For the development of new control strategies for small activated sludge systems (<500 P.E.) the oxidation-reduction potential (ORP) has been tested as a suitable parameter. Batch and continuous experiments in a pilot plant with alternating nitrification and denitrification phases proved the relation between the actual load and the behaviour of ORP in the aeration tank. Using a two-point controller, increasing load correlated with an extension of aerated phases in relation to unaerated phases. Decreasing load equalled to a decrease of the ratio of nitrification time-cycle time. On the basis of these results an ORP controller has been developed allowing both automatic setup and on-line adaption. Operational results from a pilot plant are presented, including effluent quality and energy efficiency.

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