Impacts of backwashing on micropollutant removal and associated microbial assembly processes in sand filters

Backwashing is crucial for preventing clogging of sand filters. However, few studies have investigated the effect of backwashing on micropollutant removal and the dynamic changes in the microbial community in sand filters. Here, we used a series of manganese and quartz sand filters under empty bed contact times (EBCTs) of 2 h and 4 h to explore variations in micropollutant degradation and temporal dynamics of the microbial community after backwashing. The results showed that the removal efficiencies of caffeine, sulfamethoxazole, sulfadiazine, trimethoprim, atrazine, and active biomass recovered within 2 d after backwashing in both types of sand filters at 2-h EBCT, but the recovery of sulfadiazine and trimethoprim was not observed at 4-h EBCT. Moreover, the removal efficiency of atenolol increased after backwashing in the manganese sand filters, whereas maintained almost complete removal efficiency in the quartz sand filters at both EBCTs. Pearson correlation analysis indicated that microbial community composition gradually recovered to the pre-backwashing level ( R increased from 0.53 to 0.97) at 2-h EBCT, but shifted at 4-h EBCT ( R < 0.25) after backwashing. Furthermore, the compositions of the recovered, depleted, and improved groups of microbes were distinguished by applying hierarchical clustering to the differentially abundant amplicon sequence variants. The cumulative relative abundance of recovered microbes at 2-h EBCT was 82.76 % ± 0.43 % and 46.82 % ± 4.34 % in the manganese and quartz sand filters, respectively. In contrast, at 4-h EBCT, the recovered microbes dropped to 15.55 %–25.69 % in both types of sand filters.