Basic forms of microbial aggregates generated in wastewater treatment and techniques used to study them, are described and discussed. The role of the free-setting velocity in evaluation of some physical properties of activated sludge flocs is emphasized. Several regression models were applied to correlate the flocs' settling velocity with flocs' size values. For the studied samples a simple linear model proved to be superior to a multiplicative one, and an introduction to this model of a settling shape factor function instead of constant intercepts, provided a very good correlation of the experimental data. It is expected that a difference between shape factors for the most stable conditions of the flocs and those for the flocs under the settling conditions will make it possible to determine softness or stiffness of these microbial aggregates.

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