Unraveling the microbial characteristics in different microbial habitats within the secondary water supply systems

The "one-spare and one-used" double tank configuration in secondary water supply systems (SWSSs) ensures water supply for high-rise (≥6) residential buildings. Water stagnation in storage tanks, especially spare tanks, promotes microbial growth, biofilm formation, and sediment accumulation. However, microbial characteristics across different habitats (water, loose sediment, biofilm) in these two tank types remain poorly understood. To address this gap, three residential areas along a drinking water distribution system were selected. The microbial characteristics in water, sediment, and biofilm from both used and spare tanks were systematically investigated using high-throughput sequencing and qPCR methods. Results showed slightly lower residual chlorine in spare tanks (p > 0.05). qPCR revealed high microbial biomass and diverse pathogens in sediment and biofilm from both tank types. Notably, only in spare tank biofilms was the biomass of some pathogenic microorganisms significantly higher than in used tanks (p < 0.05). In both tank types, sediment samples exhibited significantly higher Shannon and Richness indices than water and biofilm (p < 0.05), with a distinct microbial community composition. FEAST source tracking indicated low contributions of sediment and biofilm to water microbiota (6-23%), biofilm contributing about three times more than sediment. Redundancy analysis and Spearman correlation showed strong correlations between water quality parameters and bulk water microbes, but weak correlations with sediment/biofilm communities. Additionally, loose sediment was much more abundant in spare tanks, and suspended particulate matter protected adsorbed microorganisms from 0.5 mg/L residual chlorine in a simulated experiment. Given these distinct microbial characteristics, timely tank cleaning is proposed as an effective measure to ensure SWSS water safety. Overall, this study provides in-depth insights into microbial characteristics across habitats in double-tank SWSSs, offering data-driven support for water safety management.