Performance evaluation of a hydrogen-fed bioreactor treating nitrate containing wastewater under long-term operation

In recent years a number of studies on the optimization of operating parameters on hydrogen-based denitrifying system with several investigations on autohydrogenotrophic bacterial community have been carried out. However, it appears that long-term performance of a hydrogen-based denitrifying system and the characterization of microbial community under various hydrogen flow rates have not adequately been focused on yet. This study is thus aimed at evaluating denitrification efficiency of a continuous-flow autohydrogenotrophic bioreactor, and characterizing changes in influential parameters, sludge characteristics and microbial community under long-term operation at various hydrogen flow rates. A hydrogen-fed bioreactor was operated at hydrogen supply rates of 50, 90 and 30 mL min⁻¹ for 3 months. Stable and complete autohydrogenotrophic denitrification with the nitrogen loading rate in the range of 0.33–0.38 kg N m⁻³ d⁻¹ was obtained when the hydrogen supply rate was at 90 mL min⁻¹, while nitrite accumulation and strong fluctuations in total nitrogen removal efficiencies were observed at lower hydrogen levels. Changes in sludge characteristics, including the production of soluble and bound extracellular polymeric substances and microbial floc size distribution, were monitored. The diversity of the autohydrogenotrophic microbial community and changes at class and phyla levels at varying hydrogen supply rates were analyzed by means of high-throughput sequencing technology. As many as 200 observed operational taxonomic units were identified in the autohydrogenotrophic sludge. The phyla Proteobacteria, Chlorobi and Bacteroidetes were found to account for more than 95% of the denitrifying microbial community, and the classes Alphaproteobacteria, Betaproteobacteria and Deltaproteobacteria appeared as the most abundant taxa in the bioreactor.