WERF Phase 2: Factors affecting conditioning during dewatering- Influence of dewatering equipment

A study was conducted to investigate the factors that impact polymer demand during conditioning such as sludge characteristics, polymer characteristics and dewatering equipment characteristics. The central hypothesis of the research was that the polymer demand is related to all these three variables. By understanding how these variables impact conditioning, the dewatering process could be better operated. The first paper examines the role of sludge characteristics on polymer demand. This is the second paper in the series, and this phase examines the role of dewatering equipment on polymer demand. The hypothesis for this phase of research was that shear during dewatering influenced polymer demand, and an increase in dewatering shear resulted in an increase in polymer demand. It was further hypothesized that this increase in polymer demand due to equipment shear could be somewhat mitigated through appropriate selection of polymer. Dewatering shear was calibrated using a laboratory mixer that imparted a mixing gradient (G, s−1) for a defined time (t, s). The product, Gt is a dimensionless parameter for shear. An increase in Gt imparted to sludge was related to an increase in polymer demand. This study found that the Gt varied for different dewatering equipment, with belt filter press having low Gt values of 10,000 and centrifuges having high Gt values of greater than 70,000. This data suggests that centrifuges exert higher polymer demand than belt filter presses. This study found that different sludges exhibited differing ‘resistance to shear’, with sludge obtained from mesophilic anaerobic and aerobic digestion exhibiting the greatest resistance to shear, whereas undigested sludges and sludges obtained from thermophilic digestion exhibiting the least resistance to shear. For example, thermophilically digested sludges were more susceptible to shear compared to conventional anaerobically and aerobically digested sludges, as measured by a greater increase in polymer demand when more shear was applied during conditioning. This study also found that the increase in dewatering shear could be mitigated through appropriate polymer selection. It was found that the use of high molecular weight and/or branched polymers resulted in an overall increase in conditioned sludge resistance to shear. For example, branched polymers appeared to be more shear resistant than linear polymer. The equivalent shear of full-scale dewatering equipment was measured. These results were combined to demonstrate how polymer demand could be estimated for different process change scenarios, such as upgrading from a belt filter press to a high solids centrifuge.