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

Abstract

Effective biowaste treatment with renewable bioenergy production has become attractive. In this study, chain-elongation (CE) technology was applied to convert food waste (FW) and sewage sludge (SS) into high value-added 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-caproate 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.