The effect of wastewater salinity and presence of petroleum hydrocarbon on N2O emission was investigated in a membrane bioreactor, in which the anoxic and aerobic zones were put in series according to a pre-denitrification scheme. The pilot plant, was continuously fed by a mixture of real and synthetic wastewater. It was operated with a first phase of acclimation of the biomass to a given salinity by gradually increasing the salt concentration from 10 gNaCl/L to 20 gNaCl/L, and to a second phase of petroleum hydrocarbon dosing at 2 g/L (as gasoline). The first phase revealed a clear relationship between nitrous oxide emissions and salinity due to the increased NO2-N production caused by the stress induced both on autotrophic and heterotrophic biomass by the increased salinity. However, after 45 days of operation, the growth rate of the autotrophic species started to recover, indicating acclimatization of the nitrifiers. In the second phase, the hydrocarbon shock induced a temporary complete inhibition of the biological activities, as well as the temporary suppression of the N2O emissions. The observations in this study revealed that the oxic tank is the major source in terms of nitrous oxide emission flux. Indeed the aerobic tank emitted 1 or 2 order of magnitude more than the anoxic one. The reason of this is likely due to the stripping of the gas by aeration.
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