Biological stability shapes habitat-specific microbial community dynamics from treatment to distribution in non-chlorinated drinking water distribution systems

Maintaining biological stability is essential for high microbiological water quality in drinking water distribution systems (DWDS). Although treatment without residual disinfectant removes biodegradable organic matter to limit microbial regrowth, it remains unclear how improved biological stability influences microbial communities during distribution. We investigated habitat-specific microbial communities in two full-scale DWDS supplied with the same surface water source but treated differently. Conventionally treated drinking water was compared with water produced by managed aquifer recharge and recovery, and the additional effect of ultrafiltration (UF) post-treatment was evaluated. Microbial communities in habitats (bulk drinking water, biofilms and loose deposits) were characterized using 16S rRNA gene amplicon sequencing and analysed together with biological stability, physicochemical and cultivation-based measurements. Improved biological stability reduced regrowth potential and Aeromonas occurrence while altering microbial communities. Treatment established the initial microbial community, but UF-post treatment and storage reshaped this community, increasing cell numbers and promoting the dominance of Comamonadaceae and Brevundimonas . During distribution, loose deposits increasingly contributed to bulk drinking water communities, although this effect was reduced after UF. Loose deposits showed stronger seasonal variation than bulk drinking water. These findings show that biological stability shapes microbial communities and identifies loose deposits as an important habitat for understanding and monitoring regrowth.