Investigation of nitrite accumulation by hydrogenotrophic denitrification in a moving bed biofilm reactor for partial denitrification and anammox process

The partial denitrification and anammox (PDA) process has received attention for its ability to optimize treatment of wastewater containing a low NH₄⁺-N concentration. This study investigated the suitable operational conditions for NO₂^--N accumulation by hydrogenotrophic denitrification (HD) in operation of a laboratory-scale moving bed biofilm reactor, for future application in the PDA process. NO₂^--N accumulation was achieved by minimizing the H₂ flow rate under optimized conditions (i.e., 15 mL/min H₂ flow rate, 40 mg-N/L influent NO₃^--N, 7.0 h hydraulic retention time, and 2 L working volume). Hydrogenophaga comprised 39.2% of the bacterial abundance after NO₂^--N accumulated, indicating its contribution to the NO₂^--N accumulation. In addition, an intermittent H₂ supply maintained the NO₂^--N accumulation rate (NAR) and maximized the nitrite accumulation efficiency (NAE). A H₂ supply ratio of 0.7 (i.e., ON: 7 min, OFF: 3 min) was optimal, which induced increases in NAR, NAE, and the NO₃^--N removal efficiency that reached 0.07±0.01 kg-N/m³/d, 64.4±14.5%, and 89.2±8.9%, respectively. The ratio of H₂ supply rate to the NO₃^--N loading rate was calculated as 4.3 in this experiment, which may represent the optimal balance for maximization of NO₂^--N accumulation by HD.