Denitrification performance and microbial community variation during reverse osmosis concentrate treatment by sulfur denitrification process

Heterotrophic denitrification is widely used as a method for removing nitrate, but incomplete denitri-fication was reported in the case of wastewater having a low C/N ratio. Specifically, the reverse osmosis concentrate (ROC) generated during the water reuse process contains a high concentration of nitrate but a low amount of biodegradable organic carbon for heterotrophic denitrification. In this study, a sulfur denitrification process was considered as an alternative method to eliminate nitrate in ROC. We studied the possibility of microbial community formation in a continuous process and the period required for culture and stabilization. To this end, in this study, a denitrification efficiency according to the initial cultivation methods was compared to determine the effect of the reactor start-up strategy, and the optimal hydraulic retention time was obtained. The denitrification efficiency and the microbial content were very similar in both a continuous reactor and a batch reactor after 30 d. We thus con-cluded that in-situ cultivation in the continuous reactor without batch cultivation was possible. The optimal hydraulic retention time was then determined as 2 h. In order to evaluate the applicability of the ROC, the change of denitrification efficiency was monitored during reactor operation with simulated ROC containing organic matter. The denitrification efficiency significantly decreased due to the growth of heterotrophic microorganisms. However, the efficiency could be restored after 25 d of operation without organic matter. Moreover, the microbial community change was monitored for a better understanding of microbial actions in the sulfur denitrification process. Increased abundance of Thiobacillus thioparus, Sulfurimonas denitrificans, and Sulfurimonas paralvinellae were mainly associated with denitrification performance in the cultivation process.