The effect of mixing in sludge conditioning with polymers on the sludge dewaterability has been investigated by many researchers. What has been lacking so far are experimental data from a systematic variation of the rapid-mix conditions. Thus an experimental set-up was chosen to evaluate the effect of mixing by conditioning wastewater sludges in continuous mode rather than in batch-mode.

The results show that the rapid-mix conditions have a strong effect on sludge dewaterability. Variations in the rotational speed of in-line stirring devices revealed that intense mixing is mandatory when conditioning anaerobically digested sludges. The results indicate that during a period of one second after polymer addition, mixing with maximum intensity (rotational speeds up to 1700 rpm) at the proper polymer dose does not lead to a deterioration in dewaterability. These findings could be confirmed by experiments with sludges from different treatment plants. The investigations with the different sludges revealed differences in the energy required for mixing in order to obtain optimal dewaterability. The mixing requirement is, in addition, strongly dependent on the polymer dose. High polymer doses are less sensitive to mixing, while at low doses, intense mixing becomes important

Attempts have been made to describe the observed phenomena by considering the sludge rheology and the changes in rheology that take place in the reaction with the polymer.