Long-term hydrogenotrophic denitrification of high-nitrate synthetic and spring water in dual-recirculation UASB reactors

Hydrogenotrophic denitrification (H2Den) using hydrogen gas (H2) as an electron donor and anaerobic granular sludge represents an effective strategy for nitrate (NO3javax.xml.bind.JAXBElement@7f88b0ec) removal from organic carbon-deficient waters. This study investigates a novel-designed up-flow anaerobic sludge blanket (UASB) reactor, which exploits a dual-recirculation system (for both liquid and gaseous phases) to enhance the H2Den process. Two reactors were operated in biological duplicate for 420 days under varying conditions, including different hydraulic retention times (HRTs) and H2 supply amounts. Optimal conditions (i.e., 200% excess H2 and 1 day HRT) resulted in minimal nitrite (NO2-) accumulation (< 0.5 mg NO2javax.xml.bind.JAXBElement@41430cd0/L), and a NO3- removal efficiency up to 99% maintained over time, with a denitrification rate up to 166.6 ± 61.8 mg NO3-/L/d, also when validating the process in the presence of a low organic carbon-to-nitrogen spring water amended with NO3-. As regards greenhouse gas emissions, the nitrous oxide (N2O) emission rate under optimal conditions was 5.3·10-3 μg N2O/min. Microbial community revealed an enrichment of the Comamonadaceae (i.e., from 1.7% to 19.3%) family, known to be actively involved in hydrogenotrophic denitrification, as well as of Spirochaetaceae (i.e., from 3.3% to 19.3%), frequently detected in anoxic environments but with a yet unexplored potential for nitrate removal.