Wastewater Systems Research Articles

Multidimensional Insights into Organics Stress on Anammox systems: From a "Molecule-Cell-Ecology" Perspective.

Anaerobic ammonium oxidation (anammox) is efficient and cost-effective for treating high-strength ammonia wastewater, but the organics in wastewater will affect its stability. To address this challenge, it is crucial to gain a deep understanding of the inhibitory effects and mechanisms of organics stress on anammox bacteria. The review provided a comprehensive classification of organics and evaluated their specific effects on the anammox system according to their respective characteristics. Based on the micro to macro perspective, the "molecule-cell-ecology" inhibitory mechanism of organics on anammox bacteria was proposed. The molecular observation systematically summarized the binding process and action sites of organics with anammox bacteria. At the cellular observation, the mechanisms of organics effects on extracellular polymeric substances, membranes, and anammoxosome of anammox bacteria were also expounded. At the ecological observation, the dynamic changes in coexisting populations and their role in organics transformation were further discussed. Further revelations on response mechanisms and inhibition mitigation strategies were proposed to broaden the applicability of anammox systems for organic wastewater. This review offered a multidimensional understanding of the organics inhibitory mechanism of anammox bacteria and provided a theoretical foundation for anammox systems.

Successful control of an environmental reservoir of NDM-producing Klebsiella pneumoniae associated with nosocomial transmissions in a low-incidence setting

BackgroundThe hospital wastewater system has been reported as a source of nosocomial acquisition of carbapenemase producing Enterobacteriaceae (CPE) in various settings. Cleaning and disinfection protocols or replacement of contaminated equipment often fail to eradicate these environmental reservoirs, which can lead to long-term transmission of CPE. We report a successful multimodal approach to control a New Delhi metallo-beta-lactamase positive Klebsiella pneumoniae (NDM-KP) nosocomial outbreak implicating contamination of sink traps in a low-incidence setting.MethodsFollowing the incidental identification of NDM-KP in a urine culture of an inpatient, we performed an epidemiological investigation, including patient and environmental CPE screening, and whole genome sequencing (WGS) of strains. We also implemented multimodal infection prevention and control (IPC) measures, namely the isolation of cases, waterless patient care, replacement of contaminated P-traps and connecting pieces, and bleach and steam disinfection of sinks for 6 months, followed by patient and environmental screenings for eradication.ResultsBetween February and May 2022, five NDM-KP cases were identified in an eight-bed neurosurgical intermediate care unit. Among the eight sink traps of the unit, three were positive for NDM-KP. Patient and environmental isolates belonged to multilocus sequence typing ST-268. All isolate genomes were genetically very similar suggesting cross-transmission and a potential role of the environment as the source of transmissions. Following the introduction of combined IPC measures, no new case was subsequently detected and sink traps remained negative for NDM-KP within 6 months after the intervention.ConclusionThe implementation of multimodal IPC measures, including waterless patient care combined with the replacement and disinfection of P-traps and connecting pieces, was successful in the control of NDM-KP after eight months. In a low-incidence setting, this approach has made it possible to pursue the objective of zero transmission of carbapenemase-producing Enterobacteriaceae (CPE).

Sustainability Prediction by Evaluating the Emergy of a Co-Treatment System for Municipal Wastewater and Acidic Water Using Intermittent Electrocoagulation

The objective of this research was to evaluate the sustainability of a co-treatment system that combines Municipal Wastewater (MW) and Acid Mine Drainage (AMD) through the technique of intermittent electrocoagulation, applied as an advanced solution to improve contaminant removal efficiency and optimize energy balance. Four scenarios were analyzed: Treatment I (with a 1/7 ratio of urban wastewater to AMD), Treatment II (which includes an artificial wetland), Treatment IIIa (which introduces electrocoagulation to enhance sulfate removal and pH regulation), and Treatment IIIb (which employs a 1/15 ratio of AMD to eutrophic water). The methodology focused on calculating key sustainability indicators such as the Net Yield Ratio (EYR), Emergy Inversion Ratio (EIR), Environmental Loading Ratio (ELR), and Sustainability Index (SI), in order to assess the impact of each technology on the energy efficiency and environmental load of the system. The results showed that, although Treatment IIIa was effective in contaminant removal, the EIR increased to 0.18 and the ELR rose to 0.62, indicating a higher reliance on non-renewable inputs due to increased energy demand. However, Treatment IIIb, which combines electrocoagulation with eutrophic water, significantly improved the sustainability of the system, achieving an SI of 2.31 and an ELR of 1.22, reflecting a reduction in energy efficiency due to intensive use of external resources, but overall greater sustainability compared to the other scenarios. This research concludes that intermittent electrocoagulation, when integrated with synergistic resources like eutrophic water, can enhance contaminant removal efficiency and improve the use of renewable resources, minimizing environmental load and increasing the sustainability of wastewater treatment systems.

Sustainable Development in Rural Underserved Communities through Improved Responsible Management of Decentralized Wastewater Infrastructure: A Focus on the Alabama Black Belt.

Despite global efforts on meeting sustainable development goals by 2030, persistent and widespread sanitation deficits in rural, underserved communities in high-income countries─including the United States (US)─challenge achieving this target. The recent US federal infrastructure funding, coupled with research efforts to explore innovative, alternative decentralized wastewater systems, are unprecedented opportunities for addressing basic sanitation gaps in these communities. Yet, understanding how to best manage these systems for sustainable operations and maintenance (O&M) is still a national need. Here, we develop an integrated management approach for achieving such sustainable systems, taking into account the utility structure, operational aspects, and possible barriers impeding effective management of decentralized wastewater infrastructure. We demonstrate this approach through a binomial logistic regression of survey responses from 114 public and private management entities (e.g., water and sewer utilities) operating in 27 states in the US, targeting the rural Alabama Black Belt wastewater issues. Our assessment introduces policy areas that support sustainable decentralized wastewater systems management and operations, including privatizing water-wastewater infrastructure systems, incentivizing/mandating the consolidation of utility management of these systems, federally funding the O&M, and developing and retaining water-wastewater workforce in rural, underserved communities. Our discussions give rise to a holistic empirical understanding of effective management of decentralized wastewater infrastructure for rural, underserved communities in the US, thereby contributing to global conversations on sustainable development.

Wastewater flooding risk assessment for coastal Communities: Compound impacts of climate change and population growth

The study introduces a wastewater modeling framework that evaluates the compound impacts of intense rainfall, groundwater infiltration, sewer aging based roughness and population growth on wastewater systems. It integrates property and city-level flooding risk assessment and wastewater treatment plant (WWTP) capacity analysis into a single methodological approach. The framework applied to the coastal city of Charlottetown, whose population increased from 30,887 in 1981 to 42,440 in 2023, but the wastewater system did not expand accordingly, resulting in frequent sewer backups, street and basement flooding, as witnessed during the extreme wet weather event of September 2, 2021. Using PCSWMM (Personal Computer Storm Water Management Model), the study assessed that the city-wide wastewater flooding risk in Charlottetown, based on 2023 population data and historical IDF curves, affects 13.31% of the network during a 2-year storm and 18.38% during a 100-year storm. These risks increase to 14.5% and 22.6% under future IDF scenarios, reaching 17.89% and 26.4% by 2060 with projected population growth. The WWTP capacity is exceeded by 27.8% during peak wet weather flows from a 2-year storm and by 86.3% during a 100-year storm, based on 2023 population and historic IDFs. Under future IDF scenarios for 2060 population, exceedances rise to 103.6% and 169.1% respectively, for a 2-year and 100-year storm. Basement flooding risk affects 13.35% of basements during a 2-year storm and 18.31% during a 100-year storm, for 2023 population and historic IDFs. Future IDF scenarios indicate risk increasing to 17.77% and 25.80% by 2060 for a 2-year and 100-year storm respectively. The hydraulic modeling results indicate that GWI is not currently impacting the study area, nor is it expected to in near future, because the groundwater table is over 10 m deep, while wastewater pipes are no deeper than 6 m. The framework and study have significant social implications and benefits, including protecting public health, enhancing the resilience of urban infrastructure, and safeguarding the environment, ultimately improving the quality of life for residents in coastal communities like Charlottetown.

Development of decision support methods for efficient maintenance of urban wastewater systems

The sanitation network is a vital urban infrastructure that requires systematic and co-ordinated activities and practices to realize lowest life cycle cost. The sustainable management of this asset is based on the efficient acquisition of diagnostic data from the sanitation network in order to analyze the defects of each section, then to assess their condition to prioritize and consider the effective mode of maintenance of sections deemed failing. In this perspective, knowledge of the sanitation networks is essential for reliable classification of defects and effective maintenance. A numerical model based on weighted criticality is developed to evaluate and prioritize the defects in an urban sanitation network. It is based on the combination of two methods: Fuzzy Analytic Hierarchy Process (F-AHP) and Weighted Product Model (WPM). A case study of a real sanitation network in Bejaia city (Algeria), consisting mainly of non-visitable pipes evaluated from television inspections, is used to illustrate and validate the proposed model. The results show that the weighting of criticality parameters has a considerable influence on the classification of defects in a sanitation network. The weighted criticality results obtained in the present case are significantly better than those obtained using a method based on the notion of classical criticality. This clearly indicates the relevance of the proposed numerical model.

Synergistic Effect of Chemical and Physical Treatments on Azolla pinnata for Cadmium Ions Removal from Synthetic Wastewater Systems

Synergistic Effect of Chemical and Physical Treatments on Azolla pinnata for Cadmium Ions Removal from Synthetic Wastewater Systems

Sewer transport conditions and their role in the decay of endogenous SARS-CoV-2 and pepper mild mottle virus from source to collection

This study presents a comprehensive analysis of the decay patterns of endogenous SARS-CoV-2 and Pepper mild mottle virus (PMMoV) within wastewaters spiked with stool from infected patients expressing COVID-19 symptoms, and hence explores the decay of endogenous SARS-CoV-2 and PMMoV targets in wastewaters from source to collection of the sample. Stool samples from infected patients were used as endogenous viral material to more accurately mirror real-world decay processes compared to more traditionally used lab-propagated spike-ins. As such, this study includes data on early decay stages of endogenous viral targets in wastewaters that are typically overlooked when performing decay studies on wastewaters harvested from wastewater treatment plants that contain already-degraded endogenous material. The two distinct sewer transport conditions of dynamic suspended sewer transport and bed and near-bed sewer transport were simulated in this study at temperatures of 4 °C, 12 °C and 20 °C to elucidate decay under these two dominant transport conditions within wastewater infrastructure. The dynamic suspended sewer transport was simulated over 35 h, representing typical flow conditions, whereas bed and near-bed transport extended to 60 days to reflect the prolonged settling of solids in sewer systems during reduced flow periods. In dynamic suspended sewer transport, no decay was observed for SARS-CoV-2, PMMoV, or total RNA over the 35-h period, and temperature ranging from 4 °C to 20 °C had no noticeable effect. Conversely, experiments simulating bed and near-bed transport conditions revealed significant decreases in SARS-CoV-2 and total RNA concentrations by day 2, and PMMoV concentrations by day 3. Only PMMoV exhibited a clear trend of increasing decay constant with higher temperatures, suggesting that while temperature influences decay dynamics, its impact may be less significant than previously assumed, particularly for endogenous RNA that is bound to dissolved organic matter in wastewater. First order decay models were inadequate for accurately fitting decay curves of SARS-CoV-2, PMMoV, and total RNA in bed and near-bed transport conditions. F-tests confirmed the superior fit of the two-phase decay model compared to first order decay models across temperatures of 4 °C–20 °C. Finally, and most importantly, total RNA normalization emerged as an appropriate approach for correcting the time decay of SARS-CoV-2 exposed to bed and near-bed transport conditions. These findings highlight the importance of considering decay from the point of entry in the sewers, sewer transport conditions, and normalization strategies when assessing and modelling the impact of viral decay rates in wastewater systems. This study also emphasizes the need for ongoing research into the diverse and multifaceted factors that influence these decay rates, which is crucial for accurate public health monitoring and response strategies.

Removal of sulfamethoxazole using Fe-Mn biochar filtration columns: Influence of co-existing polystyrene microplastics

Emerging contaminants, particularly antibiotics and microplastics (MPs), present significant challenges in wastewater treatment and pose large ecological risks. This study investigates the removal efficiency of sulfamethoxazole (SMX) using Fe-Mn modified biochar (BFM) in fixed bed filtration columns, emphasizing the effect of the presence of polystyrene microplastics (PS-MPs) on SMX behavior in both water (pH ≈ 5.6) and selected wastewater (pH ≈ 8) systems. Batch sorption results show that 10 mg/L SMX in 50 mL water can be completely removed by 100 mg BFM sorbent. The Bed Depth Service Time model indicated the BFM column is feasible for SMX removal in scaled-up continuous wastewater flow operations, while the Yan model best elucidates SMX filtration behavior and suggests the dominant adsorption mechanisms include external mass transfer and intraparticle diffusion. The present of both 20 mg/L and 100 mg/L PS-MPs (pH ≈ 5.6) significantly reduced SMX retention due to competitive sorption. However, at pH 3.2, competitive sorption became negligible due to electrostatic interactions driving the PS-MPs sorption, while neutral charged SMX bound through hydrogen-bonds or π-π EDA interactions. Elevated pH shifted both PS-MPs and SMX sorption to non-electrostatic thus intensifying sorption competition, highlighting the influence of pH on their interaction dynamics. In wastewater, SMX filtration was slightly inhibited by 100 mg/L PS-MPs in BFM columns, whereas PS-MPs removal remained unaffected due to the high ionic strength and alkaline pH. These findings highlight the impact of MPs on pollution removal efficiency in filtration system, essential for enhancing biochar-based wastewater treatment strategies.

Development of a sustainable optimization model for planning regional wastewater systems with consideration of water quality

Development of a sustainable optimization model for planning regional wastewater systems with consideration of water quality

More than just the gene: investigating expression using a non-native plasmid and host and its impact on resistance conferred by β-lactamase OXA-58 isolated from a hospital wastewater microbiome.

With the escalation of hospital-acquired infections by multidrug resistant bacteria, understanding antibiotic resistance is of paramount importance. This study focuses on the β-lactamase gene, blaOXA-58, an important resistance determinant identified in a patient-facing hospital wastewater system. This study aimed to characterize the behaviour of the OXA-58 enzyme when expressed using a non-native plasmid and expression host. blaOXA-58 was cloned using a pET28a(+)/Escherichia coli BL21(DE3) expression system. Nitrocefin hydrolysis and antimicrobial susceptibility of OXA-58-producing cells were assessed against penicillin G, ampicillin, meropenem, and amoxicillin. blaOXA-58 conferred resistance to amoxicillin, penicillin G, and ampicillin, but not to meropenem. This was unexpected given OXA-58's annotation as a carbapenemase. The presence of meropenem also reduced nitrocefin hydrolysis, suggesting it acts as a competitive inhibitor of the OXA-58 enzyme. This study elucidates the phenotypic resistance conferred by an antimicrobial resistance gene (ARG) obtained from a clinically relevant setting and reveals that successful functional expression of ARGs is multifaceted. This study challenges the reliability of predicting antimicrobial resistance based solely on gene sequence alone, and serves as a reminder of the intricate interplay between genetics and structural factors in understanding resistance profiles across different host environments.

Natural Organic Matter Enhances Natural Transformation of Extracellular Antibiotic Resistance Genes in Sunlit Water.

Antibiotic resistance genes (ARGs) as emerging environmental contaminants exacerbate the risk of spreading antibiotic resistance. Natural organic matter (NOM) is ubiquitous in aquatic environments and plays a crucial role in biogeochemical cycles. However, its impact on the dissemination of extracellular antibiotic resistance genes (eARGs) under sunlight exposure remains elusive. This study reveals that environmentally relevant levels of NOM (0.1-20 mg/L) can significantly enhance the natural transformation frequency of the model bacterium Acinetobacter baylyi ADP1 by up to 7.6-fold under simulated sunlight. Similarly, this enhancement was consistently observed in natural water and wastewater systems. Further mechanism analysis revealed that reactive oxygen species (ROS) generated by NOM under sunlight irradiation, primarily singlet oxygen and hydroxyl radicals, play a crucial role in this process. These ROS induce intracellular oxidative stress and elevated cellular membrane permeability, thereby indirectly boosting ATP production and enhancing cell competence of extracellular DNA uptake and integration. Our findings highlight a previously underestimated role of natural factors in the dissemination of eARGs within aquatic ecosystems and deepen our understanding of the complex interplay between NOM, sunlight, and microbes in environmental water bodies. This underscores the importance of developing comprehensive strategies to mitigate the spread of antibiotic resistance in aquatic environments.

Study the Utilization of Biochar Derived from Sugarcane Waste to Enhance the Biogas Efficiency During the Anaerobic Digestion Process for Treating Pig Farm Wastewater

This experiment aims to study the use of biochar (Biochar, BC) from sugarcane trash through the pyrolysis process at different temperatures of 300, 400, 500, 600 and 700°C. Then add BC to the Anaerobic Digestion (AD) system of wastewater from pig farms, then analyze the characteristics of wastewater and the amount of biogas obtained before and after treatment and to study the characteristics of decomposition, the generation of biogas (Biogas) to the reduction of organic matter in wastewater (COD). From this experiment, it can be concluded that all 6 systems are producing biogas, such as: the normal system and the system with BC filling at different burning temperatures: 300, 400, 500, 600 and 700℃ in the amount of 0.4g into the system. .64, 46.13, 51.90 and 65.98% respectively. From the experiments of all systems, it is observed that the system filled with BC at a temperature of 700℃ is able to produce biogas the best because the high heating temperature will cause the electrons of the charcoal to break easily and cause more holes to be a place for microbes to grow well. A comparison of the conventional system and adding biochar with the same amount but different temperature into the sewage treatment system without using air can be concluded that the conventional system is the generation of biogas is still low efficiency and the system of adding BC at the temperature of 300, 400, 500, 600℃ in the same amount. Notice that the generation of biogas is higher in order and the filling of BC at a temperature of 700℃ is able to produce biogas higher than other systems.

Supporting Environmental Decision-Making in Urban Water and Wastewater Systems: Proposal for an Environmental Performance Index

Supporting Environmental Decision-Making in Urban Water and Wastewater Systems: Proposal for an Environmental Performance Index

Life-Cycle Analysis of Natural Treatment Systems for Wastewater (NTSW) Applied to Municipal Effluents

The objective of the described activity is to develop technologies or proposals that can be implemented within the cycle to enhance the relationship between climate change, water, energy, and food. The focus is on analyzing natural treatment systems for wastewater (NTSW) within the context of Macaronesia, considering factors such as life-cycle assessment (LCA), carbon footprint, impacts, and mitigation capacity. The analysis of real case data from the Canary Islands and Cape Verde will inform the development of appropriate technologies tailored to different areas and scales within Macaronesia. This work includes a comprehensive life-cycle analysis of the Santa Catarina (Cape Verde) NTSW. This analysis encompasses: (a) Inventory analysis of the construction phase: This involves the assessment of inputs and outputs associated with the construction of the NTSW, including materials, energy consumption, transportation, and waste generation. The maintenance and operation phases are then evaluated, with a focus on the ongoing maintenance and operation activities required for the NTSW, including energy consumption, water usage, chemical inputs (if any), labor, and equipment maintenance. (b) Finally, the impacts of the NTSW are evaluated. The environmental, social, and economic impacts generated by the NTSW are assessed. This includes an analysis of factors such as carbon emissions, water usage, land use, ecosystem impacts, human health effects, and economic costs. By conducting a comprehensive analysis of the Santa Catarina NTSW, the document aims to provide insights into the environmental performance and sustainability of the system. This information can then be used as a tool and experience of educational innovation for final-year undergraduate students to identify areas for improvement, develop mitigation strategies in the water sector, and inform decision-making processes regarding wastewater treatment technologies in Macaronesia. Furthermore, lessons learned from real case studies in the Canary Islands and Cape Verde can be applied to similar regions within the Macaronesia archipelago (IDIWATER project).

Resolving the dilemma of wastewater treatment plants: An empirical study on public acceptance for publicly accessible WWTPs

Wastewater treatment plants (WWTPs) play an indispensable role in the process of urbanization and sustainable development, while their construction often faces challenges due to diverse public opinions. Therefore, strategies to improve public acceptance of WWTPs are essential for their implementation and the planning of wastewater system. To resolve this dilemma, publicly accessible WWTPs designed in conjunction with public spaces have emerged globally. In order to validate its effectiveness, this research investigated the public acceptance of publicly accessible WWTPs and the determinants of public acceptance in China, where an online questionnaire survey was launched with 3000 responses and hypothesis testing was conducted using Structural Equation Modeling (SEM). The results indicated that compared to conventional WWTPs, publicly accessible WWTPs were much more preferred by the majority of respondents (92.3%). Moreover, this study revealed that factors including perceived benefit, perceived risk, and public spaces directly impact public acceptance, while trust and perceived fairness indirectly influence it. What's more, this research highlighted and validated the significant role of public spaces in enhancing public acceptance of WWTPs, and enriched the understanding of how multiple factors influence public acceptance, providing practical implications for the site selection, design and management of publicly accessible WWTPs.

Monitoring of ketamine-based emerging contaminants in wastewater: a direct-injection method and fragmentation pathway study

The ketamine (KET) and its analogs consumed by humans are becoming emerging contaminants (ECs), as they at present in surface waters after being carried through wastewater systems. Drugs in wastewater can be analyzed using the direct-injection method, a simple wastewater analysis (WWA) method that can provide objective, continuous and nearly to real-time findings. This article describes an ultra-high-pressure liquid chromatography–tandem mass spectrometry (UPLC–MS/MS) method for the simultaneous quantification and confirmation of seven KET-based ECs in wastewater by direct injection. After optimization of the UPLC–MS/MS and sample pretreatment conditions, the method was validated and applied to samples (n = 157) collected from several wastewater treatment plants (WWTPs) in southern China in which KET had the highest detection rate. The established direct-injection method was not only simple to perform but also had better sensitivity, shorter detection times, and analyzed more KET-based ECs than currently published methods, meeting the requirements for the monitoring and high-throughput analysis of common KET-based ECs. We also analyzed the fragmentation pathway of KET-based ECs to obtain product ion information on other unknown substances. Additional studies are needed to establish a comprehensive direct-injection screening method of ECs in wastewater on model-based assessment.

Integrated urban wastewater system operation: Performance evaluation and sewer network's operation implications

ABSTRACT The paper deals with the operation of the Integrated Urban Wastewater System as composed of the Sewer Network and associated Wastewater Treatment Plant. The performance of the integrated system is defined in terms of appropriate, selected, performance indicators of both subsystems. Both subsystems are connected and interact by a piping system. It is shown how the performance of the Integrated Urban Wastewater System is affected by various Sewer Network control scenarios: (1) only the Sewer Network outlet valve opening is controlled, (2) all the Sewer Network valves are controlled by a Model Predictive Control algorithm developed for minimizing the environmental impact of the subsystem and modified by constraining it in terms of Sewer Network outlet valve opening. More specific, it is proven that the Sewer Network outlet valve opening has an important role in the performances of the integrated system, and that already existing Sewer Network control strategies can be altered in such a way that they will enhance these performances.

Evaluating Life-Cycle Unit Costs of Traditional Cement Concrete and New Polymer Concrete Manholes in Wastewater Systems

Wastewater manholes are crucial infrastructure components in sewage systems. They provide necessary access points for inspection and maintenance. However, limited studies were conducted on the life-cycle cost analysis of manholes. The primary objective of this study is to compute and compare the Life-Cycle Unit Cost (LCUC) of cement concrete and polymer concrete manholes to identify a cost-effective alternative for public agencies. To achieve the objective, this study analyzed commonly used 1.83 m diameter manhole data; 343 cement concrete manhole and 88 polymer concrete manhole cost data were collected from the Clark County Water Reclamation District (CCWRD), Las Vegas, Nevada, United States. The results show that the initial costs of polymer concrete are higher than those of traditional cement concrete. Statistical tests were conducted to determine the group differences. The findings show that the LCUCs of polymer concrete manholes are significantly cost-effective when compared to traditional cement concrete manholes. Public agencies can utilize polymer concrete manholes to save costs in future water and wastewater pipeline manhole construction projects.

Estimating release of the antibiotic metronidazole into the environment from hospital effluents and assessment of the associated ecological risk: a case study in Isfahan, Iran (2023)

Hospital effluents are important sources for the release of pharmaceuticals into the environment. Metronidazole (MTZ) is one of the most popular antimicrobial agent, widely used in veterinary and human medicine. This study aims to determine the concentrations of MTZ in the effluent of hospitals and to evaluate ecotoxicological risk for aquatic organisms. Grab samples were taken from the effluents of 15 large hospitals in Isfahan City (Iran) and MTZ concentrations were analyzed using HPLC-UV. Average daily MTZ prescriptions for patients in each hospital was also determined. MTZ was detected in effluents of all 15 hospitals in the range of 0.3 to 9.5 µg/L (mean = 4.8 ± 6.4 µg/L). A significant correlation was observed between MTZ concentration in hospital effluent and MTZ prescription as well as the hospital beds. Ecological risk assessment showed MTZ contamination of hospital effluent from 7 of 15 hospitals had median risk (risk quotient of 0.1 to 1) for green algae (Chlorella sp.) while the effluent of all hospitals had minimal risk (risk quotient <0.1) for gram-negative bacteria (Vibrio fischeri) and aquatic crustaceans (Daphnia magna). It is necessary to develop efficient treatment methods for the removal of MTZ from the hospital effluents before discharging them into the municipal wastewater systems.