Potgieter, SarahOosthuizen-Vosloo, SolizeLangenfeld, KathrynDowdell, Katherine S.Vedrin, MatthewLahr, RebeccaPinto, Ameet J.Raskin, Lutgarde2026-02-022026-02-022026-03Potgieter, S., Oosthuizen-Vosloo, S., Langenfeld, K. et al. 2026, 'Biofiltration, seasonality, and distribution system factors influence nitrifier communities in a full-scale chloraminated drinking water system', Water Research, vol. 292, art. 125288, pp. 1-11, doi : 10.1016/j.watres.2025.125288.0043-1354 (print)1879-2448 (online)10.1016/j.watres.2025.125288http://hdl.handle.net/2263/107772DATA AVAILABILITY : Raw sequence reads are available on NCBI at Bioproject number PRJNA1081894.Nitrification in chloraminated drinking water systems has been widely studied, although limited information is available on the role of biofiltration in shaping the nitrifier communities within drinking water distribution systems (DWDS). Additionally, the co-occurrence of comammox and canonical nitrifiers in drinking water systems remains unclear. This study investigates how biofiltration shapes nitrifier communities in a full-scale drinking water system where chloramine is a secondary disinfectant, and biofilters are backwashed with chloraminated water. Samples were collected monthly for one year from biofilter effluent, finished water, and three DWDS sites with varying water ages, water quality, and nitrite concentrations. Nitrifier abundances were quantified using droplet digital PCR, which showed contrasting temporal trends between the ammonia-oxidizing bacteria amoA gene and both nitrite-oxidizing bacteria 16S rRNA gene and comammox amoB gene abundances. Genome-resolved quantitative metagenomics revealed Nitrosomonas cluster 6a species, canonical Nitrospira species, and Nitrospira-like comammox species as the dominant nitrifiers. The same populations were detected in biofilter effluent and across DWDS sites, indicating that biofilter operation contributed to the persistence of nitrifiers in the DWDS. Further, DWDS site-specific factors, such as water age and disinfectant degradation, influenced the presence and abundance of individual nitrifier populations. These findings advance our understanding of how upstream treatment processes influence microbial community structure and nitrifier persistence in full-scale chloraminated DWDSs, and highlight the importance of considering biofilter operation, alongside disinfection practices, within integrated nitrification control strategies. HIGHLIGHTS • Seasonal and spatial factors shaped nitrifier dynamics and nitrite accumulation. • Nitrosomonas cluster 6a, Nitrospira, and comammox co-occurred throughout the system. • Nitrifier MAGs in the biofilter effluent persisted through the distribution system. • Distribution system water age influenced the spatial patterns of Nitrosomonas MAGs. • Comammox showed seasonal variation and persisted post-chloramination.en© 2026 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Drinking water distribution systems (DWDS)BiofiltrationDroplet digital PCRChloraminationDrinking waterNitrificationQuantitative metagenomicsArticle