The dissolved oxygen (DO) probe is repeatedly subjected to a step change of the oxygen concentration with a time interval of 15-30 s. For this purpose an existing measuring technique was used in which the activated sludge was sampled continuously from the aeration tank. At each step the time constant of the probe response is estimated which provides a continuous diagnosis of the probe condition. Therefore the DO concentration measurements along the response are fitted to a first order response model of the probe. It was shown that the probe response time constant is a useful indicator for fouling of the probe membrane.

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