Insight into Greenhouse Gas Emissions and Nitrogen Removal Performance: A Comparative Study on Denitrification and Anammox Under Anoxic Conditions

Abstract

Wastewater treatment is a significant source of greenhouse gas (GHG) emissions particularly methane (CH4) and nitrous oxide (N2O). Denitrification acts both as a source of N2O emissions and a sink for its reduction, but the direct measurement of N2O reduction to N2 remains a challenge. In this study, an assay method was developed to monitor GHG emissions by comparing denitrification and partial denitrification coupled with anammox (PD/A), with natural isotope analysis used to track N2O transformation. The results showed that PD/A process had a higher nitrogen removal rate and decreased N2O emission by 94% compared to denitrification in the treatment of domestic sewage. Both PD/A and denitrification treatments exhibited similar CH4 emission factors at 0.06%. In addition, organic matter had great impact on GHG emissions. In the tests of synthetic wastewater prepared with sodium acetate, both PD/A and denitrification treatments demonstrated a two-fold increase in NO3--N removal rates, along with a 67%-78% reduction in N2O emissions and a 67%-83% reduction in CH4 emissions. Isotope analysis of N2O indicated that PD/A exhibited a higher 15N site preference and greater N2O reduction rates compared to denitrification, contributing to N2O mitigation. The analysis of microbial community showed that the synergy of denitrifiers (Denitratisoma and Dechloromonas) and anammox bacteria (Candidatus Brocadia) has the potential to enhance nitrogen removal rates and reduce N2O emissions.

Keywords: Denitrification, Anaerobic ammonia oxidation, Nitrogen removal, Natural isotope, Functional microorganism