This study aimed to evaluate the effects of residual ozone and chlorine and the backwash intervals on bacterial activities and density in pilot-scale biological activated carbon (BAC) filters by employing adenosine triphosphate (ATP) measurement and flow cytometry. The BAC filters received water treated by full-scale coagulation-filtration-ozonation processes. The attached bacterial density on the BAC increased rapidly in the first few weeks. The residual chlorine in the influent water caused ca. 1-log reduction of ATP-per-cell in the bacteria attached on the BAC and in the effluent and backwash waters. Extending the backwash interval made the bacteria attached on the BAC more resistant to ozone and chlorine, as suggested by the higher ATP-per-cell of the attached bacteria and the higher percentage of high-nucleic-acid intact bacteria in the backwash water. However, the attached bacterial density was higher for the shorter backwash interval operation than for the longer one during the high-ozone period. Although no significant difference in DOC removal rates was observed between the two BAC filters, fluorescence excitation-emission matrix analysis revealed that high residual ozone decreased the removal of aromatic proteins and soluble microbial product-like compounds. (C) 2015 Elsevier Ltd. All rights reserved.