Earthster currently uses the “Allocation, cut-off by classification” system model for all the available Ecoinvent datasets.

The following section of this article is sourced from Ecoinvent's database specification: System Models.

The system model “Allocation, cut-off by classification”, or the cut-off system model, is based on the recycled content, or cut-off, approach. In this system model, wastes are the producer’s responsibility (“polluter pays”), and there is an incentivisation to use recyclable products, that are available burden free (cut-off).

The underlying philosophy of this approach is that primary (first) production of materials is always allocated to the primary user of a material. If a material is recycled, the primary producer does not receive any credit for the provision of any recyclable materials. As a consequence, recyclable materials are available burden-free to recycling processes, and secondary (recycled) materials bear only the impacts of the recycling processes. For example, recycled paper only bears the impacts of waste paper collection and the recycling process of turning waste paper into recycled paper. It is free of any burdens of the forestry activities and processing required for the primary production of the paper.

Furthermore, producers of wastes do not receive any credit for recycling or reuse of products resulting from any waste treatment. For example, heat from the incineration of municipal solid waste can be used to heat houses or offices and therefore has value. Nevertheless, the incineration is allocated completely to the treatment of the waste; therefore, the burden is assigned to the waste producer while the heat comes burden-free. This approach to by-product allocation was also used in ecoinvent versions 1 and 2, where it was the only available system model.

In the cut-off system model, all intermediate exchanges (i.e., flows from the technosphere) in the database are classified into one of 3 categories: allocatable, recyclable or waste. This classification determines how such exchanges will be handled during allocation. The classification is at the product level, not the individual activity level: throughout the database, an intermediate exchange can be used and produced many times in many activities, but the classification of each (identified clearly by name) is consistent throughout the entire database.

A product name may imply a certain category, but classification is not always obvious. For example, waste paper may be classified as a recyclable material, yet the name contains the term waste for clarity. A full list of all products and their classifications is available on the ecoinvent website, and the datasets contain classifications of their products. The classifications are based on the perspective of the data provider and the judgement of the ecoinvent experts and editors. The choice is made based on the use and fate of the product within the ecoinvent database.

Most goods produced fall into this category. Allocatable products are ordinary (by-)products; they have economic value and therefore are included in the allocation. Examples of allocatable products include heat and electricity.

Materials with no or little economic value can serve as the input or resource for a recycling activity; therefore, interest in their collection exists. Examples include metal scraps and waste paper.

Waste products are materials with no economic value and no interest in their collection without compensation. The producer therefore generally has to pay to dispose of these materials; thus, he consumes the service of disposing of these materials. Examples are wastewater, chemically polluted soil and radioactive waste.

The cut-off system model has, broadly speaking, the effect that recyclable materials are cut off at the beginning of the treatment processes, becoming available burden-free for following uses. The treatment of waste is completely allocated to the waste producer, and all valuable by-products of waste treatment are cut off in the waste treatment and become available burden-free. Ordinary by-products are handled by allocation among products if an activity produces more than one product.

The following sections describe in more detail what happens to by-products in this system model.

Waste by-products have to be treated, and the treatment burden is allocated completely to the waste-producing activity. Therefore, wastes are linked as a negative input to the activity, representing the fact that the activity requires the service of waste disposal. Waste disposal is then provided by different treatment processes, which have inputs and emissions that add to the burden of the waste-producing activity.

Any non-waste by-products of a waste treatment process (i.e., not other waste products) are cut off and do not provide credit to the production activity. The cut-off point is therefore the end of the waste treatment, which means that the resulting products are available in the database and can be used as burden-free inputs in other activities.

Recyclable materials are cut off from their production activities through the use of special datasets, denoted as “product name, recycled content cut-off”. These datasets have no inputs or emissions and are therefore burden-free. In a production activity, the material is recorded as a negative input, as in the case of waste; however, the material is not linked to any treatment activity but simply to the empty process.

Thus, the cut-off approach is simple on the producing side. The cut-off point is at the end of the activity producing the recyclable material. The secondary use cycle begins with pick-up of the material from the producer, and transport to the processing site is the beginning of the supply chain for the secondary use. In the system model, a process requesting a recyclable material as an input, e.g., a recycling process, receives the product from its market, which links to the burden-free recycled content cut-off dataset.

Adapted from Wernet, G., Bauer, C., Steubing, B. et al. The ecoinvent database version 3 (part I): overview and methodology. Int J Life Cycle Assess 21, 1218–1230 (2016). https://doi.org/10.1007/s11367-016-1087-8

Recycling processes can occur over several linked unit processes. Often, the cut-off simply occurs at the beginning. However, recycling products are sometimes produced in different forms during a recycling process. This case is best explained with an example. Waste glass is first treated by processing it and breaking it down into glass cullets, which are recyclable materials that can be turned into glass bottles. In this example, both waste glass and glass cullets are considered recyclable materials, so other producers producing either of them will have these by-products cut off. For recycling, however, the system model maintains the recycling chain, so glass cullets will have a non-empty supply chain despite being a recyclable material. The glass cullet supply chain will lead all the way to the beginning of the secondary use cycle, with the collection of waste glass, and there is no cut-off between waste glass and glass cullets.

An example for a recycling chain as present in version 3.7.1. of the ecoinvent database.

After the handling of waste and recyclable materials, allocation occurs for all remaining allocatable by-products produced within the activity. This process uses the allocation factors defined in the dataset by the dataset author. As waste and most recyclable materials (except those within recycling chains, where they remain as products) are at this point moved to the input side of the activity, they will be considered similar to other inputs and allocated over the different co-products of the activity.