Continuous waste activated sludge and food waste co-fermentation for synchronously recovering vivianite and volatile fatty acids at different sludge retention times: Performance and microbial response

[Display omitted] •P and VFAs were recovered efficiently and stably in long-term WAS/FW fermentation.•82.88% P from WAS was recovered as high-purity vivianite (95.23%).•The efficiencies of Fe2+ and P release and VFAs recovery were highly linked to SRT.•The transformation of P forms during WAS/FW co-fermentation was analyzed.•The FeCl3 and FW enriched the functional bacteria with stimulated activities. A practical approach of synchronously recovering vivianite and volatile fatty acids (VFAs) by food waste (FW) and waste activated sludge (WAS) co-fermentation in continuous operation was investigated. Approximately 82.88% P as high-purity vivianite (95.23%) and 7894 mg COD/L VFAs were finally recovered. The simultaneous addition of FW and FeCl3 contributed to the fermentation conditions by adjusting pH biologically and increasing the concentration of organic substrates, which enhanced the Fe3+ reduction efficiency and microbial activities (e.g., hydrolases and acidogenic enzymes). Microbial analysis found the functional bacteria related to Fe3+ reduction and VFAs generation were further enhanced and enriched. Besides, results indicated that the efficiencies of Fe2+ and P release and VFAs recovery were highly linked to SRT, the satisfactory fermentation performance was obtained at SRT of 6 d. This research would provide a practical waste recycling technology to treat FW and WAS simultaneously for recovering vivianite and VFAs synchronously.