Production of medium chain fatty acids through co-fermentation of food waste and sewage sludge without external electron donors

Effective biowaste treatment with renewable bioenergy production has become attractive. In this study, chainelongation (CE) technology was applied to convert food waste (FW) and sewage sludge (SS) into high valueadded medium chain fatty acids (MCFAs) by co-fermentation, which not only reduces environmental pollution but also recovers valuable resources. Five groups of batch co-fermentation experiments were performed using FW and SS at different ratios (FW/SS based on chemical oxygen demand, COD) without external electron donors (EDs) to investigate the effects of different FW/SS ratios on the production of MCFAs. The results showed that 100 % FW (FW/SS=1:0) achieved the highest n-caproate concentration (396.37 +/- 35.98 mg COD/L), followed by 75 % FW (FW/SS=3:1) (384.42 +/- 28.00 mg COD/L). Moreover, 75 % FW (FW/SS=3:1) had higher n-caproate specificity (11.22 +/- 3.81 %) than 100 % FW (7.52 +/- 0.68 %). Thus, a semi-continuous co-fermentation experiment at a FW/SS ratio of 3:1 was performed for 90 days to explore the feasibility of long-term MCFAs production without external EDs. The results showed that the maximum MCFAs concentration, specificity, and production rate were 10441.31 mg COD/L, 58.20 %, and 250.59 mg COD/L.d(-1), respectively. Among the three MCFAs, n-capmate was the most abundant, accounting for 60.28 %. Microbial community analysis revealed that four lactate-producing bacteria (Bifidobacterium, Olsenella, Atopobium, and Proteiniphilum) co-occurred with Caproiciproducens, developing a CE system driven by lactate as the ED. This study provides new insights into the recovery of resources from FW and SS.