Post-consumer waste wood in attributive product LCA: Context specific evaluation of allocation procedures in a functionalistic conception of LCA

Background. In product life cycle assessment (LCA), the attribution of environmental interventions to a product under study is an ambiguous task. This is due to a) the simplistic modeling characteristics in the life cycle inventory step (LCI) of LCA in view of the complexity of our techno-economic system, and b) to the non-tangible theoretical nature of the product system as a representation of the processes 'causally' linked to a product. Ambiguous methodological decisions during the setup of an LCI include the modeling of end-of-life scenarios or the choice of an allocation factor for the allocation of joint co-production processes. An important criterion for methodological decisions - besides the conformity with the relevant series of standards ISO 14 040 - is if the improvement options, which can be deduced from the LCI, are perceived by the decision-maker as to redirect the material flows at stake into more sustainable paths. Methods. From this functionalistic conception of LCA, this article develops a set of wood-specific requirements, an LCI of wood products has to fulfill to give adequate decision support under Central European conditions. These requirements serve as a basis for the evaluation of different allocation procedures in a case study related to the modeling of end-of-life scenarios in a product LCA of wood products. The case study discusses how the recycling and incineration of a creosote-treated railway sleeper (Am. tie) are modeled according to various methodological propositions for the solution of the allocation problems related to recycling and final disposal. A partial life cycle model of the railway sleeper demonstrates the effect of the different allocation procedures to the over-all result. Results and Discussion. The most important conclusion - apart from proposing a functionalistic approach to solve allocation problems - is that under Central European conditions both the material and energy aspects of wood and the related substitution and opportunity effects (opportunity 'cost') should be considered for the modeling of post-consumer waste wood in attributive product LCA, even when comparing products made of different materials.