Effect of salinity on nitrous oxide emission in the biological nitrogen removal process for industrial wastewater

The effects of wastewater salinity on both nitrogen removal efficiency and N₂O emission rate were investigated in a single nitrification process, a single denitrification process and an anoxic-oxic activated sludge process. In the single nitrification process, by increasing the salt concentration from 1.0 to 2.0 wt%, the N₂O conversion ratio in the steady state increased by 2.2 times, from 0.22 to 0.48%. In the single denitrification process, a minimal change in the N₂O conversion ratio was observed in the steady state even when the salt concentration was increased from 3.0 to 5.0 wt%. From the results of the anoxic-oxic activated sludge process, it was found that a salt concentration increase from 1.6 to 3.0 wt% significantly increases the N₂O conversion ratio from 0.7 to 13%. It is suggested that an increase in salt concentration markedly influences N ₂O emission both directly and indirectly via the inhibition of N ₂O reductase activity. The indirect inhibition is due to the high concentration of dissolved oxygen which is transported from the oxic tank to the anoxic tank through the circulated liquid. Thus, the salt concentration should be maintained below 3.0% to suppress N₂O emission in an anoxic-oxic activated sludge process.