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Co-production (the combined or joint production of two or more products from the same process or system) has been seen as presenting a problem to the system modelling in life cycle assessment, and the traditional solution has been co-product allocation (the partitioning and distribution of the environmental exchanges of the co-producing process or system over its multiple products according to a chosen allocation key) in parallel to cost allocation. Compared to this traditional solution, system expansion according to ISO 14041 provides a more realistic modelling of the actual consequences of product related management decisions. Through a number of examples, including recycling of steel and aluminium, it is demonstrated how co-product allocation can be avoided in practice. The example of platinum-group metals is used to illustrate how system expansion may sometimes be used as a justification for economic allocation.
