The specific cake resistance is considered to be the key factor in the characterization of sludge dewaterability. The use of this parameter without knowing how it is derived and the conditions under which it can be used has been creating a great deal of confusion in the literature. Moreover, the lack of a universally agreed test method to determine its value causes additional problems. Hence, attempts to go from the laboratory data to the full scale industrial equipment using this concept may end up in failure.

This paper starts with the origin of the specific cake resistance in the historical perspective of filtration theory. Currently used experimental techniques in the determination of specific cake resistance are reviewed, advantages and disadvantages axe succinctly summarized. An alternative way of characterization of sludges using a resistance function rather than a specific cake resistance is proposed.

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