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.     

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.     

Case study on the use of a combined system as an intermediate solution in Brazil: cost estimate

As a developing country, Brazil has to deal with a lack of basic sanitation. One of the major challenges is the treatment of wastewater. While 94% of Brazilian municipalities have stormwater systems, only 55% are connected to a wastewater system. To help improve the current situation, this study estimates the costs for implementing an intermediate solution, which is designed to ease the transition towards a separate wastewater treatment system. This solution utilizes a combined sewer system approach in areas where stormwater systems have already been implemented. A case study was carried out assessing two possible alternatives for a wastewater treatment system with data from the city of Joinville. Both alternatives are separate systems but one has a combined system as an intermediate solution. The results show that the alternative with an intermediate combined system will reduce the short‐term financial burden on governments in comparison to the separate sewer system approach.     

Modelling of groundwater infiltration into sewer systems

Groundwater infiltration into urban sewers represents a problem that influences costs and management of technical systems. The hydrodynamic groundwater software MODFLOW is used to analyse the influencing variables of the infiltration processes. Besides the hydraulic conductivity of the soil and the piezometric head in the vicinity of the sewer pipe, properties of the sewer trench, the shape and the size of leaks are important influencing factors. A non-linear-regression method is applied to develop a one-dimensional approach in accordance with the MODFLOW results and Darcy's law. Monte Carlo simulations and the developed one-dimensional model are used to assess the leak area and the range of pressure loss in the vicinity of the pipe leaks. By additional sensitivity analysis it was found that the infiltration factor and the conductivity of the backfill are very important for the calculation of the leak area.     

Production and transport of urban wet weather pollution in combined sewer systems: the “Marais” experimental urban catchment in Paris

An experimental catchment area was set up in the centre of Paris (France) so as to follow up the quality of wet weather flows from the entry to the exit of a combined sewer network. The distinctive characteristic of this site is its location in a town centre and the extent of the equipment used to monitor the water pollution over the whole length of its course through the catchment area. The results obtained show a change in quality between the runoff entering the sewer network and the combined storm water flow at the sewer's outlet, which cannot be explained only by the mixture with domestic wastewater. In particular, an increase was observed in the concentrations of suspended solids (SS), VSS, COD, BOD and Cu, in the proportion of pollutants linked to particles and in the characteristics of the particles. A calculation of the total masses going in and out of the sewer network during a rainfall event shows that the erosion of in-sewer pollution stocks is the main source of particles and of organic matter in wet weather flows, whereas heavy metals loads originated from roof runoff, due to the corrosion of metallic roofs. Particles eroded from the sewer sediments during rain events were found to be quite different from the particles of type A deposits and organic biofilms. Nevertheless, they have mean organic and metallic loads that are of the same order of magnitude as the particles of the organic layer at water sediment interface. A change in the chemical form of heavy metals was noticed during the transport in the sewer and it is suspected that a fraction of the dissolved metals from the runoff is adsorbed on sewer sediments.     

The Form of the Core

This article assesses the public water and sewer costs associated with alternative housing patterns. These patterns are defined in terms of lot size, tract dispersion, and distance from existing water and sewer service centers. The engineering cost model presented here gives empirical evidence of how sensitive local government service costs are to the spatial pattern of single-family residential development. The results show that more spread out housing patterns are more costly to supply with public water and sewer services, but that shifting a majority of these costs to the private sector may be a relatively simple matter.     

On hydraulic and pollution effects of converting combined sewer catchments to separate sewer catchments

The overall objective of this paper is to contribute to the standing debate concerning the advantages of separate sewer systems compared to traditional combined sewer systems. By a case study this investigation reveals that transformation of part of a town from being serviced with combined sewer systems to become serviced with separate sewer systems decreases the volumes of storm water and pollutants diverted to the waste water treatment plant and discharged as combined sewer overflow. This happens at the expense of an increase in volumes of storm water and pollutant loads diverted to local receiving waters when detention ponds are not built-in the new separate sewer systems. It is concluded that consequences can be fatal for receiving waters, if no retention of pollutants is integrated into the system.     

Calibration of hydrodynamic model-driven sewer maintenance

Sewer performance is typically assessed using hydrodynamic models assuming the absence of in-sewer defects. As a consequence, hydraulic performance calculated by models is likely to be overestimated, while the real hydraulic performance of the sewer system remains unknown. This article introduces the concept of ‘hydraulic fingerprinting’ based on model calibration to identify in-sewer defects affecting hydraulic performance. Model calibration enables detection of changes in hydraulic properties of the sewer system. Each model calibration results in a set of model parameter values, their uncertainties and residuals. The model parameter values also incorporate the antecedent condition of the catchment of the event calibrated and are therefore less suitable to identify in-sewer defects. The residuals on the other hand, and more specifically their absolute values, statistical properties and the correlation between residuals at different monitoring locations are suitable as indicators of the occurrence of in-sewer defects. This allows the application of ‘hydraulic fingerprinting’ based on model calibration, where the ‘fingerprint’ is defined by the model parameters and the residuals. The concept of ‘fingerprinting’ is demonstrated for the combined sewer system ‘Tuindorp’ (Utrecht, the Netherlands). The results show that ‘hydraulic fingerprinting’ can be a powerful tool for directing sewer asset management actions.     

Wastewater quality and pollutant loads in combined sewers during dry weather periods

An observatory of urban pollutants was created in Paris in order to assess, at the urban catchment scale, the quality of sanitary sewage and pollutant loads during dry weather periods in the Paris combined sewer. Investigations were carried out for six urban catchments (varying from 42 to 2580 ha) focusing on a wide range of parameters, including: suspended solids (SS), chemical and biochemical oxygen demand (COD and BOD₅), total organic carbon (TOC), total Kjeldahl nitrogen (TKN), heavy metals (Cd, Cu, Pb, Zn), and aliphatic and polycyclic aromatic hydrocarbons (AHs and PAHs). Despite the marginal intra-site variability of some pollutants, which serves to reflect the impact of point sources, this work attests to the spatial homogeneity, at the physical scales considered, of wastewater quality and pollutant loads within the Paris combined sewer network. These results imply that similar production and transfer processes are occurring within sewers during dry weather periods and strongly suggest that data obtained on one specific catchment could be extrapolated to smaller or larger catchments that display quite similar land use and sewer characteristics.     

Stochastic modeling of total suspended solids (TSS) in urban areas during rain events

The load of total suspended solids (TSS) is one of the most important parameters for evaluating wet-weather pollution in urban sanitation systems. In fact, pollutants such as heavy metals, polycyclic aromatic hydrocarbons (PAHs), phosphorous and organic compounds are adsorbed onto these particles so that a high TSS load indicates the potential impact on the receiving waters. In this paper, a stochastic model is proposed to estimate the TSS load and its dynamics during rain events. Information on the various simulated processes was extracted from different studies of TSS in urban areas. The model thus predicts the probability of TSS loads arising from combined sewer overflows (CSOs) in combined sewer systems as well as from stormwater in separate sewer systems in addition to the amount of TSS retained in treatment devices in both sewer systems. The results of this TSS model illustrate the potential of the stochastic modeling approach for assessing environmental problems.     

基于SWMM模型的截流倍数环境效应分析

由于对改造建设的重视程度、时序安排、经济条件、改造难度等许多主观和客观因素的限制,许多城市老城区的合流制排水系统保留至今。采用SWMM水力水质模型,对城市合流区的污染物削减情况进行模拟评价,以期为合流制排水系统的截流倍数取值及建设方向提供参考。结果表明,当截流倍数n≥2时,合流制排水系统即可达到分流制排水系统的污染物截流效果;但只有当截流倍数n≥5时,合流制排水系统才能达到分流制排水系统的污染物削减效果。城市合流区需尽快进行合改分建设,或加大末端处理设施的建设。     

Towards quantification of uncertainty in predicting water quality failures in integrated catchment model studies

This paper describes the development and application of a method for estimating uncertainty in the prediction of sewer flow quantity and quality and how this may impact on the prediction of water quality failures in integrated catchment modelling (ICM) studies. The method is generic and readily adaptable for use with different flow quality prediction models that are used in ICM studies. Use is made of the elicitation concept, whereby expert knowledge combined with a limited amount of data are translated into probability distributions describing the level of uncertainty of various input and model variables. This type of approach can be used even if little or no site specific data is available. Integrated catchment modelling studies often use complex deterministic models. To apply the results of elicitation in a case study, a computational reduction method has been developed in order to determine levels of uncertainty in model outputs with a reasonably practical level of computational effort. This approach was applied to determine the level of uncertainty in the number of water quality failures predicted by an ICM study, due to uncertainty associated with input and model parameters of the urban drainage model component of the ICM. For a small case study catchment in the UK, it was shown that the predicted number of water quality failures in the receiving water could vary by around 45% of the number predicted without consideration of model uncertainty for dissolved oxygen and around 32% for unionised ammonia. It was concluded that the potential overall levels of uncertainty in the ICM outputs could be significant. Any solutions designed using modelling approaches that do not consider uncertainty associated with model input and model parameters may be significantly over-dimensioned or under-dimensioned. With changing external inputs, such as rainfall and river flows due to climate change, better accounting for uncertainty is required.     

Contribution of different sources to the pollution of wet weather flows in combined sewers

Experiments performed on "Marais" catchment, in central Paris, aimed to follow up the quality of wet weather flows from the entry to the exit of a combined sewer network. SS, VSS, COD, BOD5, Cd, Cu, Pb, Zn concentrations were measured for an important number of rain events in roof, yard, street runoff, as well as in dry and wet weather flows at the catchment outlet. Mass entry-exit totals, at the scale of the catchment, were calculated over 31 rain events in order to evaluate the contribution of different types of runoff, of sanitary sewage and of sewer sediments to the total wet weather pollutant loads at the catchment outlet. The erosion of in-sewer pollutant stocks was found to be the main source of particles and of organic matter in wet weather flows, whereas heavy metal loads mainly originated from roof runoff, due to the corrosion of metallic roofs. Particles eroded inside the sewer during rain events were found to be quite different from the particles constituting the main part of sewer sediments: they are organic and biodegradable, with rather important settling velocities and seem to accumulate during dry weather periods. A change of the chemical form of heavy metals was noticed during the transport in the sewer and it is suspected that a fraction of the dissolved metals from the runoff is adsorbed on sewer sediments.     

Source characterisation and loads of metals and pesticides in urban wet weather discharges

Contaminants in urban wet weather discharges originate from a number of sources such as materials from wet and dry atmospheric deposition, wastewaters, urban surface erosion, traffic-related activities, in-sewer deposits, etc. In the current study, four contributions (rainwater, dry atmospheric deposition, dry weather discharge and catchment surface + possible erosion of in-sewer deposits) to the total concentrations of priority substances have been assessed at the outlet of two urban catchments (one residential catchment with a combined system and one industrial area with a separate stormwater system) for 12 storm events (six for each catchment). Mass balances were calculated for seven metals and four pesticides, as well as for total suspended solids and chemical oxygen demand. The respective contributions of dry and wet atmospheric deposition, wastewater and catchment surface differ for each pollutant type, corresponding to different land use, activities, environments and sewer systems. For most of the pollutants, the catchment surface appears to be the main contribution, with significant storm event variability, excepted for atrazine in one catchment.     

Cosmoss: conceptual simplified model for sewer system simulation: A new model for urban runoff quality

In order to minimise storm water impact on receiving waters, mathematical models to simulate processes occurring in sewer systems are needed. The aim of this study is to propose a simple and reliable conceptual model for simulating suspended solids discharge during storm events. The model has been tested using experimental data acquired in Parco d'Orleans catchment, Palermo, in Fossolo catchment, Bologna, and in Milijakovac catchment, Belgrade. Results obtained show the need to extend the experimental investigations in order to obtain more reliable information about the washoff rate.     

Sewer deterioration modeling with condition data lacking historical records

Accurate predictions of future conditions of sewer systems are needed for efficient rehabilitation planning. For this purpose, a range of sewer deterioration models has been proposed which can be improved by calibration with observed sewer condition data. However, if datasets lack historical records, calibration requires a combination of deterioration and sewer rehabilitation models, as the current state of the sewer network reflects the combined effect of both processes. Otherwise, physical sewer lifespans are overestimated as pipes in poor condition that were rehabilitated are no longer represented in the dataset. We therefore propose the combination of a sewer deterioration model with a simple rehabilitation model which can be calibrated with datasets lacking historical information. We use Bayesian inference for parameter estimation due to the limited information content of the data and limited identifiability of the model parameters. A sensitivity analysis gives an insight into the model's robustness against the uncertainty of the prior. The analysis reveals that the model results are principally sensitive to the means of the priors of specific model parameters, which should therefore be elicited with care. The importance sampling technique applied for the sensitivity analysis permitted efficient implementation for regional sensitivity analysis with reasonable computational outlay. Application of the combined model with both simulated and real data shows that it effectively compensates for the bias induced by a lack of historical data. Thus, the novel approach makes it possible to calibrate sewer pipe deterioration models even when historical condition records are lacking. Since at least some prior knowledge of the model parameters is available, the strength of Bayesian inference is particularly evident in the case of small datasets.     

Reliability-based layout design of sewage collection systems in flat areas

The sewer layout in flat areas significantly influences the final design which is desired to be optimized for the construction and operational costs. This study introduces a model for designing the layout of sewer networks considering their reliability. A reliability criterion is introduced and optimized using a simple simulated annealing scheme. The best layout with the maximum reliability represents an optimum sewer layout in which clogging in a sewer has the least effect on its upstream lines. A case study is solved using the proposed model. Then, for the obtained layout, the sewer specifications are designed using a dynamic programming model. The reliability somehow reflects the operational costs that can be taken into account in the system design by the proposed model. Furthermore, it is concluded that the network's reliability and construction cost are in conflict with each other such that more reliable layouts lead to more expensive designs.     

An economic loss model for failure of sewer pipelines

Estimating the costs of failure for sewer pipelines is usually accompanied with uncertainties because of the difficulty in capturing the relationship between the physical and economical characteristics of failed pipelines. To reduce such uncertainties economic loss models are usually used to evaluate the consequences of failure. This paper presents a methodology to estimate economic loss as a result of sewer pipelines’ failure using cost benefit analysis approach. Costs of sewer pipelines’ failure in addition to costs resulting from avoiding such failures are identified and analysed. To validate the proposed methodology, actual costs from a real failure incident were compared with the proposed model outputs. The model could estimate the direct and indirect costs with a deviation ranging between 10–12% and 22–30%, respectively. By implementing the proposed methodology on two case studies, it was found that the indirect costs as a result of sewer pipelines’ failure represent a significant portion ranging between 89 and 94% of the total costs of failure. Also, it was found that costs related to environment, delays to work and traffic disruptions contribute by 12–35% to the indirect costs.     

集中式污水处理系统的最佳规模研究

综合分析污水处理厂的吨水投资、管网投资和运行费用有助于弄清城镇污水末端处理的规模效应。研究表明,中、小规模污水处理厂的吨水投资、运行费用在不同处理工艺间均存在规模效应,且相关性显著;大型和特大型污水处理厂的吨水投资和运行费用均保持在较低的范围内,且随着处理规模的增加而逐步稳定;当污水管网的收集规模<6×104m3/d时,其吨水投资的规模效应突出;随着污水管网规模的增大,吨水管网投资趋于较低的水平。当污水处理厂仅作为排水系统的末端环节时,综合考虑污水处理厂和收集管网的单位投资及运行费用等因素,最佳的处理规模为10×104m3/d左右;当污水处理厂具有污染治理和污水再生回用的双重功能而成为水资源循环的中间环节时,适宜的污水处理规模受多种因素的影响,区域性的半集中式处理模式将成为一种可行的选择。     

Optimal design program for gravity sanitary sewers

The development and implementation of a Fortran computer program called OGSDP to design a least-cost gravity sanitary sewer system are presented. OGSDP is a model that finds the least-cost design for a non-looping, gravity sanitary sewerage system for a given set of design parameters, costs and lay-out. The final design is the least-cost combination of pipe sizes, upstream and downstream invert elevations (of each pipe) and the corresponding manhole sizes. OGSDP respects most Ten States Standards' (1978) design guidelines. OGSDP determines an initial sewer system design using a heuristic procedure, and then improves the design using discrete dynamic programming with successive approximations to obtain the final least-cost design. It is a practical and efficient tool in the design of sanitary sewer systems.