Characteristics Of N₂O Emission During Autotrophic Nitritation Under Oxygen-limited Conditions

Nitrous oxide(N2O)is one of the greatest potential greenhouse gases,with an approximately 265-fold stronger global warming effect than carbon dioxide,and it is also considered to be the dominant stratospheric ozone depleting substance.The amount of N2 O emitted from wastewater sector accounts for about 3% of the total global emissions,as the sixth largest contributor.Nitrogen compounds in wastewater are mainly removed by nitirfication and denitrification.Several factors have been reported to affect N2 O production by nitrifier,such as DO concentration,NO2-levels and the ammonium(NH4+)loading rate.DO is a very important factor affecting the N2 O production.The variation of DO has a direct effect on the relative contributions of NH2 OH oxidation pathway and AOB denitrification pathway to N2 O production during nitritation.NO2-is also thought to be a critical factor improving N2 O production by AOB denitrification.NH4+is the substrate for nitrification and the electron donor of AOB denitrification.In this experiment,a lab-scale sequencing batch reactor(SBR)with enriched nitrifiers was developed under oxygen-limited conditions with synthetic wastewater(without organic carbon)to investigate the characteristics of N2 O emission.The system achieved 70%conversion of the influent NH4+ to nitrite(NO2-)and the N2 O emission factor(the ratio between N2 O nitrogen emitted and the NH4+oxidized)was about 17.0%.During oxygen-limited autotrophic nitritation,the DO concentration stayed at a plateau of 0.1 mg O2/L for the balance of oxygen delivery and consumption.When the NH4+was exhausted,DO breakthrough occurred for the rapid reduction of oxygen consumption,which implied the end of the autotrophic nitritation.The dissolved N2 O concentration dropped after the commencement of aeration.After 1.8 h of the influent injection,dissolved N2 O concentration started to increase for the consumption of NH4+.When the NH4+concentration was lower than 20 mg N/L,the dissolved N2 O concentration decreased again.The removal rate of ammonia nitrogen remained constant with the variation of influent ammonia concentration,in other words,the oxidative ammonolysis in oxygen-limited autotrophic nitritation was a zero order reaction.N2 O emission increased with the rise of influent NH4+ concentration.When the influent NH4+ concentrations were 60,120 and 240mg/L,the total N2 O emissions were 3.24,8.75 and 24.59 mg/L,respectively.The intermittent aeration model contains aeration periods and non-aeration periods.For the flow of aerator,N2 O was mainly released from aeration periods during the process ofintermittent aeration,the contributions of aeration periods and non-aeration periods to N2 O emissions were calculated as 71.32% and 28.68%,respectively.Compared to the continuous aeration mode,the intermittent aeration model(aeration/non-aeration was 2h:2h)in oxygenlimited autotrophic nitritation led to a lower N2 O emission factor.It has been widely known that there are three main routes for N2 O production during the process of biological wastewater treatment: heterotrophic denitrification,NH2 OH oxidation,and AOB denitrification.The heterotrophic denitrification was very limited for the synthetic wastewater contains no extra organic compound at all.NH2 OH oxidation was not the main contributor to N2 O production either.The ratios between the N2 O emitted by NH2 OH oxidation and the total N2 O emissions were about 25.6%,17.8% and 14.0% at the influent NH4+ of 60,120 and 240 mg N/L,respectively.AOB denitrification was identified to be an important contributor to the N2 O production in oxygen-limited autotrophic nitritation.Take all the arguments above into consideration,conclusion can be drawn that AOB denitrification is the main cause of N2 O emisson in this experiment.