Alkalinity requirements and the possibility of simultaneous heterotrophic denitrification during sulfur-utilizing autotrophic denitrification

Alkalinity requirement and the possibility of simultaneous heterotrophic denitrification during sulfur-utilizing autotrophic denitrification were evaluated with sulfur packed bed reactors (SPBRs). SPBR showed >99% NO₃^--N removal efficiency at influent NO₃^--N concentration of 1,500 mg/L, although 25-40% of the added NO₃^--N was recovered as N₂O. Complete denitrification without N₂O production was achieved when the influent NO₃^--N concentration decreased to 750 mg/L. When nitrified landfill leachate containing 602–687 mg/L of NO₃^--N was fed to SPBR, denitrification efficiency was greater than 98%. During leachate treatment, alkalinity consumption was 3.25–3.76 g CaCO₃/g NO₃^--N removed. Most of denitrification activity occurred within bottom 11.5 cm of sulfur layer, meaning that effective HRT of 2.34 hours was enough for the complete denitrification at the loading rate of 2.2 kg NO₃^--N/m³-day. Complete denitrification was also achieved when methanol was added to nitrified leachate without alkalinity addition. In this case, alkalinity produced by heterotrophs was used for sulfur-utilizing denitrification.