Often the suspended solids concentration in activated sludge treatment plants may display both large amplitudes and unnaturally fast changes. If the sensor calibration has been found satisfactory, the abnormal appearance has to be explained by the process itself. There may be three principal reasons for such a behaviour: severe hydraulic conditions, poor floc separation properties or very high sludge blanket. During these circumstances it is very informative to qualitatively examine the relationships between the suspended solids concentration variations and important influencing variables, such as the flow rates and the sludge blanket level. This kind of knowledge based diagnosis is superior to dynamic models for such odd behaviour. A simple on-line method to perform the diagnosis is presented. The method has been tested on a massive set of data from a full scale wastewater treatment plant in Sweden and shown unexpected good ability to supply early warnings. It is also shown that small hydraulic disturbances may purposefully be injected into the settler in order to analyse the risk for process failure.

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