Interaction of polyelectrolyte with digested sewage sludge and lignite in sludge dewatering

Physical conditioners, often known as skeleton builders, are commonly used for enhancing the rate and the extent of water removal of polyelectrolyte-flocculated sewage sludge. In this study, the interactions of two different cationic polyelectrolytes with lignite (a low rank coal) in conditioning of digested sewage sludge are investigated. Vacuum filtration tests, zeta potential measurements and floc population measurements are performed on flocculated sludge and mixtures of the flocculated sludge and the lignite. The results for the flocculated sludge demonstrate the classical flocculation mechanisms of charge neutralization and particle–particle bridging. However, when lignite is mixed with the flocculated sludge in the conditioning process, the lignite particles agglomerate with the flocculated sludge by polymer bridging, which improves the dewatering properties of the flocculated sludge. The lignite particles strongly agglomerate with the flocculated sludge for the high molecular weight (MW) and low charge density (CD) polyelectrolyte, but the agglomeration is weaker and shear sensitive for the low MW and high CD polyelectrolyte.