Nitrate removal with low N2O emission by application of sulfur denitrification in actual agricultural field

Sulfur denitrification was applied to the agricultural field and the characteristics of the treatment were evaluated from the viewpoints of nitrate removal efficiency and nitrous oxide (N₂O) emission. Two actual sites where sulfur denitrification was performed were surveyed. One is a valley bottom field, where groundwater contaminated with nitrate is coming up as spring water. The nitrate concentration in influent was about 45 mgN/L. The other was wastewater from a plastic greenhouse. The nitrate concentration in inflow water was about 200 mgN/L. Nitrate was almost removed by the containers packed with sulfur (S⁰)- CaCO₃ blocks in both sites. Increase of sulfate indicated that nitrate was removed by sulfur denitrification. This was also estimated stoichiometrically from the relationships between the removed nitrate and produced sulfate. The N₂O was supersaturated in water at most sampling points and the highest concentration of dissolved N₂O reached 900 μgN/L in Saitama in March. It seemed that insufficient nitrate removal caused accumulation of intermediates during denitrification, such as nitrite and N₂O, in this month. However, the emission ratio of N₂O to the removed nitrate during these processes was kept low, ranging from 0.01 to 0.19%, at both two sites throughout all surveys.