The changes of microbial population structure in biofilm caused by variations of COD/NHNH+4‐N ratio in influent were investigated in a suspended carrier biofilm reactor (SCBR). When COD/NHNH+4‐N was 10, the highest COD removal reached was about 73.8 per cent, whereas the highest NH+4‐N removal, about 55.9 per cent, was reached when COD/NH+4‐N was 5. By using fluorescence in situ hybridization (FISH) and confocal laser scanning microscopy (CLSM) observation, it was found that the thickness of the biofilm formed on the carrier was about 80 to 120 μm, and ammonia-oxidizing bacteria (AOB) and nitrifying bacteria existed mainly in the upper layer about 20 to 30 μm. The results of the quinone profile analysis showed that when the COD/NH+4‐N increased, the microbial diversity (DQ) increased, while the proportions of AOB and nitrifying bacteria in the biofilm communities decreased.

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