Restart performance of prolonged stopped down-flow hanging sponge reactor for treating of low-strength domestic sewage

The down-flow hanging sponge (DHS) reactor, a type of trickling filter used as a sponge, is a promising treatment system for low-cost sewage treatment. However, the performance of dried sponges that have been non-operational for 8 months has not been evaluated. Accordingly, herein, the re-startup process performance of a DHS reactor treating low-strength domestic sewage in Thailand that had stopped operation for over eight months was investigated. The dried sponge carrier immediately retained water in the sponge biomass-retained carrier and absorbed organic compounds. High nitrification activity of the DHS reactor was observed after 13 d, which was the same as that of the pure sponge biomass-retained carrier start-up. The tracer test results showed that the restarted DHS reactor had a high hydraulic retention time (HRT)/theoretical HRT ratio of 134.2 % at the beginning of the operation from the water retention property of the sponge carrier. Microbial community analysis showed that nitrifying microorganisms were effectively recovered in the biomass-retained sponge carrier. These results demonstrate that the sponge carriers in the DHS reactor that were left into dry condition are available for reuse and has high potential for rapid re-startup. •Restart performance of the DHS reactor was evaluated.•The sponge carrier quickly absorbed water and organic compounds.•High nitrification activity in the DHS reactor was observed after 13 days.