Combined Sewer Overflow Structures Research Articles

Effects of combined sewer overflow on water quality: a case study of Hatirjheel Lake in Dhaka

This paper presents a case study focusing on the impacts of combined sewer overflows on the water quality of the receiving water body, Hatirjheel. Hatirjheel, the largest surface water body in Dhaka City with an area of about 1.012 km2, receives discharges from nine combined sewer overflow (CSO) structures. The water quality of Hatirjheel is poor throughout the year, but particularly during the wet season (June to October) near the CSO structures through which significant rainwater-sewage overflows. The water has been found to contain high concentrations of 5-day biochemical oxygen demand (BOD5) and chemical oxygen demand; some of the BOD5 values exceed the national discharge standards for treated effluents. Total ammonia concentration in Hatirjheel water increases during the wet season, often exceeding 20 mg/l; the concentration continues to increase after the end of the wet season, most likely due to the ammonification process. Nitrate concentration in Hatirjheel water increases at the end of the wet season, possibly due to nitrification; subsequent reduction in nitrate and ammonia concentration is possibly due to incorporation of nitrogen into algal mass. Excessive phosphorus in Hatirjheel promotes eutrophication, resulting in the visible greenish color of the water. This study highlights the significant adverse impact of combined sewer overflows, particularly for a densely populated city like Dhaka, where most of the rainfall occurs within a relatively short period during monsoon.ArticleHighlightsCombined sewer overflows could significantly deteriorate the water quality of the receiving water bodies.Sewer overflows create a significant spatiotemporal variation of water quality, with higher pollution close to the overflowing combined sewer overflow structures.Because of its significant adverse impact on water quality and ecology, combined sewer systems may not be viable for high-density urban areas.

An Acoustic Sensor for Combined Sewer Overflow (CSO) Screen Condition Monitoring in a Drainage Infrastructure.

Combined sewer overflow structures (CSO) play an important role in sewer networks. When the local capacity of a sewer system is exceeded during intense rainfall events, they act as a “safety valve” and discharge excess rainfall run-off and wastewater directly to a natural receiving water body, thus preventing widespread urban flooding. There is a regulatory requirement that solids in CSO spills must be small and their amount strictly controlled. Therefore, a vast majority of CSOs in the UK contain screens. This paper presents the results of a feasibility study of using low-cost, low-energy acoustic sensors to remotely assess the condition of CSO screens to move to cost-effective reactive maintenance visits. In situ trials were carried out in several CSOs to evaluate the performance of the acoustic sensor under realistic screen and flow conditions. The results demonstrate that the system is robust within ±2.5% to work successfully in a live CSO environment. The observed changes in the screen condition resulted in 8–39% changes in the values of the coefficient in the proposed acoustic model. These changes are detectable and consistent with observed screen and hydraulic data. This study suggested that acoustic-based sensing can effectively monitor the CSO screen blockage conditions and hence reduce the risk of non-compliant CSO spills.

Evaluation of two statistical approaches for estimating pollutant loads at adjacent combined sewer overflow structures.

Quantifying pollutant loads from combined sewer overflows (CSOs) is necessary for assessing impacts of urban drainage on receiving water bodies. Based on data obtained at three adjacent CSO structures in the Louis Fargue catchment in Bordeaux, France, this study implements multiple linear regression (MLR) and random forest regression (RFR) approaches to develop statistical models for estimating emitted loads of total suspended solids (TSS). Comparison between hierarchical clustering selection and random selection of CSO events for model calibration is included in model development. The results indicate that selection of the model's explanatory variables depends on both the type of approach and the CSO structure. By using the cluster technique to select representative events for model calibration, model predictability is generally improved. For the available dataset, MLR may have advantages over RFR in terms of verification performance and lower range of error due to splitting events for calibration and verification. But RFR model uncertainty bands are considerably narrower than the MLR ones.

Estimation of combined sewer overflow discharge: a software sensor approach based on local water level measurements.

Combined sewer overflow (CSO) structures are constructed to effectively discharge excess water during heavy rainfall, to protect the urban drainage system from hydraulic overload. Consequently, most CSO structures are not constructed according to basic hydraulic principles for ideal measurement weirs. It can, therefore, be a challenge to quantify the discharges from CSOs. Quantification of CSO discharges are important in relation to the increased environmental awareness of the receiving water bodies. Furthermore, CSO discharge quantification is essential for closing the rainfall-runoff mass-balance in combined sewer catchments. A closed mass-balance is an advantage for calibration of all urban drainage models based on mass-balance principles. This study presents three different software sensor concepts based on local water level sensors, which can be used to estimate CSO discharge volumes from hydraulic complex CSO structures. The three concepts were tested and verified under real practical conditions. All three concepts were accurate when compared to electromagnetic flow measurements.

Using data from monitoring combined sewer overflows to assess, improve, and maintain combined sewer systems

Using low-cost sensors, data can be collected on the occurrence and duration of overflows in each combined sewer overflow (CSO) structure in a combined sewer system (CSS). The collection and analysis of real data can be used to assess, improve, and maintain CSSs in order to reduce the number and impact of overflows. The objective of this study was to develop a methodology to evaluate the performance of CSSs using low-cost monitoring. This methodology includes (1) assessing the capacity of a CSS using overflow duration and rain volume data, (2) characterizing the performance of CSO structures with statistics, (3) evaluating the compliance of a CSS with government guidelines, and (4) generating decision tree models to provide support to managers for making decisions about system maintenance. The methodology is demonstrated with a case study of a CSS in La Garriga, Spain. The rain volume breaking point from which CSO structures started to overflow ranged from 0.6mm to 2.8mm. The structures with the best and worst performance in terms of overflow (overflow probability, order, duration and CSO ranking) were characterized. Most of the obtained decision trees to predict overflows from rain data had accuracies ranging from 70% to 83%. The results obtained from the proposed methodology can greatly support managers and engineers dealing with real-world problems, improvements, and maintenance of CSSs.

Estimating inflow to a combined sewer overflow structure with storage tank in real time: evaluation of different approaches.

The performance assessment of storage tanks and combined sewer overflow (CSO) structures in sewer systems requires knowledge of the total inflow from the catchment during rainfall events. Many structures are, however, only equipped with sensors to measure water level and/or outflows. Based on the geometry of the tank, expressed as a level-storage relationship, inflow can be calculated from these data using a simple conceptual storage model. This paper compares a deterministic and a Bayesian approach for estimating the inflow to a CSO structure from measurements of outflows and water level. The Bayesian approach clearly outperforms the deterministic estimation which is very sensitive to measurement errors. Although computationally more demanding, the use of a simple linear storage model allows the online application of the Bayesian approach to repeatedly estimate inflow in short time intervals of a few minutes. The method could thus be used as an online software sensor for inflow to storage structures in sewer systems.

Methods to accompany and evaluate planning of combined sewer overflow treatment concepts for complex sewer systems

Simulation tools are in common use to evaluate combined sewer overflow (CSO) treatment concepts in complex sewer systems. However, the planning of CSO structures in a sewer system is a matter of local constraints, expert knowledge and trial and error. Common standards only provide general recommendations to plan CSO structures and work out management strategies. Additionally, modelling the emissions of complex sewer systems tends to result in comprehensive findings. Although, it is essential to understand local behaviour and interaction of CSO structures in a system to improve local and overall performance there is a lack of tools to illustrate comprehensive simulation results in a simple way. In this context the methods presented here are developed. These include clear illustrations of the as-is state in the catchment using Sankey diagrams to show relevant volume and pollutant flows. Furthermore, loading and treatment indicators are suggested to illustrate local loading conditions and treatment capabilities of CSO structures in relation to the overall system. Additional emission indicators provide information on local emissions and show interactions of CSO structures. The results indicate that the suggested methods contribute to an efficient evaluation of interactions and performances to improve treatment strategies in the planning phase.

Field validation of a new low-cost method for determining occurrence and duration of combined sewer overflows

Combined sewer overflow (CSO) events produced in combined sewer systems (CSS) during wet weather conditions are a threat for the receiving water bodies. The large number of CSO structures normally present in a CSS makes that the monitoring of the complete CSO network in a simultaneous way would drastically increase the investment costs. In this paper, a new methodology is presented aiming to characterize the occurrence and duration of CSO events by means of low-cost temperature sensors. Hence, a large number of CSO structures can be simultaneously monitored and the system can be characterized as a whole. The method assumes temperature differences between the overflowing mix of wastewater and stormwater and the sewer gas phase, so the temperature shift produced during a rainfall episode is related to a CSO event occurrence. The method has been tested and validated in La Garriga CSS (Spain) where the temperature at 13 CSO weirs was monitored for a period of 1year (57 rainfall episodes). For the whole set of CSO events, occurrence and duration were successfully determined in 80% of cases. Advantages, limitations and potential applications of the method are discussed at the end of the paper.

A software-based sensor for combined sewer overflows

A new methodology for online estimation of excess flow from combined sewer overflow (CSO) structures based on simulation models is presented. If sufficient flow and water level data from the sewer system is available, no rainfall data are needed to run the model. An inverse rainfall-runoff model was developed to simulate net rainfall based on flow and water level data. Excess flow at all CSO structures in a catchment can then be simulated with a rainfall-runoff model. The method is applied to a case study and results show that the inverse rainfall-runoff model can be used instead of missing rain gauges. Online operation is ensured by software providing an interface to the SCADA-system of the operator and controlling the model. A water quality model could be included to simulate also pollutant concentrations in the excess flow.

The use of CFD modelling to optimise measurement of overflow rates in a downstream-controlled dual-overflow structure

The measurement of the flow through complex combined sewer overflow structures in the frame of automated monitoring remains difficult. In this paper, a methodology based on the use of computational fluid dynamics (CFD) modelling in order to improve the instrumentation of a downstream-controlled dual-overflow structure is presented. The dual-overflow structure is composed of two combined sewer overflows (CSOs) connected by a rectangular channel and controlled by a downstream gate located at the entry of the Meyzieu waste water treatment plant (close to Lyon, France). The analysis of the CFD results provides: (i) a better understanding of the interaction between the two CSOs--that means the hydraulic operation, the hydrodynamic behaviour, the backflow effect--and (ii) an ability to optimise the location of the water depth sensor. The measured water depth is used to assess the overflow rate by means of a numerical relationship. Uncertainties are also assessed.

Implications of long-term stormwater quality modelling for design of combined sewer infrastructure

In many countries traditional simple design approaches for combined sewer overflow (CSO) structures and storage tanks are used in general practice, neglecting the significant variability observed in real systems. In this paper, two hydraulic software simulation tools and two stormwater quality model approaches are compared to assess CSO behaviour and the impact of different modelling approaches on the estimation of spilled pollutant loads in long-term simulations for an urban catchment area. A good coherence of the two runoff models is observed. The more complex stormwater quality model approach leads to significantly lower TSS interception ratios for smaller specific storage tank volumes. For both the hydraulic CSO indicators and TSS interception ratios an important annual variability is observed. Therefore, using single design value approaches or short rainfall time series in modelling can lead to significant impact on the design ratio of storage tank and CSO structures.

Groundwater infiltration, surface water inflow and sewerage exfiltration considering hydrodynamic conditions in sewer systems

Sewer systems are closely interlinked with groundwater and surface water. Due to leaks and regular openings in the sewer system (e.g. combined sewer overflow structures with sometimes reverse pressure conditions), groundwater infiltration and surface water inflow as well as exfiltration of sewage take place and cannot be avoided. In the paper a new hydrodynamic sewer network modelling approach will be presented, which includes--besides precipitation--hydrographs of groundwater and surface water as essential boundary conditions. The concept of the modelling approach and the models to describe the infiltration, inflow and exfiltration fluxes are described. The model application to the sewerage system of the City of Dresden during a flood event with complex conditions shows that the processes of infiltration, exfiltration and surface water inflows can be described with a higher reliability and accuracy, showing that surface water inflow causes a pronounced system reaction. Further, according to the simulation results, a high sensitivity of exfiltration rates on the in-sewer water levels and a relatively low influence of the dynamic conditions on the infiltration rates were found.

Combined sewer overflow over an oblique weir at Rat Creek in Edmonton, Alberta

Oblique weirs are commonly used in urban drainage systems to remove excess flow from a sewer, in particular, a combined sewer system that has limited conveyance capacity. It is important to understand the hydraulics of these weirs to properly monitor the amount of the overflows as well as to design and improve sewer systems. The Rat Creek structure in Edmonton, Alberta, is a combined sewer overflow structure with a weir at an oblique alignment to the centerline of the sewer. A physical model study of the structure was conducted. The results show that both the approach flow conditions and the chamber geometry can significantly affect the hydraulic performance of the weir and invalidate the application of standard weir equations. A unique flow regime with a linear head–discharge rating curve was observed. The effects of modifying the weir and the hanging baffle wall downstream of the weir were also studied and reported. The results of this case study help to improve the understanding of the hydraulics of oblique weirs in sewer systems.

Determining the spill flow discharge of combined sewer overflows using rating curves based on computational fluid dynamics instead of the standard weir equation

It is state of the art to evaluate and optimise sewer systems with urban drainage models. Since spill flow data is essential in the calibration process of conceptual models it is important to enhance the quality of such data. A wide spread approach is to calculate the spill flow volume by using standard weir equations together with measured water levels. However, these equations are only applicable to combined sewer overflow (CSO) structures, whose weir constructions correspond with the standard weir layout. The objective of this work is to outline an alternative approach to obtain spill flow discharge data based on measurements with a sonic depth finder. The idea is to determine the relation between water level and rate of spill flow by running a detailed 3D computational fluid dynamics (CFD) model. Two real world CSO structures have been chosen due to their complex structure, especially with respect to the weir construction. In a first step the simulation results were analysed to identify flow conditions for discrete steady states. It will be shown that the flow conditions in the CSO structure change after the spill flow pipe acts as a controlled outflow and therefore the spill flow discharge cannot be described with a standard weir equation. In a second step the CFD results will be used to derive rating curves which can be easily applied in everyday practice. Therefore the rating curves are developed on basis of the standard weir equation and the equation for orifice-type outlets. Because the intersection of both equations is not known, the coefficients of discharge are regressed from CFD simulation results. Furthermore, the regression of the CFD simulation results are compared with the one of the standard weir equation by using historic water levels and hydrographs generated with a hydrodynamic model. The uncertainties resulting of the wide spread use of the standard weir equation are demonstrated.

On the effect of spatial variances in historical rainfall time series to CSO performance evaluation

Historical, high-resolution rain series are the backbone of modern combined sewer overflow (CSO) structure design. These rain series are the input to the computational estimation of the performance of the measures with respect to CSO pollution abatement. However, those historical precipitation measurements are available at only a few locations. Frequently rain series have to be used from gauging stations at a significant distance. In order to judge and to compensate for this influence an estimate between rain characteristics and combined sewer outflow (CSO) performance indicators would be useful. In this paper such correlations have been sought for a collection of 37 rain series covering large areas of Europe. It was found that the mean annual rain volume can explain most of the variances for the performance indicators Number of overflows and CSO volume. For explaining the spatial differences in the efficiency of the CSO structure another rain characteristic, i.e. the maximum event with a return period of one year, is to be used.

UNSATISFACTORY INTERMITTENT DISCHARGES FROM COMBINED SEWER OVERFLOWS: AN ALTERNATIVE TO STORAGE SOLUTIONS

UNSATISFACTORY INTERMITTENT DISCHARGES FROM COMBINED SEWER OVERFLOWS: AN ALTERNATIVE TO STORAGE SOLUTIONSDuring wet weather conditions, wastewater flows in combined sewers systems can be enormous, resulting in untreated spills from combined sewer overflow structures to the receiving water. Where such intermittent discharges are judged to be unsatisfactory, the conventional solution has been to either increase the pass forward flow or store the wet weather overflow in large tanks for later treatment....Author(s)G. D. E. GlasgowB. J. MorrowSourceProceedings of the Water Environment FederationSubjectSession 45 Collection Systems: Singing in the Rain: Wet Weather IssuesDocument typeConference PaperPublisherWater Environment FederationPrint publication date Jan, 2003ISSN1938-6478SICI1938-6478(20030101)2003:8L.442;1-DOI10.2175/193864703784640235Volume / Issue2003 / 8Content sourceWEFTECFirst / last page(s)442 - 451Copyright2003Word count178

Urban Drainage Systems: Design and Operation

Design and operation of urban drainage systems are addressed in the context of the urban water system comprising drainage, sewage treatment plants and receiving waters. The planning and design of storm sewers are reviewed with reference to planning objectives, design objectives, flows and pollutant loads, sewer system structures and urban runoff control and treatment. The discussion of combined sewers focuses on hydraulic design of combined sewer systems, including combined sewer overflow (CSO) structures, and the use of CSO structures and storage in control of CSOs. The section on operation of sewer systems focuses on real time control, its feasibility, planning, design, operation and applications. Sewer system planning and design are generally conducted using computer modelling tools and procedures which are reviewed in the last section. A brief listing of selected models focuses on internationally used models. Finally, it was concluded that further improvements in environmental and ecological protection of urban waters is feasible only by consideration of urban drainage systems in conjunctions with sewage treatment and water quality in the receiving waters.