Continuous Caproate Production from Sludge Fermentation Liquid with Chain Elongation Enrichment in a Long-Term UASB Reactor

Upgrading short-chain fatty acids (SCFAs) to high-value medium-chain fatty acids (MCFAs) through a chain elongation (CE) microbial reaction from excess sludge has attracted considerable attention. In this study, an upflow anaerobic sludge bed reactor inoculated with CE-enriched culture was applied to produce caproate using sludge fermentation liquid as substrates, investigating the stability of CE-enriched cultures under continuous flow conditions. Under conditions of 20,000 mg/L SCOD in the filtered fermentation liquid, an ethanol/acetate ratio of 8:1, a pH of 7, and a hydraulic retention time (HRT) of 2 days, the maximum caproate concentration of 3.49 g/L was achieved. Although optimum pH and the ethanol/acetate ratio benefited CE activity, long-term stability of CE-enriched culture received a threat from hydrolytic-acidogenic bacteria in the sludge fermentation liquid. Further removal of indigenous microorganisms from the influent successfully restored the dominance of the CE-functional microorganism, enabling sustained caproate production. This study proposes feasible optimization strategies for the CE process for sludge resource recovery.