Nitrous oxide (N2O) is one of the most important greenhouse gases

about 265 times stronger than carbon dioxide (CO2)

and it may also destroy the ozone layer. In the biological wastewater treatment process

autotrophic nitrification has been thought to be the major source of N2O production. So it becomes increasingly important to prevent N2O emission from sewage treatment. In this study

by adding NH4+

NH2OH and NO2- to the effluent

the effect of NO2- on N2O production by NH4+ and NH2OH oxidation during the nitritation process was investigated in a laboratory batch-scale system with activated sludge for treating domestic wastewater. Within the first 30 min of NH4+ and NH2OH oxidation process (total test time:180 min) N2O accounted for more than 25% of the total production. As NH4+ or NH2OH was consumed completely

the amount of N2O net production reduced to less than 0.2 mg·L-1 in the last 30 min. Furthermore

the concentration of NO2--N could affect N2O production. The increase of NO2--N would promote generation of N2O. The maximum total N2O net production was 6.86 mg·L-1 when the concentration of NO2- was 60 mg·L-1. NH2OH oxidation played a key role in N2O production at the very beginning of the experiment while nitrifier denitrification became a main pathway later. When the domestic sewage was treated under DO limited conditions

due to the presence of COD

there might occur heterotrophic denitrification during the aeration phase. However

oxygen and NO2- had strong inhibition on the activity of nitrous oxide reductase (NOS)

consequently N2O could not be reduced to N2 completely. It led to more than 17% N2O was produced and the maximum total net production reached 11.07 mg·L-1. Hence

the contribution to N2O produced by denitrification could not be ignored during the domestic wastewater treatment. Besides co-existence of NH4+ and NH2OH could significantly increase N2O production and this process also emitted more N2O when the concentration of NO2- was increased.