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.     

A Bayesian network model to assess the public health risk associated with wet weather sewer overflows discharging into waterways

Overflows from sanitary sewers during wet weather, which occur when the hydraulic capacity of the sewer system is exceeded, are considered a potential threat to the ecological and public health of the waterways which receive these overflows. As a result, water retailers in Australia and internationally commit significant resources to manage and abate sewer overflows. However, whilst some studies have contributed to an increased understanding of the impacts and risks associated with these events, they are relatively few in number and there still is a general lack of knowledge in this area. A Bayesian network model to assess the public health risk associated with wet weather sewer overflows is presented in this paper. The Bayesian network approach is shown to provide significant benefits in the assessment of public health risks associated with wet weather sewer overflows. In particular, the ability for the model to account for the uncertainty inherent in sewer overflow events and subsequent impacts through the use of probabilities is a valuable function. In addition, the paper highlights the benefits of the probabilistic inference function of the Bayesian network in prioritising management options to minimise public health risks associated with sewer overflows.     

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.     

Spatial variability of the characteristics of combined wet weather pollutant loads in Paris

An on-site observatory of urban pollutant loads in combined sewers has been created in Paris in order to investigate wet weather pollutant loads at different spatial scales. This observatory is composed of six urban catchments, covering areas from 41 to 2581ha. For a wide range of parameters including suspended solids (SS), volatile suspended solids (VSS), chemical oxygen demand (COD), biochemical oxygen demand (BOD5), total organic carbon (TOC), total Kjeldahl nitrogen (TKN), metals (Cd, Cu, Pb, Zn), aliphatic hydrocarbons and polycyclic aromatic hydrocarbons (AHs and PAHs), this article serves to evaluate the potential scale effect on wet weather flow (WWF) quality. Although the extensive data set compiled herein has emphasized the high variability in WWF quality from one rain event to the next, no scale effect could be observed for a given rain event on pollutant concentrations, distributions between dissolved and particulate phases, pollutant contents and loads. Such results are of prime importance since they suggest (i) no spatial scale influence on WWF quality for the considered catchments and (ii) similar dominant sources and transfer processes at the various spatial scales.     

流域治理视角下合流制雨季超量混合污水治理策略

传统集中式城镇卫生系统在改善水环境的同时,也凸显了集中管网系统在雨季对汇水区域雨水快速收集、快速下泄导致的污水厂进水短期水量大幅提高的水力负荷冲击问题,而我国目前活性污泥系统工艺设计及运行模式无法应对雨季峰值流量,导致管网沿程溢流或厂前溢流或者超越,严重污染了城市河道、受纳水体。在分析集中式城镇排水系统应对峰值流量系统性欠缺的基础上,结合雨季峰值流量特性,总结了欧美发达国家在雨季超量混合流量处理方面主要的技术路线、解决方案和运行控制模式,并就我国流域治理大背景下实施雨季峰值流量处理在立法及排放标准方面提出了建议。     

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.     

Characteristics of wet and dry weather heavy metal discharges in the Yeongsan Watershed, Korea

A comprehensive water quality monitoring program was conducted in the Yeongsan (YS) River, Korea from 2005 to present to investigate wet and dry weather pollutant discharge in an attempt to establish point and non-point pollution management strategies. As part of this monitoring program, 11 heavy metal species were measured during dry and wet weather conditions in the YS River, where Gwangju City (GJ), a subcatchment of the YS River, was further monitored to clarify the responsibility of different metal species discharged into the mainstream. Monthly grab water samples showed that greater amounts of metals along the YS River were discharged during the wet summer months due largely to storm runoff. In addition, further monitoring results revealed that GJ, a highly urbanized area, was a significant contributor of the heavy metals being discharged into the YS River during both wet and dry weather. The most abundant metal species discharged from GJ were manganese, aluminum and iron with different contributions of wet and dry weather flows to the total discharge load. Wet weather flow was a significant contributor to the annual dissolved metal loads, accounting for 44-93% of the annual load depending on the metal species, with the exception of chromium and cadmium (9% and 27%, respectively). Mostly, metal loads during wet weather were shown to be proportional to the rainfall depth and antecedent dry period. A substantial fraction of metals were also associated with solids, suggesting that sedimentation might be an appropriate management practice for reducing the metal load generated in GJ. Overall, although dissolved metal concentrations in YS River were at an acceptable level for aquatic community protection, continual metal discharge throughout the year was considered to be a potential problem in the long-term due to gradual water quality degradation as well as continuous metal accumulation in the system.     

Contribution of domestic effluents to hydrocarbon levels of dry weather flow in combined sewers

The importance of hydrocarbon contamination of the sewer network has been reported by many authors recently. Most of the studies are focused on the introduction of such pollutants into combined sewers by street and roof stormwater, but few evaluate the contribution of domestic inputs to hydrocarbon pollution. As a consequence, this work—carried out on the framework of the OPUR (Observatory of Urban Pollutants) research program—assesses the resolved aliphatic (AH), unresolved complex mixture (UCM) and polycyclic aromatic hydrocarbon (PAH) concentrations of domestic effluents (DE) and evaluates the role played by such effluents on the hydrocarbon levels of dry weather flow (DWF) in combined sewers. Results show hydrocarbon concentrations in the 200 – 300 μg · l^?1, 300 – 1 000 μg · l^?1 and 0.9 – 1.4 μg · l^?1 ranges for AHs, UCM and the 16 PAHs of the US-EPA, respectively. The assessment of hydrocarbon fluxes conveyed by domestic effluents on the scale of the ‘Le Marais’ experimental urban catchment (42 ha, centre of Paris) reveals the predominant contribution of domestic inputs to the DWF pollution and highlights the unsuspected role of households.     

Priority pollutants in wastewater and combined sewer overflow

Implementation of the European Water Framework Directive and its affiliated directives requires Member States to improve their understanding of priority pollutants (PPs) in urban areas and obviously within wastewater systems. As a direct consequence, this study is intended to furnish data on both PP occurrence and the significance of concentrations in wastewater during dry and wet periods within combined sewers. Various sampling sites within the Paris combined sewer network were selected; for each sample, a total of 66 determinants, including metals, polycyclic aromatic hydrocarbons (PAHs), pesticides, organotins, volatile organic compounds, chlorobenzenes, phthalates and alkylphenols, were analysed. A broad range of PPs was observed in wastewater during dry as well as wet weather periods. Of the 66 elements investigated, 33 and 40 priority substances could be observed in raw sewage and wet weather effluent, respectively. As expected, a majority of metals were present in all samples, reflecting their ubiquitous nature. For both periods, chlorobenzenes and most of the pesticides always remained below the limit of quantification, while the majority of other organic pollutants assessed were identified within the microg l(-1) range. As highlighted by the larger number of substances detected in wet weather samples and the significance of their concentrations, runoff via atmospheric inputs and/or surface leaching was found to induce a wider range of PPs (n=40) and lead to higher concentrations of certain metals, PAHs, pesticides and other individual compounds. The data generated during this survey, which constitutes one of the first studies conducted in Europe to report concentrations for a variety of priority substances in wastewater within combined sewers, may be used in the future to identify PPs of potential significance for dry and wet weather periods and targeted for further investigation.     

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.     

Hydrocarbons and heavy metals in the different sewer deposits in the 'Le Marais' catchment (Paris, France): stocks, distributions and origins

The knowledge of the pollution stored in combined sewers is of prime importance in terms of management of wet weather flow pollution since sewer deposits play a significant role as source of pollution in combined sewer overflows. This work, which focused on the hydrocarbon (aliphatic and aromatic hydrocarbons) and metallic (Fe, Zn, Pb, Cu and Cd) pollution fixed to the different kinds of sewer deposits (gross bed sediment [GBS], organic layer [OL] and biofilm), was performed in order to provide a complete overview of the contaminant storage in the 'Le Marais' combined sewer (Central Paris, France). Firstly, our results have shown that, for all kinds of pollutants, a major part was stored in the GBS (87 to 98%), a lesser part in the OL (2 to 13%) and an insignificant part in the biofilm (<1%). These results demonstrated that the potential contribution of biofilm to wet weather pollution was negligible compared to the OL one. Secondly, the investigation of hydrocarbon fingerprints in each deposit has provided relevant information about contamination origins: (1) aliphatic hydrocarbon distributions were indicative of petroleum input in the GBS and reflected a mixture of biogenic and petroleum inputs in the OL and biofilm, (2) aromatic hydrocarbon distributions suggested an important pyrolytic contamination in all the deposits. Finally, the study of pollutant fingerprints in the different deposits and in the suspended solids going through the collector has shown that: (1) the suspended solids were the major component of OL and biofilm while urban runoff seemed to be the main transport mechanism introducing pollutants in the GBS and (2) the residence times in sewer of OL and biofilm were quite short compared to those for GBS.     

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.     

合流制排水管道沉积物及其模型研究进展

在合流制排水系统中,管道沉积物不但影响系统的正常运行,而且在雨天时随着溢流排放被冲刷释放,造成雨天出流污染负荷的增加,成为水体面源污染的重要组成部分。因此,开展排水管道沉积物的研究不仅有利于改善排水系统的运行状况,也有利于水体污染的控制。介绍了排水管道沉积物的来源、构成、性质以及国外管道沉积物的模型研究进展,并总结了其中的不足,以期为我国开展此方面的研究提供参考依据、基础以及研究方向。     

Modelling operational costs of a large water resource recovery facility receiving stormwater contributions

The main objective of this paper is to develop a methodology for modelling dry and wet weather operational costs applied to the Alcântara Water Resource Recovery Facility (WRRF), which is the largest Portuguese WRRF. Following data collection and intensive data treatment and validation, dry weather patterns of WRRF performance were combined with specific operational costs and described by a conceptual model. The impact of infiltration flows on operational costs was identified. The model simulations show a significant variation of distribution costs during wet weather periods compared with dry weather periods. The methodology can be used for predicting dry and wet weather operational costs and for defining dry weather tariffs and wet weather tariffs. Moreover, the analysis of the dry and wet weather period can also be used for infiltration quantification.     

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.     

上海中心城区合流制排水系统雨天溢流水质研究

合流制排水系统雨天溢流已成为上海中心城区河道的主要污染源.为此,重点研究了上海中心城区某合流制排水区域JXB系统雨天污染物溢流的出流过程和污染物的事件平均浓度,并探讨了溢流污染物的初期效应.结果表明,降雨特性对污染物溢流过程的水质影响较大;该区域雨天溢流COD、BOD5、SS、NH3-N的事件平均浓度分别为268、101、110、9.5 mg/L,SS浓度明显低于相关研究的结果;分析溢流污染物的初期效应表明,JXB系统排江污染物的初期效应较为显著,其中COD和BOD5的初期效应比SS的显著.     

An evaluation of a sanitary sewer system using a computer model

The purpose of this study was to (1) provide the city of West Lafayette, Indiana, USA, with an analysis and evaluation of its sanitary sewer system and (2) make recomendations for improving the existing system and any modifications necessary for accomodating future growth within the service area. Using a computer model, virtually every sewer line within the city and Purdue University was modelled. The computer model was calibrated for dry weather flow using water use records, while wet weather flows were checked against field observations. The analysis included both the present condition and future growth projections. The unique aspects of this study include the use and enhancement of a comprehensive, user-friendly computer program that models the stormwater runoff component and provides easy calibration and verification.     

Observations of turbidity in the Thames Estuary,United Kingdom

Two years of data of water level, salinity and turbidity have been analysed to understand the response of the estuarine turbidity maximum in the Thames to variations in tidal range and freshwater flow. We show the increase in turbidity in spring, together with a sudden decrease in autumn after fluvial flooding. In order to try to understand the mechanisms, we also present data from individual tides. During dry periods, there is a period of slack water around high tide when settling occurs. There is little equivalent settling at low tide, nor is there any significant settling during wet weather periods, pointing to the importance of tidal asymmetry at certain times of year. We also present an empirical relationship between peak tidal water level and turbidity during flood tides, which clearly shows the greater landward transport of sediment under spring tides, although this is moderated by the availability of erodible material.     

Approaches adopted by the European Union and selected Member States for the control of urban pollution

Several EU Directives have been adopted which have an influence on the control of urban pollution in particular the EU Urban Wastewater Treatment and the Integrated Pollution Control and Prevention Directive. In addition, the recently adopted Water Framework Directive (WFD) will have an additional impact on the control of urban pollution in particular related to storm overflows. The present paper provides a brief discussion of the EU legislation relevant to the control of urban pollution with special emphasis on the Urban Wastewater Treatment Directive (UWWTD) and its implementation in the Member States.The recently adopted WFD requires the achievement of good ecological and chemical status in all waters. The implementation of the UWWTD by the Member States, which should be achieved by 2005, will make a significant contribution towards achieving good ecological and chemical status. However, besides the discharge of sewage effluents, storm overflows can have a significant impact on the quality of surface waters and they will therefore require adequate control in order to achieve the final goal of good ecological and chemical status. The paper, therefore, also provides a brief analysis of the permitting requirements in the different countries and of the criteria currently applied by the EU Member States for the design of combined sewer overflow systems. Not all countries currently require a permit for storm overflows and the design criteria for combined sewer overflows are generally based on the spill frequency or treatment capacity in terms of dry weather flow, which do not take into account the effect on the receiving water. However, in the UK a method has been developed to assess the impact of combined sewer overflows on the quality of the receiving water, which is being applied to ensure the design of the storm overflow is adequate for the protection of the receiving water. Member States will need to adapt their methodologies used for the design of storm overflow to ensure the requirements of the WFD, the achievement of good ecological and chemical water quality, is being met.     

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.