International Review of Waste Management Policy - Department of ...

Aeration and the C:N ratio are believed to have an important effect on the nitrogen
conversion processes. Where composting processes have included manures,
intensive aeration in connection with low C-content has been shown to give rise to
nitrite accumulation in slurry (up to 33% of the total nitrogen content) and incomplete
ammonium oxidation. Low ventilation rates and sufficient carbon supply support the
formation of nitrous oxide during nitrification and denitrification processes.
Gronauer et al suggest that around 12% of total nitrogen escapes from the material in
the form of ammonia. 1260 This gave a figure of 0.53kg/tonne waste in raw gas, but
0.0264kg per tonne waste when the air is passed through a biofilter. Gronauer et al
also assumed that 0.15kg N2O per tonne waste would be emitted.
One Swedish study assumed the nitrogen leakage to air was 7.5% of the nitrogen
content of the feedstock. 1261 Of this leakage, it was assumed 89% was emitted as
NH3, 9% as N2O and 2% N2. The study for the Danish EPA assumed that of the total
amount of nitrogen lost as gaseous emission, 98 % was volatilised as NH3, 0.5 % as
N2O and 1.5 % as N2 (the figures relate to raw gas as opposed to gas which has been
scrubbed). 1262
Our analysis assumes nitrogenous emissions from composting processes are as
follows:
1018
� 10% of the total nitrogen content of the waste is released in some form. Of this
10%, we further assume that:
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• 88% is released as NH3;
• 2% is released as N2 (this is not a problem for the environment); and
• 10% is released as N2O.
Whilst the NH3 is released untreated in windrow systems, in vessel systems typically
make use of biofilters to convert much of this pollutant to N2O. This is discussed in
the sections that follow.
63.8.2 Climate Change Impacts of Composting Processes
Table 63-24 outlines the key assumptions used to model the climate change impacts
of composting processes, which were developed in a previous study for WRAP by
1260 A. Gronauer, M. Helm, H. Schon (1997) Verhafen und Konzepte der Bioabfallkompostierung –
Vergleich – Bewertung – Empfehlungen, Bayerische Landesanstalt fur Landtechnik der TU Munchen-
Weihenstephan
1261 Goran Finnvenden, Jessica Johansson, Per Lind and Asa Moberg (2000) Life Cycle Assessments of
Energy from Solid Waste, Forskningsgruppen for Miljostrategiska Studier, FMS 137, August 2000
1262 B. Gunnarsdotter Beck-Friis (2001) Emissions of Ammonia, Nitrous Oxide and Methane during
Composting of Organic Household Waste, Agraria 266, Doctoral Thesis, SLU, Sweden, sited in A. Baky
and O. Eriksson (2003) Systems Analysis of Organic Waste Management in Denmark, Environmental
Project No. 822, Copenhagen: Danish EPA