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Abstract

The invention provides processes and apparatus for the controlled addition of conditioning (polymer) material to sewage sludge to improve its dewaterability. These materials are expensive and overdosage is to be avoided, while underdosage causes difficulty in sludge handling. The principal problem has been the absence of a system capable of continuous determination of proper dosage. It has been found that shear stress tests with a viscometer result in characteristics that exhibit a unique peak when dosage is adequate or above. A control system controlling the rate of pumping of the polymer to mix with the sludge is operative by measuring the shear stress and increasing the dosage in the absence of a peak. Upon detection of a peak the rate of decrease after the peak is also measured and used to resolve the potential ambiguity of whether dosage should be increased, decreased or remain steady. The viscometer can be of variable shear rate type, but it is found that the control system can also employ a simple fixed rate viscometer, the shear stress then being evaluated for peak and slope over a predetermined time interval. Specific control strategies are disclosed that will reduce the rate of change of dosage as the optimum is approached to avoid overshoot, and also to increase the time interval between tests to avoid unnecessary operation.

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First Claim