Understanding nitrous oxide production by ammonia oxidizing bacteria in an enriched nitrifying sludge

Abstract

Nitrous oxide (N2O) is not only a potent greenhouse gas with approximately 300 times global warming potential of carbon dioxide (CO2), but it is also a major sink for stratospheric ozone.Wastewater treatment systems are a recognized source of N2O. During biological wastewater treatment, N2O is mainly generated from biological nitrogen removal (BNR), which involves both nitrification and denitrification. Recently, ammonia-oxidizing bacteria (AOB) are identified as the major contributor to N2O production in wastewater treatment plants. However, the mechanisms of N2O production by AOB are still not fully understood. This thesis aims to experimentally assess and mathematically model the effect of several key operational parameters on N2O production by AOB as well as the contributions of different N2O production pathways to total N2O emission.In order to achieve the aim, a nitrifying culture comprising primarily AOB and nitrite oxidizing bacteria (NOB) was enriched in a sequencing batch reactor (SBR). The reactor was fed with synthetic water containing 1 g N/L ammonium (NH4 + ) and performed complete nitrification with 100% conversion of NH4 + to nitrate (NO3 -). This thesis firstly investigated the effect of dissolved oxygen (DO) on the N2O production by the enriched nitrifying sludge. On this occasion nitrite accumulation was minimised by augmenting nitrite (NO2 -) oxidation through the addition of an enriched NOB sludge into the batch reactor. It was demonstrated that the specific N2O production rate (N2OR) increased from 0 to 1.9 ± 0.09 (n=3) mg N/hr/g VSS with an increase of DO concentration from 0 to 3.0 mg O2/L, whereas the N2O emission factor (the ratio between N2O nitrogen emitted and the ammonium nitrogen converted) decreased from 10.6 ± 1.7% (n=3) to 2.4 ± 0.1% (n=3). The site preference measurements, which is calculated as the difference between central and terminal nitrogen isotopomer signatures in N2O molecule, indicated that both the AOB denitrification and hydroxylamine (NH2OH) oxidation pathways contributed to N2O production, and DO had an important effect on the relative contributions of the two pathways. This finding is supported by analysis of the process data using an N2O model integrating both pathways. As DO increased from 0.2 to 3.0 mg O2/L, the contribution of AOB denitrification decreased from 92% − 95% to 66% − 73%, accompanied by a corresponding increase in the contribution by the NH2OH oxidation pathway. IIThe combined effect of NO2 -and DO on N2O production by AOB was further studied using the enriched nitrifying culture. At each investigated DO level, both the biomass specific N2O production rate and the N2O emission factor increased as NO2 -concentration increased from 3 mg N/L to 50 mg N/L. However, at each investigated NO2 -level, the maximum biomass specific N2Oproduction rate occurred at DO of 0.85 mg O2/L, while the N2O emission factor decreased as DO increased from 0.35 to 3.5 mg O2/L. The analysis of the process data using the two-pathway N2O model indicated that the con...