Nitrous oxide production in high-loading biological nitrogen removal process under low COD/N ratio condition

Area of Science:

• Environmental Science
• Environmental Engineering
• Wastewater Treatment

Background:

• Biological nitrogen removal processes are crucial for treating high-strength wastewater.
• Nitrous oxide (N2O) is a potent greenhouse gas, and its emission from wastewater treatment requires careful management.
• The influent chemical oxygen demand to nitrogen (COD/N) ratio is a key operational parameter influencing treatment efficiency and emissions.

Purpose of the Study:

• To investigate the impact of varying influent COD/N ratios on N2O emissions in biological nitrogen removal.
• To elucidate the underlying mechanisms of N2O production under low COD/N ratio conditions.
• To assess the role of denitrification pathways and N2O reduction capacity.

Main Methods:

• Laboratory-scale bioreactors were used to simulate intermittent aeration biological nitrogen removal.
• Experiments were conducted with high-strength wastewater at different influent COD/N ratios.
• The 15N tracer technique, measurement of reduction rates, and batch experiments under denitrifying conditions were employed to study N2O production mechanisms.

Main Results:

• Influent COD/N ratios below 3.5 resulted in significant N2O emissions, accounting for 20-30% of influent nitrogen.
• 15N tracer studies confirmed that N2O originated from denitrification during the anoxic phase.
• N2O reduction capacity consistently exceeded nitrate (NO3(-)-N) and nitrite (NO2(-)-N) reduction capacities.

Conclusions:

• High N2O emission rates at low COD/N ratios are primarily driven by endogenous denitrification, particularly involving nitrite (NO2(-)-N) accumulation.
• The observed NO2(-)-N build-up is linked to disparities between NO3(-)-N and NO2(-)-N reduction rates.
• This phenomenon, characterized by NO2(-)-N accumulation, is specific to low COD/N ratio operations in the studied system.