The present study proposes a categorization of bacteria that leak from activated sludge processes to secondary treated water (STW). Bacterial populations in primary treated water (PTW), activated sludge (AS), STW, and the 0.2 μm-filtrate of STW (FSTW) in a full-scale wastewater treatment plant with two treatment trains were observed for a period of one year using a 16S rRNA ana-lysis approach. The taxonomic groups detected were categorized as different "leak types" based on the read occupancies in PTW, AS, STW, and FSTW, where a leak type indicates the likelihood of a taxonomic group to leak to STW. Five leak types were introduced: "LTE", "LTE-I", "LTEF", "LTF", and "NLT", with "LT" for leak type, "E" for high read occupancy in STW or the effluent of secondary settling tanks, "I" for high read occupancy in PTW or influent to the AS process, "F" for high read occupancy in FSTW, and "NLT" for a smaller likelihood to leak. Representative taxonomic groups for each leak type were Neisseria and ABY1 for "LTE" Parcubacteria for "LTEF", Campylobacterota for "LTE-I", and Saccharimonadia, Bdellovibrionota, and some lineages in Comamonadaceae for "LTF". Although some taxonomic groups, such as Comamonadaceae, included different leak types, the categorization assigned to each taxonomic group was mostly consistent between the two treatment trains. The categorization scheme proposed herein may become a useful key for understanding the characteristics of bacteria that appear in AS and STW.