Nitrous oxide production under varied C/N-ratio and DO in an SBR treating digester supernatant

Nitrous oxide (N2O) is a greenhouse gas formed during biological wastewater treatment under aerobic and anoxic conditions. Two reasons for high N2O emissions are low oxygen concentration during nitrification and low carbon/nitrogen-ratio during denitrification. In this full-scale study of N2O emissions from a sequencing batch reactor for treating digester supernatant, the oxygen concentration was reduced stepwise and the carbon/nitrogen-ratio was varied to investigate how N2O emissions were influenced. N2O concentrations were measured online in water and off-gas. A distinct relationship was found between low oxygen concentrations and high N2O emissions. N2O was formed in water during both nitrification and denitrification. Decreased oxygen concentration during nitrification led to increased nitrite concentration, which in turn led to increased N2O concentration in the subsequent denitrification phase. When nitrification resumed, accumulated N2O was stripped off to the atmosphere. A too low carbon/nitrogen-ratio resulted in increased N2O emissions in off-gas under longer-term conditions and increased risk of separate peak emissions. Very high N2O concentrations, over 76,000 ppm, were measured in the off-gas. Furthermore, the maximum N2O emitted during one cycle represented 217.9% of the total nitrogen load (36.2% of total nitrogen present in the bulk liquid at cycle start), among the highest emission levels measured from a full-scale municipal plant treating digester supernatant.