The effects of specific oxygen uptake rate (sOUR) ratio between AOB and NOB on seasonal N2O gas production from full-scale wastewater treatment plant (WWTP)

Abstract

Nitrous oxide (N2O) is one of the main greenhouse gases from wastewater treatment plants (WWPTs). It has 298 carbon dioxide (CO2) equivalents of global warming potential with a long atmospheric lifetime (~120 years). The 90 % of N2O is produced at biological nitrogen removal (BNR) system. The nitrification process in aerated zones is predominant at N2O production than denitrification. The mechanism of N2O production during nitrification can be classified with ammonia oxidizing bacteria (AOB) denitrification and hydroxylamine (NH2OH) oxidation, which are oxygen-independent. The factors, which affects to N2O production by AOB denitrification and NH2OH oxidation, are nitrite (NO2-) concentration and ammonium (NH4+) oxidation rate. If the activities of AOB is higher than that of nitrite oxidizing bacteria (NOB), accumulation of NO2- and NH4+ oxidation rate will be promoted. To investigate the N2O production from WWTP by activity of nitrifying bacteria, the WWTP, which use sequencing batch reactor (SBR) for BNR system, was selected. During the cycle of SBR, dissolved oxygen (DO) concentration was maintained higher than 3 mg-O2/L, thus the nitrification was assumed as predominant process than denitrification process. Specific oxygen uptake rate (sOUR) was used as the representation of activities of AOB and NOB, and NO2- concentration and NH4+ oxidation rate were also considered. At the beginning of the cycle, NH2OH oxidation is seems to be predominant than AOB denitrification, on the other hands, the AOB denitrification is more dominant after the accumulation of NO2-. In cycle variation, the sOUR ratio between AOB and NOB was increased with NH4+ oxidation rate, and NO2- accumulation was occurred. After the NH4+ concentration was decreased, the sOUR ratio and NO2- concentration was also decreased. The N2O production rate from SBR was also shown with similar pattern with sOUR ratio, NH4+ oxidation rate and NO2 concentration. In seasonal variation, the sOUR ratio was also shown with similar pattern of N2O production rate. NH4+ loading rate and temperature are considered as the factors, which are affects to sOUR ratio and seasonal N2O emission. It is suggested by this study that the sOUR ratio can represent N2O production during nitrification as a new indicator.