Sodium citrate enhances anaerobic fermentation of granular sludge: the multifaceted roles of structure disruption and metabolic regulation

Anaerobic fermentation is an efficient approach for recovering organic carbon and other valuable resources from waste sludge, yet its efficiency is constrained by the structural stability of extracellular polymeric substances (EPS), especially for aerobic granular sludge (AGS). Despite the abundant physical-chemical pre-treatment approaches for enhancing EPS dissolution, biocompatible strategies coordinating structural disruption with metabolic regulation remain unexplored. Herein, sodium citrate (SC) was used to enhance the performance of anaerobic fermentation of AGS. The results suggested that SC significantly enhanced the hydrolysis efficiency and volatile fatty acids (VFAs) production of AGS. Despite the direct conversion of SC to acetate, indirect enhancement played more important roles in AGS fermentation. Mechanism analysis indicated that SC disrupted granular sludge structure by chelating Ca²⁺ and facilitated the release of EPS and hydrolytic enzymes, which was conducive to sludge hydrolysis and acidification. At the level of microbial community, SC facilitated the accumulation of VFAs by enriching the acid-producing microorganisms and inhibiting the acid-consuming microorganisms. Furthermore, SC regulated the genes involved in the direct generation of acetate and pyruvate-centric metabolism, resulting in the massive accumulation of VFAs. Finally, the economic benefits resulting from increased VFA production versus SC costs. Overall, SC enhanced the anaerobic fermentation of AGS by simultaneously affecting EPS structure and regulating metabolism, and this study provided efficient methods for AGS anaerobic treatment.