Alkaline fermentation of refinery waste activated sludge mediated by refinery spent caustic for volatile fatty acids production

Volatile fatty acids (VFA), produced from waste activated sludge (WAS), provide unique opportunities for resource recovery in wastewater treatment plants. This study investigates the potential of refinery spent caustic (RSC) on VFA production during refinery WAS (RWAS) alkaline fermentation. The highest VFA yield was 196.3 mg/g-VS at a sludge retention time of 6 days. Amplicon sequencing revealed the enrichment of Soehngenia (20.21%), Bacilli (11.86%), and Brassicibacter (4.17%), which was associated with improved activities of protease (626%) and α-glucosidase (715%). Function prediction analysis confirmed that acetyl-CoA production and fatty acid biosynthesis were enhanced, while fatty acid degradation was inhibited. Accordingly, hydrolysis, acidogenesis, and acetogenesis were improved by 6.87%, 10.67%, and 28.50%, respectively; whereas methanogenesis was inhibited by 28.87%. The sulfate and free ammonia in RSC likely contributed to increased acetic acid production. This study showcases that RWAS alkaline fermentation mediated by RSC for VFA production is the practicable approach.