fractions of glass and metal, and of different types of metal, are reported
at a national level only. This leaves large uncertainties in
assessments at a municipality or settlement level, which will affect
the results of an accounting LCA where we have an estimated
source-separation ratio. However, uncertainty in waste composition
concerning metals is of little importance if the source-separated
amounts are known.
Food waste is the next largest fraction after paper for all cities.
We found the nutrient enrichment impact category to be especially
sensitive to changes in this fraction. The challenge with food waste
is often the way in which it is reported. In some waste composition
analyses food waste is reported together with garden waste in a
single organic waste category, while others report the food waste
category separately (Bernstad et al., 2011; Gentil et al., 2009). It
is important to be aware of this difference, as food waste and garden
waste often have different routes in the waste management
system, and therefore also different environmental impacts.
While in accounting-LCA we can find relatively large effects of
uncertainty in the waste composition, comparisons of the effect
of increased source-separation or introduction of food waste sorting
provide more robust results. The total impact is then of little
importance, it is the difference between the impacts that decides
which solutions are the most desirable. Although there were some
ranking issues, these were small. This is supported by findings by
Eriksson and Baky (2010) and Christensen et al. (2009).