Novel insight into sludge dewaterability mechanism using polymeric aluminium ferric chloride and anaerobic mesophilic digestion treatment under ultrahigh pressure condition

Abstract

There are significant challenges associated with treating sewage sludge with high water content. In this study, polymeric aluminium ferric chloride (PAFC)/anaerobic mesophilic digestion (AMD) conditioning combined with ultrahigh pressure filter press (UPFP) dewatering was investigated as a novel and feasible technique to improve sludge dewatering. Response surface methodology experiments found that the optimal operating conditions were at an AMD temperature of 42.6 °C, an AMD reaction time of 240.00 min, a PAFC dose of 150.00 mg/g dry solid, and filter pressure of 5.37 MPa. Under these optimal conditions, the water content of the dewatered cake was reduced by 54.54 wt%. The study explored the detailed mechanisms involved with the conditioning and dewatering process, by tracing the sludge, filter cloth, and filtrate characteristics. First, during the combined PAFC/AMD conditioning process, the treatment effectively removed the protein-like and polysaccharide-like materials in the extracellular polymeric substances of sludge, forming large flocs. Over time, the sludge had a lower viscosity and a higher flowability, promoting the release of bound water. Second, during the UPFP dewatering process, the filter cloth maintained its high permeability, and water easily passed through it. Third, high levels of dissolved organic matter were released after the dewatering process, ultimately forming a porous and rigid sludge cake. Furthermore, the study found strong correlations between sludge dewaterability and sludge, filter cloth, and filtrate characteristics. The experiment involving recycling filter cloth demonstrated that the combined treatment effectively prevented and reduced clogging of the filter cloth, reducing costs.