Effect of pH on sodium citrate pretreatment to promote volatile fatty acids production during anaerobic fermentation of waste activated sludge

Abstract

The efficiency of sodium citrate (SC) pretreatment in enhancing volatile fatty acids (VFAs) generation from waste activated sludge (WAS) has been demonstrated, yet the yield remains limited. Therefore, this study investigated the influence of pH regulation on the efficiency of SC pretreatment by combining different initial acid-base conditions with SC. The results indicated that the acidic environment coupled with SC led to a tighter extracellular polymeric substances (EPS) structure and hindered the organic matter solubilization or VFAs production. Conversely, the alkaline environment combined with SC demonstrated a positive synergistic effect on sludge hydrolysis and VFAs production, with the synergy becoming more pronounced as alkalinity increases. When the initial pH was 11, the sludge disintegration degree (DDSCOD) and DNA solubilization were 1.67 and 3.76 times higher than those of SC alone, respectively. Moreover, the maximum VFAs production of 759.75 mg COD/g VSS (volatile suspended solids) was achieved at day 4 of fermentation. Microbial analysis showed that acidic conditions combined with SC favored hydrogen-producing bacteria (Clostridium_sensu_stricto) enrichment, while alkaline conditions combined with SC promoted the substantial enrichment of hydrolytic acid-producing bacteria with higher tolerance and adaptability (Macellibacteroides, Proteiniclasticum, and Acetoanaerobium). This may be the fundamental reason for the improvement of VFAs production.