The relationship between filtrate flow rate, cake liquid content, and cake structure compression is investigated for compressible structures in cake filtration. It has been observed that when the pressure is above a critical value, the filtrate flow rate is nearly independent of filtration pressure for highly compressible cake structures. In this study the specific resistance of waste-activated sludge is investigated and the critical filtration pressure, above which filtrate how rate is independent of pressure, is found to be 0.02 bar, a value well below that used in dewatering processes. Local values of any point within the cake of specific resistance and liquid content are important to characterize the behavior of compressible structures in cake filtration. It is, however, difficult to make local measurements in the cake structure. Therefore, the local properties are often determined from overall sludge filtration parameters by calibration of mathematical models for cake filtration. These models require, however, knowledge of local functional relationships between structure stress, how resistance, and liquid content. This requirement is a problem in the calibration of the models. The methods developed in this study avoid the requirement of local relationships but allow the determination of relationships between local structure stress, specific resistance, and liquid content from overall measurements. The values of local specific resistance and void ratio are determined for waste-activated sludge using the relationships. The relationships determined for waste-activated sludge show the logarithmic local specific resistance to be proportional to the logarithmic structure stress. The local void ratio is shown to decrease rapidly for increasing structure stress which corresponds to an extremely compressible behavior.
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