The release of heterotrophic plate count (HPC) and particles in ozonation and biological activated carbon (O-3-BAC) processes and related influential factors were studied using a laboratory-scale apparatus in this paper. The results showed that pre-ozonation of BAC influent had a positive effect on the decrease of the particle counts and HPC of BAC effluent. We also found BAC filtration influent particle counts had no effect on the effluent. While the particle counts of the BAC filtration influent varied between 42-58, 170-184 and 248-269 Count/mL (CNT/mL), the particle counts of effluents remained stable in the range 19-23 CNT/mL. In addition, the BAC filtration velocity had little influence on the release of HPC and on the particle counts from BAC, but a sudden increase in the BAC filtration velocity lead to a significant release of HPC and increased particle counts. These hydraulic shocks also made particles with sizes of 5-10 mu m more likely to leak from the BAC filtration. We also found that the maximum particle count and HPC in water initially filtered with BAC were as high as 311 CNT/mL and 4.7 log. These levels took approximately 30 min to decrease to a stable and a normal level, which were approximately 20 CNT/mL and 3.2 log. Finally, the leakage of HPC and the number of particles decreased with increasing depth, with the highest amount of leakage occurring at the top layer of the BAC filter. (C) 2015 Elsevier B.V. All rights reserved.