Sewer Overflow Volume Research Articles

Predictive heuristic control: Inferring risks from heterogeneous nowcast accuracy

Urban Drainage Systems can cause ecological and public health issues by releasing untreated contaminated water into the environment. Real-time control (RTC), augmented with rainfall nowcast, can effectively reduce these pollution loads. This research aims to identify key dynamics in the nowcast accuracies and relate those to the performance of nowcast-informed rule-based (RB)-RTC procedures. The developed procedures are tested in the case study of Rotterdam, the Netherlands. Using perfect nowcast data, all developed procedures showed a reduction in combined sewer overflow volumes of up to 14.6%. Considering real nowcast data, it showed a strong ability to predict if no more rain was expected, whilst performing poorly in quantifying rainfall depths. No relation was found in the nowcast accuracy and the consistency of the predicted rainfall using a moving horizon. Using the real nowcast data, all procedures, with the exception of the one predicting the end of the rainfall event, showed a significant risk of operative deterioration (performing worse than the baseline RB-RTC), linked to the relative performance of the nowcast algorithm. Understanding the strengths of a nowcast algorithm can ensure the reliability of the RB-RTC procedure and can negate the need for detailed modelling studies by inferring risks from nowcast data.

Studying the effect of bioswales on nutrient pollution in urban combined sewer systems

The objective of this study was to evaluate the impact of bioswales on nutrient pollution in an urban combined sewershed. This evaluation was based on two criteria: the ability of bioswales to (1) remove nutrient pollution from stormwater runoff directly and (2) decrease sewer overflow volumes, which indirectly reduces total sewershed nutrient pollution during a storm event. Bioswales' direct nutrient removal was determined by analyzing nitrogen and phosphorus levels in water samples at seven bioswales located in the Bronx, New York City (NYC) over 42 storm events, while a bioswale's indirect nutrient removal through combined sewer overflow reduction was estimated by quantifying water retention at one of the bioswales. The study results indicated that: 1) the bioswale retained about 40% of stormwater conveyed to it from a drainage area 231 times its size, 2) bioswales leach nutrients into the subsurface, and 3) nitrogen leaching from bioswales varied seasonally, while phosphorus leaching decreased steadily over the study period. Although the studied bioswales leached a median 1.3 kg nitrogen per year into the subsurface, they provided an aggregate decrease in watershed nutrient pollution, from 7.7 to 6 kg nitrogen per year, due to their reduction of combined sewer overflow via stormwater retention.

A Methodology for the Design of RTC Strategies for Combined Sewer Networks

While real-time control (RTC) is considered an established means of performance improvement for existing urban drainage networks, practical applications are frequently only documented for large case studies, and many operators are still reluctant to adopt RTC into their own systems. The purpose of the presented study is to highlight the potential of RTC also for smaller networks by the example of five representative catchments in Flanders, Belgium, and to demonstrate a novel methodology for the automated design of control strategies. This method analyses a given sewer network for the identification of suitable existing and new control locations. The gathered information is used in a second step for the design of control algorithms according to generic control concepts documented in the literature, such as e.g., “Equal Filling Degree”. The resulting RTC strategy uses sensible default parameters, and can form a starting point for further refinement through optimization or manual tuning. With a modelled total combined sewer overflow volume reduction of 20% to 50%, the created strategies showed generally good performance for the tested catchments. The method proved to be applicable for all tested networks. Its use for the real-life implementation of RTC is currently under way for 10 other Flemish cases.

Quantifying Uncertainty in Simulation of Sewer Overflow Volume

AbstractEnvironmental regulators frequently stipulate the modeling approaches required for water utilities managing sewer networks to demonstrate regulatory compliance. The performance of drainage ...

Modelling Earthquake Effects on Wastewater Overflow Pollutants

Earthquakes can cause a variety of damage in a wastewater network. One of the key impacts of an earthquake is the increase in wastewater overflow due to high inflow and infiltration. Earthquake impacts on sewer overflow volume and sewer overflow water pollutants in Christchurch are reported in this paper. Two hydraulic models (the 2010 pre-earthquake model and the 2011 post-earthquake model) were used as part of this investigation. In Christchurch, earthquakes caused a 30% increase in wastewater overflow volume in a 1 in 3 year's rainfall event whereas the sewage overflow volume had increased by around 18% and 15% during the 1 in 5 years and 1 in 10 year's rainfall events respectively. The amount of water pollutants discharged via sewer overflow increased by around 40%, 12% and 8% in the 1 in 3 year's, 1 in 5 years and 1 in 10 year's rainfall events respectively. Though there was a significant increase in sewer overflow volume in the post-earthquake network, it did not mean water pollutants discharged via the sewer overflow points also increased significantly. The concentration of water pollutants decreased as the sewer overflow volume increased.

Temporal rainfall disaggregation using a multiplicative cascade model for spatial application in urban hydrology

Rainfall time series of high temporal resolution and spatial density are crucial for urban hydrology. The multiplicative random cascade model can be used for temporal rainfall disaggregation of daily data to generate such time series. Here, the uniform splitting approach with a branching number of 3 in the first disaggregation step is applied. To achieve a final resolution of 5 min, subsequent steps after disaggregation are necessary. Three modifications at different disaggregation levels are tested in this investigation (uniform splitting at Δt = 15 min, linear interpolation at Δt = 7.5 min and Δt = 3.75 min). Results are compared both with observations and an often used approach, based on the assumption that a time steps with Δt = 5.625 min, as resulting if a branching number of 2 is applied throughout, can be replaced with Δt = 5 min (called the 1280 min approach). Spatial consistence is implemented in the disaggregated time series using a resampling algorithm. In total, 24 recording stations in Lower Saxony, Northern Germany with a 5 min resolution have been used for the validation of the disaggregation procedure. The urban-hydrological suitability is tested with an artificial combined sewer system of about 170 hectares. The results show that all three variations outperform the 1280 min approach regarding reproduction of wet spell duration, average intensity, fraction of dry intervals and lag-1 autocorrelation. Extreme values with durations of 5 min are also better represented. For durations of 1 h, all approaches show only slight deviations from the observed extremes. The applied resampling algorithm is capable to achieve sufficient spatial consistence. The effects on the urban hydrological simulations are significant. Without spatial consistence, flood volumes of manholes and combined sewer overflow are strongly underestimated. After resampling, results using disaggregated time series as input are in the range of those using observed time series. Best overall performance regarding rainfall statistics are obtained by the method in which the disaggregation process ends at time steps with 7.5 min duration, deriving the 5 min time steps by linear interpolation. With subsequent resampling this method leads to a good representation of manhole flooding and combined sewer overflow volume in terms of hydrological simulations and outperforms the 1280 min approach.

Green-blue water in the city: quantification of impact of source control versus end-of-pipe solutions on sewer and river floods.

Urbanization and climate change trends put strong pressures on urban water systems. Temporal variations in rainfall, runoff and water availability increase, and need to be compensated for by innovative adaptation strategies. One of these is stormwater retention and infiltration in open and/or green spaces in the city (blue-green water integration). This study evaluated the efficiency of three adaptation strategies for the city of Turnhout in Belgium, namely source control as a result of blue-green water integration, retention basins located downstream of the stormwater sewers, and end-of-pipe solutions based on river flood control reservoirs. The efficiency of these options is quantified by the reduction in sewer and river flood frequencies and volumes, and sewer overflow volumes. This is done by means of long-term simulations (100-year rainfall simulations) using an integrated conceptual sewer-river model calibrated to full hydrodynamic sewer and river models. Results show that combining open, green zones in the city with stormwater retention and infiltration for only 1% of the total city runoff area would lead to a 30 to 50% reduction in sewer flood volumes for return periods in the range 10-100 years. This is due to the additional surface storage and infiltration and consequent reduction in urban runoff. However, the impact of this source control option on downstream river floods is limited. Stormwater retention downstream of the sewer system gives a strong reduction in peak discharges to the receiving river. However due to the difference in response time between the sewer and river systems, this does not lead to a strong reduction in river flood frequency. The paper shows the importance of improving the interface between urban design and water management, and between sewer and river flood management.

Development of probability distributions for urban hydrologic model parameters and a Monte Carlo analysis of model sensitivity

AbstractThis paper proposes an approach to estimating the uncertainty related to EPA Storm Water Management Model model parameters, percentage routed (PR) and saturated hydraulic conductivity (Ksat), which are used to calculate stormwater runoff volumes. The methodology proposed in this paper addresses uncertainty through the development of probability distributions for urban hydrologic parameters through extensive calibration to observed flow data in the Philadelphia collection system. The established probability distributions are then applied to the Philadelphia Southeast district model through a Monte Carlo approach to estimate the uncertainty in prediction of combined sewer overflow volumes as related to hydrologic model parameter estimation.Understanding urban hydrology is critical to defining urban water resource problems. A variety of land use types within Philadelphia coupled with a history of cut and fill have resulted in a patchwork of urban fill and native soils. The complexity of urban hydrology can make model parameter estimation and defining model uncertainty a difficult task. The development of probability distributions for hydrologic parameters applied through Monte Carlo simulations provided a significant improvement in estimating model uncertainty over traditional model sensitivity analysis. Copyright © 2013 John Wiley & Sons, Ltd.

CSO Modelling Considering Moving Storms and Tipping Bucket Gauge Failures

The simulation of combined sewer overflow volumes and loads is important for the assessment of the overflow and overflow load to the receiving water to predict the hydraulic or the pollution impact. Hydrodynamic models are very data-intensive and time-consuming for long-term quality modelling. Hence, for long-term modelling, hydrological models are used to predict the storm flow in a fast way. However, in most cases, a constant rain intensity is used as load for the simulation, but in practice even for small catchments rain occurs in rain cells, which are not constant over the whole catchment area. This paper presents the results of quality modelling considering moving storms depending on the rain cell velocity and its moving direction. Additionally, tipping bucket gauge failures and different corrections are also taken into account. The results evidence the importance of these considerations for precipitation due the effects on overflow load and show the difference up to 28% of corrected and uncorrected data and of moving rain cells instead of constant raining intensities.

Statistical Modeling of the Serviceability of Sewage Pumps

Sewage pumping stations represent an element of the sewer system, which is directly responsible for affecting serviceability; i.e., failing pumps may result in combined sewer overflows or flooding. However, failures of sewage pumps are not yet incorporated in sewer assessments due to lack of data. This paper presents the analysis of pump failure data provided by two sewer management authorities in The Netherlands. Pump failures have been studied accounting for the nature of the failures, the operation and maintenance procedures of the management authority, the aging of the pumps, and the changes in the environment of pumps. The analysis shows that sewage pumps fail relatively often due to the composition of sewage and the discontinuous operation of the pumps. The interarrival time and the duration of failures are highly variable and independent of the specific function of the pump. Resulting pump failure characteristics are applied in a Monte Carlo simulation to calculate the impact of failures on combined sewer overflow volumes. The results indicate that the serviceability of sewer systems is significantly affected by failing pumps. Therefore, including pump availability in sewer system assessments should be considered.

A method for the evaluation of pollution loads from urban areas at river basin scale

The European water framework directive (WFD) requires the identification of significant human pressures and impacts on water bodies. This is part of the wider analysis of river basins required by Article 5 (and Annex II) of the WFD, which must be completed by the end of 2004. The human pressures on surface waters include point and diffuse source pollutions, water abstraction, water flow regulation, morphological alterations and land use patterns. In order to be able to complete the analysis of significant pressures and impacts before the WFD deadline it is necessary to maximise the use of existing information. A method using only existing information for the evaluation of point source pollution at river basin scale, especially the pollution loads from drained urban areas into surface waters was therefore proposed. This method includes an approach to estimate imperviousness in urban areas and to calculate sewer overflow volume at river basin scale. The imperviousness in urban areas is estimated using information on land use in CORINE Landcover or ATKIS (Official Topographic–Cartographic Information System). Data used for the evaluation of sewer overflows, such as rainfall depth, sewer lengths, dry weather flow volume and total volume of combined wastewater discharged to wastewater treatment plants, are available from German Weather Survey, or they are, like in North Rhine-Westphalia, stored in databases. Imperviousness in the Wupper basin and nutrient inputs from urban areas into the Wupper river system were calculated in a case study and compared with the results of other investigations.

Storm water management in an urban catchment: effects of source control and real-time management of sewer systems on receiving water quality

For the examination of the effects of different storm water management strategies in an urban catchment area on receiving water quality, an integrated simulation of the sewer system, wastewater treatment plant and receiving water is carried out. In the sewer system real-time control measures are implemented. As examples of source control measures the reduction of wastewater and the reduction of the amount of impervious surfaces producing storm water discharges are examined. The surface runoff calculation and the simulation of the sewer system and the WWTP are based on a MATLAB/SIMULINK simulation environment. The impact of the measures on the receiving water is simulated using AQUASIM. It can be shown that the examined storm water management measures, especially the source control measures, can reduce the combined sewer overflow volume and the pollutant discharge load considerably. All examined measures also have positive effects on the receiving water quality. Moreover, the reduction of impervious surfaces avoids combined sewer overflow activities, and in consequence prevents pollutants from discharging into the receiving water after small rainfall events. However, the receiving water quality improvement may not be seen as important enough to avoid acute receiving water effects in general.

Comparison of conventional rule based flow control with control processes based on fuzzy logic in a combined sewer system

While conventional rule based, real time flow control of sewer systems is in common use, control systems based on fuzzy logic have been used only rarely, but successfully. The intention of this study is to compare a conventional rule based control of a combined sewer system with a fuzzy logic control by using hydrodynamic simulation. The objective of both control strategies is to reduce the combined sewer overflow volume by an optimization of the utilized storage capacities of four combined sewer overflow tanks. The control systems affect the outflow of four combined sewer overflow tanks depending on the water levels inside the structures. Both systems use an identical rule base. The developed control systems are tested and optimized for a single storm event which affects heterogeneously hydraulic load conditions and local discharge. Finally the efficiencies of the two different control systems are compared for two more storm events. The results indicate that the conventional rule based control and the fuzzy control similarly reach the objective of the control strategy. In spite of the higher expense to design the fuzzy control system its use provides no advantages in this case.

Interaction between the treatment plant and the sewer system in Halmstad: integrated upgrading based on real time control

In Halmstad, Sweden great efforts have been made during the 1990's to improve the functionality and to reduce the environmental impact of the sewer system and the wastewater treatment plant. The investment and rehabilitation program includes to a great extent an effective use of existing resources. The wastewater treatment plant is reconstructed to meet increased nutrient removal demands. A five year rehabilitation plan for the sewer system is under completion, where the measures mainly are motivated by the aim to reduce the combined sewer overflow volumes and to minimize the risk of local flooding. It was soon realized that an integrated use of storage volumes at the wastewater treatment plant and within the sewer system could improve the general conditions for the treatment at the plant. To implement this strategy a real time control system was introduced by installing controllable weirs and flow control devices in the main sewer. The article describes the background of the sewerage master plan, how the upgrading work has been carried out by means of simulations and measurement, gives examples of some expected potential benefits, and outlines plans for the future.

Interaction between the treatment plant and the sewer system in halmstad: Integrated upgrading based on real time control

In Halmstad, Sweden great efforts have been made during the 1990's to improve the functionality and to reduce the environmental impact of the sewer system and the wastewater treatment plant. The investment and rehabilitation program includes to a great extent an effective use of existing resources. The wastewater treatment plant is reconstructed to meet increased nutrient removal demands. A five year rehabilitation plan for the sewer system is under completion, where the measures mainly are motivated by the aim to reduce the combined sewer overflow volumes and to minimize the risk of local flooding. It was soon realized that an integrated use of storage volumes at the wastewater treatment plant and within the sewer system could improve the general conditions for the treatment at the plant. To implement this strategy a real time control system was introduced by installing controllable weirs and flow control devices in the main sewer. The article describes the background of the sewerage master plan, how the upgrading work has been carried out by means of simulations and measurement, gives examples of some expected potential benefits, and outlines plans for the future.

A methodology for the assessment of frequency distributions of combined sewer overflow volumes

A stochastic approach for the calculation of the frequency distribution of overflow volumes is presented. The methodology has been tested with a data set comprising rainfall, overflow volumes and pollutant loads. The stochastic approach involves a multiplication of two stochastic variables, rainfall intensity and duration. Correlation between these variables leads to erroneous results. It is shown that an improvement can be obtained by subclassification of the data set, in order to remove the effect of this interdependency. However, the correlation between input variables cannot be removed entirely by the method of subclassification.