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Biological activated carbon (BAC) filtration is an effective technology for the removal of natural organic matter. However, one potential drawback of BAC, especially old BAC, is that effluents can contain soluble microbial products released from the biofilm, which are recognized as more toxic nitrogenous DBPs (N-DBPs) precursors. So far, limited studies reported the risk of DBP formation potentials (FPs) increase caused by the microbial leakage of BAC. This study compared removal differences of DBP FPs between two BAC filters operated for 1 year and 8 years in a drinking water plant. The results showed that the total summed haloacetic acid FPs and trihalomethane FPs decreased by 34.31% from chlorination, and 55.01% of the total summed halogen acetonitrile FPs from chloramination were removed by the new BAC. However, Chlorinated haloacetonitriles FPs increased by 2.33% after old BAC filtration. To sum up, BAC filtration decreased most DBP FPs, but a potential risk regarding more toxic N-DBP FPs from old BAC should receive more attention.