In an activated sludge process for nitrogen removal, nitrate may be reduced to nitrogen gas by facultative heterotrophic bacteria in an anoxic environment. In order to guarantee sufficient supplies of readily biodegradable carbon compounds, an external carbon source often needs to be added. In this paper, an automatic control strategy for controlling the nitrate level using an external carbon source is presented. The external carbon source is added in the first anoxic zone to control the nitrate concentration in the last anoxic zone. Key process parameters are estimated on-line for a simplified Activated Sludge Model No 1. The estimated parameters are used for updating a linear quadratic controller. The strategy is illustrated in a simulation study with realistic influent data and is shown to perform very well.

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