4. DiscussionWe wanted to study the

4. Discussion
We wanted to study the effect of uncertainty in waste composition
in a waste management system comprising the recycling
of paper, plastic, glass and metals, and the incineration of residues
with heat recovery. We have combined waste composition
analyses and the annual amounts of source-separated waste fractions
reported by the relevant municipal authorities to estimate
waste compositions for five cities. There can be many reasons for
uncertainty in the estimated waste composition, stemming from
uncertainty in both the composition analyses and in reported
waste amounts, and from the combination of these two sources
of information. A Norwegian study compared the composition
analysis of 52 municipalities and found an uncertainty in each
waste fraction of between three and six per cent (hazardous waste
and textiles had higher uncertainty) (Skullerud et al., 2010). Gentil
et al. (2009) found large variation in literature data on waste compositions
when performing an assessment of the waste systems in
several European countries, and Dahlen et al. (2009) discuss uncertainty
in public waste data. Large difference can be found if we
compare the five municipalities included in the present study. Sogndalen
has a paper fraction with a percentage by weight of 28.7,
while the paper percentage by weight for Trondheim is 37. For
plastic the percentage by weight is 8.2 in Skien, while in Sogndalen
it is 17.4. The reason for these differences could be due to the different
sizes of the cities, their geographical location, seasonal variations
influencing waste composition analysis, etc. A comparison
of waste compositions in Norwegian cities found large cities to
have a larger percentage by weight of paper, hazardous waste
and other non-burnable waste, and less metal, food and other
burnable waste (Skullerud et al., 2010). We will, however, not go
further into the reasons for uncertainty in waste composition,
but rather discuss the effect of such uncertainty on modelling of
waste system