Conditioning of sewage sludge via combined ultrasonication-flocculation-skeleton building to improve sludge dewaterability

Strong water trapping ability of polyelectrolyte flocculated sludge and high compressibility of sludge filter cake greatly hindered the further improvement of sludge dewaterability. Ultrasound (US) was used to strip the water from the sludge flocs through sludge disintegration, traditional cationic polyacrylamide (CPAM) was applied to reconstruct the sludge aggregates through flocculation, and rice husk (RH) was utilized to improve the permeability of sludge filter cake as skeleton builders in this paper. The feasibility of combined US-CPAM-RH sludge conditioning was demonstrated, and single and co-conditioning processes were compared to explore the synergetic effect and mechanisms of US-CPAM-RH sludge conditioning. The results indicated that the sludge dewaterability was considerably improved by the combined US-CPAM-RH conditioning compared with that of raw sludge. The optimal ultrasonication condition was 0.3 W/ml for 12 s with an Ultrasound frequency of 22 kHz, and the RH and CPAM dosage was 50.0 wt% and 20 mg/L, respectively, which resulted in a shortest time to filter (TTF) of 43 s and a lowest moisture content of sludge filter cake of 62.22%. The US mainly stripped the extracellular polymeric substances and water from the sludge flocs, leading to an obvious increase of concentration of proteins and polysaccharides in sludge filtrate. The CPAM and RH addition resulted in a remarkable sludge floc growth to a mean diameter d(0.5) of 128.85 µm after combined US-CPAM-RH conditioning. Moreover, scanning electron microscopy (SEM) results clearly showed that the combined US-CPAM-RH conditioning formed a more porous structure of sludge filter cake with a porosity increase of 98.80% and a compressibility decrease of 37.81% compared with that of raw sludge. Thus, the US-CPAM-RH co-conditioning could significantly improve the sludge dewaterability.