Effect of inoculum percentage and hydrogen supply on hydrogenotrophic denitrification driven by anaerobic granular sludge

Hydrogenotrophic denitrification (H2Den) is an encouraging biological technology to remove nitrate (NO3−) from supply water with a low carbon/nitrogen ratio or in the absence of organic carbon. This study provides important insights into the use of anaerobic granular sludge for NO3− removal from a synthetic water with an initial concentration of 200 mg NO3− L−1 (i.e., 45.2 mg NO3−-N L−1). This study investigated the effect of the inoculum input, expressed as percentage of reactor filling, i.e., 10% vs. 20% vs. 40% (v/v) by the anaerobic granular sludge, as well as the hydrogen (H2) supply, i.e., stoichiometric vs. 50% excess vs. 100% excess, on the H2Den process. Coupling 10% (v/v) inoculum percentage with 100% excess of H2 supply was the most favourable condition, ensuring a NO3− removal efficiency of up to 96%. Indeed, a 10% (v/v) inoculum percentage ensured the maximal denitrification rate, reaching 6.0 mg NO3− g−1 VS d−1, which was further enhanced when increasing the H2 dosage. Despite the great potential, this study also highlighted possible drawbacks of the anaerobic granular sludge-driven H2Den process, such as nitrite (NO2−) accumulation as a denitrification intermediate. On the other hand, the release of gaseous denitrification intermediates such as N2O and NO was negligible under most of the investigated experimental conditions.