Copyright @ IF AC Advanced Control of Chemical Processes Pisa, Italy, 2000 ' NONLlNEAR PROPORTIONAL INTEGRAL CONTROLS ROBUSTLY STABILISE BIOLOGICAL REACTORS R. Antonelli and A. Astolfi Centre for Process Systems Engineering, Imperial College of Science, Technology and Medicine, London SW7 2BY, UK FAX:+44-171-5946606, E-mail:{ r.antonelli,a.astolfi}@ic.ac.uk Abstract : Biological processes are often encountered in industrial applications. Nev- ertheless several control problems are still unsolved, because of the intrinsic non- linear behaviour, lack of measurements and large model uncertainties. In this work Lyapunov-based methods are used to design nonlinear output feedback Proportional Integral (PI) control laws yielding robust stability and guaranteed performance. It is shown that the proposed control algorithm is able to cope with input constraints and requires only a very limited knowledge of the process parameters . Applications to a wastewater treatment reactor are described in detail and simulations are enclosed. Copyright ©2000 IFA C Keywords: Lyapunov Methods, Output Feedback, PI Control, Biological Reactors, Wastewater Treatment Plant 1. INTRODUCTION The selection of effective methodologies for the supervision and control of complex processes is essential for achieving optimal results. Feedback stabilisation of biological reactors has become an active area of research in recent years (Bastin and Dochain, 1990; Daoutidis et al. , 1990; Hen- son and Seborg, 1992; Bastin, 1992; Bastin and Van Impe , 1995; Soroush, 1997). The classical approach to the bioreactor control problem has focused on biological modelling as a tool to appro- priately describe the nonlinearities and to predict their behaviour with time . Even though many models with various degrees of complexity have been proposed, it is well recognized that these models are not easy to develop and often provide solutions that are not satisfactory. The main diffi- culty lies in the comparison between noisy experi- mental data and nonlinear models with uncertain parameters . Nowadays, continuous and fed-batch biological reactors have been extensively studied 105 mainly because of their prevalence in biological wastewater treatment plants (WTPs) . Wastewa- ter containing pollutants and organic materials is generally treated in aerated tanks where it is mixed with biological particles constituting the biomass. After a sufficiently long contact time, this mixture is discharged to a settler where the suspended biomass is separated from the treated water. A small amount of the biomass is purged while the rest is recirculated into the tank . A biological reactor, like any other process in which living micro-organisms are involved, constitutes a complex process which is difficult to control. The main issues are the lack of detailed information of the process; the variations of the reactor inflow, both in quantity and in quality ; the variation of the population of the catalytic bacteria in quan- tity and in number of species; the unavailability of reliable analyzers; the difficulty in the interpreta- tion of the experimental data. Because of all these factors control of biological reactors represents a very interesting and challenging problem.