The concept of waste
The concept of "waste" takes on different specific meanings in various legislative contexts. Focusing on physical modeling, it is advisable to avoid the direct use of the term "waste." In the context of consequential modeling, no distinction is made between by-products and waste; all intermediate outputs that do not lead to the production of products according to the same rules and procedures are treated in the same way. A general procedure for managing both by-products and waste involves modeling them as negative inputs rather than positive outputs. This implies that the determining product becomes the only residual intermediate output. Thanks to this methodology, consequential modeling avoids the need to allocate co-products, while ensuring the maintenance of mass and energy balances in the context of co-production activities.
ISO 14044 standard defines this procedure as "system expansion", as it considers the subsequent fate of by-products and waste, along with their changes (replacements), as an integral part of the product system. Instead of excluding these elements from allocation, the system expansion method incorporates them. However, since the expression "system expansion" suggests that the original product system was incomplete, the more neutral term "substitution method" is often used as a synonym.
By-products and waste are influenced by markets, which determine the consequences of variations in their demand and supply. The specific nature of by-products or waste is crucial for modeling such markets. The focus of modeling is whether the output (i.e., the negative input) is considered a "material for treatment" (MFT). A "material for treatment" is defined as a by-product or waste that is not a positive determining product for any other activity in the same geographical area. Therefore, it cannot directly replace a determining product as an input for an activity. A material for treatment thus requires specific treatment:
- In a disposal activity, i.e., waste treatment.
- In a recycling activity, i.e., a treatment that transforms the material into a product capable of replacing a determining product.
By-products or waste that do not fall into the category of "materials for treatment" can serve as a direct substitute input for a determining product of an activity. Consequently, they can be directly linked to a demand-driven market without requiring preliminary treatments. This market link resulting from the negative input represents a decrease in market demand, similar to how a link from a positive input increases demand. The reduction in market demand also leads to a decrease in the required input for the market and all related activities. These activities can therefore be relocated or "avoided." It is important to note that a by-product or waste can never constitute the marginal supply for a demand-driven market that has other unconstrained suppliers. This is because the supply of the by-product or waste is constrained by the demand for the determining product in the supply activity. Additional (marginal) demand in these markets cannot be met by increasing the supply of by-products or waste but will instead be met by unconstrained suppliers present on the markets.
Markets for treatment materials
Markets for treatment materials share the same function as other markets but stand out because they operate on products related to negative impacts, namely the treatment or disposal services of the material. These markets are not driven by material demand but by supply, particularly by the need to manage that supply. The flow of the material opposes the flow of the service. The determinants of treatment activities and their related markets can also be expressed in terms of negative quantities of materials to be treated, or the service of removing specific quantities of material.
In the case of recycling activity, the key product is the service of removing the material to be treated (potentially referred to as a negative quantity of materials to be treated). The quantity of material to be treated represents a constraint on the volume of recycling activity. The output after recycling thus becomes a by-product of the treatment activity. Like other by-products, this recycled material is treated as a negative input in the recycling activity and connected to the market as a negative flow, indicating a decrease in demand. Additional (marginal) demand in this market cannot be satisfied with an increase in the supply of recycled products but will instead be satisfied by suppliers not constrained to this market.
An exception occurs when an activity depends on the input of a by-product/waste, meaning it cannot function with a substitute of this material without that input being a constraint for the activity. For example, an activity that produces products labeled based on a specific percentage of recycled material is considered a "special production" and has a positive determining product (not the service of removing the material for treatment). An increase in (marginal) demand from specialty production will effectively lead to an increase in the supply of by-products/waste for specialty production, but this will come at the expense of reduced supply in marginal activities of treating this by-product/waste (which could be recycling activities).
If there is not enough demand for recycled material, the material to be treated is not fully used for recycling, and recycling activity is not the marginal treatment activity for this material. This material to be treated can also be characterized as a “near-waste” since additional (marginal) supply of the material to be treated will be destined for disposal (waste treatment). Additional demand for recycled material will replace disposal. The marginal supplier of the material to be treated can therefore be referred to as “avoided disposal”.
Next lesson - “Multi-determining products"
