Nitrous oxide emissions from an aerobic granular sludge system treating low-strength ammonium wastewater

Aerobic granular sludge is a promising technology in wastewater treatment process. Its special microorganism structure could make the emissions of greenhouse gas nitrous oxide (N₂O) more complicated. This study investigated the N₂O emissions from a batch-fed aerobic granular sludge system during nitrification of low-strength synthetic ammonium wastewater. The N₂O emission was 2.72 ± 0.52% of the oxidized ammonium during the whole anoxic-oxic sequencing batch reactor (SBR) cycle. Under nitrification batch test with sole ammonium substrate (50 mg N/L), N₂O emission factor was 1.82% (N₂ON/NH₄⁺-N_ox) and ammonia-oxidizing bacteria (AOB) was the responsible microorganism. The presence of high ammonium concentration (or high ammonium oxidation rate (AOR)) and accumulation of nitrite would lead to significant N₂O emissions. AOB denitrification pathway was speculated to contribute more to the N₂O emissions under nitrification conditions. While under simultaneous nitrification and denitrification condition with carbon source of 500 mg COD/L, the N₂O emission factor increased to 2.76%. Both AOB and heterotrophic denitrifiers were responsible for N₂O emission and heterotrophic denitrification enhances N₂O emission. Step feeding of organic carbon source declined N₂O emission factor to 1.60%, which underlined the role of storage substance consumption in N₂O generation during denitrification.