OPTIMAL-CONTROL OF N-REMOVAL IN ASPS

Optimisation of the operation strategy of N-removing activated sludge processes (ASPs) contributes to improved effluent quality and/or costs savings. This paper deals with the development of an aeration strateg y yielding optimal N-removal in continuously mixed, continuously fed A SPs. First, optimal control theory is applied to the generally accepte d ASM no.1 model (Henze et al., 1987). This study reveals that, from a n removal point of view, both alternating nitrification/denitrificatio n and simultaneous nitrification/denitrification at limiting DO-levels might be optimal, depending on the uncertain oxygen half-saturation c onstants of autotrophic and heterotrophic biomass. Hence, taking into consideration the risk of sludge bulking at limiting DO-levels, an alt ernating anoxic/aerobic strategy is favoured. A Receding Horizon Optim al Control (RHOC) strategy using NH, and NO, measurements is developed enabling feedback control of the alternation between anoxic and aerob ic phases with the explicit objective of optimal N-removal. Simple rul es are given for straightforward tuning of this controller. The contro ller successfully passed several tests both in simulation and in appli cation to a pilot plant continuously fed with presettled domestic wast ewater. (C) 1998 Elsevier Science Ltd. All rights reserved.