Combined Sewer Overflow Management Research Articles

Ecological assessment of combined sewer overflow management practices through the analysis of benthic and hyporheic sediment bacterial assemblages from an intermittent stream

Combined sewer overflows (CSO) are used to avoid overloading unitary sewers and wastewater treatment plants. Following the European Council Directive on Urban Wastewater Treatment (UWT), CSO discharges are regulated using guidelines that aim to reduce their ecological impact on aquatic systems. A model CSO, which is part of a long-term experimental field observatory, was modified according to these guidelines and used to evaluate the benefits of compliance through analyses of the bacteriological and chemical states of the receiving intermittent stream. The benthic and hyporheic sediments of similar geomorphic units located upstream and downstream of a monitored CSO outlet were compared before and after changes in CSO regimes. Hydrological, pollutants (Metal Trace Elements, MTE; Polycyclic Aromatic Hydrocarbons, PAH; fecal indicator bacteria, FIB), and tpm-based DNA meta-barcoding datasets resolving the occurrences of >700 bacterial species of nearly 200 genera were studied. The frequency of overflow was confirmed to have significantly decreased following the application of the UWT guidelines. Overflows became almost limited to periods of heavy summer thunderstorm events. These changes were not associated with a significant decrease in most of the surveyed MTE, PAH, and FIB among stream sediments, except for chromium. Ecological benefits were highlighted by significant changes in tpm-based meta-barcoding community patterns between the UWT compliant sampling period and the previous one. Bacterial community change point analyses confirmed this segregation in the meta-barcoding dataset according to hydrological indices such as the number of CSO events and discharged volumes. A significant decline in CSO bacterial taxa in the benthic and hyporheic sediments was observed. Thirty-four CSO indicator species were identified, including Aeromonas caviae, Aeromonas media, and Pseudomonas oleovorans. These indicators, often documented as opportunistic pathogens (to humans, animals or plants) and/or pollutant degraders, were proposed as ecological sentinels for the assessment of CSO impacts.

Smart management of combined sewer overflows: From an ancient technology to artificial intelligence

AbstractSewer systems are an essential part of sanitation infrastructure for protecting human and ecosystem health. Initially, they were used to solely convey stormwater, but over time municipal sewage was discharged to these conduits and transformed them into combined sewer systems (CSS). Due to climate change and rapid urbanization, these systems are no longer sufficient and overflow in wet weather conditions. Mechanistic and data‐driven models have been frequently used in research on combined sewer overflow (CSO) management integrating low‐impact development and gray‐green infrastructures. Recent advances in measurement, communication, and computation technologies have simplified data collection methods. As a result, technologies such as artificial intelligence (AI), geographic information system, and remote sensing can be integrated into CSO and stormwater management as a part of the smart city and digital twin concepts to build climate‐resilient infrastructures and services. Therefore, smart management of CSS is now both technically and economically feasible to tackle the challenges ahead. This review article explores CSO characteristics and associated impact on receiving waterbodies, evaluates suitable models for CSO management, and presents studies including above‐mentioned technologies in the context of smart CSO and stormwater management. Although integration of all these technologies has a big potential, further research is required to achieve AI‐controlled CSS for robust and agile CSO mitigation.This article is categorized under: Engineering Water > Sustainable Engineering of Water Science of Water > Water and Environmental Change

Research on the Comprehensive Regulation Method of Combined Sewer Overflow Based on Synchronous Monitoring—A Case Study

Combined sewer overflow pollution has gradually become the limiting factor for the further improvement of river water quality during rain events. Setting up a comprehensive regulation method based on synchronous monitoring is essential for combined sewer overflow management. However, current studies mainly focus on single monitoring and lack a correlation between control objectives and control effects. This study establishes a new aspect of a comprehensive regulation and control method based on overflow characteristic analysis, a calculation model, and control target determination. Through synchronous monitoring of the pipe network, the sewage treatment plant, and the river course in the Liangshui River basin of China, rainfall thresholds of outlets in a combined pipe network, pre-treatment overflow, and simple-treatment overflow were 14, 9, and 16 mm, respectively, and the overflow volume was positively correlated with the rainfall. The COD (chemical oxygen demand) concentration from the pre-treatment overflow was much higher than that from the combined pipe network, and the EMC (event mean concentration) in heavy rain was higher than in rainstorms. The shortest time exceeding the water quality by overflow pollution was 1 h, and the longest time was more than 7 days. Overflow load proportions of the three links were 43.4%, 32.8%, and 23.8%, accounting for 66.3% of the total pollutant load of the river, and the best scheme of input–output ratio was to regulate the first three outlets of overflow load. Our results provide comprehensive guidance and a systematic approach for the monitoring and control of combined sewer overflow.

Costs and benefits of combined sewer overflow management strategies at the European scale.

Combined sewer overflows (CSOs) may represent a significant source of pollution, but they are difficult to quantify at a large scale (e.g. regional or national), due to a lack of accessible data. In the present study, we use a large scale, 6-parameter, lumped hydrological model to perform a screening level assessment of different CSO management scenarios for the European Union and United Kingdom, considering prevention and treatment strategies. For each scenario we quantify the potential reduction of CSO volumes and duration, and estimate costs and benefits. A comparison of scenarios shows that treating CSOs before discharge in the receiving water body (e.g. by constructed wetlands) is more cost-effective than preventing CSOs. Among prevention strategies, urban greening has a benefit/cost ratio one order of magnitude higher than grey solutions, due to the several additional benefits it entails. We also estimate that real time control may bring on average a CSO volume reduction of just above 20%. In general, the design of appropriate CSO management strategies requires consideration of context-specific conditions, and is best made in the context of an integrated urban water management plan taking into account factors such as other ongoing initiatives in urban greening, the possibility to disconnect impervious surfaces from combined drainage systems, and the availability of space for grey or nature-based solutions.

Evidence that watershed nutrient management practices effectively reduce estrogens in environmental waters

We evaluate the impacts of different nutrient management strategies on the potential for co-managing estrogens and nutrients in environmental waters of the Potomac watershed of the Chesapeake Bay. These potential co-management approaches represent agricultural and urban runoff, wastewater treatment plant effluent, and combined sewer overflow replacements. Twelve estrogenic compounds and their metabolites were analysed by gas chromatography–mass spectrometry. Estrogenic activity (E2Eq) was measured by in vitro bioassay. We detected estrone E1 (0.05–6.97 ng L−1) and estriol E3 (below detection-8.13 ng L−1) and one conjugated estrogen (estrone-3-sulfate E1-3S; below detection-8.13 ng L−1). E1 was widely distributed and positively correlated with E2Eq, water temperature, and dissolved organic carbon (DOC). Among nonpoint sources, E2Eq, and concentrations of E1, soluble reactive phosphorus (SRP) and total dissolved nitrogen (TDN) decreased by 51–61%, 77–82%, 62–64%, 4–16% in restored urban and agricultural streams with best management practices (BMPs) relative to unrestored streams without BMPs. In a wastewater treatment plant (Blue Plains WWTP), >94% of E1, E1-3S, E3, E2Eq and TDN were removed while SRP increased by 305% during nitrification/denitrification as a part of advanced wastewater treatment. Consequently, E1 and TDN concentrations in WWTP effluents were comparable or even lower than those observed in the receiving stream or river waters, and the effects of wastewater discharges on downstream E1 and TDN concentrations were minor. Highest E2Eq value and concentrations of E1, E3, and TDN were detected in combined sewer overflow (CSO). This study suggests that WWTP upgrades with biological nutrient removal, CSO management, and certain agricultural and urban BMPs for nutrient controls have the potential to remove estrogens from point and nonpoint sources along with other contaminants in streams and rivers.

Catchment-wide validated assessment of combined sewer overflows (CSOs) in a mediterranean coastal area and possible disinfection methods to mitigate microbial contamination

The first phase of this study aimed to evaluate the environmental impact of combined sewer overflow (CSO) events originated from 35 spillways on the Rio Vallescura catchment (Central Italy) and to understand their contribution to the deterioration of the coastal bathing water quality. A specific analytical campaign was carried out in the sewer system and a dynamic rainfall-runoff simulation model was developed and integrated with a water quality model and further validated. The simulations led to identify the most critical spills in terms of flow rate and selected pollutant loads (i.e. suspended solids, biochemical oxygen demand, chemical oxygen demand, total Kjeldahl nitrogen, Escherichia coli). Specifically, the E. coli release in the water body due to CSO events represented almost 100% of the different pollutant sources considered. In the second phase, the applicability of various disinfection methods was investigated on the CSOs introduced into the catchment. On site physical (UV) and lab-scale chemical (peracetic acid (PAA), performic acid (PFA), ozone) disinfectant agents were tested on microbial indicators including E. coli and intestinal enterococci. PFA and ozone were more effective on the removal of both bacteria (above 3.5 log units) even at low concentration and with short contact time; whereas, PAA showed a moderate removal efficiency (around 2.5 log units) only for E. coli. The highest removal efficiency was achieved in the on-site UV unit and none of the indicator bacteria was detected in the final effluent after the sand filtration and UV treatment. Finally, potential scenarios were developed in comparison to the baseline scenario for the management and treatment of CSOs where a mitigation of E. coli loads from 28% to 73% was achieved on the receiving water body, and a comparative cost assessment of the disinfection methods was provided for in situ treatment of the most critical spillway.

The Role of Sewer Network Structure on the Occurrence and Magnitude of Combined Sewer Overflows (CSOs)

Combined sewer overflows (CSOs) prevent surges in sewer networks by releasing untreated wastewater into nearby water bodies during intense storm events. CSOs can have acute and detrimental impacts on the environment and thus need to be managed. Although several gray, green and hybrid CSO mitigation measures have been studied, the influence of network structure on CSO occurrence is not yet systematically evaluated. This study focuses on evaluating how the variation of urban drainage network structure affects the frequency and magnitude of CSO events. As a study case, a sewer subnetwork in Dresden, Germany, where 11 CSOs are present, was selected. Scenarios corresponding to the structures with the lowest and with the highest number of possible connected pipes, are developed and evaluated using long-term hydrodynamic simulation. Results indicate that more meshed structures are associated to a decrease on the occurrence and magnitude of CSO. Event frequency reductions vary between 0% and 68%, while reduction of annual mean volumes and annual mean loads ranged between 0% and 87% and 0% and 92%. These rates were mainly related to the additional sewer storage capacity provided in the more meshed scenarios, following a sigmoidal behavior. However, increasing network connections causes investment costs, therefore optimization strategies for selecting intervention areas are needed. Furthermore, the present approach of reducing CSO frequency may provide a new gray solution that can be integrated in the development of hybrid mitigation strategies for the CSO management.

Network Topology and Rainfall Controls on the Variability of Combined Sewer Overflows and Loads

AbstractWater and pollutant fluxes from combined sewer overflows (CSO) have a significant impact on receiving waters. The random nature of rainfall forcing dominates the variability of sewer discharges, pollutant loads, and concentrations. An analytical model developed here shows how sewer network topology and rainfall properties variously impact the stochasticity of CSO functioning. Probability distributions of sewer discharge and concentration compare well with the results from a calibrated Storm Water Management Model in an application to a sewershed located in Dresden, Germany. The model is determined by only four parameters, three of which can be predicted a priori, two from the rainfall record and one from the network topology using geomorphological flow recession theory, while the fourth can be estimated from a short discharge time series. The sensitivity of CSO and wastewater treatment loads to network structure suggests simple topologies may be more vulnerable to poor performance. The analytical model is useful for evaluating various CSO management strategies to reduce adverse impacts on receiving waters in a probabilistic setting.

Exploring the performances of a new integrated approach of grey, green and blue infrastructures for combined sewer overflows remediation in high-density urban areas

Most sewage collection systems designed between 19th and early to mid-20th century use single-pipe systems that collect both sewage and urban runoff from streets, roofs and other impervious surfaces. This type of collection system is referred to as a combined sewer system. During storms, the flow capacity of the sewers may be exceeded and the overflow discharged into a receiving water body (RWB) through spillways without any control and remediation. Combined sewer overflows (CSOs) may, therefore, produce serious water pollution and flooding problems in downstream RWBs. Methodologies for a rational management of CSOs quantity and quality share many commonalities, and these two aspects should be considered together in order to maximize benefits and promote local distributed actions, especially in high urban density areas where the space availability for the construction of CSO storage tanks is often a limiting factor. In this paper, a novel strategy to control downstream flow propagation of a CSO as well as to improve its quality is tested on a real case study in the area of the metropolitan city of Milan. The approach is based on the combination of grey, green and blue infrastructures and exploits the integrated storage and self-depuration capacities of a firstflush tank, a constructed wetland and a natural stream to obtain admissible flow rates and adequate water quality in the RWB. The results, evaluated through a modelling framework based on simplified equations of water and pollutants dynamics, show excellent performances for the integrated system, both in terms of flow control and pollution mitigation. The pollution, using biological oxygen demand concentration as a proxy of the whole load, was decreased by more than 90% and downstream flooding situations were avoided, despite the spillway was not regulated. Concerning the economic point of view, from a rough estimate of the costs, the system allows reducing the investment of 30 to 50% in respect to the traditional CSO controls based solely on flow detention tanks. The proposed approach, as well as the modelling framework for its effective implementation, appear strongly scalable in different world contexts and aim to fill the gap between urban and rural environments in the management of stormwater and CSOs, promoting the involvement of the water managers, the irrigation-reclamation agencies and regional authorities.

New York City's Wait… Pilot Program: An Integrated Approach to Water Quality Improvement

A new program tested in a New York city sewershed encourages water users to reduce water use during rainstorms, and thereby improve management of combined sewer overflows.

Online monitoring and conditional regression tree test: Useful tools for a better understanding of combined sewer network behavior

A good knowledge of the dynamic of pollutant concentration and flux in a combined sewer network is necessary when considering solutions to limit the pollutants discharged by combined sewer overflow (CSO) into receiving water during wet weather. Identification of the parameters that influence pollutant concentration and flux is important. Nevertheless, few studies have obtained satisfactory results for the identification of these parameters using statistical tools. Thus, this work uses a large database of rain events (116 over one year) obtained via continuous measurement of rainfall, discharge flow and chemical oxygen demand (COD) estimated using online turbidity for the identification of these parameters. We carried out a statistical study of the parameters influencing the maximum COD concentration, the discharge flow and the discharge COD flux. In this study a new test was used that has never been used in this field: the conditional regression tree test. We have demonstrated that the antecedent dry weather period, the rain event average intensity and the flow before the event are the three main factors influencing the maximum COD concentration during a rainfall event. Regarding the discharge flow, it is mainly influenced by the overall rainfall height but not by the maximum rainfall intensity. Finally, COD discharge flux is influenced by the discharge volume and the maximum COD concentration. Regression trees seem much more appropriate than common tests like PCA and PLS for this type of study as they take into account the thresholds and cumulative effects of various parameters as a function of the target variable. These results could help to improve sewer and CSO management in order to decrease the discharge of pollutants into receiving waters.

Improving the microbiological quality of the Ruhr River near Essen: comparing costs and effects for the reduction of Escherichia coli and intestinal enterococci.

After rain events in densely populated areas, combined sewer overflows (CSOs) can have severe health-related effects upon surface water quality, as well as diffuse overland runoff and wastewater treatment plants (WWTPs). All of these sources emit pathogens and fecal indicator bacteria into the surface water, which the EU Bathing Water Directive addresses by giving threshold values for the indicators Escherichia coli and intestinal enterococci. This study presents a comparison between 21 scenarios of costs for processes that reduce the load of Escherichia coli and intestinal enterococci into the Ruhr River during and shortly after rain events. The methods examined include UV irradiation for WWTPs, integrated sewer management and treatment of CSOs with vertical flow constructed wetlands or performic acid. For pollution by diffuse overland flow, we evaluated organizational measures. The treatment of only diffuse pollution shows merely a slight effect on intestinal enterococci, but none on Escherichia coli and hence, was not considered further. Combining all three CSO reduction methods with the irradiation of WWTP effluent provides the best simulation results in terms of reducing both indicator bacteria.

Cumulative effects of fecal contamination from combined sewer overflows: Management for source water protection

The quality of a drinking water source depends largely on upstream contaminant discharges. Sewer overflows can have a large influence on downstream drinking water intakes as they discharge untreated or partially treated wastewaters that may be contaminated with pathogens. This study focuses on the quantification of Escherichia coli discharges from combined sewer overflows (CSOs) and the dispersion and diffusion in receiving waters in order to prioritize actions for source water protection. E. coli concentrations from CSOs were estimated from monitoring data at a series of overflow structures and then applied to the 42 active overflow structures between 2009 and 2012 using a simple relationship based upon the population within the drainage network. From these estimates, a transport-dispersion model was calibrated with data from a monitoring program from both overflow structures and downstream drinking water intakes. The model was validated with 15 extreme events such as a large number of overflows (n > 8) or high concentrations at drinking water intakes. Model results demonstrated the importance of the cumulative effects of CSOs on the degradation of water quality downstream. However, permits are typically issued on a discharge point basis and do not consider cumulative effects. Source water protection plans must consider the cumulative effects of discharges and their concentrations because the simultaneous discharge of multiple overflows can lead to elevated E. coli concentrations at a drinking water intake. In addition, some CSOs have a disproportionate impact on peak concentrations at drinking water intakes. As such, it is recommended that the management of CSOs move away from frequency based permitting at the discharge point to focus on the development of comprehensive strategies to reduce cumulative and peak discharges from CSOs upstream of drinking water intakes.

Coordinated management of combined sewer overflows by means of environmental decision support systems

During heavy rainfall, the capacity of sewer systems and wastewater treatment plants may be surcharged producing uncontrolled wastewater discharges and a depletion of the environmental quality. Therefore there is a need of advanced management tools to tackle with these complex problems. In this paper an environmental decision support system (EDSS), based on the integration of mathematical modeling and knowledge-based systems, has been developed for the coordinated management of urban wastewater systems (UWS) to control and minimize uncontrolled wastewater spills. Effectiveness of the EDSS has been tested in a specially designed virtual UWS, including two sewers systems, two WWTP and one river subjected to typical Mediterranean rain conditions. Results show that sewer systems, retention tanks and wastewater treatment plants improve their performance under wet weather conditions and that EDSS can be very effective tools to improve the management and prevent the system from possible uncontrolled wastewater discharges.

Optimal design of multi-storage network for combined sewer overflow management using a diversity-guided, cyclic-networking particle swarm optimizer – A case study in the Gunja subcatchment area, Korea

Multiple small-scale, distributed storage facilities have recently received much attention owing to their effectiveness for combined sewer overflow (CSO) mitigation. In this line of research, designing the optimal configuration of storage tanks in a sewer network is very challenging, and thus relatively few studies have been made to this day. To solve such a large-scale complex multimodal optimal design problem, a meta-heuristic particle swarm optimization-based design methodology of complex sewer networks for CSO management is developed. This search engine includes two mechanisms: a diversity-guided three-phase velocity update law and restricted social best searching based on the cyclic network structure. It allows regions of the design space to be explored efficiently by driving each particle to share information in switching the velocity update mechanism only with a set of neighboring particles via a fixed near-neighbor interaction structure. Therefore, the movement of a particle is no longer driven by the global best position of the entire swarm, which enhances the diversification attitude of the scheme. Its implementability under an actual environment is demonstrated by applying it to a combined sewer network case study of a complex large-scale multi-storage network in the Gunja subcatchment area located in Seoul, Republic of Korea. The simulation results indicate that the developed particle swarm optimization–based design methodology exhibits not only superior reliability but also high practicality, simplicity, and implementability for optimal planning of real-life CSO storage facilities.

Chemical disinfection of combined sewer overflow waters using performic acid or peracetic acids

We investigated the possibility of applying performic acid (PFA) and peracetic acid (PAA) for disinfection of combined sewer overflow (CSO) in existing CSO management infrastructures. The disinfection power of PFA and PAA towards Escherichia coli (E. coli) and Enterococcus was studied in batch-scale and pre-field experiments. In the batch-scale experiment, 2.5mgL−1 PAA removed approximately 4 log unit of E. coli and Enterococcus from CSO with a 360min contact time. The removal of E. coli and Enterococcus from CSO was always around or above 3 log units using 2–4mgL−1 PFA; with a 20 min contact time in both batch-scale and pre-field experiments. There was no toxicological effect measured by Vibrio fischeri when CSO was disinfected with PFA; a slight toxic effect was observed on CSO disinfected with PAA. When the design for PFA based disinfection was applied to CSO collected from an authentic event, the disinfection efficiencies were confirmed and degradation rates were slightly higher than predicted in simulated CSO.

A Point Source of a Different Color: Identifying a Gap in United States Regulatory Policy for “Green” CSO Treatment Using Constructed Wetlands

Up to 850 billion gallons of untreated combined sewer overflow (CSO) is discharged into waters of the United States each year. Recent changes in CSO management policy support green infrastructure (GI) technologies as “front of the pipe” approaches to discharge mitigation by detention/reduction of urban stormwater runoff. Constructed wetlands for CSO treatment have been considered among suites of GI solutions. However, these wetlands differ fundamentally from other GI technologies in that they are “end of the pipe” treatment systems that discharge from a point source, and are therefore regulated in the U.S. under the National Pollution Discharge Elimination System (NPDES). We use a comparative regulatory analysis to examine the U.S. policy framework for CSO treatment wetlands. We find in all cases that permitting authorities have used best professional judgment to determine effluent limits and compliance monitoring requirements, referencing technology and water quality-based standards originally developed for traditional “grey” treatment systems. A qualitative comparison with Europe shows less stringent regulatory requirements, perhaps due to institutionalized design parameters. We recommend that permitting authorities develop technical guidance documents for evaluation of “green” CSO treatment systems that account for their unique operational concerns and benefits with respect to sustainable development.

Evaluation of ecotoxicological risks related to the Discharge of Combined Sewer Overflows (CSOs) in a periurban River.

Discharges of Combined Sewer Overflows (CSOs) into periurban rivers present risks for the concerned aquatic ecosystems. In this work, a specific ecotoxicological risk assessment methodology has been developed as management tool to municipalities equipped with CSOs. This methodology comprises a detailed description of the spatio-temporal system involved, the choice of ecological targets to be preserved, and carrying out bioassays adapted to each compartment of the river receiving CSOs. Once formulated, this methodology was applied to a river flowing through the outskirts of the city of Lyon in France. The results obtained for the scenario studied showed a moderate risk for organisms of the water column and a major risk for organisms of the benthic and hyporheic zones of the river. The methodology enabled identifying the critical points of the spatio-temporal systems studied, and then making proposals for improving the management of CSOs.

CSO management from an operator's perspective: a step-wise action plan

Combined sewer overflows (CSO) are the "ugly ducklings" of urban wastewater infrastructure, rather unknown, rarely loved. Contrary to wastewater treatment plants (WWTP) where, commonly, process parameters are carefully registered, still relatively little is known about the spill behaviour of CSOs. In spite of the considerable bandwidth of pollution magnitude reported in literature, it can be generally accepted that CSO impact certainly is not negligible. As the sole wastewater utility in Flanders (6,100,000 inhabitants), Aquafin operates, next to 230 WWTP's and almost 1,000 pumping stations, some 3,100 CSOs. In search for an optimum CSO management strategy, Aquafin prospected several modelling and monitoring possibilities. As a result, a CSO action plan could be drawn up tailored to the estimated relevance of the CSO under consideration. In a further step a cost-benefit analysis of several static and dynamic remediation measures was undertaken. Real Time Control (RTC) strategies turned out to be the most cost efficient solution for CSO impact mitigation.

Management of combined sewer overflows based on observations from the urbanized Liguori catchment of Cosenza, Italy

This paper examines an urbanized catchment in Cosenza, Italy where an off-line basin intercepting CSOs was studied to illustrate reduction in CSO discharges to the Crati River. While the hydrologic transport of pollutant mass is never known a-priori and can be flow-limited, the volumetric requirements of the basin were modeled based on the classic assumption that wet weather flows transport urban and sewer loads in a mass-limited (first-flush) delivery. The volumetric capacity of the basin was varied from 10 to 50 m(3)/ha. Operational basin control was simulated with historical datasets from the Liguori catchment, event-based loading data, and continuous simulation modelling with SWMM. Utilizing data from the catchment, the SWMM simulations were conducted considering the storage basin with or without sedimentation treatment. Results illustrate the potential benefits of the off-line operation for the system with respect to the volume and mass reduction of CSOs into the Crati River. Results demonstrate the importance of particle size distribution (PSD) as an index of basin efficiency, coupled with analysis of the hydrodynamic response of the basin. The basin model attenuated influent PSDs, separating the coarser fraction of the PSD, and reduced the load of influent particulate matter (PM).