Urban stormwater contamination by polychlorinated biphenyls (PCBs) and its importance for urban water systems in Switzerland

Like other persistent organic pollutants (POPs), polychlorinated biphenyls (PCBs) are still present in the environment despite their almost worldwide prohibition. A study was conducted over 1 year in Switzerland to analyze the source and load of PCBs in urban stormwater and their importance in urban water systems. The mean PCB concentrations of 89 rain events were determined in five different strictly separate drainage systems (three in Lausanne and two in Geneva). The mean concentrations of PCBs in stormwater ranged from values below the detection limit (0.11-0.24 ng/l) to 403 ng/l. A model for the wet and dry deposition of PCBs based on these results estimates that their concentration in rainwater has remained practically constant (35 ng/l) over a period of 12 years. A mass balance of Swiss levels estimates a total input load of PCBs in the urban water systems at 110-125 kg/year. The contribution of stormwater is 75-94 kg/year in combined sewer systems and 23-34 kg in separate sewer systems. This mass balance illustrates that a major part of the PCB load in the environment or in WWTP sludges originates from urban stormwater. This result was confirmed by a fingerprint of PCB congeners in stormwater, which showed a pattern identical to that found in sewage sludge in combined sewer systems.     

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。     

Contributions of wastewater, runoff and sewer deposit erosion to wet weather pollutant loads in combined sewer systems

An observatory of urban pollutants was created in Paris for the purpose of assessing the dynamics of wastewater and wet weather flow (WW and WWF) pollutant loads within combined sewers. This observatory is composed of six urban catchments, covering land areas ranging in size from 42 ha to 2581 ha. For a wide array of parameters including total suspended solids (TSS), chemical and biochemical oxygen demand (COD and BOD₅), total organic carbon (TOC), total Kjeldahl nitrogen (TKN), heavy metals (Cu and Zn) and polycyclic aromatic hydrocarbons (PAHs), this article is intended to evaluate the contributions of wastewater, runoff and in-sewer processes to WWF pollutant loads through the use of an entry–exit mass balance approach. To achieve this objective, a total of 16 rain events were sampled on these sites between May 2003 and February 2006. This study has confirmed that at the considered catchment scale (i.e. from 42 ha to 2581 ha) the production and transfer processes associated with WWF pollutant loads do not vary with basin scale. Entry–exit chemical mass balances over all catchments and for a large number of rain events indicate that wastewater constitutes the main source of organic and nitrogenous pollution, while runoff is the predominant source of Zn. For Cu, PAHs and TSS, the calculation underscores the major role played by in-sewer processes, specifically by sediment erosion, as a source of WWF pollution. A significant loss of dissolved metals was also observed during their transfer within the sewer network, likely as a consequence of the adsorption of dissolved metals on TSS and/or on sewer deposits. Moreover, the nature of eroded particles was examined and compared to the various sewer deposits. This comparison has highlighted that such particles exhibit similar organic and PAH contents to those measured in the organic layer, thus suggesting that the deposit eroded during a wet weather period is organic and of a nature comparable to the organic layer. Despite the extent of initial field investigations, no organic deposit was observed to be present on sewer lines within the catchments, which implies that this organic deposit is probably present in another form or to be found elsewhere in the main trunks.     

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.     

Grey-box modelling of pollutant loads from a sewer system

Using a compact measuring unit with on-line meters for UV absorption and turbidity, it is possible to determine concentrations of organic load (chemical oxygen demand (COD) and suspended solids (SS)) anywhere in a sewer system. When measurements of the flow are available as well, the pollutant mass flow at the measuring point can be calculated.The measured data are used to estimate different models describing the load of pollutants in the sewer. A comparison of the models shows that a grey-box model is most informative and best in terms measured by the multiple correlation coefficient. The grey-box model is a state-space model, where the state represents the actual amount of deposition in the sewer, and the output from the model is the pollutant mass flow to the wastewater treatment plant (WWTP). The model is formulated by means of stochastic differential equations. Harmonic functions are used to describe the dry weather diurnal load profiles. It is found that the accumulation of deposits in the sewer depends on previous rain events and flows.By means of on-line use of the grey-box models, it is possible to predict the amount of pollutants in a first flush at any time, and hence from the capacity of the plant to decide if and when the available detention basin is to be used for storage of wastewater. The mass flow models comprise an important improvement of the integrated control of sewer and WWTP including control of equalisation basins in the sewer system. Further improvements are expected by the introduction of an additive model where dry weather situations and storm situations are modelled separately before addition to the resulting model.     

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.     

Energy optimization of the urban drainage system by integrated real-time control during wet and dry weather conditions

An extension of the frequently applied equal filling degree control algorithm is proposed for integrated control of sewer system and wastewater treatment plant (WWTP). Under dry weather conditions (DWC), this control synchronizes sewer storage volume activation at pumping stations (PS) throughout the sewer system with the intermittent aeration at the WWTP. This is in contrast with the frequently advocated influent load equalization at WWTPs. The concept is demonstrated on a case study using an integrated system composed of detailed models for sewer system and WWTP where it reveals considerable potential for energy savings and effluent quality improvement. Moreover, the integrated control allows for the reduction of combined sewer overflows during wet weather events and decreased sedimentation potential in the sewer system. Neither weather predictions nor structural changes at the WWTP or in the sewer system are required. The control concept is applicable to about 50 Flemish WWTPs.     

Continuous simulation of CSO emissions

Modelling is an essential method to predict the behaviour of various water systems in urban areas. A common problem of modelling these systems is that it is hardly possible to model them fully in a detailed way. The applications of existing hydrodynamic models are very data-intensive and time-consuming, particularly for continuous simulation using a long rainfall record. It is thus necessary to develop a new modelling approach which is correct in representing and efficient in dealing with the processes involved in urban water systems. SewSim is such a conceptual combined sewer overflow (CSO) emission model. SewSim has been developed using a dynamic programming package MATLAB and its toolbox SIMULINK. This paper presents some case studies using this model for continuous simulation.     

Optimal pollution control for management of large interceptor sewer systems

An optimal pollution control (OPC) model for interceptor sewer systems has been developed to minimize total pollution overspill load to receiving waters. This derives from a formal optimization applied to a simplistic hydraulic representation of the sewer, under a so-called 'slug flow' approach. Combined sewer overflow (CSO) structures are represented and the model tracks the pollution concentrations in the CSOs as well as incoming pollution loads in arriving at its optimal spill strategy at each such structure. In essence, the strategy strives to defer overspill to river as far as is possible and, when unavoidable, to first release sewage carrying the lowest pollution loads for minimum receiving water impact. This paper describes an academically driven study on the application of the OPC model to part of the Liverpool Interceptor Sewer system during a typical year of rainfall. The results show that the OPC model considerably reduces pollution overspill load compared to fixed local control strategies, by as much as 70% in the case study presented. Furthermore, the approach is computationally efficient and therefore holds promise for application in real time.     

The importance of gully flow modelling to urban flood simulation

ABSTRACT

Urban flood simulation often ignores or simplifies the function of underground gully systems due to data and computational limitations. To discover the influence of gullies on urban flooding, a novel approach is proposed in this study, by fully-coupling a 1D gully flow model (GFM), a 1D sewer flow model (SFM), and a 2D overland flow model (OFM) to simultaneously simulate the flow exchanges between surface, gullies and sewer pipes. This fully-coupled approach is compared with a simplified approach which directly introduces surface water into sewer pipes without being via gullies. The validation results show that the fully-coupled approach considerably reduces the underestimation of flood extent by 27% compared with the simplified approach. Without considering the capacity of lateral tubes between gullies and sewer pipes, the simplified approach over-drains the surface water into sewer pipes. The modelling of gully flow is crucial for correctly evaluating the efficiency of drainage systems.     

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.     

雨污分流改造保留合流管为雨水管的工程实例

在将城市排水合流管改造为雨、污分流管道的过程中，如条件允许可将合流管局部或整体保留用做雨水管，再另外新建污水管道。结合苏州市中心城区完善污水管网工程，介绍了在该市吴县新村内的应用实例。实践表明，保留的合流管能有效地排放正常年份的降雨，甚至是台风暴雨时产生的地面径流。该措施不仅能节约工程投资、缩短工期，还能减少因地面开挖对居民生活造成的不利影响。     

Probabilistic modelling of overflow, surcharge and flooding in urban drainage using the first-order reliability method and parameterization of local rain series

Failure of urban drainage systems may occur due to surcharge or flooding at specific manholes in the system, or due to overflows from combined sewer systems to receiving waters. To quantify the probability or return period of failure, standard approaches make use of the simulation of design storms or long historical rainfall series in a hydrodynamic model of the urban drainage system. In this paper, an alternative probabilistic method is investigated: the first-order reliability method (FORM). To apply this method, a long rainfall time series was divided in rainstorms (rain events), and each rainstorm conceptualized to a synthetic rainfall hyetograph by a Gaussian shape with the parameters rainstorm depth, duration and peak intensity. Probability distributions were calibrated for these three parameters and used on the basis of the failure probability estimation, together with a hydrodynamic simulation model to determine the failure conditions for each set of parameters. The method takes into account the uncertainties involved in the rainstorm parameterization. Comparison is made between the failure probability results of the FORM method, the standard method using long-term simulations and alternative methods based on random sampling (Monte Carlo direct sampling and importance sampling). It is concluded that without crucial influence on the modelling accuracy, the FORM is very applicable as an alternative to traditional long-term simulations of urban drainage systems.     

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.     

Loss rates of urban biocides can exceed those of agricultural pesticides

Biocides and pesticides are used to control unwanted organisms in urban and agricultural areas. After application, they can be lost to surface waters and impair water quality. Several national consumption studies have shown that urban and agricultural use may be in the same range. It is difficult to judge whether this results in similar loadings of surface waters because there is a lack of sound, comparative studies addressing urban and agricultural losses simultaneously.The aim of this study is thus to relate the biocide and pesticide loads found in surface waters to their respective urban and agricultural usage (loss rates). To simultaneously assess the loss rates, we conducted a comprehensive field study in a catchment of mixed land use on the Swiss Plateau. The study area was divided into four sub-catchments with different degrees of urban and agricultural land use. In addition, we studied the only wastewater treatment plant, a combined sewer overflow and a storm sewer within the area. Rain events were sampled at high temporal resolution from March to November, 2007. Information on agricultural applications was gained from local farmers. For urban uses, consumption estimations were conducted based on statistical and product information.Despite substantially lower amounts used, the measured loads of urban biocides were in the same range as the most widely-used agricultural pesticides. The lower usage was compensated by urban loss rates that were up to ten times higher than agricultural ones (0.6 to 15% for urban, 0.4 to 0.9% for agricultural compounds). For most biocides and pesticides, the loads were controlled by rain events. Besides the rain-controlled losses, some urban-used biocides (e.g. diazinon) showed a continuous load independent of rain events and season. This study demonstrates that in catchments with mixed land use, mitigation strategies have to pay sufficient attention to the urban sources.     

Quantification of groundwater infiltration and surface water inflows in urban sewer networks based on a multiple model approach

The management of sewer systems requires information about discharge and variability of typical wastewater sources in urban catchments. Especially the infiltration of groundwater and the inflow of surface water (I/I) are important for making decisions about the rehabilitation and operation of sewer networks. This paper presents a methodology to identify I/I and estimate its quantity. For each flow fraction in sewer networks, an individual model approach is formulated whose parameters are optimised by the method of least squares. This method was applied to estimate the contributions to the wastewater flow in the sewer system of the City of Dresden (Germany), where data availability is good. Absolute flows of I/I and their temporal variations are estimated. Further information on the characteristics of infiltration is gained by clustering and grouping sewer pipes according to the attributes construction year and groundwater influence and relating these resulting classes to infiltration behaviour. Further, it is shown that condition classes based on CCTV-data can be used to estimate the infiltration potential of sewer pipes.     

Sediments in Sewers

A recent survey has suggested that up to 25 000 km of UK sewerage networks may be affected by in-pipe sediment deposits. Field evidence indicates that, typically, up to 90% of the pollution load discharged from storm sewage overflows may be derived from the erosion of accumulated in-system sediment, commonly referred to as silt.
A sewer flow quality simulation model is being developed under the aegis of the WRc/Water Industry Collaborative River Basin Management research programme. Such a model will enable sewerage engineers and water quality planners to produce more effective designs for sewerage rehabilitation schemes to control river pollution. In order to produce this model it is necessary to understand the nature, characteristics and controlling mechanisms of in-pipe sediment deposits.
Field observations, coupled with sampling and analysis of combined sewer sediment deposits, have produced a five category classification for such sediments. Each category has distinctive characteristics in terms of appearance, composition and polluting potential.     

Quantifying costs and lengths of urban drainage systems with a simple static sewer infrastructure model

A generic model is introduced that is capable of quantitatively representing the combined sewer infrastructure of a settlement. It consists of a catchment area module, which calculates the length and size distribution of the needed sewer pipes on the basis of rain, housing densities and area size. These results are fed into the sewer construction costs module in order to estimate the combined sewer costs of the entire catchment area.

The model could be successfully fitted to existing Swiss sewer systems, indicating that it can emulate their principal characteristics. It could also identify fundamental differences in sewer designs in cities with historic roots. The results confirm that there are economies of scale for combined sewer systems in Switzerland. The modelling approach proved to be an effective tool for understanding the factors underlying the cost structure for water network infrastructures.     

城市污水管网信息资料的管理

简要介绍了城市污水管网信息资料管理的现状和GIS的发展及特点,分析了城市污水管网信息资料的记录内容、特点和城市污水管网地理信息系统总体框架。论述了GIS应用于城市污水管网管理具有重要的现实意义和很好的应用前景。