System Models in ecoinvent 3

Allocation cut-off by classification

The system model "Allocation, cut-off by classification", or cut-off system model in short, is based on the Recycled Content, or Cut-off, approach.

Introduction to the Cut-Off System Model

Classification of Products

Handling of By-Products by Classification

Introduction to the Cut-Off System Model

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 re-use of products resulting out of any waste treatment. For example, heat from the incineration of municipal solid waste can be used to heat houses or offices, and therefore has a value. Nevertheless, the incineration is allocated completely to the treatment of the waste, and therefore the burdens lay with the waste producer. The heat comes burden-free. This approach to by-product allocation has also been used in ecoinvent versions 1 and 2, where it was the only available system model.

Classification of Products

In the cut-off system model all intermediate exchanges (i.e. technospheric flows) in the database are classified into one of 3 categories: ordinary by-product, recyclable material or waste. This classification decides how they will be handled during allocation. The classification is on the level of products, not individual activities: throughout the database, an intermediate exchange can be used and produced many times in many activities, but the classification of each (identified clearly by the name) is always 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 datasets contain the classifications of their products. The classifications are based on the perspective of the data provider and the judgment of the ecoinvent LCI expert group. The classification of products from version 2.2 has generally been maintained, with some changes only due to data updates.

Ordinary By-Products


The most common category within the database. Most goods produced fall into this category. They could simply be called "by-products", but as that term is also used to differentiate between reference products and by-products, the addition of "ordinary" makes the category specific. Furthermore, this is only the category name, and you may find e.g. a dataset with reference product classified as "ordinary by-product".

Recyclable Materials


Materials with no or little economic value that can serve as the input or resource for a recycling activity. There is therefore an interest in their collection. Examples are scrap metals or waste paper.

Waste Products


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, so in effect he consumes the service of disposing of these materials. Examples are wastewater, chemically polluted soil or radioactive waste.

Handling of By-Products by Classification

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 wastes 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 allocating between the products if an activity produces more than one of them.


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

Handling of Waste Products


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

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

Handling of Recyclable Materials


Recyclable materials are cut off from their producing activities. This is achieved through the use of special datasets, always called "product name, Recycled Content cut-off". These datasets are without any inputs or emissions and therefore burden-free. In the producing activity, the material is recorded as a negative input, just as in the case of a waste. It is however not linked to any treatment activities, but simply to the empty process.

Thus, the cut-off approach is simple on the producing side. The point of cut-off 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. 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 will link to the burden-free Recycled Content cut-off dataset.

Special Case: Recycling Chains

Recycling processes can take place over several unit processes, one linked to the next. Often, the cut-off simply takes place in the beginning. However, sometimes recycling products are produced in different forms along a recycling process. This is best explained with an example: Waste glass is first treated by processing it and breaking it down into glass cullets. These are also a recyclable material, which 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 the 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.

Handling of Ordinary By-Products


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