Reduction of greenhouse gas emissions from closed activated sludge- to aerobic granular sludge-based biosystems via gas circulation

Greenhouse gas (GHG) emissions from biological treatment units are challenging the wastewater treatment plants (WWTPs) due to their wide applications and global climate change. This study aimed to reduce GHG emissions, especially N2O, from closed sequencing-batch reactors (SBRs) by circulating gases when the biological unit varies from activated sludge (AS) to aerobic granular sludge (AGS). Results show that gas circulation can facilitate the development of regular granules and the accumulation of GHGs in the bulk liquor. However, N2O emissions increased from AS- to AGS-based SBRs due to slowing nitrification processes. Compared to the open systems, the net N2O emissions (0.07-0.86%, N2O-N/loaded N) and CO2 emissions (0.3 kg-CO2/kg-COD) from the closed systems decreased by 55 % and 62 %, respectively. N2O reduction was mainly contributed by denitrifying bacteria that eliminated the N2O generated. This study provides a feasible strategy to control N2O emissions from the biological WWTPs