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Fast freeze/thaw treatment on excess activated sludges: floc structure and sludge dewaterability.

The effects of a fast freeze/thaw treatment (with freezing speed about 40 mm/h) on activated sludge dewaterability were demonstrated by conducting drying, centrifugal settling, and vacuum filtration tests. After a freeze/thaw treatment, the BOD and COD values of the sludge supernatant were found to raise significantly, the moisture movement resistance and the bound water content of the sludge cake both decreased, the floc volume and sludge compressibility were reduced, and the sludge filterability was improved greatly. The fractal dimension of the original and frozen sludges was measured via the size-density relationship. A simple structural interpretation based on fractal geometry was proposed to explain the experimental findings.