Influence of polyelectrolyte characteristics on pressure-driven activated sludge dewatering

Activated sludge conditioning and dewatering is a complex process involving many different parameters with varying degrees of importance. Due to the biological nature of the sludge, which impedes extensive testing in time on the same sample, a restriction of parameter values for testing has to be made. Based on a well-considered experimental design, this work has thoroughly investigated the influences of polyelectrolyte molecular weight and charge density on the dewatering performance with respect to both filtration and expression of activated sludge. Sludge samples were thereby collected in time with different solids concentrations and surface charge characteristics. molecular weight polyelectrolytes were shown to yield clear flocculation already at low doses, but tend to create elastic cakes presumably due to high internal electrostatic repulsion, as shown by the filter cake rebound after pressure release. As a further consequence of this high internal repulsion, the water retention of the flocs is high, which causes a high susceptibility to compression and limited dewaterability. Lower molecular weight polyelectrolytes require higher doses to become effective flocculants. In sufficient dose, these allow considerable water release from the sludge, enhancing the filtration phase. Thanks to the high degree of charge neutralisation, the formed flocs tend to form less elastic filter cakes. Furthermore, it seemed that a charge neutralization mechanism led to less water retention of the flocs formed and a more solid structure, making these polyelectrolytes also perform best in the expression phase.