The paper fraction is the largest fraction in all cities. This fraction
is important for the global warming impact category due to
the avoided impact of paper production when recycled and the
heating value when incinerated. We found that changes in paper
content had the largest effect on global warming impact and that
the cities with the largest paper fractions (Trondheim, Arendal
and Skien) had the most avoided impact. There is, however, an
additional reason for these results. When there is a large paper
fraction, the plastic content of the waste is low. Increased plastic
content has the opposite effect on the global warming category;
it will decrease the avoided impact. A 15% change in paper waste
will give a larger effect on the total waste composition than a
15% change in plastic content. When we, however, compared the
different increased recycling rates, we found that increasing the
plastic source-separation from 25% to 50% would be more important
than increasing the paper source-separation from 75% to
90%. One exception is the town of Skien where the plastic content
of the waste is very low. When we systematically modified the
waste composition the largest change in global warming, 43 kg
CO2-eq per tonne, was found when we had a large paper fraction
(and thereby low plastic fraction) compared to the average waste
composition. A comparison of the impact from the five cities suggested
that the difference in impact could be up to 105 kg CO2-
eq per tonne waste. Christensen et al. (2009) found a difference
in global warming impact in the order of 100–200 kg CO2-eq per
tonne waste when applying an average EU composition, a typical
‘northern European’ waste composition and a typical ‘southern
European’ waste composition, to 40 generic waste management
scenarios. The present study confirms the importance of using a
representative waste composition when estimating the global
warming impact of waste management systems.
Recycling of metals, especially aluminium, proved to be very
important for impact categories such as resource depletion and human
toxicity via water. The cities with the largest metal content
have the largest avoided impact, especially for the human toxicity
category, and increased source-separation of metals is important in
all cities for resource depletion, human toxicity via water, and even
for global warming. The human toxicity via water category is very
sensitive to changes in metal content, especially aluminium content.
In Norway glass and metals are collected together and separated
in a centralised plant serving the entire country. The actual