Mechanistic Insights into Chemical Conditioning by Polyacrylamide with Different Charge Densities and Its Impacts on Sludge Dewaterability

Abstract While chemical conditioning has been widely used to improve the sludge dewaterability, the knowledge gap in understanding the detailed interaction between the agent and sludge as well as the structural properties of the conditioned flocs remains. In this study, we therefore investigate the mechanisms of sludge conditioning with a widely used organic flocculant, polyacrylamide (PAM), by using a suite of time-resolved characterization technologies. Compared to PAM with a low charge density, the high charge-density PAM (70%) improves the sludge flocculation and apparent filtration performance as a result of the enhanced structural strength. This is mainly attributed to the bonding between the biomolecules and PAM, resulting in the aggregation of flocs, squeezing of the water dispersed among the sludge particles and finally improvement of the dewaterability. The interaction between sludge and PAM is dynamically assessed by multiple light scattering spectroscopy and quartz crystal microbalance with dissipation. Results demonstrate that the use of high charge-density PAM quickly generates flocs with a rigid and dense structure that suffer less from the hydraulic turbulence. These findings provide mechanistic insights into the interaction between sludge and flocculant/coagulant (of different charge densities) and its impacts on the floc properties, which is beneficial to the process design and optimization for sludge conditioning, pipeline transport and dewatering in high-pressure systems.