The connection between aeration regimes and EPS composition in nitritation biofilm

The effects of aeration regimes (intermittent and continuous aeration) on nitritation performance and biofilm EPS composition were evaluated in moving bed biofilm reactors (MBBRs), and a hypothesis that the aeration regimes affect EPS composition by affecting the microbial activity and sludge discharge content was proposed. The effluent NO₂^-/NH₄⁺ ratio corresponded to that of an anammox reaction (1.07 ± 0.20) for the MBBR with continuous aeration (MBBR_con.), while that in the MBBR with intermittent aeration (20 min on/15 min off) (MBBR_int.) was relatively lower (0.75 ± 0.19). Furthermore, the activity of ammonia-oxidizing bacteria in MBBR_con. was 0.4-7.9 mg-N·L^-1·h^-1 more than that in MBBR_int., which was consistent with the lower proportion of dead cells in MBBR_con. compared with MBBR_int. (9.4% vs. 31.8%). The higher microbial activity in MBBR_con. led to more sludge discharge than MBBR_int., which was reflected in the higher biofilm detachment rate in MBBR_con. compared with MBBR_int. (0.15 ± 0.02 vs. 0.11 ± 0.02 g m^-2·d^-1). The ratio of humic substances to polysaccharides in the EPS was high (0.96 ± 0.08) in the detachment biomass, while the ratios in the nitritation biofilm on carriers from MBBR_con. and MBBR_int. were 0.52 ± 0.13 and 0.72 ± 0.16, respectively. We hypothesized that biofilm matrix with high ratios of humic substances to polysaccharides are structurally unstable and prone to fall off. In addition, the higher proportion of dead cells in MBBR_int. made the proportion of humic substances in EPS high. Meanwhile, less sludge discharge in MBBR_int. than MBBR_con. caused more humic substances to accumulate in the biofilm. These was responsible for the higher ratio of humic substances to polysaccharides in MBBR_int. compared with MBBR_con. The findings elucidate the connection between aeration regimes and biofilm EPS composition, and guide the choice of aeration regimes in the design of biofilm reactors for partial nitritation.