SETTLING PROPERTIES OF DIGESTED-SLUDGE PARTICLE AGGREGATES

A multi-exposure photographic method and an image analysis system were used to measure the terminal settling velocities and some geometric characteristics of digested sludge particle aggregates larger than 35 mum (>90% of the sludge mass). It was found that the aggregates larger than 150 mum showed low settling shape factor values, indicating their irregular structure. The density of the aggregates decreased with the increase in their size and was calculated by neglecting or accepting the irregularity of their structure. The terminal settling velocities of the aggregates, in a settling medium of filtered supernatant diluted 1:2 with tap water, were proportional to their diameter rising to the power of 0.67. A correction factor, incorporating the settling shape factor, was formulated for the determination of the hydrodynamic properties of the aggregates studied. Their solid volume fraction was expressed by a three-parameter empirical model.