Sewer Flow Quality Model Research Articles

Applying global sensitivity analysis to the modelling of flow and water quality in sewers

While several approaches for global sensitivity analysis (GSA) have been proposed in literature, only few applications exist in urban drainage modelling. This contribution discusses two GSA methods applied to a sewer flow and sewer water quality model: Standardised Regression Coefficients (SRCs) using Monte-Carlo simulation as well as the Morris Screening method. For selected model variables we evaluate how the sensitivities are influenced by the choice of the rainfall event. The aims are to i) compare both methods concerning the similarity of results and their applicability, ii) discuss the implications for factor fixing (identifying non-influential parameters) and factor prioritisation (identifying important parameters) and iii) rank the important parameters for the investigated model. It was shown that both methods lead to similar results for the hydraulic model. Parameter interactions and non-linearity were identified for the water quality model and the parameter ranking differs between the methods. For the investigated model the results allow a sound choice of output variables and rainfall events in view of detailed uncertainty analysis or model calibration. We advocate the simultaneous use of both methods for a first model assessment as they allow answering both factor fixing and factor prioritisation at low computational cost.

Single- and Multievent Optimization in Combined Sewer Flow and Water Quality Model Calibration

Over the last three decades, storm-water quality modeling has been used increasingly commonly to describe the general system behavior and assess the pollution loads transferred in and spilled out of combined sewer systems. The calibration of quality models is, in most cases, based on conventionally obtained calibration data, e.g., by automated sampling. Long-term high-resolution online measurement data are available for the Graz West catchment (Graz, Austria), allowing an assessment of the full dynamics of discharge and pollution concentrations. This paper focuses on the application and comparison of single-event and two different multievent optimization schemes for sewer-water quality model calibration. While both single- and multievent optimization lead to satisfying results for the calibration events in discharge calibration, it is shown that validation events are better reproduced by using multievent calibration. Single- and multievent autocalibration of pollution concentration is based on the best dataset obtained from the discharge calibration. As for discharge, the pollutographs are reproduced satisfactorily, and multievent calibration is more stable. In all cases, the two multievent approaches performed equally well.

USE OF A FULL‐SCALE TEST FACILITY TO EXAMINE SEWER‐SEDIMENT MOBILITY

ABSTRACTThe mobility of granular in‐sewer sediments was measured in a study which was carried out at the National Combined Sewer Overflow Test Facility, Wigan. It is thought that these measurements are unique in that the facility permits tests using crude sewage and real in‐sewer sediments to be conducted under full‐scale controlled hydraulic conditions. Tests were carried out in which the bedload transport rates, resulting from different steady‐flow discharges and flow depths, were measured. The results were compared with transport‐rate predictions made using relationships which are recommended by the Construction Industry Research and Information Association. These comparisons indicated that, for the sewer sediments, the existing prediction methods are likely to significantly under‐predict the mobility of in‐sewer sediments — particularly if a range of grain sizes is present. This might have important implications both for the design of sewerage systems and for sewer flow quality modelling.

The effect of extended in-sewer storage on wastewater treatment plant performance

A project funded by UKWIR is under way in the UK to develop a relatively simple methodology whereby the effects of the introduction of extended in-sewer storage at CSOs on downstream sewerage and treatment can be assessed. Recent legislation (UK and European) has compelled many sewer system operators to introduce systems which increase in-sewer retention times, and also retain more flow and load within sewer networks. The project has reviewed existing knowledge about the interaction between in-sewer flow and treatment plants, together with available models. The study is utilising a "benchmark" of 3 configurations of treatment plant and dynamic simulation using the WRc STOAT software, with minor modifications to ensure that effects on odour generation and nutrient removal processes are adequately modelled. As no existing sewer flow quality model can represent the range of conditions possible in sewer networks, a combined application of the Hydroworks model and a new model developed at Aalborg University is being used for this part of the study.

Computing near-bed solids transport in sewers and similar sediment-carrying open-channel flows

Near-bed solids (NBS) in sewers are known to convey a large part of the particulate polluting material and to introduce serious bias in sewer flow quality modelling when their existence is not properly acknowledged. It is proposed that a suspended-load approach should be used to characterize this material and the related flow pattern. A two-layer suspension model (proposed as an alternative to the classical Rouse equation) is applied to combined sewer flows displaying a NBS pattern, but also to capacity suspension flows carefully controlled under laboratory conditions. Both applications are conclusive and stress the importance of a good knowledge of particle settling velocity in ambient (turbulent) conditions. In combined sewers, the marked increase of effective shear velocity associated with rain runoff events is understood to lift the coarse settleable material which ordinarily remains in a concentrated NBS flow zone, into the upper part of the flow.

The erosion of cohesive mixed deposits: implications for sewer flow quality modelling

The movement of sediments in sewers has been widely recognised as being strongly linked with poor sewer flow quality. Currently, empirically calibrated transport capacity relationships are used to estimate the movement of in-sewer sediments. The majority of these relationships were developed and calibrated with laboratory data sets that used uniformly sized granular sediment. In many combined sewers, there are both organic and inorganic sediments. The fine-grained organic sediments are thought to be able to develop considerable cohesive strength under certain conditions. This paper describes the results from laboratory experiments in which the erosion of non-homogeneous in-pipe sediment deposits was investigated. The types of sediment mixtures used ensured that a range of cohesive deposit strengths was developed. The measured bed load and suspended load were compared with transport capacity relationships currently used for sewer design. Significant divergence between the transport capacity predictions and experimental observations was discovered. It was seen that it is unwise to model in-sewer deposits as homogeneous and granular and that it is important to take into account the impact of deposit cohesion.

Reliability of sewer flow quality models - results of a North Rhine-Westphalian comparison

In North Rhine-Westphalia, Germany a project for the evaluation of sewer flow quantity and quality models was carried out with the aid of measured data in 1986 to 1991. Measurement projects in two real subcatchments in the cities of Solingen and Düsseldorf delivered the data for model calibration and verification. 10 models with different approaches for quantity and quality simulation took part in a relative and an absolute comparison. This report focuses on some selected results of the absolute comparison and discusses the reliability and accuracy of model application.

Reliability of sewer flow quality models - Results of a North Rhine-Westphalian comparison

In North Rhine-Westphalia, Germany a project for the evaluation of sewer flow quantity and quality models was carried out with the aid of measured data in 1986 to 1991. Measurement projects in two real subcatchments in the cities of Solingen and Düsseldorf delivered the data for model calibration and verification. 10 models with different approaches for quantity and quality simulation took part in a relative and an absolute comparison. This report focuses on some selected results of the absolute comparison and discusses the reliability and accuracy of model application.

The influence of near bed solids transport on first foul flush in combined sewers

The problems associated with deposited sediments in sewers, and their transport through sewer systems have been the subject of detailed fieldwork programmes in the UK, and elsewhere in Europe. Existing laboratory, and some field based research exercises have focused on the relatively small, discrete particles. It is clear, however, that combined sewer systems have inputs which comprise of a significant proportion of large organic solids (faecal and food wastes), as well as the finer range of particle sizes. The increased concern regarding CSO spills into the environment has fuelled the recent development of sewer flow quality models, such as HYDROWORKS QM and MOUSETRAP, some of which make no attempt to represent the transport of these larger organic particles. Herein, the results of a collaborative research programme undertaken between three UK universities and a water authority are discussed. Transport at the bed in sewers, as “near bed solids”, is defined. Based on a comprehensive data collection program undertaken in the Dundee combined sewerage system, a method is presented which may be used to estimate the rate of sediment transport near the bed in sewers. The influence that solids in transport near the bed have on first foul flush in combined sewers is discussed. A methodology is proposed which may be used to estimate the extent to which sediment in transport near the bed in sewers contributes to first foul flush phenomena, by describing the movement of a storm wave along a conceptual sewer length.

The influence of near bed solids transport on first foul flush in combined sewers

The problems associated with deposited sediments in sewers, and their transport through sewer systems have been the subject of detailed fieldwork programmes in the UK, and elsewhere in Europe. Existing laboratory, and some field based research exercises have focused on the relatively small, discrete particles. It is clear, however, that combined sewer systems have inputs which comprise of a significant proportion of large organic solids (faecal and food wastes), as well as the finer range of particle sizes. The increased concern regarding CSO spills into the environment has fuelled the recent development of sewer flow quality models, such as HYDROWORKS QM and MOUSETRAP, some of which make no attempt to represent the transport of these larger organic particles. Herein, the results of a collaborative research programme undertaken between three UK universities and a water authority are discussed. Transport at the bed in sewers, as “near bed solids”, is defined. Based on a comprehensive data collection program undertaken in the Dundee combined sewerage system, a method is presented which may be used to estimate the rate of sediment transport near the bed in sewers. The influence that solids in transport near the bed have on first foul flush in combined sewers is discussed. A methodology is proposed which may be used to estimate the extent to which sediment in transport near the bed in sewers contributes to first foul flush phenomena, by describing the movement of a storm wave along a conceptual sewer length.

Sewer flow quality modelling

Within the framework of an EC-funded SPRINT-project, a sewer flow quality model of a typical rural Flemish catchment was set up. The applicability of such a model is demonstrated. Furthermore a methodology for model building, data collection and model calibration and verification is proposed. To this end an intensive 9 month measuring campaign was undertaken. The hydraulic behaviour of the sewer network was continuously monitored during those 9 months. During both dry weather flow (DWF) and wet weather flow (WWF) a number of sewage samples were taken and analysed for BOD, COD, TKN, TP and TSS. This resulted in 286 WWF and 269 DWF samples. The model was calibrated and verified with these data. Finally a software independent methodology for interpretation of the model results is proposed.

Sewer flow quality modelling

Within the framework of an EC-funded SPRINT-project, a sewer flow quality model of a typical rural Flemish catchment was set up. The applicability of such a model is demonstrated. Furthermore a methodology for model building, data collection and model calibration and verification is proposed. To this end an intensive 9 month measuring campaign was undertaken. The hydraulic behaviour of the sewer network was continuously monitored during those 9 months. During both dry weather flow (DWF) and wet weather flow (WWF) a number of sewage samples were taken and analysed for BOD, COD, TKN, TP and TSS. This resulted in 286 WWF and 269 DWF samples. The model was calibrated and verified with these data . Finally a software independent methodology for interpretation of the model results is proposed.

The diversity of sewer sediments and the consequences for sewer flow quality modelling

As a consequence of recent EU Directives and a substantial increase in the environmental awareness of the general public, practising engineers and researchers are now more than ever required to collaborate to provide the higher standards of water quality being demanded. The engineers role is to implement a cost effective, practical and sustainable solution; the researcher must provide the engineer with the tools and ‘know how’ to allow this solution to be attained. Recent research endeavours in the UK have resulted in a philosophy known as Urban Pollution Management. This philosophy allows the engineer to solve a drainage catchment problem utilising either simple or complex approaches, depending upon the ability of the receiving watercourse to assimilate discharges from CSOs and WTP's. The backbone of the complex approach is the sewer flow quality model. Consequently for a ‘realistic’ solution to be implemented, the sewer flow quality model must represent reality accurately. Work carried out in the City of Perth, has shown that there are significant limitations to the accuracy of a typical sewer flow quality model; limitations which cannot be attributed to the mathematics or the limitations of the particular package, but are due to the complexity and diversity of the phenomena encountered within sewerage systems, particularly the effects of the sediments. This paper questions whether in fact the recent UK research endeavour has actually provided engineers with a tool/philosophy which can provide cost effective, practical and sustainable solutions to complex drainage catchment problems.

The diversity of sewer sediments and the consequences for sewer flow quality modelling

As a consequence of recent EU Directives and a substantial increase in the environmental awareness of the general public, practising engineers and researchers are now more than ever required to collaborate to provide the higher standards of water quality being demanded. The engineers role is to implement a cost effective, practical and sustainable solution; the researcher must provide the engineer with the tools and ‘know how’ to allow this solution to be attained.Recent research endeavours in the UK have resulted in a philosophy known as Urban Pollution Management. This philosophy allows the engineer to solve a drainage catchment problem utilising either simple or complex approaches, depending upon the ability of the receiving watercourse to assimilate discharges from CSOs and WIP's. The backbone of the complex approach is the sewer flow quality model. Consequently for a ealistic’ solution to be implemented, the sewer flow quality model must represent reality accurately. Work carried out in the City of Perth, has shown that there are significant limitations to the accuracy of a typical sewer flow quality model; limitations which cannot be attributed to the mathematics or the limitations of the particular package, but are due to the complexity and diversity of the phenomena encountered within sewerage systems, particularly the effects of the sediments. This paper questions whether in fact the recent UK research endeavour has actually provided engineers with a tool/philosophy which can provide cost effective, practical and sustainable solutions to complex drainage catchmcnt problems.

Mousetrap: modelling of real sewer sediment characteristics and attached pollutants

A dynamic, pollutograph based sewer flow quality simulation model is an essential tool to aid in the control of intermittent pollution from combined sewer overflows (CSOs). Such a model can be used in conjunction with river impact models to provide an objective methodology for the control of intermittent sewage discharges. A new component to the MOUSE package has been developed, called MOUSETRAP, which is composed of a series of deterministic modules to represent the quality of surface run-off, sediment and pollutant transport within the pipe network, and the biological and chemical reactions within the sewer system. MOUSETRAP can be used to predict sewer flow quality and, hence, CSO spill concentrations and loads over time. The results of recent research into the behaviour of sediments and associated pollutants in sewers have shown that these aspects are crucial to the development of a reliable, deterministic sewer flow quality model. MOUSETRAP, by representing the physical and chemical characteristics and behaviour of real sewer sediment types has the ability to simulate storm event pollutographs incorporating first foul flush phenomena. MOUSETRAP achieves this by representing pollutants associated with dissolved phase liquid transport and pollutants associated with sediments including bed deposits, bed load and settleable suspended solids. The results of data collection are used to characterise sewer sediments and a theoretical basis for the modelling of sediments and sediment attached pollutants in MOUSETRAP is considered. Finally, the model default values for characteristics of the surface, deposited foul flow and in-pipe sediment types in MOUSETRAP are presented.

MOUSETRAP: Modelling of real sewer sediment characteristics and attached pollutants

A dynamic, pollutograph based sewer flow quality simulation model is an essential tool to aid in the control of intermittent pollution from combined sewer overflows (CSOs). Such a model can be used in conjunction with river impact models to provide an objective methodology for the control of intermittent sewage discharges. A new component to the MOUSE package has been developed, called MOUSETRAP, which is composed of a series of deterministic modules to represent the quality of surface run-off, sediment and pollutant transport within the pipe network, and the biological and chemical reactions within the sewer system. MOUSETRAP can be used to predict sewer flow quality and, hence, CSO spill concentrations and loads over time.The results of recent research into the behaviour of sediments and associated pollutants in sewers have shown that these aspects are crucial to the development of a reliable, deterministic sewer flow quality model. MOUSETRAP, by representing the physical and chemical characteristics and behaviour of real sewer sediment types has the ability to simulate storm event pollutographs incorporating fast foul flush phenomena MOUSETRAP achieves this by representing pollutants associated with dissolved phase liquid transport and pollutants associated with sediments including bed deposits, bed load and settleable suspended solids.The results of data collection are used to characterise sewer sediments and a theoretical basis for the modelling of sediments and sediment attached pollutants in MOUSETRAP is considered. Finally, the model default values for characteristics of the surface, deposited foul flow and in-pipe sediment types in MOUSETRAP are presented.

Pollutant Release from Sediments in Sewer Systems and Their Potential for Release into Receiving Waters

A programme of work funded by the UK Water Research Centre (WRc) has been carried out to investigate the nature, occurrence and re-entrainment into the sewage flow of sewer sediments and their associated pollutants to provide data for the development of UK sewer flow quality models, MOSQITO and MOUSETRAP. Methodologies for the preparation of sewer sediment samples prior to chemical analysis are described. These methods have been developed to determine the level of pollutants released from sewer sediments which will be representative of the conditions found in the sewer environment. Application of progressively lower shear stresses to samples of sediment, from an extreme, via blending, to less severe preparative methods by means of a stirring technique, fractionate sediment samples so that the polluting potential may be assessed separately for both dissolved and resuspendable phases. Stirring preparative methods release less pollutants from a sediment sample than from the blending procedures, with the level of pollutant release being dependent upon the shear stress being applied to the sample, with higher shear values releasing as would be expected, more pollutants.

Development, Calibration and Further Data Requirements of the Sewer Flow Quality Model Mosqito

MOSQITO is the initial version of a sever flow quality model being developed by Hydraulics Research Ltd and the Water Research Centre as part of the UK River Basin Management programme. MOSQITO I simulates the time-varying behaviour of suspended solids, biochemical oxygen demand, chemical oxygen demand, ammoniacal nitrogen and hydrogen sulphide on catchment surfaces and in sewer systems. The model produces discharge pollutographs for these determinands which can be used as input to a river water quality model. MOSQITO consists of four sub-models which represent washoff from catchment surfaces, foul water inflow, pollutant behaviour in pipes and channels, and pollutant behaviour in ancillary structures within drainage systems. These sub-models are linked to the flow simulation model incorporated in the WALLRUS package which is the latest computer implementation of the Wallingford Procedure. The rationale behind the model, its structure and its operational basis have been discussed elsewhere (Moys and Henderson, 1988) and are therefore described briefly so that emphasis can be placed on the aspects which follow. Calibration and verification of the model are being carried out using data from a variety of experimental catchments in the UK. These catchments have been selected to exhibit a wide range of characteristics and include separate and combined sewer systems. Results of the calibration work are presented together with illustrations of the performance of the various sub-models and the overall model.