Narrow host range phages infect essential bacteria for water purification reactions in groundwater-fed rapid sand filters

•Small yet abundant core phageome detected in biofiltration units.•DWTP phageome influenced by source water type and geographic location.•Phages in Danish RSFs fed by groundwater exhibit a narrow host range.•Phages primarily infect bacteria involved in key biogeochemical reactions in RSFs.•DWTP phages exert top-down control over microbial populations through phage predation.