Assessment of N2O emissions from an SBR plant with aerobic granular sludge technology on pilot scale


Wastewater treatment plants (WWTPs) are evolving towards a more sustainable manner, by which not only the effluent quality and operational costs but also the greenhouse gases (GHG) potential is incorporated into the assessment inventory. GHG emissions from the WWTPs include CH4, CO2 and N2O, of which the N2O is of special interest due to 265-fold CO2-equivalent. Thus, even a low amount of N2O is undesired. Aerobic granular sludge (AGS) is a promising biological nutrient removal technology due to considerable structural and microbiological distinctions compared with conventional activated sludge (CAS) flocs, leading to huge improvements of carbon footprint saving. Nevertheless, the N2O formation from the AGS reactor is likely higher than that from the CAS, in terms of sequence batch reactor (SBR) configuration and inherent complex mechanism. In addition, there wasn’t any long-term monitoring campaign on the AGS reactor focusing on N2O emissions so far.This study focusses on a N2O emission online monitoring campaign, which was carried out in a Nereda® AGS reactor treating domestic wastewater from the Berlin region, lasted more than 6 months, including two different phases, namely feeding with pre-treated and raw wastewater after aerated sand trap and 2mm sieve box. The off-gas was sucked from the top of the SBR reactor and measured with online gas analyzer. Then the emission factor (EF) was calculated based on the correlated influent nitrogen load, which was converted from the influent NH4-N concentration by fixed ratio of 0.8. During the first phase, the EF was equal to 2.97%, while during the second phase, the EF was equal to 4.52%. Generally, the EF calculated in terms of both phases was 3.71%. Compared with other long-term campaigns based on CAS and SBR processes, higher GHG potentials could be induced, which also challenges the predominance of the AGS reactors from the perspective of minimizing GHG when only considering the energy consumption into scope. In-depth analysis indicated that the hydroxylamine oxidation pathway was the most likely over the monitoring course and EF calculated during main aeration incorporate negligible fraction of N2O produced from the pre-denitrification phase. Correlation test combining two specific time frames showed the moderate positive correlation between temperature and EF, which was in contrast to what has been assumed before but coincided with the inference from the micro-level analysis of our study. The weak negative correlation ship of COD/N ratio and EF was reported for each individual phase. Due to the insignificant impact from pre-denitrification and exclusion of the post-denitrification phase, it could not be considered as reliable. In terms of narrow range of DO and no accumulation of nitrite, the weak negative correlation ship of DO and EF could not infer to any further conclusion. In addition, it should be noted that some uncertainties may distract the reliability of our results. High resolution online measurement should be applied for the determination of off-gas flow, COD and TNb concentration, instead of correlation method or infrequent laboratory analysis. The detection of dissolved N2O along the course are needed to provide more insights about the N2O formation during the process and to distinguish the contribution between aerated phase and non-aeration phase. At last, more frequent monitoring of the significant precursor nitrite and hydroxylamine is demanded to figure out the dominant pathway for AGS reactors.

Master Thesis. FG Siedlungswasserwirtschaft. Technische Universität Berlin