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Simulated drinking water distribution system (DWDS) treated with O3-BAC-Cl2 (ozonebiological activated carbon-chlorine) was constructed to study its effects on the regrowth of five typical opportunistic pathogens (OPs).It was found that O3-BAC-Cl2 could significantly reduce the regrowth of target OPs in the effluents of DWDS compared with Cl2 and O3-Cl2 with the same residual chlorine levels.However,the effect of O3-BAC-Cl2 on the average numbers of target OPs gene markers in the biofilms of DWDS was not apparent,suggesting that OPs in the biofilms of DWDS were tolerant to the upstream disinfection process.The quantification of target OPs in the BAC-filter column demonstrated that OPs decreased with the increase of depth,which was likely due to the organic nutrient gradient and microbial competition inside the BAC-filter.Increase in the ozone dose could further reduce the OPs at the bottom of the BAC-filter.Spearman correlation analysis demonstrated that some significant correlations existed between target microorganisms,suggesting potential microbial ecological relationships.Overall,our results demonstrated that the BAC-filter may act as a “battlefield” suppressing the OPs through microbial competition.O3-BAC-Cl2 could be an effective multibarrier process to suppress the proliferation of OPs in the bulk water of DWDS.However,OPs protected by the biofilms of DWDS should receive further attention because OPs may be detached and released from the biofilms.