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 strategy yielding optimal N-removal in continuously mixed, continuously fed ASPs. First, optimal control theory is applied to the generally accepted ASM no. 1 model. This study reveals that, from an N-removal point of view, both alternating nitrification/denitrification and simultaneous nitrification/denitrification at limiting DO-levels might be optimal, depending on the uncertain oxygen half-saturation constants of autotrophic and heterotrophic biomass. Hence, taking into consideration the risk of sludge bulking at limiting DO-levels, an alternating anoxic/aerobic strategy is favoured. A Receding Horizon Optimal Control (RHOC) strategy using NH4 and NO3 measurements is developed, enabling feedback control of the alternation between anoxic and aerobic phases with the explicit objective of optimal N-removal. Simple rules are given for straightforward tuning of this controller. The controller successfully passed several tests both in simulation and in application to a pilot plant continuously fed with presettled domestic wastewater.
Optimal control of N-removal in ASPs
L. J. S. Lukasse, K. J. Keesman, A. Klapwijk, G. van Straten; Optimal control of N-removal in ASPs. Water Sci Technol 1 August 1998; 38 (3): 255–262. doi: https://doi.org/10.2166/wst.1998.0219
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