Enhancing volatile fatty acids accumulation through anaerobic co-fermentation of excess sludge and sodium citrate: Divalent cation chelation and carbon source supplement

This work developed an innovative anaerobic co-fermentation strategy of excess sludge to enhance volatile fatty acids (VFAs) production and carbon recovery relying on the sodium citrate (SC) functions of divalent cation bridging impairing and biodegradable substrate supplement. It was demonstrated that SC greatly improved rate constant and equilibrium performances of both sludge hydrolysis and VFAs accumulation, suggesting that the 0.4 g/g SS was the optimum SC dosage. The SC was partly absorbed in sludge solid and chelated structural divalent cations, which greatly facilitated sludge hydrolysis with the SCOD/TCOD of 28.20%. Meanwhile, the residual SC in liquid phase contributed to VFAs bio-generation as carbon substrates. Thereby, the VFAs accumulation was improved to 387.72 mg COD/g VSS in a short-term anaerobic co-fermentation (2 d), approximately 7.64 times higher than that without SC addition. The VFAs were mainly composed by acetate (58.86-81.76%) and propionate (7.97-16.28%). In the whole anaerobic co-fermentation process, remarkable carbon source of 1785.74 mg/L was released. Acetate was the dominant composition in total soluble carbon sources with the percentage of 44.45-83.98%, manifesting the bio-availability of fermentative liquid. In this case, the bioavailable carbon recovery rate of 38.04% was achieved. Moreover, mechanism of the enhanced anaerobic cofermentation was illustrated and the economic benefits were clarified.