Nitrous oxide emission and nutrient removal in aerobic granular sludge sequencing batch reactors

Area of Science:

• Environmental Science
• Environmental Engineering
• Microbiology

Background:

• Aerobic granular sludge (AGS) offers superior settling and microbial concentration for wastewater treatment.
• The complex structure of AGS can lead to incomplete denitrification and the emission of nitrous oxide (N2O), a potent greenhouse gas.

Purpose of the Study:

• To investigate the impact of varying aeration rates and Chemical Oxygen Demand (COD) to Nitrogen (N) ratios on N2O emissions from AGS.
• To quantify N2O generation during denitrification and nitrification processes under different operational conditions.

Main Methods:

• Three laboratory-scale aerobic granular sludge sequencing batch reactors (SBRs) were operated at 14 ± 4 °C.
• Synthetic wastewater, mimicking pig manure digestate and municipal wastewater, was treated.
• Experiments were conducted using three aeration rates (0.2, 0.6, 1.0 L air/min) and three COD:N ratios (1:0.22, 1:0.15, 1:0.11).

Main Results:

• N2O emission proportions decreased with increased aeration rates and lower COD:N ratios, ranging from 8.2% to 2.2%.
• Specific N2O generation rates during denitrification were higher when nitrite (NO2-) was the nitrogen source compared to nitrate (NO3-).
• N2O generation from ammonium (NH4+-N) nitrification was largely insensitive to aeration rates.

Conclusions:

• Optimizing aeration rates and COD:N ratios in AGS systems is crucial for mitigating N2O emissions.
• Denitrification using nitrite as a substrate contributes significantly to N2O production in AGS.
• Further research into nitrification pathways is needed to fully understand N2O formation mechanisms.