STRUCTURE COMPRESSION IN CAKE FILTRATION

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 ab ove a critical value, the filtrate flow rate is nearly independent of filtration pressure for highly compressible cake structures. In this s tudy the specific resistance of waste-activated sludge is investigated and the critical filtration pressure, above which filtrate how rate i s 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 t he cake of specific resistance and liquid content are important to cha racterize the behavior of compressible structures in cake filtration. It is, however, difficult to make local measurements in the cake struc ture. Therefore, the local properties are often determined from overal l sludge filtration parameters by calibration of mathematical models f or 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 t he models. The methods developed in this study avoid the requirement o f local relationships but allow the determination of relationships bet ween local structure stress, specific resistance, and liquid content f rom overall measurements. The values of local specific resistance and void ratio are determined for waste-activated sludge using the relatio nships. The relationships determined for waste-activated sludge show t he logarithmic local specific resistance to be proportional to the log arithmic structure stress. The local void ratio is shown to decrease r apidly for increasing structure stress which corresponds to an extreme ly compressible behavior.