Allocation Issues Related To Recycling At The End-Of-Life
There are several important methodological elements to a life-cycle assessment (LCA), but one that is keeping the LCA community engaged is the aspect of allocation. This topic is not limited to just pavement LCA, but is relevant to all LCA studies. This article on allocation is focused on material recycling performed at the end of the life of the pavement (discussed in Chapter 8 (.pdf, 3 mb) of the Reference Document).
Whenever a system of production yields multiple products or services, the environmental inputs and outputs of the system have to be assigned to each product and service, referred to as co-products. The ISO 14040 standards for LCA prescribe a hierarchical preference for how to assign, or allocate, environmental flows that occur in the modeling of the LCA. These allocations must be assigned whenever a production system boundary is crossed. For example, when one pavement life cycle ends and another begins, allocation must be utilized when assigning environmental impact to the material that is recycled from the pavement.
A general consensus among LCA practitioners and those involved in evaluating products and systems is that allocation rules should be set up to:
- Incentivize practices that reduce environmental impact.
- Prevent double counting of credits or the omission of important items.
- Provide fairness between industries by reflecting as closely as possible what is actually happening.
- Be transparent so that all parties can understand how allocation is applied and how it influences the results.
In addition, ISO standards, such as ISO 14044 for LCA, require sensitivity analysis to evaluate the impact of allocation rules to determine how they might change the final results of the assessment. According to ISO standards, the preference for treating co-products is to first try to avoid allocation by either 1) subdividing the production system into processes that can be assigned wholly to a single co-product, or 2) expanding the scope of interest to include the processes that seem to need allocation, thereby removing the need for allocation (this is referred to as system expansion). System expansion is more or less equivalent to displacement or substitution, where co-products are modeled as if they are displacing equivalent products in the marketplace. Thus, the system of production is credited with avoiding the need for producing these equivalent products. This approach is often used in consequential LCA approaches, as described earlier.
In most pavement LCA studies, the boundaries for the system of production are crossed (and thus allocation is necessary) in three situations:
- Multi-output situations like manufacturing processes with co-products (e.g., oil refineries).
- Reuse of components and recycling of materials after initial use, such as steel rebar, reclaimed asphalt pavement, coal combustion co-products from power generation, or use of discarded tires in asphalt binder.
- Multi-input situations like waste treatment processes, such as incineration and landfilling.
Reuse of Components and Recycling of Materials after Initial Use
When using a material from another product, pavement or system, several approaches for allocation have been and are being tried to ensure that the "benefits" of using secondary materials or fuel resources are properly reflected in an LCA.
Most environmental product declaration (EPD) approaches use a strict and conservative approach: all processes and transportation needed to reuse or recycle the material are assigned to the product utilizing the recycled content, but the production of the original product is assigned to the product's life cycle where it was first used, which is typically not included when EPDs are the focus of LCA. The same is true of reused or recycled materials that are used in pavement projects, such as the secondary content in steel, recycled aggregate from building waste, rubberized asphalt binder containing recycled tires, recovered binder from asphalt shingles, and SCMs derived from other industrial processes. Furthermore, materials that become available for reuse or recycling at the end of the pavement life cycle, such as RAP, RCA, and reinforcing steel, are also allocated in this manner.
An important element in this discussion is whether a material is defined as a waste or a product. If an economic approach is used to define a resource as a waste or a co-product, the following reasoning can be used:
- Where a waste flow material has value, it is considered a co-product and needs to have "production" processes allocated to it for the life cycle that is using the material. In essence, as soon as a waste flow has positive economic value, it is considered a co-product and should be treated as such.
- Where a waste flow material has a negative cost but becomes an economically valuable product through processing, the impact of processing and handling is allocated based on the difference between the cost (assigned to the producing life cycle where the waste occurred) and the positive value (assigned to the receiving life cycle where the co-product is used). An example of this is concrete waste that requires an acceptance fee at a crushing facility where it is processed (crushed and sized), and then sold back to the market at a price.
- Where the waste remains a cost regardless of processing, all environmental burdens of the processes are assigned to the producing life cycle; in this case, it essentially stays a waste and never becomes a co-product. The life cycle that uses materials like this are essentially part of the waste treatment process and receive the material "for free" apart from handling and transportation to be able to use the material.
Other approaches assign a "value" to the recycled materials and include credits for preventing the need for new primary materials for the new application. This is referred to as substitution, and must be considered cautiously and aligned with the approach for the receiving product system. Double counting of credits should be prevented.
One variation of assigning credits for recycling is the modeling of multiple life cycles to reflect repeated recycling benefits. It can also be useful in policy LCA where the impacts on the broader economy are studied. This approach is typically used to assign future recycling credits to the current product. Caution and extreme care is required for these studies and approaches. There are examples reported where an infinite number of life cycles are modeled to show the benefits of recycling, which can extend time periods that are irrelevant on a human scale. This is not considered good LCA practice, particularly given that modeling a pavement over a period in the range of 50 to 75 years is methodologically challenging enough as it is.
See Chapter 10 (.pdf, 5 mb) of the Reference Document for more details.