Analysis of nitrous oxide emissions from aerobic granular sludge treating high saline municipal wastewater

Highlights: • At 3/d the N₂O emission factor from AGS and CAS were similar. • Emissions were higher from AGS at loadings >0.6 kg COD/m³/d when compared to CAS. • Functional nitrifying bacterial groups showed no large changes between AGS and CAS. Conventional activated sludge (CAS)-based wastewater treatment processes have the potential to emit high concentrations of nitrous oxide (N₂O) during nitrification and denitrification, which can significantly impact the environmental performance and carbon footprint of wastewater treatment operations. While N₂O emissions from CAS have been extensively studied, there is little knowledge of N₂O emissions from aerobic granular sludge (AGS) which is now an increasingly popular secondary treatment alternative. The N₂O emissions performance of AGS needs to be investigated to ensure that the positive benefits of AGS, such as increased capacity and stable nutrient removal, are not offset by higher emissions. This study quantified N₂O emissions from a pilot-scale AGS reactor operated under a range of organic loading rates. A second CAS pilot plant was operated in parallel and under identical loading rates to allow for side-by-side comparison of N₂O emissions from floc-based activated sludge. Under low loadings of 3/d the N₂O emission factor from AGS and CAS were similar, at around 1.46 ± 0.1% g N₂Oemitted/g ammonium loaded. A step increase in the organic loading rate increased N₂O emissions from AGS more so than CAS which appeared to be attributed to the reactor feeding strategy that was required for AGS formation. The use of a separate anaerobic feeding phase which was followed by the aeration phase, resulted in extended periods of low dissolved oxygen (DO) concentrations combined with an initial high biomass ammonium loading rate, which favours N₂O production and was exacerbated at higher organic loads. Conversely, the combined feeding plus aeration operation (aerobic feed) employed by the CAS system enabled a more even biomass ammonium loading rate and DO supply. This work has shown that while AGS has many operational benefits, the impacts that aeration profile, loading rate and feeding strategy have on N₂O emissions must be considered.