Enhancing treatment performance of anaerobic bioreactor under short hydraulic retention time mediated by chitosan

Abstract

Short hydraulic retention time (HRT) treatment operation offers promising techno-economic advantages; however, the treatment effect on extracellular polymeric substance (EPS), mass transfer, microbial structure, and metabolism still needs further understanding. We examined the treatment performance and microbial community structure using a spiral symmetry stream anaerobic bioreactor (SSSAB) at varied HRT conditions. After each operating period, water quality results indicated that phases I (HRT 12 h), II (HRT 6–2 h), and III (HRT 3.2–5 h) achieved chemical oxygen demand removal efficiencies of 89.7 ± 1.1%, 94.8 ± 0.3%, and 78.9 ± 1.6%, respectively. The NH3–N increasing rates observed in phases I, II, and III were 34.5 ± 5.5%, 33.4 ± 5.7%, and 18.2 ± 2.7%, indicating limited NH3–N removal efficiency in phases I and II. The EPS experimental results revealed that chitosan addition in SSSAB reduced the EPS secretion, notably polysaccharides. This reduction led to an increase in the protein/polysaccharide ratio within the microbial matrix. The altered EPS composition increased the mass transfer resistance between microbes and substrates. Our findings suggest that chitosan acts as a regulatory agent, influencing both EPS composition and microbial metabolism, potentially offering a strategy for controlling microbial activity in bioreactors. The analysis from high-throughput amplicon sequencing revealed that Geobacter and Methanosaeta dominated all bioreactor compartments. Chitosan addition did not inhibit Geobacter and Methanosaeta direct interspecies electron transfer for substrate digestion and methanogenesis.