Mitigating NO and N2O emissions from a pilot-scale oxidation ditch using bioaugmentation of immobilized aerobic denitrifying bacteria

Nitrous oxide (N2O) emission from wastewater treatment plants (WWTPs) requires urgent mitigation because of its significant contribution to the greenhouse effect. In this study, bioaugmentation was applied in a pilot-scale oxidation ditch with the aerobic denitrifying bacteria strain PCN-1 immobilized on polyurethane biocarriers, which demonstrated effective N2O mitigation. Microbial community analysis suggested that the bioaugmentation facilitated a symbiotic relationship of the bacterial populations between the activated sludge and the biocarriers. The denitrifying bacteria with well-known N2O reducing capabilities predominated on the biocarriers. Correspondingly, the increases of denitrifying genes and NO and N2O reductase provided evidence for the enhanced genetic potential for NO and N2O reduction. Besides, the enriched comammox Nitrospira on the biocarriers is proposed as another significant driver for N2O mitigation by avoiding nitrite accumulation. In addition, the bioaugmentation enhanced the stability and recovery capability of the system in the ammonia overload and aeration failure shock tests.