Three cationic polyelectrolytes with different charge densities were used to condition activated sludge samples from a municipal plant during a period when the sludge characteristics changed strongly. Initially the sludge had high residual turbidity after sedimentation, high filtration resistance, low floc strength and high amounts of extractable extracellular polymers (ECP). These problems could be related to the snow melting that was going on. When that ceased the sludge recovered as shown by decreased residual turbidity, filtration resistance and amount of extractable ECP and increased floc strength.

The polyelectrolytes with 40 and 100 % charge density gave good and similar filterabilities during the whole investigation period. With the 10 % charged polymer the filtration resistance was generally higher and especially so during the snow melting period. The reason is that low charge density polymers flocculate with a bridging mechanism. This gives a flexible floc structure which allows the formation of dense filter cakes and also to some extent preserves the original structure. High charge density polymers give closer contacts between the particles and thus a more inflexible and open structure which is favourable in filtration.