Effects of the Food-to-Microorganism (F/M) Ratio on N2O Emissions in Aerobic Granular Sludge Sequencing Batch Airlift Reactors

The present study investigated the effect of the food-to-microorganism (F/M) ratio on nitrous oxide (N2O) emissions in aerobic granular sludge sequencing batch airlift reactors. Three identical sequencing batch airlift reactors were fed with sodium acetate-based wastewater at different chemical oxygen demand (COD) concentrations, resulting in F/M ratios from 0.2 to 0.67 g COD/gSS. The results indicated that N2O emissions increased with an increase of the F/M ratio. N2O emissions at the high F/M ratio of 0.67 g COD/g SS were the highest (4.4 +/- 0.94 mg/d). The main source of the high N2O emissions at the F/M ratio of 0.67 g COD/g SS was nitrifier denitrification, rather than heterotrophic denitrification, confirmed by the qPCR (quantitative real-time PCR) results. The heterotrophic denitrification was destroyed by the DO (dissolved oxygen) diffusing into the sludge particles with porous structures. This study offers theoretical support to study the N2O emissions in aerobic granular sludge, and can provide guidance for conducting risk assessment and enhancing our ability to predict N2O production in aerobic granular sludge at different F/M ratios.