Multivariable PID control of an Activated Sludge Wastewater Treatment Process

In general, wastewater treatment plant (WWTP) consists of several stages before it is released to a receiving water body. There are, preliminary and primary treatment (mechanical treatment), a secondary treatment (biological treatment) and a tertiary treatment (chemical treatment). In this chapter, since the work involve of identification and control design of activated sludge process to improve the performance of the system, and most of the control priorities are centred on the biological treatment process, only the secondary treatment will be highlighted. System modelling and identification of the activated sludge process has provided a wider understanding and a powerful tool to predict the behaviour of the system under different conditions. In control design, system modelling and identification are the most important parts which need be taken into account. Often, models developed for controller design have to be as simple as possible. The simplicity means models can be obtained directly from input-output (or experimental) data and used for control design of WWTP. This type of model is called black box or data-driven model, see for example Box Jenkins (El-Din et al., 2002) and Artificial Neural Network (ANN) proposed by (Cote et al., 1995). It will be shown that the identified data-driven control model describes the activated sludge wastewater system well, at least around an operating point. One of the popular techniques used in the system identification is the subspace identification algorithm and this algorithm is used for the design of control in WWTP. Another approach to modelling is to use model reductions or simplifications. The reduced order (linear) model can be later used for controller design and/or stochastic simulation, see for example (Robertson and Cameron, 1996). The biochemical processes involved in the activated sludge wastewater treatment process are complex and their understanding was very limited. However, due to the importance of providing concise and efficient information in describing a complicated set of activated sludge system behaviours, several mathematical models have been developed for gaining a better understanding of a real system. In the late 80s, a more scientific perspective of this biotechnology process was achieved by the first development of International Association for Water Quality (IAWQ) Activated Sludge Model no.1(ASM1) proposed by (Henze et 1