Sewer Overflows Research Articles

Impacts of Extreme Rainfalls on Sewer Overflows and WSUD-Based Mitigation Strategies: A Review

Extreme rainfall events cause an increase in the flow into aging sewer networks, which can lead to Sanitary Sewer Overflows (SSOs). This literature review presents a complete assessment of the application of Water Sensitive Urban Design (WSUD) approaches as mitigation strategies for reducing rainfall-induced SSOs. The review highlights the various WSUD techniques identified in past studies for reducing sewer overflows. In these studies, it was identified that permeable pavements, green roofs, raingardens/bio-retention cells and rainwater tanks were the most popular WSUD strategies that have been extensively used in the past for the mitigation of sewer overflows. WSUD or “green” approaches also have enormous environmental, social and economic benefits when compared to the conventional “gray” approaches for sewer overflow mitigation. However, there have been limited studies conducted in the past that highlight and quantify the benefits of WSUD approaches for sewer overflow mitigation, particularly when such strategies are applied at a large scale (e.g., city scale). This review has identified the modelling software, SWMM, to be the most widely applied tool that has been used in the literature for WSUD modelling. It was also identified that with climate change-induced extreme rainfall events on the increase, WSUD-based “green” strategies alone may not be enough for the mitigation of sewer overflows. A suitable sewer overflow mitigation strategy could be green or a hybrid green-gray strategy, which would need to be identified based on a detailed context specific analysis.

Microbial community composition and function in an urban waterway with combined sewer overflows before and after implementation of a stormwater storage pipe.

When the wastewater volume exceeds the sewer pipe capacity during extreme rainfall events, untreated sewage discharges directly into rivers as combined sewer overflow (CSO). To compare the impacts of CSOs and stormwater on urban waterways, we assessed physicochemical water quality, the 16S rRNA gene-based bacterial community structure, and EcoPlate-based microbial functions during rainfall periods in an urban waterway before and after a stormwater storage pipe was commissioned. A temporal variation analysis showed that CSOs have significant impacts on microbial function and bacterial community structure, while their contributions to physicochemical parameters, bacterial abundance, and chlorophyll a were not confirmed. Heat map analysis showed that the impact of CSO on the waterway bacterial community structure was temporal and the bacterial community composition in CSO is distinct from that in sewers. Hierarchical clustering analysis revealed that the waterway physicochemical water qualities, bacterial community composition, and microbial community function were distinguishable from the upper reach of the river, rather than between CSO and stormwater. Changes in the relative abundance of tetracycline resistance (tet) genes-especially tet(M)-were observed after CSOs but did not coincide with changes in the microbial community composition, suggesting that the parameters affecting the microbial community composition and relative abundance of tet genes differ. After pipe implementation, however, stormwater did not contribute to the abundance of tet genes in the waterway. These results indicate that CSO-induced acute microbial disturbances in the urban waterway were alleviated by the implementation of a stormwater storage pipe and will support the efficiency of storage pipe operation for waterway management in urban areas.

A bilevel data-driven method for sewer deposit prediction under uncertainty

Deposit accumulation is one of the predominant causes of sewer blockage and overflow. Nevertheless, the traditional detection methods are costly and time-consuming, and the accuracy of the mathematical models for deposit prediction is usually affected by some uncertain factors (e.g., pipe properties and flow velocity of water). This paper proposes a framework of global sensitivity analysis (GSA) to identify the most sensitive indicators for sewer deposit prediction by (i) developing a data-driven bilevel (i.e., catchment level and segment level) model to map the relation between input and output indicators and (ii) employing three different GSA methods, namely, the Morris method, Sobol method, and Borgonovo index method to identify the indicators as important or unimportant (insensitive). The results show that the likelihood of combined sewer overflow occurrences (LCSOO), pipe age (PA), and pipe material (PM) are influential parameters for the thickness of deposits. Here, we pay close attention to the most influential parameters, which can help improve forecast prediction accuracy.

A Longitudinal Assessment of Benthic Macroinvertebrate Diversity and Water Quality along the Bronx River

The Bronx River is an urban waterway with a long history of anthropogenic disturbance. We conducted a longitudinal assessment of the Bronx River's water quality by measuring benthic macroinvertebrate diversity at 6 sites along the river. We integrated long-term water-quality data collected by the New York State Department of Environmental Conservation. We found that the overall water quality of the river has remained moderately impacted over different timepoints throughout the past 22 years. The study site upstream of combined sewage overflows and municipal separate stormwater systems exhibited healthier biological profiles, whereas the most-downstream sites exhibited slight declines in water quality. The most recent survey of the Bronx River (2020) revealed that high invasive species dominance was associated with benthic macroinvertebrate communities that were less healthy. Notably, one invasive species not documented in historical surveys, Corbicula fluminea (Asian Clam), was sampled in 5 of 6 study sites during the 2020 surveys. Moreover, no species were sampled from the order Ephemeroptera (mayflies) in 2020 despite being present in previous surveys. These results can be used to guide the management of urban rivers.

Combined sewer overflows: relating event duration monitoring data to wastewater systems' capacity in England

Water pollution caused by the frequent use of combined sewer overflows (CSOs) has been attracting increased media and political coverage in England as in other places in the world.

Study on urban combined sewer overflows and self-purification capacity load of river

Study on urban combined sewer overflows and self-purification capacity load of river

Dynamics of estrogenic activity in an urban river receiving wastewater effluents: effect-based measurements with CALUX

Estrogenic substances (ES) in an urban river Zenne (BE) dominated by wastewater effluents were assessed over the course of one year. To measure the bioequivalent (BEQ) 17 β-estradiol (E2) concentrations of ES, the biological effect-based methodology - the Chemical-Activated LUciferase gene eXpression (CALUX) bioassay was used. Daily water discharges were collected from January 2015 to February 2016 at or near the sampling stations in the Brussels Capital Region. An annual water budget shows that approximately 50% of the Zenne River flow downstream is from wastewater effluent. The estrogenic activity and yearly average ES load in influents and effluents of wastewater treatment plants (WWTPs) located in the North and South, combined sewer overflows (CSOs) and the Zenne River, were assessed for upstream and downstream of two WWTPs of Brussels. Both WWTPs with activated sludge treatment remove more than 90% of the ES. The influent concentrations of ES at the South and North WWTPs ranged from 30-359 and 18-55 ng E2 eq./L, respectively. The effluent concentrations of ES ranged from 1.0-2.1 and 1.1-6.6 ng E2 eq./L at WWTP-S and -N, respectively. The yearly average ES loads were 0.05-0.14 and 0.39-1.5 g E2 eq./d for WWTP-S and -N, respectively. The temporal variation of E2-eq concentrations at the river stations Z3 and Z5 (upstream) ranged from 1 to 2 ng E2 eq./L, while the ES activity at sites Z9 and Z11 (downstream) varied from 2-17 ng E2 eq./L and from 1-8 ng/L ng E2 eq./L, respectively. The relative ES loads to the Zenne River are as follows: WWTPs (31%), CSOs (27%), upstream Zenne (15%), a missing source (14%), and local tributaries (13%). ES in the Zenne River behave in a pseudo-persistent manner because of continuous input from the WWTPs and slow degradation in the 18 km river stretch. The BEQ concentration of E2 exceeds the EU environmental quality standards (EQS) of 0.4 ng E2/L throughout the Zenne River.

Implementing green infrastructure for stormwater management and combined sewer overflow control

Implementing green infrastructure for stormwater management and combined sewer overflow control

Study on Combined Sewer Overflow (CSO) Pollution and Its Control Method Based on Sewage Interception Along Rivers

Combined sewer overflow (CSO) pollution is the main factor that plagues the water environment in the area with combined sewer system in most cities of China. This paper analyzed CSO pollution, and reviewed the measures to solve CSO pollution and the latest research progress. The problems of traditional river interception projects were analyzed, and optimization and improvement ideas were proposed. Finally, a combined optimization scheme of source reduction-process control-end storage was proposed to control CSO pollution, which could provide reference for urban CSO pollution control in China.

Optimal siting of rainwater harvesting systems for reducing combined sewer overflows at city scale

The installation of green infrastructure (GI) is an effective approach to manage urban stormwater and combined sewer overflow (CSO) by restoring pre-development conditions in urban areas. Research on simulation-optimization techniques to aid with GI planning decision-making is expanding. However, due to high computational expense, the simulation-optimization methods are often based on design storm events, and it is unclear how much different rainfall scenarios (i.e., design storm events vs. long-term historical rainfall data) impact the optimal siting of GI. The Parallel Pareto Archived Dynamically Dimensioned Search (ParaPADDS) algorithm in a novel simulation-optimization tool OSTRICH-SWMM was used to leverage distributed computing resources. A case study was conducted to optimally site rainwater harvesting cisterns within 897 potential subcatchments throughout the City of Buffalo, New York. Seven design storm events with different return periods and rainfall durations and a one-month historical rainfall time series were considered. The results showed that the optimal solutions of siting cisterns using event-based scenarios, though less computationally expensive, may not perform well under continuous rainfall scenarios, suggesting design rainfall scenarios should be carefully considered for optimizing GI planning. The impact of rainfall scenarios was particularly significant in the middle region of the Pareto front of multi-objective optimization. Utilizing high-performance parallel computing, OSTRICH-SWMM is a promising tool to optimize GI at large spatial and temporal scales.

Spatiotemporal analysis of fluorescent dissolved organic matter to identify the impacts of failing sewer infrastructure in urban streams

Failing sewer infrastructure introduces raw wastewater into streams. We used fluorescence excitation-emission matrix (EEM) spectroscopy and parallel factor analysis (PARAFAC) to track hotspots of raw wastewater in low- and medium-order urban streams that do not receive wastewater effluent but are impacted by sanitary sewer overflows, septic systems, and sewer exfiltration. After analyzing 296 surface water samples from 27 sites in two watersheds over a one-year period, we proposed that the (i) area-normalized ratio of soluble microbial product-like to humic acid-like fluorescence (R4/R5 ≥ 0.85) and (ii) ratio of EEM-PARAFAC components with tryptophan-like and fulvic acid-like fluorescence (C4/C3 ≥ 1.45) could distinguish when and where untreated wastewater is introduced to urban streams. The proposed ratios were validated by co-detection of contaminants of emerging concern, such as sucralose, antibiotics, and UV filters, at concentrations as high as 1354, 108, and 212 ng L−1, respectively. Based on the aggregate data, we identified three sites in rural/suburban areas that were impacted by septic systems and ten sites in urban sections affected by sanitary sewer overflows and/or sewer exfiltration. Moreover, the ratiometric C4/C3 and R4/R5 parameters were immune to dilution effects caused by rain events. Impacts on upstream sites were mostly identified in spring and early summer, but urban hotspots occurred in almost every month. These findings confirmed the potential for EEM-PARAFAC-based wastewater indicators as a quick, easy, cost-effective, and scalable technique to screen for failing sewer infrastructure in low-order streams.

Assessing the impact of climate change on Combined Sewer Overflows based on small time step future rainfall timeseries and long-term continuous sewer network modelling

The evolution of the climate in the future will probably lead to an increase in extreme rainfall events, particularly in the Mediterranean regions. This change in rainfall patterns will have impacts on combined sewer systems operation with a possible increase of spilled flows, leading to an increase of untreated water volumes released to the receiving water. Due to the impact of overflows on the water cycle, local authorities managing combined sewer systems are wondering about the extent of these changes and the possibility of taking it into account in stormwater management structure design. To do this, rainfall data with a fine time step are required to better master the shape of the hyetographs that are crucial to get a relevant rainfall/runoff relationship in an urban environment. However, there are currently no simulations of future rainfall series available at a time step compatible with the needs in urban drainage field. In this work, future rainfall time series with a fine time step are elaborated with the aim to be used in urban hydrology. The proposed approach is based on simulations results from five regional climate models in the framework of the Euro-Cordex program. It consists in a spatial downscaling step followed by a temporal disaggregation. The rainfall time series obtained are then used as input for a calibrated and validated hydrological model to investigate the evolution of annual CSO volumes and frequencies by 2100. The results show an increase of annual spilled volumes between 13% and 52% according to the considered climate model. This increase will most likely be a problem regarding compliance of sewer networks in line with the water framework directive, particularly the current French regulations. No clear trends were observed on the CSO frequencies. If there is a consensus for all the carried-out simulations to conclude that the CSO volumes will increase, we must remember that actual regional climate models suffer from limited spatial and temporal resolution and don't explicitly solve convection processes. Due to this point uncertainty concerning the evolution rate remains important particularly for intense rainfall episodes. New generations of climate models are needed to accurately predict intense episodes.

Cost to society from infiltration and inflow to wastewater systems

Water from infiltration and inflow to wastewater systems is an extensive problem causing costs to society in various ways. Comprehensive methods for supporting decisions on how to efficiently mitigate the problems in a sustainable manner are, however, missing today. This paper presents a novel risk-based model to assess the cost to society from infiltration and inflow to wastewater systems by monetising effects related to treatment of wastewater, pumping, combined sewer overflows, and basement flooding. The present value is calculated for a specified time horizon and discount rate, using a cost-benefit analysis approach. To acknowledge the various uncertainties, a probabilistic approach is applied where probability distributions represent the input variables. The model is shown to be applicable by illustrating its use in a case study area in Gothenburg, Sweden. Main results from the case study show that most of the costs are related to investments at the wastewater treatment plant and restoration due to basement flooding events. Sensitivity analyses show that the result is highly dependent on factors such as the volume of infiltration and inflow water, the share of basement flooding, and the discount rate. Using expert elicitation to quantify input data is also illustrated and shown to be a valuable method. The presented model fills an important research knowledge gap and will facilitate a more sustainable and comprehensive handling of water from infiltration and inflow.

Towards coordinated and robust real-time control: a decentralized approach for combined sewer overflow and urban flooding reduction based on multi-agent reinforcement learning

The real-time control (RTC) of urban drainage systems can make full use of the capabilities of existing infrastructures to mitigate combined sewer overflow (CSO) and urban flooding. Despite the benefits of RTC, it may encounter potential risks and failures, which need further consideration to enhance its robustness. Besides failures of hardware components such as sensors and actuators, the RTC performance is also sensitive to communication failures between the devices that are spatially distributed in a catchment-scale system. This paper proposes a decentralized control strategy based on multi-agent reinforcement learning to enhance communication robustness and coordinate the decentralized control agents through centralized training. To investigate different control structures, a centralized and a fully decentralized strategy are also developed based on reinforcement learning (RL) for comparison. A benchmark drainage model and a real-world drainage model are formulated as two cases, and the control agents are trained to control the orifices or pumps for CSO or flooding mitigation in each case. The three RL strategies reduce the CSO volume by 5.62-9.30% compared with a static baseline in historical rainfalls of the benchmark case and reduce the CSO and flooding volume by 14.39-21.36% compared with currently-used rule-based control in synthetic rainfalls of the real-world case. Benefitting from centralized training, the decentralized agents can achieve similar performance to the centralized agent. The decentralized control also enhances the communication robustness with smaller performance loss than the centralized control when observation communication fails, and provides a robust backup at the local level to limit the uncertainties when action commands from the centralized agent are lost. The results and findings indicate that multi-agent RL contributes to a coordinated and robust solution for RTC of urban drainage systems.

Real-time control of combined sewer systems: Risks associated with uncertainties

Model Predictive Control (MPC) of combined sewer systems can reduce environmental degradation caused by uncontrolled overflows. However, practical uncertainties are often neglected when assessing the potential of MPC strategies. This paper aims to understand the risks associated with using a non-perfect internal MPC-model, real precipitation forecast, and realistic dynamic system capacity fluctuations. An MPC with the objective to reduce the total combined sewer overflow (CSO) volume was implemented in the case study of Eindhoven in the Netherlands where highly sensitive waterways receive the sewer overflows. Two types of risks were identified: relative system performance loss and operative deterioration. The former entails a practical decrease in efficacy of controlling CSO spills compared to the theoretical situation, whereas the latter describes the aggravation of environmental pollution compared to a static form of system operation. The results obtained demonstrate that precipitation forecast uncertainty is associated with a small relative system performance loss. Opposite to this, significant performance loss was observed as a consequence of uncertainties in the internal MPC model and the actual sewer system capacity available. The latter caused additional combined sewer overflows compared to a statically optimised control for smaller precipitation events.

Enhanced removal of typical odor compounds in constructed wetlands integrated with iron carbon and tidal flow

The use of constructed wetlands (CWs) has become increasingly popular in pollutant removal in combined sewer overflows, while the performance in dealing with odor compounds in CWs has remained an open question. In this study, Tidal flow Wetland (TW), Iron-carbon Wetland (IW) and Tidal flow Iron-carbon Wetland (TIW) were parallel constructed, where novel substrates modified with iron-carbon and different hydrodynamics in optimizing removal efficiency were compared. The degradation kinetics of geosmin (GSM), a typical kind of odor compounds, were selected to indicate the removal efficiency. The biological features in different constructed wetlands were investigated, in terms of microbial community distribution, functional genes and metabolic pathways. In contrast to TW, IW and TIW exhibited enhanced performance in degrading of GSM. At the steady stage, 19.74 % and 23.19 % of the GSM was removed in a 24-h cycle for IW and TIW respectively. The degradation process followed the pseudo-first-order kinetics. Cyanobacteria, which produced GSM as parts of their metabolites, and Flavobacterium and Sphingomonas, which acted as the degrader on the other hand, together contributed to the variation of GSM concentration in the constructed wetlands. The adding of iron-carbon in the substrates increased the abundance of Flavobacterium and Sphingomonas, while decreased the growth of Cyanobacteria, and weakened the biosynthetic pathway of terpenoids. Tidal flow further enhanced the aerobic environment of the wetland and stimulates the metabolic activities of microorganisms, therefore enhanced the pathway of metabolism and degradation of odor compounds. Consequently, such combination further favored the degradation of GSM and the final removal efficiency.

Neonatal and child health crises due to recent floods in Pakistan.

Neonates and children are more vulnerable to the negative impact of flood-related changes and may have a variety of detrimental negative impacts on their health. They are more prone to get various infectious diseases. They are also more vulnerable to malnutrition during floods. Flooding limits access to clean water as sewage overflows and contaminates nearby water sources. The polluted setting in the flood-affected area makes it difficult to ensure the hygiene of feeding equipment used to prepare infant formula. Breastfeeding may also become less effective due to the lack of privacy for women to breastfeed their kids while living in temporary shelters with other flood victims. In addition, milk production decreases and might even cease due to mothers' reduced food intake and increased stress levels. Flooding may also cause supplemental feeding to deteriorate. The mothers and other primary caregivers usually lack the resources in affected areas to prepare supplemental diets for their kids, which further harm the babies. There is mounting evidence that children are more likely to develop clogged noses, itchy eyes, hoarseness, skin complications, and sneezing while living in humid areas.

Characteristics of antibiotic resistance gene distribution in rainfall runoff and combined sewer overflow.

Rainfall runoff and combined sewer overflow (CSO) converge with organic waste, nutrients, and microbes from the ground and wastewater. These pollutants promote the spread and transformation of antibiotic resistance genes (ARGs). In this study, four rainfall runoff and one CSO outfall were chosen, and samples were collected to explore the occurrence and distribution of ARGs. The ARGs were extracted from suspended solids and analyzed using metagenomic sequencing. A total of 888 ARG subtypes, belonging to 17 ARG types, were detected in all samples. Eleven ARG types were shared by all the samples. Multidrug resistance genes had the highest relative abundance. Their total relative abundance reached 1.07 ratio (ARG copy number/16S rRNA gene copy number) and comprised 46.6% of all the ARGs. In all samples, the CSO outfall had the highest total relative abundance (8.25 × 10-1 ratio) of ARGs, with a ratio ranging ND (not detected)-3.78 × 10-1 ratio. Furthermore, the relationship between ARG types and environmental factors was determined using redundancy analysis. The results showed that chemical organic demand (COD) and bacterial abundance were positively correlated with most ARG types, including multidrug, bacitracin, aminoglycoside, β-lactam, tetracycline, and sulfonamide. NH3-N, TN, and TP were positively correlated with rifamycin, fosmidomycin, and vancomycin resistance genes. The relationship among the ARG subtypes was investigated using network analyses. The multidrug resistance gene subtypes had the highest frequency of co-occurrence. This study provides insights into the occurrence and distribution of ARGs under non-point source pollution and may contribute to the control of ARGs.

Optimal Positioning of RTC Actuators and SuDS for Sewer Overflow Mitigation in Urban Drainage Systems

Real-time control (RTC) and sustainable drainage systems (SuDS) can be implemented simultaneously to enhance the performance of existing urban drainage systems (UDS). However, significant challenges arise when choosing the optimal locations due to hydraulic interactions between the different interventions and the high number of possible configurations. This paper presents a novel optimization–simulation framework to optimize the spatial allocation of flow control devices (FCDs) combined with SuDS for combined sewer overflow (CSO) spill mitigation within UDS. Optimal intervention schemes were identified by a genetic algorithm (GA), combining different numbers of FCDs installed in existing manholes with simplified SuDS implemented in different portions of the catchment. The methodology was tested on two case-study catchments with different characteristics to mitigate CSO spills during synthetic storm events. FCD-SuDS configurations were then validated during continuous rainfall series, resulting in CSO spill volumes reduction ranging between 11% and 45% compared to the baseline networks. The results demonstrate how the GA-based method can efficiently identify optimal placement schemes within UDS characterized by different distributions of in-pipe storage potential as well as hydrological responses to rainfall-runoff events, enhancing the combined benefits of the two decentralized solutions for mitigating CSO spills.

Abatement of Sewer Overflow Pollution Based on Distributed Optimal Control Approach

Abatement of Sewer Overflow Pollution Based on Distributed Optimal Control Approach