What are unit and system processes?

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In SimaPro desktop, the ecoinvent libraries are divided into unit and system processes. In this article, you will learn what unit and system processes are and what the differences between them are.

What is a unit process?

A unit process is the smallest element in the life cycle inventory analysis for which input and output data are quantified (ISO 14040). Unit processes describe a distinct part of a life cycle, not a whole life cycle in themselves.  The scope of a unit process can vary, and there are often several possible approaches to defining the scope and level of detail of life cycle elements. Many unit processes do not represent the smallest known process step. After all, adding more detail does not always offer significant value. In a database such as ecoinvent, all unit processes are documented to describe their scope.

What is a system process?

There are also aggregated versions of processes. These are known as system processes. System processes are the result of the compilation and quantification of inputs and outputs for a product throughout its life cycle (ISO 14040:2006). In other words, a system process is a single, cradle-to-gate aggregation of all environmental flows caused by the provision of the reference product. For this reason, they are also known as an aggregated life cycle inventory (LCI).

What is the difference between unit and system processes?

A system process can be calculated out of unit processes and is not an independent dataset. a system process only contains inputs and outputs to and from the biosphere per reference product, so per individual output to technosphere. Whereas a unit process contains only emissions and resource inputs from one process step, plus references to input from other processes.

System versus unit processes

Example: Making steel

The unit process for making steel contains transport of hot metal, and other input materials, to the converter, the steel-making process and casting. This means the unit process starts at the point where molten iron comes in from another process. The environmental load connected to making the hot iron is described in a series of other unit processes. These unit processes can easily be distinguished as the process record contain links to the relevant processes.
If you select the steel-making unit process when you want to do a LCA of a product that includes steel, SimaPro will of course automatically include all these upstream processes. However, if you take the system process version of this same activity, you will find that all emissions from mining up to making steel are already in the process record. You will also find that there are no links to other processes, only substance and emissions. The system process is experienced as a black box – you cannot see easily which steps from previous processes are included. One could say that the system process is the result of an overall LCA on making steel.

If you are unsure which library to choose, please see this article, which gives some guidance on which to choose.