Incorporating concepts from physical theory into stochastic modelling of urban runoff pollution

On evaluating the present or future state of integrated urban water systems, sewer drainage models, with rainfall as primary input, are often used to calculate the expected return periods of given detrimental acute pollution events and the uncertainty thereof. The model studied in the present paper incorporates notions of physical theory in a stochastic model of water level and particulate chemical oxygen demand (COD) at the overflow point of a Dutch combined sewer system. A stochastic model based on physical mechanisms has been formulated in continuous time. The extended Kaiman filter has been used in conjunction with a maximum likelihood criteria and a non-linear state space formulation to decompose the error term into system noise terms and measurement errors. The bias generally obtained in deterministic modelling, by invariably and often inappropriately assuming all error to result from measurement inaccuracies, is thus avoided. Continuous time stochastic modelling incorporating physical, chemical and biological theory presents a possible modelling alternative. These preliminary results suggest that further work is needed in order to fully appreciate the method's potential and limitations in the field of urban runoff pollution modelling.     

广州市中心城区典型合流制排水口污染负荷排放规律分析

随着广州市中心城区河道水环境治理力度的加大,合流制溢流污染问题越来越突出,对城市河道水体构成了日益严重的威胁。本文选取广州市中心城区典型合流制排水口进行了18场降雨的径流污染监测,总结了在典型小雨、典型中雨及典型大雨场次下排水口各水质指标随降雨历时的变化规律,同时分析了18场降雨的排水口最高浓度、平均浓度与总降雨量的关系,研究成果可为合流制污染控制工程的设计提供参考。     

Correlation of combined sewer overflow reduction due to real-time control and resulting effect on the oxygen concentration in the river

Real-time control of the sewer system is a frequently applied measure for the abatement of pollution caused by urban runoff in the receiving water. Although the goal is an improvement of the water quality the actual aim of real-time control is usually formulated as the reduction/avoidance of combined sewer overflow. However, testing a virtual drainage system by means of a three-month rain series, hardly any correlation between the combined sewer overflow reduction and the resulting effect on the oxygen concentration in the river has been found. The efficiency of real-time control for pollution abatement by means of artificial performance criteria has to be doubted.     

分流制雨水与合流制溢流水质的比较

排水管道系统是城市基础设施的重要组成部分,它的完善程度影响着整个污水处理和水污染控制系统的运行状况和效率.而排水体制的选择需要结合当地的地形、降雨类型、管理情况等实际情况出发,综合分析确定.从实测数据和理论计算两个方面对分流制雨水和合流制溢流的浓度和污染负荷量进行了初步的比较,简要地讨论了两种排水系统的适用条件,为排水体制的选择和城市水污染控制的战略决策提供参考.     

Frequency analysis of river water quality using integrated urban wastewater models

In recent years integrated models have been developed to simulate the entire urban wastewater system, including urban drainage systems, wastewater treatment plants, and receiving waterbodies. This paper uses such an integrated urban wastewater model to analyze the frequency of receiving water quality in an urban wastewater system with the aim of assessing the overall system performance during rainfall events. The receiving water quality is represented by two indicators: event mean dissolved oxygen (DO) concentration and event mean ammonium concentration. The compliance probability of the water quality indicators satisfying a specific threshold is used to represent the system performance, and is derived using the rainfall events from a series of 10 years' rainfall data. A strong correlation between the depth of each rainfall event and the associated volume of combined sewer overflow (CSO) discharges is revealed for the case study catchment, while there is a low correlation between the intensity/duration of the rainfall event and the volume of the CSO discharges. The frequency analysis results obtained suggest that the event mean DO and ammonium concentrations have very different characteristics in terms of compliance probabilities at two discharging points for CSO and wastewater treatment plant effluent, respectively. In general, the simulation results provide an understanding of the performance of the integrated urban wastewater system and can provide useful information to support water quality management.     

Influence of systematic errors from tipping bucket rain gauges on recorded rainfall data

Tipping bucket rain gauges (TBR) are widely used in urban hydrology. The present study investigated the uncertainties in recorded rainfall intensity induced by the following properties of the TBR: depth resolution i.e. the bucket volume, calibration parameters, wetting and evaporation losses and the method of data recording (time between tips or tips per minute). The errors were analysed by means of a TBR simulator i.e. a simulation program that models the behaviour of a TBR. Rainfall data disaggregated to 6 seconds from measured 1-min data and randomly varied were taken as input to the simulator. Different TBR data series were produced by changing the properties of the simulated rain gauge. These data series together with the original rainfall events were used as input to a rainfall-runoff model. Computed overflow volume and peak discharge from a combined sewer overflow (CSO) weir were compared. Errors due to depth resolution (i.e. the bucket size) proved to be small. Therefore TBRs with a depth resolution up to 0.254 mm can be used in urban hydrology without inducing significant errors. Wetting and evaporation losses caused small errors. The method of data recording had also little influence. For larger bucket volumes variable time step recording induced smaller errors than tips per minute recording.     

Soft sensing of water depth in combined sewers using LSTM neural networks with missing observations

Information and communication technologies combined with in-situ sensors are increasingly being used in the management of urban drainage systems. The large amount of data collected in these systems can be used to train a data-driven soft sensor, which can supplement the physical sensor. Artificial Neural Networks have long been used for time series forecasting given their ability to recognize patterns in the data. Long Short-Term Memory (LSTM) neural networks are equipped with memory gates to help them learn time dependencies in a data series and have been proven to outperform other type of networks in predicting water levels in urban drainage systems. When used for soft sensing, neural networks typically receive antecedent observations as input, as these are good predictors of the current value. However, the antecedent observations may be missing due to transmission errors or deemed anomalous due to errors that are not easily explained. This study quantifies and compares the predictive accuracy of LSTM networks in scenarios of limited or missing antecedent observations. We applied these scenarios to an 11-month observation series from a combined sewer overflow chamber in Copenhagen, Denmark. We observed that i) LSTM predictions generally displayed large variability across training runs, which may be reduced by improving the selection of hyperparameters (non-trainable parameters); ii) when the most recent observations were known, adding information on the past did not improve the prediction accuracy; iii) when gaps were introduced in the antecedent water depth observations, LSTM networks were capable of compensating for the missing information with the other available input features (time of the day and rainfall intensity); iv) LSTM networks trained without antecedent water depth observations yielded larger prediction errors, but still comparable with other scenarios and captured both dry and wet weather behaviors. Therefore, we concluded that LSTM neural network may be trained to act as soft sensors in urban drainage systems even when observations from the physical sensors are missing.     

An investigation on suspended solids sources in urban stormwater runoff using 7Be and 210 Pb as tracers

Radionuclides 7Be and 210Pb were used as tracers to identify suspended solid sources and transport pathways in the storm runoff events from urban catchments. Water samples were collected in runoff of storm events in Wuhan City, China. Suspended solids, COD, TN, TP, and the 7Be and 210Pb activities in the suspended solids were analyzed. Following the pathway of urban runoff pollution, the rain precipitation, urban ground dust, gutter sediments, and sewer deposit samples were analyzed for 7Be and 210Pb activities. The results show that the 7Be/210Pb ratio decreased through the system from a value of 0.86+/-0.44 in ground dust, to 0.63+/-0.18 in suspended solids in storm runoff from the sewer outlet, to 0.55+/-0.31 in gutter sediments, and to 0.41+/-0.13 in combined sewer deposits. The 7Be/210Pb ratio decrease suggests that 60+/-12% of suspended solids at sewer outlet originated from the drainage system sediments, the rest was from the wash-off of urban ground dust during the rainfall events. The 7Be and 210Pb trace approach can give insight into the short-term source and transport of pollutant during storm runoff in urban drainage systems and it can help to develop management strategies.     

“灰绿”协同措施对银川市合流制溢流污染的影响EI北大核心CSCD

针对合流制管网系统在雨天溢流污染严重,造成城市水体黑臭现象的问题,以银川市某高密度城区合流制管网系统为例,基于SWMM模型,在短历时设计降雨和长历时设计降雨两种条件下,模拟分析了合流制溢流(CSO)调蓄池、雨污管道混错接改造、绿化带海绵化改造等“灰绿”协同措施对CSO污染的影响。结果表明:CSO调蓄池、雨污管道混错接改造、绿化带海绵化改造及“灰绿”措施结合4种方案在短历时、长历时设计降雨条件下,随着降水量的增加,溢流水量及溢流污染物负荷均增加,溢流削减率均逐渐减小,其中“灰绿”措施结合方案对溢流污染的削减效果最为显著;重现期小于5 a时,溢流水量削减率与溢流污染物负荷削减率基本达到80%;降雨条件为中雨时,污染物负荷削减率基本达到75%;重现期为20 a时,溢流水量削减率及TSS、COD、TP、NH^(+)_(4)-N负荷削减率分别达到64%、70%、70%、70%、70%;降雨条件为大雨时,溢流水量削减率及TSS、COD、TP、NH^(+)_(4)-N负荷削减率分别达到28%、32%、26%、31%、33%。     

An integrated approach for improving the wastewater discharge and treatment systems

Since treatment plants have been built all over Germany during the last decades, the water quality of receiving streams has been improved remarkably. But there are still a lot of quality problems left, which are caused e.g. by combined sewer overflows (CSO), treatment plant effluents or rainwater discharges from separate sewer systems. At present different efforts are undertaken to control sewer systems in order to improve the operation of urban drainage systems or more generally, design processes. The Emschergenossenschaft and Lippeverband (EG/LV) are carrying out research studies, which are focusing on a minimization of total emissions from sewer systems both from wastewater treatment plant (WWTP) effluents and from CSO. They consider dynamic interactions between rainfall, resultant wastewater, combined sewers, WWTP and receiving streams. Therefore, in an advanced wastewater treatment, a model-based improvement of WWTP operation becomes more and more essential, and consequently a highly qualified operational staff is needed. Some aspects of the current research studies are presented in this report. The need and the use of an integrated approach to combine existing model components in order to optimize dynamic management of combined sewer systems (CSS) with a benefit for nature are outlined.     

Impact of dimension uncertainty and model calibration on sewer system assessment.

Assessments of sewer performance are usually based on a single computation of CSO (combined sewer overflow) volumes using a time series of rainfall as system loads. A shortcoming of this method is that uncertainties in knowledge of sewer system dimensions are not taken into account. Moreover, sewer models are rarely calibrated. This paper presents the impacts of database errors and model calibration on return periods of calculated CSO volumes. The impact of uncertainties is illustrated with two examples. Variability of calculated CSO volumes is estimated using Monte Carlo simulations. The results show that calculated CSO volumes vary considerably due to database errors, especially uncertain dimensions of the catchment area. Furthermore, event-based calibration of a sewer model does not result in more reliable predictions because the calibrated parameters have low portability. However, it enables removal of database errors harmonising model predictions and 'reality'.     

银川市某高密度城区合流制溢流污染特性研究

针对合流制管网系统溢流污染严重且易造成受纳水体出现黑臭现象等问题,以银川市第一污水处理厂片区为例,基于雨洪管理模型(SWMM),在短历时设计降雨和典型年长历时降雨条件下,模拟分析了研究区域溢流排口溢流量及溢流污染物负荷对不同降雨条件的响应关系及其对受纳水体水质的影响。结果表明：雨天合流制管网系统的溢流量及溢流污染负荷量较大,且无论是短历时还是长历时降雨条件下,均具有随着降雨量的增加而增大的特点;厂前溢流污染浓度大,超过受纳水体自净能力,短期内水体水质处于黑臭状态。本研究对银川市黑臭水体治理及溢流污染防治方案的制订具有重要的参考价值。     

排水系统截流调蓄设施运行效率的概率分析方法

介绍了一种适用于规划层面的评价调蓄设施长期平均运行效率的方法.该方法以降雨特性参数的变差系数表征降雨事件的可变性、间歇性的概率特征,以质量负荷去除率作为评价指标,评价不同运行方式下调蓄设施的长期调蓄效率.在分析上述方法的基础上,以上海市成都北泵站调蓄系统为例,通过上海市降雨特性参数,具体计算了成都北泵站截流及调蓄池的综合长期运行效率.     

Rainfall data monitoring and application for urban hydrology in the Czech Republic

With respect to increasing number of advanced technology applications for urban drainage projects in the Czech republic the requirements for relevant rain data become more important Two research projects related to measurement, evaluation and application of rainfall data for urban hydrology purposes were launched. Suitable measuring equipment and their reliability have been studied and evaluated. The needs of high density of rain gauges for single event investigation of sewer system have been verified. The spatial distribution of rainfall doesnt influence statistical evaluation of the effects obtained by long term simulation significantly. The needs for rainfall data will be proposed for the master plan of urban drainage in Prague.     

Development of the real-time control (RTC) system for Tokyo sewage system.

Tokyo Metropolitan government has decided to make the maximum possible use of the existing facilities while ensuring safety against inundation and to promote measures also from a software approach by introducing a system capable of minimizing combined sewer overflow, the real-time control system (RTC). A pilot RTC system was installed in August 2002 for the Shinozaki Pumping Station. The RTC system monitors the precipitation volume and the water level in the pipe. Simulations were carried out on the basis of these data. From the results, it was found that with the use of the RTC it is possible to reduce CSO by roughly 50% for small rainfalls with a total precipitation level of 20 mm or less by strong rainwater in the pipe routes at the beginning of the rain. It has also been shown that CSO can be reduced by about 80% through the use of rainfall forecasting.     

A software-based sensor for combined sewer overflows

A new methodology for online estimation of excess flow from combined sewer overflow (CSO) structures based on simulation models is presented. If sufficient flow and water level data from the sewer system is available, no rainfall data are needed to run the model. An inverse rainfall-runoff model was developed to simulate net rainfall based on flow and water level data. Excess flow at all CSO structures in a catchment can then be simulated with a rainfall-runoff model. The method is applied to a case study and results show that the inverse rainfall-runoff model can be used instead of missing rain gauges. Online operation is ensured by software providing an interface to the SCADA-system of the operator and controlling the model. A water quality model could be included to simulate also pollutant concentrations in the excess flow.     

深圳城市两级排水系统设计暴雨重现期的衔接关系

城市内涝防治采用市政排水与水利排涝两级排涝模式,针对城市两级排水系统标准无法衔接的问题,以深 圳市大空港区为例,考虑城市两级排水系统可能遭遇的两类衔接风险,通过构建雨量结构关系分别得到以短历时 暴雨为主和以长历时暴雨为主的设计暴雨重现期衔接关系。同时利用芝加哥雨型和珠江三角洲雨型分别推求市 政排水与水利排涝设计暴雨过程,以探讨城市两级排水系统设计暴雨雨峰之间的衔接关系。最终通过 Spearman 相关系数评估截留河流域各条支流的衔接关系与地理参数之间的相关性。结果表明,水利排涝与市政排水设计 暴雨重现期的衔接关系受降雨强度和降雨历时共同作用,且设计暴雨雨峰衔接关系存在地区差异性。以德丰围 涌为例,在市政排水设计暴雨重现期为 X 年一遇时,水利排涝设计暴雨重现期应至少设计为（2~3）X 年一遇才能 实现城市两级排水系统的衔接。     

基于MIKE URBAN的西安市中心城区雨洪过程模拟

城市排水管网的分布与建设是城市基础设施建设的重要组成部分,城市的排水能力关系到城市服务功能的正常运转。通过城市雨洪过程的分析,可以评价城市洪涝灾害。以西安市中心城区为研究对象,利用MIKE URBAN构建排水管网模型,根据模拟结果对研究区管网排水能力进行评估,分析易涝成因。结果表明:MIKE URBAN能够较好地模拟城市管网水位、流量变化及易涝点的分布情况。根据西安市暴雨强度公式,设计不同重现期(1、2、3、5a)的降雨过程,在1a降雨过程下,研究区90%以上管道处于满流状态,60%以上的检查井发生溢流,满流管段数和溢流井个数会随着降雨频率的增加而增加,但增幅相对减少;管道设计标准普遍偏低、下垫面不透水率增大、地形等因素是导致地面积水的主要原因。研究成果可为城市内涝防治及海绵城市建设提供理论基础和技术支撑。     

Surface models for coupled modelling of runoff and sewer flow in urban areas.

Traditional methods fail for the purpose of simulating the complete flow process in urban areas as a consequence of heavy rainfall and as required by the European Standard EN-752 since the bi-directional coupling between sewer and surface is not properly handled. The new methodology, developed in the EUREKA-project RisUrSim, solves this problem by carrying out the runoff on the basis of shallow water equations solved on high-resolution surface grids. Exchange nodes between the sewer and the surface, like inlets and manholes, are located in the computational grid and water leaving the sewer in case of surcharge is further distributed on the surface. Dense topographical information is needed to build a model suitable for hydrodynamic runoff calculations; in urban areas, in addition, many line-shaped elements like houses, curbs, etc. guide the runoff of water and require polygonal input. Airborne data collection methods offer a great chance to economically gather densely sampled input data.