Unit Processes: The Building Blocks of Life Cycle Assessments

In the last post, we explored how reference flows are calculated, by making careful, thought-out assumptions and well-applied research. In our exploration of the Ecosystem Analytics Inc. LCA on cloth handkerchief use versus disposable facial tissues, we demonstrated how reference flows for the end of life for facial tissues were calculated. We calculated the amount of paper facial tissues, cardboard box, and plastic box insert that need to be landfilled and incinerated to fulfill the functional unit for an average American living in New England. But what do we do with these amounts? We need a way to model the effects of landfilling and incineration, but it is highly unlikely that an LCA practitioner can go to a municipal landfill or incinerator to collect data on air, water, and land impacts and the energy produced or used. Even if a local landfill or incinerator was generous enough to allow access, the costs of conducting such an inventory for each project would be exorbitant. Luckily, publically-available, peer-reviewed, and well-researched databases of industrial processes and impacts exist for the LCA practitioner to use when conducting an LCA. One of the best LCA databases is the Ecoinvent database, developed by the Swiss Federal Institute of Technology Zürich (ETH Zurich) and Lausanne (EPF Lausanne), the Paul Scherrer Institute (PSI), the Swiss Federal Laboratories for Materials Testing and Research (Empa), and the Swiss Federal Research Station Agroscope Reckenholz-Tänikon (ART). Most of the data was acquired by studying Swiss and European manufacturing plants, energy production centers, and waste disposal locations, and quantifying the types of chemicals used and the emissions to the air, water, and land.

The Ecoinvent database is a database of unit processes. According to the ISO 14040 LCA standards, a unit process is the “smallest element considered in the life cycle inventory analysis for which input and output data are quantified.” So, for our example here, the amount of facial tissue, cardboard, and plastic packaging to be landfilled and incinerated is the smallest element we can calculate an input for. Therefore, they are the unit processes for the end of life for the facial tissues. We looked through the Ecoinvent database to find unit process listings that matched our needs and found these:

• Disposal, paper, 11.2% water, to sanitary landfill/ CH U
• Disposal, paper, 11.2% water, to municipal incineration/ CH U
• Disposal, polyethylene, 0.4% water, to sanitary landfill/ CH U
• Disposal, polyethylene, 0.4% water, to municipal incinerator/ CH U

We aggregated the reference flows for facial tissues and cardboard to fit the paper unit process. These reference flows and unit processes form the inputs to our LCA model.

• Disposal, paper, 11.2% water, to sanitary landfill/ CH U, 551 g
• Disposal, paper, 11.2% water, to municipal incineration/ CH U, 693 g
• Disposal, polyethylene, 0.4% water, to sanitary landfill/ CH U, 3.48 g
• Disposal, polyethylene, 0.4% water, to municipal incinerator/ CH U, 4.37 g

In the next post, we will discuss the assumptions made in picking unit processes for the LCA model – specifically how to simulate geographic relevance in the LCA.