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

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.