Factors determining polyelectrolyte choice and dosage in conditioning of activated sludge were investigated. It was found that due to differences in particle size distribution electrophoretic mobility did not correlate with optimal dosages. Instead the polyelectrolyte consumption to a large degree could be explained by a linear model containing the variables: sludge volume index, rest turbidity, floc break up factor obtained by CST-stirring test and CST (Capillary Suction Time) of unconditioned sludge.

For the investigated sludges biopolymers and colloidal particles were the main consumers of polyelectrolyte which however also was needed to build the existing flocs to optimum properties. The model was statistically less significant in the cases of high stirring times and high sludge volume indexes.

Sludges with high amounts of dispersed particles were best conditioned by a high molecular weight and moderately charged polymer while for well flocculated sludges with high sludge volume index a high charge density polymer was most suitable.

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