The bacterial community composition of the full-scale biologically active, granular activated carbon (BAC) filters operated at the St. Paul Regional Water Services (SPRWS) was investigated using Illumina MiSeq analysis of PCR-amplified 16S rRNA gene fragments. These bacterial communities were consistently diverse (Shannon index, >4.4; richness estimates, >1,500 unique operational taxonomic units [OTUs]) throughout the duration of the 12-month study period. In addition, only modest shifts in the quantities of individual bacterial populations were observed; of the 15 most prominent OTUs, the most highly variable population (a Variovorax sp.) modulated less than 13-fold over time and less than 8-fold from filter to filter. The most prominent population in the profiles was a Nitrospira sp., representing 13 to 21% of the community. Interestingly, very few of the known ammonia-oxidizing bacteria (AOB; <0.07%) and no ammonia-oxidizing Archaea were detected in the profiles. Quantitative PCR of amoA genes, however, suggested that AOB were prominent in the bacterial communities (amoA/16S rRNA gene ratio, 1 to 10%). We conclude, therefore, that the BAC filters at the SPRWS potentially contained significant numbers of unidentified and novel ammonia-oxidizing microorganisms that possess amoA genes similar to those of previously described AOB. P ublic water utilities use surface water to produce high-quality, potable drinking water for almost 70% of the people residing in the United States (1). The conventional process for treating surface water involves a series of unit operations that include coagulation, flocculation, sedimentation, filtration, and disinfection (2). The application of these technologies has generally proven effective for protecting public health (2), although numerous exceptions have been reported (3). Given the presumptive safety of public water supplies, water consumers are increasingly concerned about the taste and odor of public water supplies as well as other esthetic concerns (4).Public water utilities, therefore, have begun to augment their treatment processes to specifically remove taste-and odor-causing compounds, such as geosmin and 2-methylisoborneol (5-7). Although a few alternative technologies can be used, filtration using biologically active, granular activated carbon (BAC) has proven successful because of its simplicity (i.e., it is easily retrofitted into preexisting operations) and effectiveness (8, 9). Filtration with BAC differs from filtration with conventional granular media (e.g., sand and anthracite) because of its high sorptive capacity (6, 8) as well as the active biofilm that grows on its surface (10-13). The biofilm on the BAC filter medium provides direct biodegradation of dissolved geosmin as well as biological regeneration of the filter medium by metabolizing the initially sorbed geosmin as it desorbs from the carbon (8,14).In this study, we investigated the bacterial community dynamics of the full-scale BAC filters at the Saint Paul Regional Water Surfaces (SPRWS) in St. Pau...