To close or open the tank input water valve: Secondary water-supply systems with double tanks will induce a higher microbial risk.

Secondary water supply systems (SWSSs) are widely used to supply water to high-rise households in urban residential buildings. A special mode of double tanks with one used while another was spared was noted in SWSSs, which would facilitate microbial growth due to longer water stagnation in the spare tank. There are limited studies on the microbial risk of water samples in such SWSSs. In this study, the input water valves of the operational SWSSs consisting of double tanks were artificially closed and opened on time. Propidium monoazide-qPCR and high-throughput sequencing were performed to systematically investigate the microbial risks in water samples. After closing the tank input water valve, it may take several weeks to replace the bulk water in the spare tank. The residual chlorine concentration in the spare tank decreased by up to 85 % within 2-3 days compared with that in the input water. The microbial communities in the spare and used tank water samples clustered separately. High bacterial 16S rRNA gene abundance and pathogens-like sequences were detected in the spare tanks. Most antibiotic-resistant genes (11/15) in the spare tanks showed an increase in their relative abundance. Moreover, when both tanks within one SWSS were in use, the water quality of the used tank water samples deteriorated to varying degrees. Overall, running SWSSs with double tanks will reduce the replacement rate of water in one storage tank, and consumers who use taps served by the presented SWSSs may have a higher microbial risk.