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

Abstract

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.