Multidimensional insights into biofilm ecology and function of a denitrifying anaerobic membrane biofilm reactor with limited hydrogen supply

The anaerobic membrane biofilm reactor (An-MBfR) using dead-end hollow fiber membranes (HFMs) inevitably suffers the limited supply of gaseous electron donors to biofilms, as a result of the back-diffusion of inert gases. The microbial mechanisms, underlying the biofilm formation and decontamination performance of the An-MBfR disadvantaged by limited active gas supply, are still obscure in the literature. Herein, we investigated the evolution laws of biofilm ecology and function in a denitrifying H₂-based An-MBfR, from a multidimensional perspective. Results showed that despite the operating parameters of the reactor were set at the optimal values, the ununiform biofilms were developed on the HFMs, exhibiting a variation trend that with increasing distance from the near-gas end, the thickness and biomass of biofilms were decreased accompanied by their morphological change from the compacted to loosened. As hydrogenotrophic denitrifying bacteria (DNB) suffered limited H₂ supply to the biofilm, they could not produce abundant extracellular polymeric substances (EPS) and result in a high ratio of protein/polysaccharide (PN/PS) ratio in the EPS to facilitate the biofilm growth; their proliferation slowed down, especially in the outer layer of the biofilm at the far-gas end. The propagation of heterotrophic DNB was more active in the outer layer rather than inner layer of biofilms, ascribed to the abundant presence of PN and PS as well as increased NO₃⁻ availability. The variation trends in abundance of functional genes pertinent to the biofilm formation and NO₃⁻ reduction coincided well with the evolution laws of biofilm characteristics and DNB distribution. The findings provided mechanistic insights into the biofilm structure and microbial interaction in the denitrifying An-MBfR.