Influence of polyelectrolyte characteristics on pressure-driven activated sludge dewatering

Higher 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.