A by-product/waste is any Activity output that is neither a Determining product nor an Elementary exchange. This term serves as an umbrella classification for all intermediate outputs from human activities that fall outside the two primary output categories used in Life Cycle Assessment modelling.
To understand what constitutes a by-product/waste, it is essential to recognise what it is not. Determining products are those product outputs for which changes in demand directly affect an activity's production volume. These are the main products driving production decisions. Elementary exchanges, on the other hand, represent flows between an activity and the natural, social, or economic environment, such as emissions to air or water, unprocessed resource extraction, or working hours under specified conditions.
By-products/wastes thus encompass all remaining activity outputs that flow between unit processes within a product system but do not determine production volumes. Whilst ISO 14040 defines these flows as intermediate exchanges, the by-product/waste classification specifically excludes determining products from this category, focusing on the dependent outputs and materials requiring treatment or disposal.
This category includes several distinct subcategories. Waste represents outputs with zero or negative market or non-market value. Material for treatment comprises by-products/wastes that cannot substitute determining products as inputs to activities within the same geographical area. Recycling activities handle certain by-product/waste streams that yield outputs capable of displacing determining products from other activities. Finally, Final waste refers to materials entering final disposal activities with no further product outputs beyond the disposal service itself.
The by-product/waste classification is fundamental to LCA system modelling, as it determines how different material flows are handled within product systems and influences the allocation procedures or system expansion approaches applied during inventory analysis.
