Investigation of volatile fatty acids production in biological hydrolysis of waste activated sludge via microbial community network and fermentation pathway analyses

The functional pathways of volatile fatty acids production at different biological hydrolysis temperatures and times were investigated in terms of microbial profile, network analysis, and phylogenetic investigation of communities by reconstruction of unobserved states. The principal component analysis result showed that biological hydrolysis at 35 degrees C and 42 degrees C were good for propionate production, and biological hydrolysis at 55 degrees C facilitated acetate production within 4.5 days of biological hydrolysis time but increased butyrate production with the biological hydrolysis time beyond 4.5 days. Network analysis revealed that the increased abundance of Chitinophagales and Flavobacteriales reduced the amount of propionate, butyrate, and iso-butyrate but increased the amount of acetate at biological hydrolysis 35 degrees C, 42 degrees C, and 55 degrees C. Moreover, compared to Clostridiales, Bacteroidales is more specific for other bacteria in the biological hydrolysis of waste activated sludge. Thirty-three enzymes were identified in the primary volatile fatty acid metabolism by phylogenetic investigation of communities by reconstruction of unobserved states. Acetate was produced dominantly through acetyl-CoA pathway and regulated by the functional genes atoA and frhG, and propionate was regulated by the functional gene sucD and acs via succinyl-CoA pathway; Butyrate was generated by enriching the functional genes ptb, buk, and atoA. These genes were regulated by biological hydrolysis temperatures, resulting in variations in the distribution of acetate, propionate, and butyrate under different temperature conditions. This study is significant for the optimization of biological hydrolysis for the pretreatment of waste activated sludge.