Life cycle assessment (LCA) is a popular tool used to evaluate the environmental performance
of municipal solid waste (MSW) management systems. Although reviews of LCAs of MSW
have been undertaken to assess the validity of the 'waste hierarchy,' a recent review
of the goal, scope and results of LCAs of mixed-material MSW management systems has
yet to be performed. This paper is a comparative analysis of 20 process-based LCAs
of MSW published between 2002 and 2008 in a total of 11 English-language peer-reviewed
journals. It quantifies the methodological transparency of the studies and the frequency
of use of particular system boundaries, types of data sources, environmental impact
categories, impact weightings, economic valuations, sensitivity analyses, and LCA
computer models. Net energy use (NEU), global warming potential (GWP), and acidification
potential (AP) values for various types of MSW management systems are also compared
using statistical indicators. The reviewed LCAs differ substantially in their system
boundaries. Half or more of the LCAs either do not mention or are unclear in whether
or not life cycle emissions from energy inputs or capital equipment are included in
the calculation of results. Only four impact categories are common to more than half
of the reviewed LCAs. The human and ecological toxicity impact categories are much
less common than global warming potential, acidification, and eutrophication. A financial
life cycle costing is present in eight of the reviewed LCAs, while an economic valuation
of the environmental impacts is observed in five. Explicit sensitivity analyses are
present in 4/20 of the studies, although many more LCAs evaluate the effects of varying
model parameters by increasing the number of waste management scenarios. There is
no consensus on whether or not to use the marginal or average source of electricity
in calculating environmental impacts. Eight out of the 20 do not mention this source
while the remaining LCAs are evenly split between the marginal and average electricity
source. One quarter of the reviewed LCAs supply weighted results for the overall environmental
performance of MSW management scenarios. All but one of these concurred with the 'hierarchy
of waste' that the environmental performance of landfilling is lower than that of
all the other treatment methods, and that thermal treatments are inferior to recycling.
The comparative analyses of the NEU, GWP and AP results are based on 37, 45, and 42
MSW management scenarios, respectively. As measures of statistical dispersion, the
interquartile ranges of the NEU, GWP and AP values are lowest for the landfilling
(AP, NEU) and thermal treatment (GWP) scenarios. The results of the statistical analysis
of the NEU, AP and GWP values appear to indicate that thermal treatment scenarios
have a better environmental performance than landfilling, while the results for mixed
treatment scenarios are less obvious. A comparison of the relative environmental performances
of MSW treatment scenario types within each study did not provide a clear confirmation
or repudiation of the waste hierarchy. This paper concludes that many recently published
LCAs do not ensure that the methodological assumptions are made clear to the reader.
Lack of transparency makes the results difficult to interpret, and hampers meaningful
comparisons between the LCA results. A convergence in the adoption of particular assumptions
that are more representative of MSW management systems would facilitate the comparison
of the results.