Virus removal in a membrane bioreactor (MBR) by gravity drain was studied. Coliphage f2 (mean size of 25 nm), which is similar in size to human enteric pathogenic viruses, was selected as a model virus. Two microfiltration membrane modules with pore sizes of 0.22 μm and 0.1 μm were applied to investigate the effects of membrane pore size on the virus rejection. The MBR with these modules could reject virus in a range of 2.6–5.1 logs. The experimental results showed that the mechanisms of virus removal in the MBR involved rejection and inactivation. The virus rejection depended mainly on the dynamic layer on the membrane surface (not membrane itself) because virus rejection by the membrane modules with pore sizes of 0.1 μm and 0.22 μm were similar. The microbial activity and the aeration oxidation were the two important factors for the virus inactivation. It was found that the inactivation of coliphage f2 was much more rapidly in activated sludge mixed liquor than in clean water, and the effect of aeration was significant.

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