Catchment & sewer network simulation model to benchmark control strategies within urban wastewater systems

Highlights

• Plant-wide Benchmark simulation model is extended to include the catchment and sewer system.

• The extended model describes the generation and transport of wastewater.

• New evaluation criteria are defined for benchmarking control strategies/structural modifications.

• Case studies depicting the usefulness/application of the model are performed.

Abstract

This paper aims at developing a benchmark simulation model to evaluate control strategies for the urban catchment and sewer network. Various modules describing wastewater generation in the catchment, its subsequent transport and storage in the sewer system are presented. Global/local overflow based evaluation criteria describing the cumulative and acute effects are presented. Simulation results show that the proposed set of models is capable of generating daily, weekly and seasonal variations as well as describing the effect of rain events on wastewater characteristics. Two sets of case studies explaining possible applications of the proposed model for evaluation of: 1) Control strategies; and, 2) System modifications, are provided. The proposed framework is specifically designed to allow for easy development and comparison of multiple control possibilities and integration with existing/standard wastewater treatment models (Activated Sludge Models) to finally promote integrated assessment of urban wastewater systems.

Introduction

It has become increasingly clear that wastewater treatment plants (WWTPs) are strongly interconnected to other elements (sewer network, receiving media) within the urban wastewater system (UWS) and the evaluation of WWTP control strategies should be tackled in a more holistic manner (Rauch et al., 2002, Bach et al., 2014). For this reason, there is a need to move “outside the fence” of the WWTP and develop integrated tools for model-based evaluation and control of the UWS (Benedetti et al., 2013). This goal has inspired a large number of scientific contributions that attempt to investigate different aspects of integrated modelling. For example, Benedetti et al. (2004) and Vanrolleghem et al. (2005) tackled important issues such as model integration and model compatibility. Another important aspect has been model complexity reduction to allow for long term simulations (Erbe and Schütze, 2005, Fu et al., 2009a). The latter and the increase in computational power promoted the use of Monte Carlo simulations and the study of input uncertainty propagation through the model either during the model development process or during model use (e.g. Astaraie-Imani et al., 2012, Benedetti et al., 2008, Benedetti et al., 2010, Freni et al., 2011, Fu et al., 2009b). Long term simulations can be conducted as well, including the study of integrated control (e.g. Fu and Butler, 2012, Weijers et al., 2012). Finally, studies of the fate of particular compounds such as sulfur compounds (Jiang et al., 2010), greenhouse gas emissions (Guo et al., 2012) and micro-pollutants (Vezzaro et al., 2014, Snip et al., 2014) were also performed.

One of the major areas of application for integrated models is control. Integrated control has been studied for some years and the main benefits of using such an approach are demonstrated in several studies (e.g. Harremoës et al., 1994, Schütze et al., 2002, Vanrolleghem et al., 2005, Langeveld et al., 2013). With the future clearly pointing towards integrated management of the UWS, the need for development of efficient integrated control strategies is growing. In this context, we believe that a benchmarking tool can be extremely beneficial to develop and test control strategies in the UWS. Within sewer systems, Borsányi et al. (2008) conducted a benchmarking study using real-time control strategies applied to two virtual sewer systems. In the WWTP community, benchmarking control strategies has been very successful. Benchmark Simulation Models (BSM1, BSM1-LT and BSM 2) and associated spin-off products (influent generator, ADM1 implementation, sensor models, evaluation criteria etc.) have demonstrated to be valuable tools in the field of WWTP optimization and have been widely used in both industry and academia (Gernaey et al., 2014). Nevertheless, there is a lack of benchmarking tools that allow objective comparison of control strategies in urban catchments and sewer systems. Therefore: 1) Rigorous development/evaluation of control strategies in the WWTP (Gernaey et al., 2014) is based on influent generators (Gernaey et al., 2011, Flores-Alsina et al., 2014, Martin and Vanrolleghem, 2014), and such influent generators are not suitable for modelling control strategies upstream of the WWTP; and, 2) In many cases, integrated UWS control strategies cannot be developed and evaluated on a single simulation platform.

The objective of this paper is to develop a catchment and sewer network model to benchmark control strategies. The catchment model reproduces the generation of wastewater through the combination of four different sub-models (Domestic (DOM), Industrial (IND), Stormwater (SW) and Infiltration to sewers (INF)). The sewer model describes wastewater transport (TRANSPORT) as well as the sudden increase of particulates during the beginning of a rain event following a period of drought (FIRST FLUSH) and the retention of wastewater (especially during rain events) using storage tanks to avoid combined sewer overflows (STORAGE). A set of evaluation criteria are used to assess the overflow discharged into the receiving waters. The criteria can be applied for a specific overflow location (Local) or for the entire system (Global). The criteria can be further classified into those describing: 1) cumulative effects; and, 2) acute effects on the receiving water system. As a receiving water model is not used in this study, these criteria are only indirect indicators of the effect of overflow discharges on river systems. Additionally, case studies demonstrating the possible applications of the tool for analysing the impact of: 1) local/global control strategies; and, 2) system modifications, are presented and discussed in detail. The proposed framework is specifically designed to allow for development and comparison of multiple control strategies, and allows easy interfacing with existing wastewater treatment (benchmark) models to finally promote integrated assessment of catchment, sewer network and WWTP performance.