Microbial safety evaluation for recycling of sand-filter backwash water in a water plant in Southern China

Sand-filter backwash water (SFBW), a byproduct of filtration, retains particles from the filter, raising microbial safety concerns despite the water conservation benefits. This study investigated the microbial safety implications of recycling SFBW over a year in a conventional drinking water treatment plant in Southern China. The results indicated that the SFBW recycling increased microbial counts, particularly E. coli and opportunistic pathogens (OPs) including Legionella spp., Mycobacterium avium-intracellulare complex, Aeromonas spp., and Pseudomonas aeruginosa. Peak concentrations of E. coli and the OPs in the SFBW reached 3700 MPN/100 mL and 6.34, 5.19, 7.46, and 5.68 log gene copies/L, respectively. Turbidity, dissolved organic carbon, and residual chlorine were crucial water parameters influencing microbial counts. Microbial community analysis revealed more diverse OPs, including Pseudomonas, Novosphingobium, Flavobacterium, Comamonas, and Sphingomonas. Quantitative microbial risk assessment showed a significant increase in annual infection risks for E. coli, MAC, and Pseudomonas aeruginosa in SFBW, exceeding the WHO benchmark (10−3), suggesting potential health risks associated with pathogens. While finished water meets the Standard for Drinking Water Quality (GB 5749-2022), these findings emphasize the need for better management of SFBW recycling, enhanced monitoring of OPs, and prevention efforts to safeguard the health of DWTP operators.