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

Table 63-33: Emissions of Gas-fuelled and Diesel-fuelled Buses
1030
29/09/09
Emissions Emissions g g / / km
km
NMHC NMHC
NOx NOx
PM PM
Diesel 0.12 8.50 0.10
Gas 0.03 4.60 0.01
Source: N. Nylund, K. Erkkilä, M. Lappi, and M. Ikonen (2004) Transit Bus Emission Study: Comparison
of Emissions from Diesel and Natural Gas Buses, VTT Processes, October 2004
The data in Table 63-33 suggests the use of biogas to fuel vehicles should result in
improvements to air quality, particularly with regard to the NOx emission.
63.9.3 Compost Produced by AD Process
The residues can either be dewatered, creating a solid and a liquid fraction, or used
directly on land as a slurry, sometimes using flocculants in the process.
Whilst there may be some arguments for direct spreading, not least that of cost, it is
considered better practice to stabilise the solid residues (following dewatering
depending upon the materials and the process) through an aerobic stage so as to
produce a compost. One of the reasons for stabilising the solids from digestion is to
reduce the potential for nitrogen to leach following application – the stabilisation
process makes for an amendment with high organic matter content, but with reduced
availability of nitrogen. The level of nutrients such as phosphorous is much lower than
in, say, sludge based materials, making it possible to apply more organic material
without creating problems of groundwater pollution / surface water run-off.
In this study, we have assumed that the digestion process is followed by an aerobic
treatment phase, and that the early stages of the aerobic treatment employ a
biofilter. This adds slightly to the costs of treatment, and some equipment suppliers
prefer to make the slurry available without such a treatment step.
Additional emissions of CO2 and NH3 are assumed to occur in the aerobic phase, and
some additional CH4 is assumed to be emitted in the transition from the anaerobic to
the aerobic phase. The use of the biofilter necessitates the use of some additional
electricity.
Emissions offsets attributed to the use of the compost produced in this way are
calculated using the same methodology described in Section 63.8.4.
63.9.4 Summary – Damage Costs Associated with AD Processes
Table 63-34 summarises the damage costs associated with treating source
separated food waste in AD systems.