Understanding the mechanism of how anaerobic fermentation deteriorates sludge dewaterability

Abstract Anaerobic fermentation is a promising process to recover volatile fatty acids from waste activated sludge, but this process seriously deteriorates sludge dewaterability. Although several efforts were made in the past, the mechanism of how anaerobic fermentation deteriorates the dewatering performance remains largely unknown. This work therefore aims to fill the gap by exploring the variations in both liquid and solid phases before and after alkaline anaerobic fermentation and identifying their impacts on dewatering. Experimental results showed that 1418.6 ± 15.4 s of capillary suction time and (50.08 ± 1.37) × 1012 m/kg of specific resistance to filtration were determined in fermented sludge, which were 20.3- and 5.1-fold of those measured in raw sludge. The mechanism explorations revealed that anaerobic fermentation increased proteins, polysaccharides, humic acids and PO43− concentrations but decreased Ca2+ amount in liquid phase. The increases of proteins and humic acids increased the viscosity but decreased the zeta potential of liquid. The decline of Ca2+ not only decreased the zeta potential but also deteriorated the flocculability. The increase of PO43− decreased the zeta potential while the increase of polysaccharides increased the viscosity. Among these components, the release of humic acids was identified to the major contributor to the deterioration of sludge dewatering. Besides, anaerobic fermentation increased loosely bound extracellular polymeric substances, which benefited fermented sludge to wrap water especially bound water and enhanced sludge viscosity. It was also found that anaerobic fermentation caused transformation of microbes from Gram-negative to Gram-positive, which increased the resistance of mechanical forces and negative charges on sludge flocs.