Nitrous oxide emissions during the removal of nitrogen from wastewater treatment plants in the Paris metropolitan area: current state and forecasts
The Seine, one of Europe’s rivers most affected by intense human pressures, receives wastewater discharges from almost fifteen million inhabitants, equal to a quarter of the French population, among which ten million are concentrated in the Paris conurbation. In the framework of the European directive 91/271, wastewater treatment in the Paris conurbation is being reorganized and improved. The nitrogen load, still discharged to the Seine River, will be gradually treated by fixed biological culture and activated sludge processes, allowing nitrification and denitrification of nitrogen pollution. It is now accepted that nitrous oxide (N2O), a greenhouse gas that induces the destruction of stratospheric ozone, is emitted during biological nitrogen treatment in wastewater treatment plants (WWTPs) and may be a significant source to the atmosphere. These emissions have been quantified under experimental conditions, depending on the nitrogen load treated and the treatment used. These results have allowed the estimation of N2O emissions from all WWTPs in the Paris conurbation for present treatments and also for future conditions, with new treatments being scheduled in 2006‐2008 and then in 2012‐2015. We estimate present N2O emissions to be 60‐120 kg N‐N2O/d, which would increase to 320‐480 kg N‐N2O/d in 2006‐2008 and reach 370‐750 kg N‐N2O/d in 2012‐2015. The emission levels could be limited to the lower range of values if the nitrification of nitrogen were performed at an oxygenation level higher than 2 mg O2/L and if denitrification were performed under total anoxic conditions and with a methanol addition, allowing a 100% reduction of the nitrate load. According to our results found for Paris WWTPs (60‐120 kg N‐N2O/d) and those obtained in the lower Seine River, which receives effluents not treated for nitrogen (90‐200 kg N‐N2O/d), N2O emissions amount to 150‐320 kg N‐N2O/d. This increase of N2O emissions by a factor of less than 2 is a rather low value taking into account the range of our estimates. Furthermore, at the scale of the Seine River basin, nitrous oxide emissions from nitrogen treatments in Paris conurbation WWTPs (at present or in the future) would continue to represent a low proportion, 1 to 10%, in comparison with the emissions from agricultural lands (4100‐8200 kg N‐N2O/d).