Unwanted mainstream nitritation–denitritation causing massive N2O emissions in a continuous activated sludge process

Abstract

Nitrous oxide emissions can contribute significantly to the carbon footprint of municipal wastewater treatment plants even though emissions from conventional nitrogen removal processes are assumed to be moderate. An increased risk for high emissions can occur in connection with process disturbances and nitrite (NO2−) accumulation. This work describes the findings at a large municipal wastewater treatment plant where the levels of NO2− in the activated sludge process effluent were spontaneously and strongly increased on several activated sludge lines which was suspected to be due to shortcut nitrogen removal that stabilized for several months. The high NO2− levels were linked to a dramatic increase in nitrous oxide (N2O) emissions. As much as over 20% of the daily influent nitrogen load was emitted as N2O. These observations indicate that highly increased NO2− levels can occur in conventional activated sludge processes and result in high nitrous oxide emissions. They also raise questions concerning the risk of increased greenhouse gas (GHG) emissions of the nitritation–denitritation processes – although the uncontrolled nature of the event described here must be taken into consideration – and underline the importance of continuous monitoring and control of N2O emissions. HIGHLIGHTS NO2− can play a significant role in N2O emissions also in conventional activated sludge processes.; Variations in NO2− levels can explain poorer than expected negative correlation between total nitrogen removal and N2O.; Nitritation-denitritation can stabilize for a long period also in conditions that do not typically promote it.; Monitoring N2O emissions is crucial in operating nitritation–denitritation processes.;

Keywords

• activated sludge process
• ghg
• n2o
• nitritation–denitritation
• nitrous oxide
• no2−