Spatial and daily variations of nitrous oxide emissions from biological reactors in a full-scale activated sludge anoxic/oxic process

Nitrous oxide (N2O) is an important greenhouse gas that can be emitted from wastewater treatment plants (WWTPs). Such emissions are reportedly process specific and related to operational parameters. This study was conducted to clarify spatial and daily variations of N2O in a full-scale activated sludge anoxic/oxic process that consisted of an anoxic tank and three oxic tanks (oxic-1, oxic-2 and oxic-3), all of which except the final sedimentation tank were fully covered. Higher dissolved N2O (D-N2O) loading and gaseous N2O (G-N2O) emissions were observed for oxic-3 than for the anoxic, oxic-1, and oxic-2 tanks, implying that there was higher N2O production potential via nitrification in the latter stage of the oxic tank. Moreover, the sudden decrease in dissolved oxygen concentration after the peak was found to lead to abrupt production of D-N2O at oxic-3 in the anoxic/oxic process. The increases in AOB amoA, AOB nirK and the following AOB norB gene transcripts at the end of the oxic-2 tank suggested that nitrifier denitrification occurred to produce N2O under low dissolved oxygen conditions when the N2O peak was observed. Additionally, the much lower transcription levels of the two nosZ genes suggested lower N2O consumption. The N2O emission factors ranged from 0.087% to 0.302%, and lower N2O emission factors were observed during summer.