Uncertainty and limitations – a brief overview
For the calculation of the GHG emission factors presented in the
sections above, numerous assumptions had to be made. Each of
these assumptions might have introduced a margin of error. From
the emission factors calculated for recycling, the one factor derived
from the least number of assumptions and the most measured
parameters is the emission factor for glass. The ones with the most
assumptions used are the GHG emission factors for the different
plastic types. In particular, the use of European data on plastic production
when calculating GHG from substitution of raw plastics is
an underestimation of the likely South African emissions, which
means in reality GHG savings are higher. This is because the South
African electricity mix is different from the European one and has a
higher GHG burden. Electricity is used in the production of all
types of plastics.
Representativeness of compost calculations is another limitation
which has to be noted. Composting studies rely on specific
parameters derived from individual studies which then are generalised
for a waste management system of a country and region.
This is the case also for the South African situation, in which composting
has been studied for garden waste from the eThekwini
Municipality, which is situated in a subtropical location. The vegetation
and the other parameters (most importantly temperature
and moisture) might not be representative for other parts of the
country, which have arid conditions. However, until more detailed
composting studies are done, covering more climatic areas in the
country, the factors calculated in this paper are considered more
representative than the theoretical, international ones published
in the literature.