LAUSR

Abstract Flocculent sludge system usually shows a low treatment efficiency and bad resistance to the feeding shock in the hydrolysis acidification. Integrated floating-film and activated sludge (IFFAS) is a favored system to resist shock, and zero-valent iron (ZVI) is an attractive material to enhance hydrolysis acidification. In this study, an IFFAS equipped with ZVI composite carriers was used to stably increase the degree of acidification (DA) in the anaerobic treatment of dairy wastewater. At the influent organic loading rate of 24.0 kg COD/m3/d with the hydraulic retention time of 3 h, the DA in the reactor filled with ZVI composite carriers was as high as 51.6%, which increased by 9.5% and 7.7%, respectively, comparing with the reactor without biofilm carriers and the reactor filled with polyethylene biofilm carriers. Also, the ZVI composite carriers optimized the biofilm formation and enhanced the activity of protease, pyruvate oxidoreductase, and acetate kinase. Microbial community analysis revealed that hydrolysis acidification bacteria of Comamonas (47.0%) and Petrimonas (2.6%), and acetotrophic methanogens of Methanosarcina (47.1%) and Methanothrix (16.0%) were enriched in the biofilm of ZVI composite carriers. Apart from the main enhancement of mediated interspecies electron transfer, a potential direct interspecies electron transfer also might be established for the presence of Comamonas, Petrimonas, Methanosarcina, and Methanothrix, accelerating the syntrophic production of short-chain volatile fatty acids and methane.