PARTIAL NITRIFICATION UNDER OXYGEN LIMITED CONDITIONS RESULTS IN SIGNIFICANT GREENHOUSE GAS PRODUCTION

Biological nitrogen removal based on partial nitrification and denitrification via nitrite results in significant reductions in operating cost in aeration power (nitrification) and in electron donor (denitrification) relative to conventional nitrification and denitrification via nitrate. Partial nitrification has most commonly been achieved by controlling operating conditions such as dissolved oxygen (DO at 1 mg O2/L), solids retention time (SRT at 1–2 d), temperature (at 25–37°C) and pH (at 7.5–8.0) (Ciudad et al.; Garrido; Hao and van Loosdrecht; Jianlong and Ning). Despite the obvious benefits of partial nitrification, very few studies have systematically investigated the triggers for possible deleterious generation of greenhouse gases such as nitrous oxide (N2O), nitric oxide (NO) and nitrogen dioxide (NO2) from partial nitrification reactors. In our study, long term (t=232d) partial nitrification was achieved with stable nitrite accumulation (80±18%, n=97) and ammonia removal (82±18%) at reactor DO between 1.0 and 2.2 mg-O2/L and at operating SRT of 3d. During stable partial nitrification, specific ammonia oxidation activity measured as specific oxygen uptake rate (sOUR) was consistently 4–30 times as high as sOUR for nitrite oxidation suggesting significant NOB limitation. In concurrence, using quantitative polymerase chain reaction (qPCR) AOB were shown as the predominant microbial population in the partial nitrification reactor, varying in the range 0.86–11.4 ug/ml as total DNA. Significant NO production as much as 0.9 ppm was observed during steady state partial nitrification. Reactor nitrite concentrations were the strongest positive trigger for biological NO generation. In contrast, reactor operating DO concentrations were a weaker and negatively correlated with NO generation. Interestingly, we also showed that abiotic chemo-denitrification did not contribute significantly to NO production. Therefore, although more cost effective compared to conventional BNR, partial nitrification based BNR can result in significant emission of nitric oxide and similar nitrogenous greenhouse gases, likely due to ammonia oxidizing bacteria activity).