-anaerobic conditions. In most cases, the presence of oxygen can help with hydrocarbon metabolism. Some contaminants, such as petroleum hydrocarbons in wastewater, inhibit bacterial growth by reducing compressed oxygen and electron acceptors [ 227 ]. Although shaking can improve oxygenation, delivering enough oxygen for the biodegradation of organic pollutants is a part of an operational issue and is costly [ 228 ].

An optimum quantity of nutrients and other chemicals is important for microbial metabolism [ 229 ]). Additional input of nutrients changes the nutrient balance for microbial growth, affecting the rate and effectiveness of biodegradation [ 230 ]. Microbes require various nutrients, including carbon, nitrogen, and phosphorus, to survive and continue their metabolic activities [ 231 ]. Varjani et al. [ 232 ] identified phosphorus as a critical factor in microbe growth.

5.1.1. Contaminant concentration

The type and number of contaminants can have an impact on biodegradation. A high biodegradation rate can be achieved by increasing the contaminant concentration [ 233 ]. Heavy contaminants, such as oil petroleum-containing wastewater, have been fatal to the microbial community and negatively impact their biocatalytic activity. Low molecular weight contaminants with simple structures can achieve a high bioremediation rate [ 234 ]. Kerosene, for example, can be completely biodegraded at optimal concentrations due to its simple structure and low molecular weight [ 235 ].

5.1.2. Salinity

According to [ 236 ] organic pollutants present in wastewater which contain alkaline chemicals biodegrade very slowly due to their ability to persist in waste. Contaminants with a high salt content may reduce biodegradation activity by inhibiting biological movement [ 237 ].

5.2. Biological factors

Biological factors influence the breakdown of organic pollutants as microorganisms compete for limited carbon sources, and antagonistic interactions between microbes exist. Major biological factors that affect the bioremediation activity of microbes include enzyme activity, interactions (competition, succession, and predation), population size and composition, mutation, etc. [ 238 ]. The rate of biodegradation is dependent on the substrate as well as the biocatalyst [ 239 ] and the specificity of the enzyme. Inhibition of enzymatic activities due to several factors like competition for carbon and nutrient sources can affect the biodegradative activity of microbes [ 240 ].

6. Microbial consortium: emerging technology

The microbial consortium is the emerging biotechnology-based green approach. Using a single microbe strain to treat wastewater may not give effective results, and efficiency can be compromised. Thus, many research findings have proposed applying microbial consortia [ 241 , 242 ]. Consortia comprising different groups of environmental microbes capable of degrading pollutants in wastewater can be an effective choice. Such consortiums have many advantages over the application of a single strain like fast removal, assistance in secondary application of treated wastewater, along with promoting ecological sustainability.

In the natural habitat, biofilm is formed by aggregating different groups of microbes attached through exopolymeric substances. The whole system is synergistic with microbial partners’ contributing toward forming a strong community [ 243 ].

The development of consortia is an emerging approach for wastewater treatment. The algal-bacterial consortium has many advantages owing to its biomass refiniability and reduced power consumption [ 244 ]. The fundamental principle in the microbial community is utilizing beneficial relationships which are promoting in pollutant removal from wastewater. The synergy is observed in the relationship wherein bacteria are involved in BOD removal, and algae remove nitrogen and phosphorus by absorption [ 245 ]. The relationship established between algae and bacteria provides a suitable ground for bioremediation [ 246 ]. Photosynthesis is undertaken by cyanobacteria bacteria and converts inorganic carbon present in wastewater to organic carbon [ 247 ]. The CO 2 produced by bacterial oxidation serves as the carbon source for photosynthetic algae. Decomposers like Acinetobacter can remove BOD and oxidize organic carbon sources into CO 2, which serves in algae growth [ 248 ].

Extensive research findings have supported microbial consortium as a potential candidate for wastewater treatment [ 249 ]. Recent experimental studies conducted by [ 250 ] revealed the application of Ecobacter bacterial consortium facilitated the bioaugmentation for the biological removal of nitrogen compounds; showing ammonium was transformed by the microorganism reduction reaction; thus, presented decrease in the concentration of ammonium at the end of the treatment period. In their studies, Qi et al. [ 251 ] proved effectively that a well-established microbial (algal-bacterial) consortium in the phycosphere can be optimized and used in advanced wastewater treatment. The results of the research conducted by [ 252 ] showed that treatment of paper pulp wastewater by microbial consortium between microalgae and bacteria allowed good efficiency in removal of organic matter and nutrients. Rehman et al. [ 253 ] studied microbial consortium with Klebsiella sp. LCR187, Bacillus subtilis LOR166, Acinetobacter sp. BRS156 and Acinetobacter junii TYRH47 and Typhadomingensis and Leptochloafusca to treat oil field wastewater. Tara et al. [ 254 ] reported greater than 90% removal efficiency of pollutants from textile wastewater using microbial consortium. Leong et al. [ 255 ] reported 94% pollutant removal efficiency from municipal wastewater using microalgae consortium with bacteria. Microbes carry out the degradation through the secretion of various enzymes and organic acids [ 256 ]. Monica et al. [ 257 ] used Effective Microorganism (EM), which comprises Lactobacillus , Aspergillus, Pseudomonas , Streptomyces , and Saccharomyces , for biodegradation of sewage load in the water. Lactobacillus does the breakdown of lignin and cellulose in this consortium. Pseudomonas releases bioactive compounds which act on sewage and detoxifies or precipitate the metal. Aspergillus decomposes organic matter rapidly, producing alcohol and esters. Table 10 shows the effective utilization of consortia between microbes for treating wastewater from various sources.

Microbial consortium for treatment of wastewater.

7. Conclusion

Water contamination from various sources has become a serious problem around the world. The use of microbes as a treatment for water pollution is a viable alternative. Microbial remediation is an evolutionary and revolutionary technique for wastewater treatment that is currently in use. Microbes (bacteria, fungi, algae, and yeast) are naturally occurring and thus offer a long-term solution to the problem of water pollution. Lack of appropriate information about microbes’ metabolic capacity to degrade contaminants and a lack of controlled conditions such as temperature, pH, the appropriate number of contaminants, nutrients, and more time consumption are all possible limitations with the use of microbes in the treatment process. If the process is not controlled, contaminants may not be completely degraded, resulting in toxic byproducts. As a result, appropriate inside characterization can be an effective way to overcome the drawbacks of microbial-assisted wastewater remediation. Success in microbial wastewater treatment can also be attributed to advances in genetic engineering. Engineered microbial strains with high metabolic potential and well-understood detoxification pathways will undoubtedly aid in combating the wastewater threat to the greatest extent possible. Although several studies have been performed on the use of microbial consortium like microalgae-bacterial systems for the treatment of waste water, but still there is a need for further research in optimizing parameters for large-scale units. Maintaining the stability of the consortium is the main challenging task. More emphasis should be placed on some parameters viz. on selection of capable microbial strains, modeling the system in the long run and optimizing operational parameters, techno economic feasibility, etc. The present era demands to develop environmentally friendly technology that is also commercially viable. Engineered microbes must be integrated from the scientific stage to the practical and pilot stage in order to make significant advances in the use of microbes in the wastewater treatment process. Effective coordination across various disciplines and updated technologies are required to develop better environmental management in the near future.

Acknowledgments

Authors acknowledge to Department of chemical engineering IIT (BHU) Varanasi and PGDAV Delhi University for providing facilities.

Funding Statement

The author(s) reported that there is no funding associated with the work featured in this article.

Disclosure statement

No potential conflict of interest was reported by the authors.