Contribution of combined sewer overflows to trace contaminant loads in urban streams

The present study examines the contribution of combined sewer overflows (CSO) to loads and concentrations of trace contaminants in receiving surface water. A simple method to assess the ratio of CSO to wastewater treatment plant (WWTP) effluents was applied to the urban River Spree in Berlin, Germany. The assessment indicated that annual loads are dominated by CSO for substances with removal in WWTP above ∼95%. Moreover, it showed that substances with high removal in WWTP can lead to concentration peaks in the river during CSO events. The calculated results could be verified based on eight years of monitoring data from the River Spree, collected between 2000 and 2007. Substances that are well removed in WWTP such as NTA (nitrilotriacetic acid) were found to occur in significantly increased concentration during CSO, while the concentration of substances that are poorly removable in WWTP such as EDTA (ethylenediaminetetraacetic acid) decreased in CSO-influenced samples due to dilution effects. The overall results indicate the potential importance of the CSO pathway of well-removable sewage-based trace contaminants to rivers. In particular, high concentrations during CSO events may be relevant for aquatic organisms. Given the results, it is suggested to include well-removable, sewage-based trace contaminants, a substance group often neglected in the past, in future studies on urban rivers in case of combined sewer systems. The presented methodology is suggested for a first assessment, since it is based solely on urban drainage data, which is available in most cities.     

Hydrological assessment of flow dynamic changes by storm sewer overflows and combined sewer overflows on an event-scale in an urban river

In addition to assessing the impacts of water quality changes in urban rivers caused by storm water sewer overflows (SWO) and combined sewer overflows (CSO), the extent to which flow dynamics are changed by these structures must be understood in order to define hydrological assessment criteria to guide sustainable water management strategies as required by the European Community (EC) Water Framework Directive. In this study, the quantitative impacts of SWOs and CSOs on the flow dynamics of an urban river and their variability are investigated. For four single runoff events, hydrological measurements were accomplished in the River Dreisam, upstream and downstream of the city of Freiburg, in southwest Germany. As the catchment is widely free of urban areas upstream of the city, comparison with downstream locations allowed quantification of Freiburg's effects on the changes in the hydrograph on an event scale. The proposed hydrological parameter—flow acceleration, peak discharge, and discharge dosage—were shown to be appropriate to assess the impacts of SWOs and CSOs on flood hydrographs in urban rivers.     

Computational fluid dynamics based assessment of discharge-water depth relationships for combined sewer overflows

This article describes a general method to assess discharge-water depth relationships for combined sewer overflow (CSO) chambers. This method is particularly suitable for: complex geometries, unavailable univoc relationship between upstream and downstream flow and complex downstream hydraulic conditions. The methodology presented in this article is based on computational fluid dynamics. Its aim is to propose a relationship between the water depths in the CSO chamber and the overflow discharge. This study focuses on the evaluation and integration of the uncertainties in the determination of the most suitable depth-overflow relationship. Uncertainties related to numerical model, boundary conditions and water depth have been taken into account in order to elaborate water depth-overflow rate relationship. This approach is illustrated through the example of the ‘Milan’ CSO of Mulhouse city, France.     

Characteristics of combined sewer overflows in Shanghai and selection of drainage systems

The overflow frequencies of combined sewer systems (CSSs) along Suzhou Creek in Shanghai are definitely lower compared with low interception ratio and design expectation. This noteworthy phenomenon was analysed and the results indicate that large sewer storage capacity due to a dense interconnection between sewer systems, flat pipe slope and pump drainage pattern, as well as the spatial–temporal difference of rainfall on a large catchment that is integrated by the connected network, lead to the overflow frequencies of combined sewer overflows (CSOs) along Suzhou Creek that are much less than those of designed and of Japanese counterparts. The drainage hydraulic model was used to simulate the performance of typical CSSs in Shanghai centre area to further explain the phenomenon. Meanwhile, as separate systems have serious illicit connections and are subject to heavy wet weather pollution in Shanghai, separate systems did not show the expected advantage in nonpoint pollution control. Thus, keeping old CSSs in the Shanghai downtown area seems to be of great value with respect to the control of urban non-point pollution.     

Selecting a series of storm events for a model-based assessment of combined sewer overflows

The hydraulic verification of combined sewer systems as well as the assessment of combined sewer overflows (CSOs) can be conducted using a hydrodynamic model. Unfortunately, long-term simulations with hydrodynamic models for the assessment of CSOs can cause unacceptably long computation times. Using only a series of storm events instead of a precipitation continuum can reduce this time and enables parallel simulation of single storm events. We introduce a method to select this series of storm events. The method’s parameters have been optimized to replicate the overflow volume of the continuous simulation and to minimize the overall computation time. This optimization revealed a generally applicable parameter set that results in series of storm events that are shorter than the precipitation continuum by 86.2–95.2% for the investigated cases. Additionally, the deviation of overflow volume between continuous simulation and series simulation ranges between only 0.1% and 4.1%.     

Mathematical modeling of sewers under surcharge for real time control of combined sewer overflows

The efficiency of real time control (RTC) to decrease storm overflows from combined sewers to receiving waters was demonstrated in several case studies. Global predictive control (GPC), an effective strategy to perform real time management of sewers, requires simulation models to anticipate the future state of the system (flow rates and water levels). To compute these predictions, a hydraulic model using an implicit discretization scheme to solve simplified continuity and momentum equations was developed. The performance of the model is evaluated by comparison with the USEPA EXTRAN model results. This comparison shows that the model gives results similar to those of the EXTRAN model, with the advantage of producing faster and more stable results regardless of space and time steps used. Additional validation with the SUPERLINK model is also presented.     

Using the duration of combined sewer overflow events for the calibration of sewer hydrodynamic models

This paper compares two calibration approaches to enhance the ability of hydrodynamic models to describe the performance of combined sewer overflow (CSO) structures. One approach is based on the duration of CSO events (monitored by low-cost sensors) and the other focuses on the overflow volume. Both were applied to the West-Graz catchment model to simulate the discharge from the CSO overflow channel under small, medium and large rain episodes. The methodology includes sensitivity analysis of the model parameters and automatic calibration through optimization. The results revealed that the calibration using the CSO duration led to results similar to those of the approach that used overflow volume, with less than 12% error differences between approaches for medium and large rain episodes.     

Improved understanding of combined sewer systems using the Illinois Conveyance Analysis Program (ICAP)

Understanding the conveyance of sewer networks is vital, especially in cases of great variability in flow rates, such as in combined sanitary and storm sewer systems. Conventional conveyance studies in sewer systems often have extended computation times due to complexity of the solution, or alternatively make assumptions that ignore the water-surface profile within a pipe. In previous research, the hydraulic performance graph (HPG) was successfully used for open-channel capacity determination. The HPG summarizes the results of many backwater calculations for a reach so that these calculations do not need to be repeated. This article describes algorithms utilized by the Illinois Conveyance Analysis Program that uses the HPGs to describe the conveyance of a system, identify bottlenecks for varying conditions, conserve mass by tracking outflow and overflows under stepwise steady flow conditions. The software is freely available at https://github.com/obergshavefun/icap/wiki.     

Condition grading for dysfunction indicators in sewer asset management

Asset management is an increasing concern for the water and wastewater industry. Condition assessment of sewer segments is an important component of sewer asset management and relies mostly on visual inspection. Observed defects are translated into a score for each segment. Although most protocols give a segment a condition grade by comparing its score with a subjective scale of numerical values, we propose a protocol to calibrate thresholds for each asset stock. Thresholds are calculated according to two sets of parameters: overall condition of the asset stock in question (estimated by a representative sample or provided by the utility manager) and assignment-error weighting (determined by the utility manager) linked to either over-estimation or under-estimation of condition grade. This method is applied to 150 km of sewers from the Greater Lyon asset stock. Sensitivity analyses of these parameters are then implemented. Three hypotheses about overall condition of the asset stock are combined with three matrices of assignment-error weights. Both parameters influence thresholds and change the assessment of the studied segments. The synthesis of such sensitivity analyses can be used to prioritise complementary investigations.     

Numerical modeling of tight fitting flexible liner in damaged sewer under earth loads

Finite element analysis and closed form solutions are used to investigate and interpret tests conducted on a damaged rigid pipe fitted with an HDPE pipe liner. Details of the analysis are described, including the modeling of local interactions between the liner and the host pipe, different segments of the damaged host pipe across longitudinal fractures, and the host pipe and the surrounding ground. Laboratory tests on a lined sewer pipe under earth loads are studied to establish whether finite element and closed form solutions for the repaired pipe system are able to effectively represent the observed behaviour.

The deteriorated rigid pipe is fractured into four fragments, and these interact to impose pairs of closely spaced vertical line loads at the crown and invert of the liner. The finite element analysis demonstrates that significant local bending develops in the liner under these contact forces. Very little thrust develops in the liner, which is almost in pure bending provided the host pipe can still carry hoop thrust across fractures. The angular expansion of the fractures at crown and invert had a negligible effect on the local bending in the liner, allowing the use of theory for rings under parallel plate loading to provide simple calculations of liner response.

Nonlinear finite element analyses indicate that the interface between the liner and the host pipe is close to the full-slip (or smooth) condition. Analysis results for that condition are within 10% of the measured values. The finite element procedure also indicates that the host pipe and the liner within it deform under the effect of the full overburden pressure if the host pipe fractures after the liner is installed. Safe liner design could be achieved by ensuring that local bending associated with host pipe fracture and deformation under the full overburden pressures does not exceed tensile capacity of the polymer (in addition to liner design to resist buckling under external fluid pressure, in accordance with previous liner stability studies). Alternatively, measures might be used to avoid local bending in the liner by preventing deterioration of the host pipe and the surrounding soil during the design life of the repair.     

Impact of combined sewer overflow on urban river hydrodynamic modelling: a case study of the Chicago waterway

Combined sewer overflow (CSO) can be a critical inflow source for urban rivers during storm events. This paper presents a case study of the Chicago waterway. A three-dimensional (3D) river hydrodynamic model was developed and integrated with an urban rainfall-runoff model using the Open Modelling Interface (OpenMI). Both the effects of CSO discharge on river and river water levels on CSO outlets were considered by the integrated model. A historical storm, which was similar to a 100-year return period rain event, was simulated and compared with field measurements. This study highlights the necessity of quantifying CSO for hydraulic modelling of urban rivers under extreme storm event conditions, and shows that an integrated hydrologic and hydraulic approach can be used to address this challenge. The 3D river hydrodynamic model can deal with the complex hydrodynamics at river confluences and provide better hydrodynamic results for water quality modelling in the future.     

The potential to retrofit sustainable drainage systems to address combined sewer overflow discharges in the Thames Tideway catchment

Experience of retrofitting sustainable drainage systems (SuDS) in the United Kingdom is limited, and there are no well‐established procedures for evaluating the feasibility, value or cost‐effectiveness of doing this, particularly at the catchment scale. This paper demonstrates a two‐phase process for evaluating the potential to retrofit SuDS to address combined sewer discharges in three subcatchments within the Thames Tideway catchment of London. The first phase evaluates what might be achieved with various levels of disconnection (‘global’ disconnection scenarios) using hydraulic models, while the second phase considers how disconnection might practically be achieved. High levels of disconnection are technically possible but practicably difficult. In selected cases, and with aggressive implementation of SuDS, combined sewer overflow CSO discharges could potentially be eliminated or reduced to acceptable levels without the need for any modifications to underground assets. However, retrofit SuDS could not eliminate the requirement for some form of sewer modification in any subcatchments.     

Innovative tracer methods for sewer infiltration monitoring

Two innovative tracer methods for the quantification of extraneous discharges in sewers have been proposed as routine applications within the scope of the European research project APUSS (Assessing Infiltration and Exfiltration on the Performance of Urban Sewer Systems). In order to improve the accuracy of wastewater hydrograph separation compared to conventional practice, the novel approaches use intrinsic physicochemical characteristics (stable isotopes, pollutographs) of the wastewater as a natural tracer for the differentiation of its constituting components. The methods were evaluated and tested under field conditions and have been validated in the course of a comparative experimental study. We analyse the state of development achieved and demonstrate the application of the protocols with data from a reference experiment conducted in a combined sewer catchment housing 5400 inhabitants. The article details the requirements for suitable boundary conditions and presents a comprehensive assessment of measurement uncertainties.     

HAZard and OPerability (HAZOP) analysis for identification of information requirements for sewer asset management

Asset management is a prerequisite for maintaining the required level of serviceability of urban drainage systems. The required asset management effort to achieve a certain level of service is unclear due to limited knowledge on sewer systems process and structure complexity. Sewer failure mechanisms explain the structural/operational failures of sewer elements. This paper describes a method for assessing sewer failure mechanisms as a first step to identify information needed for sewer asset management. In order to be able to identify the main processes and defects responsible for the structural/operational failures of sewer elements, as well as the possibility of obtaining the information about them, a HAZard and OPerability (HAZOP) approach was applied. HAZOP results were, due to the nature of the analysis, rearranged in such a way that they can be applied in Failure-Tree Analysis (FTA) for the purpose of further risk estimation.     

Benefits of using basic,imprecise or uncertain data for elaborating sewer inspection programmes

One key goal of sewer inspection programmes is to target segments in the worst condition. Despite the development of deterioration models, the influence of available data on models’ predictive power has not been studied in depth yet. In this article, numerical experiments have been conducted to answer three main questions: (1) How can the data most probably available within a utility be used to define an effective inspection programme? (2) Can we use an auxiliary variable in order to compensate effects of missing data on inspection programmes? (3) Is it worth to accept a degree of uncertainty within data instead of not having them? In other words, is it preferable to have uncertainty instead of incompleteness within utility database? In order to respond to these questions, we considered an asset stock and then degraded the information by introducing uncertainty, imprecision and incompleteness within, to form a utility's database. The results show that significant improvement of inspection programmes could be achieved by using the most probably available data within utilities. We also show that using the notion of ‘district’ can provide efficient results when the most informative factor ‘age’ is not available. Finally, it is shown that having uncertain data is preferable to having incompleteness.     

Analysing the importance of variables for sewer failure prediction

When defining the variables to predict sewer failure and therefore optimise sewer systems maintenance, it is important to identify the ones that most significantly influence the quality of the predictions or to define the smallest number of variables that is sufficient to obtain accurate predictions. In this study, three different statistical variable selection algorithms are applied for the first time to identify the most important variables for sewer failure prediction: the mutual information indicator, the out-of-bag samples concept, based on the random forest algorithm, and the stepwise search approach. The methods were applied to a real data-set that consists of the categorisation of sewer condition and associated physical characteristics. The mutual information and the stepwise search methods provided good predictions while those obtained using out-of-bag samples based on random forest were somewhat different, justified by the lack of robustness to imbalanced class distributions.     

A numerical approach for analysing the performance of a sewage screening chamber

The flow within a sewage screening chamber can be highly complex due to high-flow conditions and the geometrical constraints imposed by the screening chamber itself. This paper presents an application of a two-dimensional depth-averaged shallow flow model for flow simulations in a sewage screening chamber. For this purpose, the shallow flow model is improved by including a simple turbulence model. The improved model is able to reproduce the main features of the flow field, as confirmed by comparing with the measurements resulting from a scale-down laboratory model. The potential of numerical model for analysing the performance of the screening chamber is then demonstrated by testing a new design of including flow dissipating blocks to (i) re-distribute more evenly the flow through different flow passages installed with screens and (ii) reduce maximum mean flow velocity approaching the screens, so as to reduce the risk of damaging the screens during high-flow conditions.     

Statistical comparison of the performance of data-based models for sewer condition modeling

Abstract

The present article proposes a methodology to consider the uncertainty intrinsic to data-based models when comparing their performance. The goal is to provide a quantification of the variability of this type of models due to the random nature of the calibration process and enable a statistical comparison of the models’ performance when attempting to identify the best. The methodology proposed doesn’t provide an alternative metric to determine the models’ performance, but it expands the traditional deterministic comparison to a stochastic comparison. The methodology builds on the current standard approach for developing data-based model and its application is demonstrated to model sewer condition using data from 4 trunk sewers of the SANEST – Saneamento da Costa do Estoril sewer system, corresponding to 25?km of sewer pipes. The data-based models were developed using artificial neural networks, support vector machines, bootstrapping aggregation and least squares support vector machines. For the case study, the highest and average misclassification performance records are similar for all models (23% to 24% and 31% to 33%, respectively) but the lowest performance varied more significantly (39% to 62%). This demonstrates that selecting a model based on its maximum single realisation performance alone may be misleading.     

液压挖掘机工作装置运动与动力综合优化

根据液压挖掘机工作装置的特点，应用Denavit-Hartenberg齐次变换矩阵建立运动学方程，以最大挖掘力为优化目标，进行了运动与动力综合性能优化求解，为进一步研究实现挖掘机自动控制奠定了理论计算基础，具有工程应用价值。