The outline of a novel sensor for the secondary clarifier based on the combination of in-line sensor principles and image analysis is presented. The sensor consists of a model secondary clarifier of 10 litres from which samples can be taken at different heights, running parallel with the full-scale plant. These samples are examined in an image analysis system, incorporating a flow through cell, a microscope and specific hardware. The measurements that can be obtained with this apparatus are multifarious: a one-dimensional concentration profile (and a fortiori the sludge blanket height) and the morphology and size distribution of the sludge flocs. Results show that it is possible to characterize the settling properties of the sludge automatically. Furthermore, the information is extended by using a mathematical model to describe the concentration profile. Lab-scale tests demonstrate that the information obtained could be uses to optimize the operation of the plant through suitable process control.

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