Manfred Schütze

Integrating simulation models with a view to optimal control of urban wastewater systems

Abstract Rising expectations concerning levels of service, protection of the environment and enhanced sustainability are putting increasing demands on the urban wastewater system. The conventional approach to design and operation of the system, considering each component separately, cannot provide the gains in performance required. However, with recent gains in the understanding and modeling of the system it is now possible to represent the system as a whole and to optimize its performance. This paper describes the development and benefits of integrated modelling of the operation and control of urban wastewater systems and describes the development of a simulation package SYNOPSIS. SYNOPSIS encompasses sub-models of the sewer system, treatment plant and river, based on commonly accepted modeling approaches. The sub-models are linked in a parallel way allowing interactions between these sub-systems to be fully considered. This feature, used in concert with a control module implemented in the software, proves to be of particular importance for the development and analysis of integrated real-time control strategies. The model is applied to a case study site where it is clearly shown that river quality can be significantly improved by implementing an integrated control strategy as compared with the conventional passive or local control scenario. It is also shown how the application of conventional criteria (e.g. overflow volumes, discharged pollutant loads) can result in misleading conclusions when assessing the performance of the urban wastewater system under various scenarios. The paper argues that it is no longer necessary to use such simplified criteria, but using the software and techniques outlined, it is now feasible to operate complete urban wastewater systems to maximize river water quality directly. Furthermore, simulation results indicate the clear potential of integrated control even for catchments where local control provides hardly any benefits.

Modelling, simulation and control of urban wastewater systems

1. Introduction.- 1.1 Motivation of this Book.- 1.1.1 Administrative Responsibilities.- 1.1.2 Standards.- 1.1.3 Computer Software.- 1.1.4 Design and Operation.- 1.2 Outline of Chapters.- 2. The State of the Art.- 2.1 Components of the Urban Wastewater System: Basic Processes and Modelling Concepts.- 2.1.1 Urban Catchment Runoff and Sewer System.- 2.1.1.1 Surface Runoff in Urban Areas.- 2.1.1.2 Flow in the Sewer System.- 2.1.1.3 Pollutant Transport in the Sewer System.- 2.1.1.4 Biochemical Transformations in the Sewer System.- 2.1.1.5 Storage Tanks.- 2.1.2 The Wastewater Treatment Plant.- 2.1.2.1 Storm Tank.- 2.1.2.2 Primary Clarification.- 2.1.2.3 The Activated Sludge Process.- 2.1.2.4 Secondary Clarification.- 2.1.3 Rivers.- 2.1.3.1 River Flow.- 2.1.3.2 Pollutant Transport in the River.- 2.1.3.3 Biochemical Transformations in the River.- 2.2 Impact of Storm Events on the Urban Wastewater System.- 2.2.1 Impacts on Sewer Systems.- 2.2.2 Impacts on Treatment Plant Performance.- 2.2.3 Impacts on the Receiving River.- 2.2.4 Criteria for the Assessment of River Water Quality.- 2.2.5 The Dilemma of Control of the Urban Wastewater System.- 2.3 Integrated Modelling Approaches.- 2.4 Operational Management of Wastewater Infrastructure.- 2.4.1 General Concepts.- 2.4.2 Real-time Control of Sewer Systems.- 2.4.3 Development of Control Strategies - Exemplified for Sewer Systems.- 2.4.3.1 Off-line Development of Strategies.- 2.4.3.2 On-line Development of Strategies.- 2.4.4 Operation of Wastewater Treatment Plants.- 2.4.5 Real-time Control of Receiving Rivers.- 2.4.6 Integrated Real-time Control.- 2.4.7 Concluding Remarks.- 2.5 Mathematical Optimisation Techniques.- 2.5.1 Definition of the Optimisation Problem.- 2.5.2 A Review of Optimisation Methods.- 2.5.2.1 Local Optimisation.- 2.5.2.2 Global Optimisation.- 2.6 Conclusion.- 3. Development of the Integrated Simulation and Optimisation Tool SYNOPSIS.- 3.1 Requirements on the Simulation Tool.- 3.2 Modules Simulating the Parts of the Urban Wastewater System.- 3.2.1 Implementation of the Sewer System Module.- 3.2.2 Implementation of the Treatment Plant Module.- 3.2.2.1 The Original Implementation of Lessard and Becks Treatment Plant Model.- 3.2.2.2 Modifications of the Treatment Plant Model.- 3.2.3 Implementation of the River Module.- 3.3 Assembling the Integrated Simulation Tool.- 3.3.1 Integration of the Simulation Software.- 3.3.2 Variables in SYNOPSIS.- 3.3.3 Auxiliary Routines Necessary for Simulation.- 3.4 Implementation of Control in SYNOPSIS.- 3.5 Optimisation Algorithms in SYNOPSIS.- 3.5.1 Controlled Random Search.- 3.5.2 A Genetic Algorithm.- 3.5.3 Powells Local Optimisation Method.- 3.5.4 Interfacing the Simulation Tool with the Optimisation Routines.- 3.6 Summary: Overview of the Integrated Simulation and Optimisation Tool SYNOPSIS.- 4. Simulation of the Urban Wastewater System Using SYNOPSIS.- 4.1 Definition of a Case Study Site.- 4.1.1 Existing Data Sets.- 4.1.2 Definition of the Sewer System.- 4.1.3 Definition of the Wastewater Treatment Plant.- 4.1.4 Definition of the River.- 4.1.5 Overview of the Case Study Site Defined.- 4.2 Simulation of Dry-weather Flow.- 4.3 Simulation of a Rainfall Time Series.- 4.4 Analysis of the Control Devices of the Urban Wastewater System.- 4.5 Potential of Reduction in Simulation Time by Selective Simulation.- 4.5.1 Separation of Rainfall Events.- 4.5.2 Potential Savings in Simulation Time.- 4.5.3 Selective Versus Continuous Simulation.- 4.5.4 Conclusions.- 5. Analysis of Control Scenarios by Simulation and Optimisation.- 5.1 Definitions and Methodology.- 5.2 Analysis of Strategy Parameters - an Example.- 5.2.1 Definition of a Strategy Framework.- 5.2.2 Exploring the Parameter Space by Gridding.- 5.2.3 Optimisation of Strategy Parameters.- 5.3 A Top-down Approach to the Definition of Control Strategies.- 5.3.1 Definition of Various Frameworks.- 5.3.2 Evaluation of the Optimisation Algorithms.- 5.3.3 Conclusions.- 5.4 A Bottom-up Approach to the Definition of Control Strategies.- 5.4.1 Towards a Systematic Definition of Frameworks.- 5.4.2 Analysis of Frameworks Involving Several Controllers.- 5.5 Integrated Versus Local Control.- 5.6 Further Aspects.- 5.6.1 Sensitivity of Solutions.- 5.6.2 Multi-objective Optimisation.- 5.6.3 Simulation Period Required for Optimisation.- 5.6.4 Control Potential of Various Case Study Sites.- 6. Conclusions and Further Research.- 6.1 Summary.- 6.2 Suggestions for Further Research.- Appendix A. Overview of Existing Software.- A.1 Software for Simulation of Sewer Systems.- A.2 Software for Simulation of Activated Sludge Wastewater Treatment Plants.- A.3 Software for Simulation of Rivers.- Appendix B. Parameters of the Treatment Plant Model.- Appendix C. Rainfall Data Used in This Study.- Appendix D. Detailed Results of Optimisation Runs Presented in Chapter 5.- References.

Is combined sewer overflow spill frequency/volume a good indicator of receiving water quality impact?

It is often assumed that the frequency or volume of combined sewer overflow (CSO) spill is a good indicator of receiving water pollution impact. Whilst this assumption would appear to be true, recently there have been challenges to its veracity. To test this basic premise, an integrated model (SYNOPSIS) has been applied to the urban wastewater system of a semi-hypothetical catchment. By increasing the storage volume at a single downstream tank in the drainage system, the CSO spill frequency and volume was reduced. River water quality criteria, based on UPM standards, were calculated and related to spill frequency and volume over a series of long-term simulation runs. It was found that, up to certain storage volume levels, decreasing overflow frequency improved river DO and BOD and total ammonia. Beyond these volumes, however, there was no further improvement in DO/BOD and an increase in total ammonia. It is concluded that overflow frequency/volume can be used as a performance indicator for receiving water quality, provided its significant limitations are understood.

Assessment of the effects of greywater reuse on gross solids movement in sewer systems

Onsite greywater reuse (GWR) and installation of water-efficient toilets (WETs) reduce urban freshwater demand and thus enhance urban water use sustainability. Research on GWR and WETs has generally overlooked their potential effects on municipal sewer systems: GWR and WETs affect the flow regime in sewers, and consequently also influence gross solids transport. To asses these impacts, a gross solids transport model was developed. The model is based on approaches found in the literature. Hydrodynamic calculations of sewage flow were performed using the SIMBA6 simulator and then used for the gross solid movement models. Flow characteristics in the up- and downstream sections of the sewer network differ. Therefore different approaches were used to model solids movement in each of these two parts. Each model determines whether a solid moves as a result of a momentary sewage flow, and if it moves, calculation of its velocity is possible. The paper shows the adoption and implementation of two gross solids transport models using SIMBA6 and depicts the results of the effects of various GWR and WET scenarios on gross solids movement in sewers for a real case study in Israel.

Impacts of onsite greywater reuse on wastewater systems

Together with significant water savings that onsite greywater reuse (GWR) may provide, it may also affect the performance of urban sewer systems and wastewater treatment plants (WWTPs). In order to examine these effects, an integrated stochastic simulation system for GWR in urban areas was developed. The model includes stochastic generators of domestic wastewater streams and gross solids (GSs), a sewer network model which includes hydrodynamic simulation and a GS transport module, and a dynamic process model of the WWTP. The developed model was applied to a case study site in Israel. For the validation of the sewer simulator, field experiments in a real sewer segment were conducted. The paper presents the integration and implementation of these modules and depicts the results of the effects of various GWR scenarios on GS movement in sewers and on the performance of the WWTP.

Simulation of the Urban Wastewater System Using SYNOPSIS

Following the description of the integrated simulation and optimisation tool SYNOPSIS in the previous chapter, this chapter prepares the studies to be carried out in Chapter 5 in various respects. Section 4.1 discusses the availability of data and defines the case study site used throughout this work as well as the relevant input data. Sections 4.2 and 4.3 present results of simulations of dry-weather flow and of a rainfall series, respectively, in order to illustrate the capabilities of the simulation package. Another example of the application of the simulation part of SYNOPSIS is provided in Section 4.4. Here, various settings of some of the control devices available in the urban wastewater system and their impact on receiving water quality are assessed. This prepares the analyses of control strategies in the subsequent chapters. Since the application of optimisation procedures (in Chapter 5) will be potentially demanding in terms of computing time, Section 4.5 analyses to what extent continuous long-term simulations can be substituted by simulations of series of individual events, in order to potentially reduce the time required for simulations.

Analysis of Control Scenarios by Simulation and Optimisation

This chapter applies the simulation and optimisation tool SYNOPSIS to the evaluation of various control scenarios for the semi-hypothetical case study defined in the previous chapter. Some fundamental defmitions are given in Section 5.1. Section 5.2 provides an example of the application of the simulation and optimisation procedure to the determination of the parameters of a control strategy.

Development of the Integrated Simulation and Optimisation Tool SYNOPSIS

The main processes in the urban wastewater system and their representation in models have been presented in the previous chapter. The present chapter outlines the development of the integrated simulation and optimisation tool (named SYNOPSIS - “Software package for synchronous optimisation and simulation of the urban wastewater system”), which is to be used for the studies described in later chapters of this book. The development of the tool starts with the definition of a list of requirements (Section 3.1), which are based on the discussion of the previous chapter. According to these requirements, three existing software packages are selected for implementation in SYNOPSIS (Section 3.2). Section 3.3 describes their integration to an integrated simulation tool. This section also discusses the problems encountered when connecting the simulation programs. Implementation of control options is detailed in Section 3.4. Section 3.5 discusses the implementation of the optimisation procedures selected in Section 2.5 and their interfaces with the simulation tool.

Conclusions and Further Research

In Chapter 1, the ultimate goal of this book has been defined as the assessment of the potential of integrated control of the urban wastewater system. This objective has been approached in two main steps. The first subgoal was the assembly of a simulation tool, which is capable of the simulation of the water flow and quality processes in sewer system, treatment plant and receiving river. After having achieved this subgoal, the tool developed was linked to a variety of optimisation procedures to form the program package SYNOPSIS (“software package for synchronous pptimisation and simulation of the urban wastewater system”) and subsequently used for the evaluation of control strategies.