A hybrid life cycle assessment model for comparison with conventional methodologies in Australia

Background, aim, and scopeOne barrier to the further implementation of LCA as a quantitative decision-support tool is the uncertainty created by the diversity of available analytical approaches. This paper compares conventional (‘process analysis’) and alternative (‘input–output analysis’) approaches to LCA, and presents a hybrid LCA model for Australia that overcomes the methodological limitations of process and input–output analysis and enables a comparison between the results achieved using each method. A case study from the water industry illustrates this comparison.Materials and methodsWe have developed a tiered hybrid model for calculating the life cycle impacts of a system. In so doing, we have developed a novel way of overcoming a key methodological issue associated with this method: avoiding double counting. We calculate ‘system incompleteness factors’ and use these to delete the lower-order burdens in the input–output inventory according to the depth of production taken into account in the process inventory. We apply this method to a case study of Sydney Water Corporation. The functional unit is the provision of water and sewerage services to residential, industrial, and commercial customers in the city of Sydney in the year 2002/03.Results and discussionWe analysed the case study using three methods: process analysis, input–output analysis, and hybrid analysis. In each case, we obtained results for eight impact categories: water use; primary energy use; global warming potential; carcinogenic and non-carcinogenic human toxicity potentials; and terrestrial, marine and freshwater ecotoxicity potentials. Although the process analysis has a relatively shallow investigative depth, it shows good system coverage (i.e. a small truncation error) for most indicators. The truncation errors for all of the indicators except marine aquatic ecotoxicity potential compare favourably with predicted truncation errors for the relevant industry sector. This suggests that the truncation error of a particular process analysis cannot be accurately predicted using generic system completeness curves, and implies that the truncation error of a typical process analysis may be less severe than is commonly generalised by the proponents of input–output analysis.ConclusionsThe case study supports the largely theoretical claims in the literature about the relative merits and drawbacks of process and input–output analysis. Each method has the potential to highlight different aspects of the system. By estimating the truncation error of the process analysis independently of the relationship between the results obtained using the other methods, our hybrid model enhances the ability to investigate the differences between results and thus adds considerable value to such a study.Recommendations and perspectivesInput–output LCA has become more popular as computational tools have become more accessible. We directly compare input–output, process and hybrid LCA and recommend that, from an environmental analysis perspective, it would be beneficial to consider the three methodologies in parallel. We highlight the potential for misinterpretation of differences between methods that rely on different reporting frameworks, and recommend that LCA method and practice continue to emphasise the role of careful interpretation.