Development of a hydrogenotrophic denitrification system for recirculating aquaculture

Hydrogenotrophic denitrification (HD) utilizes H₂ as an electron donor, and its application in recirculating aquaculture systems (RASs) can generate a denitrification system with low toxicity to fish. However, only a limited number of studies have applied HD to RASs; therefore, major challenges in developing HD systems remain. We developed a closed, unpressurized, trickling filter-type HD system and evaluated its efficacy for NO₃^--N removal and fish growth in a lab-scale RAS culturing adult Oncorhynchus mykiss. The maximum nitrogen removal rate was 504.0 g N/m³/d at a hydraulic retention time of 1 h. Under these conditions, the HD system required 5.5 L of H₂ and 2.1 L of CO₂ to remove 1 g of nitrogen. Thauera and Dechloromonas were the dominant taxa in the HD system, accounting for 30.6 % and 33.4 % of the total bacterial abundance after the experiment, respectively. Moreover, the denitrification-related genes narG, nirK, nirS, and nosZ exhibited increased abundance, indicating the dominant bacterial contribution to denitrification. Furthermore, recirculation of the HD system effluent to the fish culture tank did not negatively affect fish growth because the total fish weight increased and mortality was not detected during the approximately 100-day culturing experiment. These results can facilitate the development of a new biocompatible denitrification system based on HD for RASs.