Quantification of nitrous oxide emissions during biological nitrogen removal from waste water due to nitrifier denitrification - a systems comparison.

There is strong evidence that the biological removal of nitrogen from waste water emits considerable amounts of nitrous oxide (N2O) gas. Up to 80 per cent of removed nitrogen is emitted in the form of N2O, which is a strong greenhouse gas and ozone depletant. This fact is well reported for treatment of agricultural wastes but not for the treatment of communal/industrial waste water. This difference between reports on agricultural and commercial waste water treatment may be due to the fact that, conventionally, the emission of nitrous oxide is seen to be cause by different bacteria involved in the water treatment process. Inventories of gaseous emissions from commercial treatment works thus may have missed out a potentially significant source of this gas. The proposed research aims to investigate, under controlled conditions in a laboratory, whether the treatment of communal/industrial waste water for nitrogen removal can principally lead to significant releases of nitrous oxide. Comparison between these results and those available for the agricultural sector will establish if there are generic differences in N2O emission potential between these two sectors. The results will be used in the formulation of a strategy for further research into i) the quanitification and abatement of N2O from industrial/communal waste waters or ii) further reduction of N2O from agricultural waste water treatment, respectively.

For biological nitrogen removal from agricultural waste waters (nitrification and denitrification), significant releases of nitrous oxide (N2O) have been reported. Some of this release has been attributed to nitrifier denitrification under oxygen limiting aeration conditions. However, there is also strong evidence of significant release of N2O under high-aeration conditions; also employed in the treatment of municipal or industrial waste waters. Increasingly tight legislation on water protection will lead to stricter treatment regimes, and thus improve the quality of surface waters, but might also cause significant contribution to global warming and ozone depletion: emissions of nitrous oxide might not be taken into account in the design and monitoring of treatment works. The work proposed here will address this problem by investigation of the potential, in principle, for the release of nitrous oxide during the denitrification of municipal / industrial waste waters. Treatment regimes with both simultaneous and separate nitrification and denitrification will be conducted in laboratory scale and nitrous oxide emissions measured. Quantification of the proportion of nitrogen removed via N2O emissions will be achieved by conventional mass balance.