Medium chain fatty acids production from anaerobic fermentation of waste activated sludge

The utilization of waste activated sludge (WAS) to recover energy in the form of methane or short-chain fatty acids (SCFAs) is generally restricted by the low energy density of products and poor degradability of WAS. Herein, this study reported a novel alternative WAS fermentation technology to produce high-energy medium-chain fatty acids (MCFAs) from WAS in a one-stage anaerobic fermentation system using ethanol as electron donor. The MCFAs production and WAS degradation at different ethanol levels were investigated. The increased ethanol levels resulted in increasing MCFAs production (from 1875 to 6115 mg chemical oxygen demand (COD)/L) and selectivity (from 30.3 to 56.2%). The main MCFAs products were n-caproate and n-caprylate at lower level of ethanol, while n-caproate was the sole MCFA product at higher level of ethanol with longer chain alcohol (i.e., n-hexanol) produced as well. The ethanol markedly increased WAS degradation, with the greatest degradation (0.72 g COD/g volatile solids (VS)) being 1.9 times of that without ethanol (0.38 mg COD/mg VS, at 0 mmol/L), which was ascribed to the advancement of sludge solubilization, hydrolysis and acidification. Microbial community revealed that the ethanol participation induced the community shift to the favorable direction for hydrolysis-acidification and chain elongation in anaerobic WAS fermentation.