Impacts of floc breakage on dewaterability of chemically conditioned sludges and implications on practical conditioning strategies

Abstract

While chemical conditioning is the most commonly used conditioning process for sludge dewatering, the dewaterability of chemically conditioned sludge is found to be significantly affected by shear force. However, relevant studies are still lacked, which obstructed an in-depth understanding on sludge conditioning. In this study, we systematically investigated the impacts of floc breakage induced by shearing on the sludges conditioned with polyacrylamides (PAMs)/inorganic salts. Although PAM-conditioned sludges had better structural strength and dewaterability than the ones conditioned with inorganic salts, they exhibited a drastically deteriorated dewaterability upon floc breakage after shearing. Further analysis revealed that the PAM-conditioned flocs were mainly wrapped by polymer chains and thus floc breakage would lead to a substantial release of extracellular polymeric substance (EPS) and exposure of massive non-neutralized negative charge. In comparison, the coagulation by inorganic salts was mainly completed by charge neutralization and compression of EPS, which resulted in a conditioned sludge with a much less deteriorated dewaterability after shearing. Accordingly, PAMs are suggested as the chemical agents if screw pump is used for the sludge conditioning, while extra shearing should be minimized in the conditioning process. Inorganic salts are more applicable for the conditioning layout with a flexibly located equalization tank due to their relatively low coagulation kinetics but well-maintained sludge dewaterability. This study fills some critical knowledge gaps in the fundamental understandings on chemical conditioning process and offers a guideline for the selection of practical conditioning strategy.