Nitrous oxide emission in a University of Cape Town membrane bioreactor: The effect of carbon to nitrogen ratio

The effect of the carbon-to-nitrogen (C/N) ratio in the influent on the nitrous oxide (N₂O) emission from a University of Cape Town Membrane BioReactor pilot plant was investigated. The membrane was located in a separate tank to single out the production of N₂O due to the biological processes from N₂O stripping as a result of the extra aeration needed for the mitigation of membrane fouling. The experimental campaign was divided into two phases, each characterized by a different C/N ratio (namely, 10 and 5 mgCOD/mgTN, Phase I and Phase II, respectively). The decrease of the C/N ratio promoted the increase of N₂O emissions in both gaseous and dissolved phases, mainly due to a decreased nitrification/denitrification capacity of the system. The highest N₂O concentration in the dissolved phase was found in the permeate. This result suggests that the dissolved N₂O in the permeate stream discharged from a MBR cannot be neglected. The total N₂O emission was approximately of 0.01% and 0.1% of the total influent nitrogen load for the Phase I and Phase II, respectively. The findings suggest that the C/N ratio represents an indirect cause of N₂O emission; the low C/N value (Phase II) led to the increase of pH and free ammonia causing a stress effect on the growth of nitrifying species increasing the N₂O emission.