Qualitative and quantitative analysis of impacts of microporous, dense and composite membranes on nitrifier/denitrifier distribution and performance of MABRs

Membrane types are known to have a decisive effect on the operational efficiency of membrane-bound reactors. The gas-permeable membrane, serving as the biofilm growth and oxygen transfer medium, plays a vital role in the membrane-aerated biofilm reactor (MABR). However, few studies have thoroughly investigated how membrane types affect MABR performance. Four parallel MABRs were equipped with different membrane types: microporous PTFE (R1), dense silicone (R2), composite A (R3), and composite B (R4) membranes. Results showed that the R3 and R4 exhibited comparable and superior COD and nitrogen removal efficiency relative to R1, maintaining stable transmembrane pressure. XDLVO theory was used to assess the interfacial interaction energy between sludge flocs and membrane surfaces. Polar forces were crucial for sludge adhesion, affecting biofilm formation and growth. Composite B exhibited the lowest energy barrier (2529 KT), leading to the fastest biofilm formation and thicker biofilms, which enhance nitrogen removal. PTFE exhibited the highest energy barrier (9655 KT), resulting in the most extended biofilm cultivation period and the thinnest biofilm. Based on fluorescent in-situ hybridization (FISH), membrane types significantly affect the distribution of nitrogen-removing microbial communities. As the biofilm thickened, the stratification of the nitrifying bacterial population became increasingly pronounced, forming distinct layers within the biofilm. Nitrosomonas and Nitrobacter dominated the biofilm near the membrane, while Nitrospira was most abundant in the outer biofilm. The outermost biofilm layer was predominantly composed of heterotrophic denitrifying bacteria (HDB). The research has shown that different biofilm types exhibit heterogeneity stratification in nitrifying bacteria and HDB. 16S rRNA genes sequencing revealed that Nitrospira was the predominant nitrifying genus within the heterotrophic/autotrophic biofilm.