Understanding The Mechanism Of How Anaerobic Fermentation Deteriorates Sludge Dewaterablity

Recovery of volatile fatty acids from waste activated sludge by anaerobic fermentation is a promising process for sludge disposal,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 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 were20.3-and 5.1-fold of those measured in raw sludge.The mechanism explorations revealed that anaerobic fermentation increased proteins,polysaccharides,humic acids and PO₄^3-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 PO₄^3-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,extracellular polymeric substances analyses showed that anaerobic fermentation increased loosely bound extracellular polymeric substances,which benefited fermented sludge to wrap water especially bound water and enhanced sludge viscosity.Succession analysis of microbial community further exhibited 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.Acid treatment has been widely used in sewage treatment plants due to the low cost and simple operation.However,previous studies have focused on wasted activated sludge,and few have studied the effect of acidic conditioning on the dewatering performance of fermentation sludge.To fill this gap,the effect of acidic treatment on dewaterability of fermented sludge as well as its mechanism was investigated in this study.Experimental results showed that acidic treatment dramatically enhanced the dewatering performance of fermented sludge.At p H 4.0,the fermented sludge showed better dewatering performance.Compared with the control,the capillary suction time and specific resistance to filtration decreased by93.8%and 69.5%,respectively.Mechanism investigation revealed that the acidification eluted bound extracellular pol ymeric substances and further destroyed cells in fermented sludge,causing the release of hydrolytic enzymes and bound water contained in sludge.Besides,acidic treatment caused protonation of negatively charged functional groups in sludge,which reduced the surface charge of sludge and electrostatic repulsive interactions between sludge particles,contributing to their flocculation.