Electron competition among nitrogen oxides reduction during methanol-utilizing denitrification in wastewater treatment

•Electron distribution among the nitrogen oxide reductases is affected by the carbon loading rate.•A lower fraction of electrons is distributed to the N2O reductase with reduced carbon loading rate.•Electron competition could happen in denitrification even when the carbon source is in excess.

Limited availability of carbon sources has been regarded as an important factor leading to N2O accumulation during denitrification in wastewater treatment. By varying the carbon (methanol) loading rate to a methanol utilizing denitrifying culture in the presence of various electron acceptors (nitrate, nitrite, N2O and their combinations), this study quantitatively investigated the electron distribution among different nitrogen oxide reductases during denitrification. The results showed that electron competition occurs under not only carbon limiting but also carbon abundant conditions. The electron distribution among the nitrogen oxide reductases is affected by the carbon loading rate, with a lower fraction of electrons distributed to the N2O reductase with reduced carbon loading rate. N2O accumulation occurs when the electron flux going to nitrite reduction is higher than that going to N2O reduction. The study also showed that, for the culture investigated, the carbon to nitrogen ratio is not a key factor leading to N2O accumulation.

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