Nitrous oxide emissions from aerobic granular sludge: A review

Nitrous oxide (N2O) is a significant greenhouse gas. Wastewater treatment plants, especially biological treatment units, are among the major sources of N2O emissions. Traditional biological treatments, including activated sludge (AS) and biofilm processes, have been extensively studied and reviewed, to analyze N2O emissions. New processes, such as aerobic granular sludge (AGS), are efficient, energy-saving, and stable sustainable development processes. Extensive research has also been conducted on N2O emissions from AGS. This review consolidates prior research on N2O emissions, specifically from AGS. The dense granule structure in AGS allows for aerobic/anoxic/anaerobic coexistence, enabling N2O production from multiple pathways, including hydroxylamine oxidation, nitrifier denitrification, heterotrophic denitrification, and abiotic/hybrid processes. In contrast to the simple structure of AS, the layered structure of AGS alters the fate of N2O emissions, with mass transfer resistance potentially prolong N2O residence time inside granules and improve the conversion of N2O. Summarizing the effects of various factors on N2O production and emission from AGS reveals that NO2- plays a central role in stimulating N2O production. Discrepancies in ammonia-oxidizing bacteria and nitrite-oxidizing bacteria distributions of result in NO2- accumulation in granules. Factors such as dissolved oxygen and substrate load mutually restrict NO2- accumulation and consequently affect N2O production. Furthermore, the emission factor (EF) of N2O production from AGS is summarized, with an average EF of 2.53 +/- 2.18%. Finally, the strategies to mitigate N2O emissions from AGS are discussed and some research blanks are presented. This is the first review that summarizes all the existing studies on N2O production and emissions from AGS and provides valuable suggestions for further studies on the sustainable development of AGS.