Response from ecoinvent, June 4th, 2019:
In the context of the system model, “Allocation, cut-off by classification”, an exchange may be classified as being either an “ordinary by-product”, a “waste” or a “recyclable material” (for information on how these different classes are handled in the cut-off system model, please refer to the ecoinvent website). Exchange classification is first recommended by the data provider based on expert judgement and then confirmed following review by one or multiple external “expert” reviewers and the ecoinvent team (as well as, in certain cases, the ecoinvent LCI expert group). The classification is then applied consistently throughout the database to all instances of that exchange. Exchange classification is subjective but unfixed; classification may be changed from one release to another if it is deemed appropriate to do.
Taking the example of electricity generation from municipal waste incineration in Switzerland, electricity, from municipal waste incineration to generic market for electricity, medium voltage [CH] is supplied (via a market activity) with electricity from 42 activities, each of which representing the “treatment” (incineration) of a specific by-product, e.g. treatment of waste building wood, chrome preserved, municipal incineration with fly ash extraction [CH]. Of those supplying activities, 39 are for the incineration of by-products that have been classified as “waste”. In these cases, the burdens of the incineration activity are allocated to the upstream waste producer, with the generated electricity provided to downstream users burden-free. However, three of the supplying activities are for the incineration of by-products that have been classified as “recyclable materials”: digester sludge, waste newspaper, and waste packaging paper. In these cases, and in-keeping with the convention for handling “recyclable materials” in the Cut-off system model, the activity of incinerating these materials is essentially considered as “recycling”, rather than waste disposal, and the burdens of this “recycling” activity are allocated to the downstream user of the secondary product (electricity, in this case. The figure below illustrates the points of cut-off for treating “wastes” and “recycling materials” in the cut-off system model. Note how, in the case of the electricity, from municipal waste incineration to generic market for electricity, medium voltage [CH], the market is being supplied with electricity from both sources.
Download the figure here
If the market were supplied exclusively with electricity from incineration activities that treated “waste”, then the burdens of this market activity would, indeed, be zero. However, as the market for electricity, for reuse in municipal waste incineration only [CH] is being supplied by a mix of incineration activities treating “recyclable materials” and “wastes”, it follows that the burdens of this market are not zero. This is a direct consequence of the by-product classification procedure and the fact that each exchange can only be given one classification, which must be applied consistently throughout the database. So why were these three by-product exchanges (digester sludge, waste newspaper, and waste packaging paper) classified as “recyclable materials” rather than “wastes”? Because in the “real world” (in this case, the Swiss MSW system) these materials are far more commonly recycled than they are incinerated at their End of Life; as reflected by their fates in the ecoinvent database.