By studying the drying characteristic curve, the moisture in sludge is classified into four categories: free moisture, interstitial moisture, surface moisture and bound moisture. The effects of three dewatering procedures: gravity drainage, vacuum filtration, and centri-fugation, as well as chemical conditioning by a cationic polymer, and physical conditioning by freeze-thaw on the moisture distribution, are investigated. It is found that the dewatering procedures studied can only remove part of the free and interstitial moisture; surface and bound moisture are not affected. Part of the free and interstitial moisture also remains with the sludge cake, which can be considered as the inherent inefficiency of the dewatering process. The complete removal of all the free moisture can be considered as the theoretical best performance any mechanical dewatering device can achieve. Polymer addition is found to improve the dewaterability of the sludge by reducing the inherent inefficiency, primarily by reducing the amount of free moisture retained in the cake. Freeze-thaw is found to effect the removal of free, interstitial and surface moisture, bringing about a dramatic increase in dewaterability. The treated sludge cake drains readily and retains no free moisture.

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