The source and abatement of nitrous oxide emissions produced from the aerobic treatment of pig slurry to remove surplus nitrogen

The removal of surplus nitrogen from pig slurry can be necessary in order t o avoid pollution such as nitrate leaching. However, the treatment itself c an create significant pollution; up to 20% of the removed slurry nitrogen h as been shown to be released as nitrous oxide (N2O), which contributes to g lobal warming and the breakdown of the ozone in the stratosphere. Avoiding such emission requires conditions that encourage the complete conversion of the nitrogen to the environmentally safe di-nitrogen gas (N-2), and a clea r understanding of the underlying biochemistry; for example, whether the ni trous oxide is the bi-product of incomplete nitrification (chemical oxidati on) or denitrification (chemical reduction). The stable isotope of nitrogen (N-15) was used in this investigation as a label. Results indicated a new route for substantial release of N2O: via nitrification (rather than denitr ification), caused by a combination of high aeration levels and the presenc e of nitrification products. Sequential aeration, which leads to a cycling between nitrification and complete denitrification, was proposed as an abat ement in view of this new mechanism. This process achieved 89% removal of a mmoniacal nitrogen in laboratory scale treatment, with 94% of the nitrogen removed in the form of N-2. These findings suggest that the possibility of N2O emissions from nitrification be considered in the design of treatment s chemes. Increased aeration would be the intuitive response to incomplete ni trification. However, the results of this study suggested that although thi s response can increase nitrogen removal, this may be as N2O rather than N- 2.