Allocation at the point of Substitution (APOS) is an allocation approach that uses expansion of product systems to avoid allocating within treatment systems. Some activities, e.g. collecting of municipal solid waste and driving it to a disposal site, take place within treatment systems, and finding appropriate allocation factors such as revenues for these individual steps can be challenging. APOS was designed to avoid such allocations in general, instead allocating the valuable by-products of treatment systems together with the activity that produced the material for treatment. Such an expansion of the product system is often impossible in foreground systems due to the scope of a given study. ecoinvent does not make any recommendations to users on which modelling to apply to their foreground system. We recommend making a choice appropriate to the goal and scope definition of the study at hand.
APOS creates datasets for by-products of waste treatments for each instance of a waste, e.g. the incineration of municipal solid waste (MSW) creates electricity as a by-product. Therefore, the resulting system would contain many datasets – one electricity dataset per instance of MSW in the database: "electricity production, from municipal solid waste incineration, from wheat production" or "electricity production, from municipal solid waste incineration, from mine operation" and many others. To reduce the complexity, these datasets are merged into one dataset, called simply "electricity production, from municipal solid waste incineration". This therefore provides the average electricity of the different MSW incineration supply chains.
However, as a merged dataset this dataset contains flows from several original datasets – you would find both wheat seeds and mine equipment as inputs in the example above. Therefore, it may be simplest to treat such datasets as aggregated datasets unless the details of the by-product supply chain are relevant for a given study.
APOS System Model