Nitrifier assemblages and dynamics in secondary water supply systems: Predominance of comammox – Nitrospira in tank biofilm and sediment

Nitrifiers in chloraminated drinking water distribution systems can trigger severe nitrification, resulting in subsequent water quality deteriorations. However, the occurrence and dynamics of nitrifiers in secondary water supply systems (SWSSs), an important water supply component beyond the distribution mains, remain largely unexplored. This study investigated the density, distribution, and diversities of nitrifiers in different microbial habitats (water, biofilm, and sediment) in SWSSs with different characteristics. Quantitative polymerase chain reaction analysis indicated higher gene copy numbers of ammonia-oxidizing archaea, ammonia-oxidizing bacteria, comammox clade A, and strict nitrite-oxidizing bacteria in SWSSs relative to the distribution mains (1.3–2.1 log10GC/mL increase, P < 0.05). Comammox were preferentially enriched in biofilm and sediments of SWSS tanks, highlighting biofilm and sediment as important ecological niches for comammox. Illumina sequencing of 16S rRNA genes revealed the presence of diverse nitrifiers (e.g., Nitrospira, Nitrosomonas, Nitrotoga, and Nitrobacter) in SWSSs, with distinct bacterial nitrifier communities noted in SWSS water compared to the distribution main water (analysis of similarity (ANOSIM), P < 0.05). Different bacterial nitrifier communities were noted between water and sediments in SWSS tanks, indicating niche segregations of nitrifiers (ANOSIM, P < 0.05). The results highlight SWSS as an important reservoir of nitrifiers and provide insights into nitrification monitoring and control strategies in SWSSs.