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.     

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.     

Probabilistic model for the annual number of storm overflow discharges in a stormwater drainage system

The paper presents an attempt to develop a probabilistic model for predicting an annual number of storm overflow discharges. Forecasting the occurrence of an overflow discharge event involved the application of the logistic regression, which does not require the development of complex hydrodynamic catchment models. The performed calculations showed that the logistic regression model can be successfully used to evaluate the performance of the emergency overflow weir. The resultant logit model eliminates the necessity to develop hydrodynamic models, to conduct continuous measurements of the flow intensity in the stormwater drainage system and to collect detailed information on the characteristics of the subcatchments within the analyzed catchment. The hydrodynamic model was used to simulate the annual number of discharges. The analysis of the results demonstrated that they are in the range of stochastic values, which indicates an application-related character of the method.     

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.     

A geospatial approach to assessing microbiological water quality risks associated with irrigation abstraction

All crops that are eaten raw can present a microbiological risk to consumers. Disease outbreaks in the United Kingdom and United States have illustrated that ready‐to‐eat crops can be a vehicle for the transmission of gastrointestinal disease. Irrigation water has been implicated as a possible source of microbiological contaminants. Over two‐thirds of irrigation water applied to UK salad crops is abstracted from rivers and streams. Many of these are subject to a continuous input of faecal contamination from sewage treatment works as well as intermittent inputs from livestock and sewer overflows. In this paper, we show how geospatial techniques can help to assess the relationships between treated effluent discharges and abstractions and thus provide a new insight into local‐scale assessments of irrigation water quality. The extent to which the approach can inform risk assessments and decision‐making at the farm scale is demonstrated using a case‐study catchment in eastern England.     

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.     

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.     

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。     

River flow and associated transport of sediments and solutes through a highly urbanised catchment, Bradford, West Yorkshire

The hydrological characteristics of catchments become drastically modified in response to urbanisation. The total contributions and dynamics of runoff, suspended sediment and solutes may change significantly and have important implications downstream where they may affect flooding, instream ecological habitat, water quality and siltation of river channels and lakes. Although an appreciation of the likely hydrological changes is crucial for effective catchment management they are still poorly understood. In this paper we present data from a network of river monitoring stations throughout the heavily urbanised Bradford catchment, West Yorkshire. Sites are upstream, within and downstream of the highly urbanised central part of the catchment. Flow, turbidity (calibrated to suspended sediment concentration) and specific conductance (surrogate for solute concentration), logged at 15-min intervals, are presented for a 12-month period (June 2000 to June 2001). The total amounts and dynamics of flow, solute and suspended sediment transport were investigated. Estimated total flow and suspended sediment transport for the monitoring period were found to be high in response to the high total rainfall. Flow and sediment transport regimes were extremely 'flashy' throughout the catchment and became increasingly flashy in a downstream direction. Suspended sediment discharged from the Bradford subcatchment makes an important contribution to downstream sediment transport on the river Aire at Beal. Data suggest that the urbanised part of the Bradford catchment is extremely important in contributing solutes to the Beck (river). Although flow and sediment are also contributed to the Bradford Beck in the urbanised part of the catchment the data suggest that significant amounts may enter the combined sewer system and bypass the river. Understanding the spatial and temporal variations of flow and the transport of suspended sediment and solutes in rivers in urbanized subcatchments is crucial to their effective management and monitoring. Furthermore, this knowledge may be extremely important to the management and monitoring of downstream rivers in large scale mixed catchments.     

城市雨水径流污染的初期弃流控制

结合理论分析及实际监测结果明确了雨水径流的初期冲刷规律是客观存在的，并在此基础上讨论了影响冲刷规律的主要因素，分析了导致一些有争议现象产生的原因。分析结果表明，初期弃流控制量应根据汇水面或管道性质、采用的处理或利用工艺系统及针对目的等具体条件而定。此外，介绍了几种常见的初期弃流控制方法和装置，重点介绍了一种高效率初期弃流装置。该装置具有容积弃流法和小管弃流法的优点，在较大汇水面和管道雨水的污染控制以及雨水利用项目中应用具有较高的投资效益。     

Controlling sewer systems – a critical review based on systems in three EU cities

The term Real Time Control (RTC) is widely used to describe all types of control systems in sewer systems. Today the term covers everything from the simplest to the most advanced types of control systems, making it difficult to communicate about sewer system control in a precise manner, as well as search and find specific types of control systems for comparison. Through a survey of implemented control systems in three EU cities today and with the perspectives of current research within the field of sewer system control, the needs for a new control system design framework is identified. With the basis of existing frameworks for control system design, a new time-scale dependent framework is proposed. We believe this comprehensive time-scale dependent framework can help water utilities to retrofit and design new control solutions and facilitate knowledge sharing about existing designs.     

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.     

Willingness to pay for sustainable drainage systems in a highly urbanised city: a contingent valuation study in Hong Kong

Sustainable drainage systems (SuDS) reduce surface runoff and pollution from developed areas using various techniques such as green roofs and porous pavements. These systems are becoming popular worldwide, except in highly urbanised cities. This study used contingent valuation to investigate city dwellers’ perceptions of SuDS and their willingness to pay (WTP). Face‐to‐face interviews were conducted with 600 respondents in Hong Kong. In general, the participants’ understanding of stormwater management was not strong, although they supported SuDS. The mean and the median WTP were 8.8 and 15.0%, respectively, of the current government spending on stormwater management for public places, and 1.8 and 2.5%, respectively, of the property price for private properties. These WTP were lower than those reported in the previous overseas studies. Respondents who were relatively younger, better educated, had higher income, and came from a previously flooded district had higher WTP. These results facilitated strategic SuDS implementation in highly urbanised cities.     

Removal of organic matter in stormwater ponds: a plug-flow model generalisation from waste stabilisation ponds to shallow rivers

ABSTRACT

This study investigates the effect of flow conditions on the organic matter concentration, removal efficiency, and reaction kinetics in a stormwater pond in Fortaleza, Brazil. As a result of unauthorized sewage discharges, BOD and COD concentrations were similar to those of combined sewer systems. The concentrations remained roughly the same during the rainy season, which was attributed to sewage network overflow. Removal efficiencies ranged from 70–90%, similar to primary facultative ponds. Fitting different hydraulic models to the field data, it was possible to obtain BOD and COD removal rates that could be described as functions of the Reynolds number. The best fit was achieved considering the plug-flow assumption, and a general BOD model including data reported in the literature for waste stabilisation ponds, stormwater ponds and shallow rivers was derived. Lastly, simulations with this general model were performed to assess the impact of remediation measures on the studied pond.     

A model to study the effects of time-variable pollutant loads on stream quality

A deterministic computer model has been proposed to study the effects of discharges from combined sewer drainage areas on receiving bodies of water. The model provides capability for studying the effects of varying treatment plant capacity, interceptor sewer capacity and storage on stream quality. The operating parameters for the storage vessel are defined in detail. A cost function for all system components is included in the model.

The variation of total mass phosphate discharge, peak instantaneous phosphate concentration and cost with the variation of the various system parameters is reported for a series of model runs on three drainage areas.     

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.     

Impacts of climate change on urban drainage systems – a case study in Fredrikstad,Norway

Norway has frequently encountered flood damage in urban areas during recent years. In this paper the authors, taking Veumdalen catchment in Fredrikstad as an example, simulated the possible consequences in the sewer system, in the present, predicted and artificial climate scenarios. Indicators that describe (1) surface flooding, (2) surcharging sewers, (3) basement flooding and (4) combined sewer overflow (CSO) are defined to represent the adverse effects of climate change. It is concluded from the annual-based simulation that the total volume of water spilling from the flooding manholes will increase 2–4-times the increase in precipitation, and the total CSO will increase 1.5–3-times as much as the increase in precipitation. The simulation results also show that the number of flooding manholes and number of surcharging sewers may change dramatically and irregularly with a slight change of precipitation, and vary with events and durations.     

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.     

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.     

Biodegradability of organic matter associated with sewer sediments during first flush

The high pollution load in wastewater at the beginning of a rain event is commonly known to originate from the erosion of sewer sediments due to the increased flow rate under storm weather conditions. It is essential to characterize the biodegradability of organic matter during a storm event in order to quantify the effect it can have further downstream to the receiving water via discharges from Combined Sewer Overflow (CSO). The approach is to characterize the pollutograph during first flush. The pollutograph shows the variation in COD and TSS during a first flush event. These parameters measure the quantity of organic matter present. However these parameters do not indicate detailed information on the biodegradability of the organic matter. Such detailed knowledge can be obtained by dividing the total COD into fractions with different microbial properties. To do so oxygen uptake rate (OUR) measurements on batches of wastewater have shown itself to be a versatile technique. Together with a conceptual understanding of the microbial transformation taking place, OUR measurements lead to the desired fractionation of the COD. OUR results indicated that the highest biodegradability is associated with the initial part of a storm event. The information on physical and biological processes in the sewer can be used to better manage sediment in sewers which can otherwise result in depletion of dissolved oxygen in receiving waters via discharges from CSOs.