the food content of the waste has the largest effect on this category
because we get a combination of more impact from the incinerator
due to more NOx-forming waste and because a 15% change in food
content has a large impact on how much paper is recycled. The
nutrient enrichment category should, given this large percentage
change in impact, be seen as very sensitive to the percentage
weight of food waste. However, the importance of this category
for the total environmental impact of the waste management system,
after normalisation, is low.
The human toxicity category aggregates all toxic emissions
potentially affecting human health into units of m3 water. This impact
category contains the largest savings, after normalisation. The
reduction in impact is due to substitution of virgin aluminium
production. Fig. 3 shows how a ±15% change in each component
of the waste in Case 1 will either give a result equal to the impact
of the reference composition or provide a 20% increase in savings.
The 20% increase in total avoided impact is due to a change in aluminium
content from 0.5% to 0.6%. The human toxicity category is
therefore very sensitive to the content of aluminium in the waste,
and most importantly to how much of the available aluminium is
recycled. The changes in total impact are much smaller for Case
2. This is because the quantity of aluminium recycled is approximately
the same in all scenarios. However, due to the level of accuracy
in the model used, there are some small differences in the
quantity of aluminium recycled, even in Case 2. Although these differences
are less than 10 kg recycled aluminium per tonne total
waste, they are nevertheless the reason for the variation in the
Case 2 results.