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The performance of a biofilter for off-gas treatment relies on the activity of microorganisms and adequate O2 and H2O.In present study,a microelectrode was applied to analyze O2 in polyurethane foam cubes (PUFCs) packed in a biofilter for SO2 removal.The O2 distribution varied with the density and water-containing rate (WCR) of PUFCs.The O2 concentration dropped sharply from 10.2 to 0.8 mg/L from the surface to the center of a PUFC with 97.20％ of WCR.The PUFCs with high WCR presented aerobic-anoxic-aerobic areas.Three-dimensional simulated images demonstrated that the structure of PUFCs with high WCR consisted of an aerobic shell and an anoxic core,with high-density PUFCs featuring a larger anoxic area than low-density PUFCs.Moreover,the H2O distribution in the PUFC was uneven and affected the O2 concentration.Whereas aerobic bacteria were observed in the PUFC surface,facultative anaerobic microorganisms were found at the PUFC core,where the O2 concentration was relatively low.O2 and H2O distributions differed in the PUFCs,and the distribution of microorganisms varied accordingly.