Question 1 of 5: LCA Fundamentals – Boundaries, Life-Cycle Phases, Impact Assessment, Benchmark Compounds

Question 1: LCA Fundamentals – Boundaries, Life-Cycle Phases, Impact Assessment, Benchmark Compounds (28 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) System boundaries and the functional unit

A Life Cycle Assessment's conclusions are only as trustworthy as the boundary drawn around the system being studied and the basis chosen for comparing alternatives. The system boundary fixes which life-cycle stages, unit processes, and geographic/temporal scope enter the inventory — common choices are cradle-to-gate (raw-material extraction through the factory gate), gate-to-gate (a single process step), and cradle-to-grave (extraction through end-of-life disposal, reuse, or recycling). The functional unit is the quantified basis of equivalent service on which competing systems are compared, and it must reflect the service each alternative actually delivers rather than a convenient physical count of one of them.

Worked example — single-use plastic shopping bag vs. reusable cotton tote bag. If the boundary is drawn narrowly as gate-to-gate bag manufacture and the functional unit is naively taken as "one bag produced," the comparison looks lopsided in the plastic bag's favour: a polyethylene bag requires only a few grams of resin and a fraction of a second of extrusion energy, while a woven cotton tote requires far more raw material and processing energy per unit. A study stopped there would recommend the plastic bag. But the plastic bag is normally used once and discarded, while the cotton tote is designed for repeated reuse over its service life. Extending the boundary to cradle-to-grave (including upstream cotton farming — itself water- and pesticide-intensive — and end-of-life fate) and correctly re-scaling the functional unit to "carrying 1000 grocery trips' worth of goods" reverses the conclusion: published comparative LCAs typically find a cotton tote must be reused on the order of 100–150 times (far fewer for a polypropylene "bag for life") before its higher embodied-energy and water footprint is repaid relative to the equivalent number of single-use plastic bags, but once that break-even reuse count is exceeded — entirely plausible over a multi-year ownership — the reusable system's per-trip impact falls well below the plastic bag's. This demonstrates that boundary and functional-unit selection are not bookkeeping details: they determine which alternative appears superior, and an LCA report that omits or buries these choices, or compares "per bag" instead of "per unit of carrying service delivered," is not defensible engineering judgement — a concern directly relevant to Canadian municipalities that have legislated single-use plastic bag bans on the strength of exactly this kind of comparative claim.

(b) Life-cycle phases

The general phases associated with a product's development and distribution are: (1) raw material acquisition (extraction/harvesting of virgin or recycled feedstocks); (2) materials processing (converting feedstocks into engineering materials, e.g. resin pellets, woven cotton fabric); (3) manufacturing/assembly of the product itself; (4) distribution/transportation to the point of sale or use; (5) use phase (operation, reuse, laundering by the end user); and (6) end-of-life management (reuse, recycling, energy recovery, or landfill/litter fate).

Continuing the bag example, consider the raw material acquisition phase. Inputs to investigate for the plastic-bag route include crude oil/natural gas feedstock and the associated extraction and cracking energy to produce polyethylene resin; outputs include the greenhouse-gas emissions of upstream oil and gas production and the fossil-carbon embodied in the resin itself. For the cotton-tote route, inputs include irrigation water (cotton is among the most water-intensive common fibre crops), agricultural pesticides and fertilizer (with associated eutrophication runoff as an output), and the mechanical/chemical energy of ginning, spinning, and weaving; a key output to track is the crop's land-use footprint, since cotton farming competes directly with food-crop land in many growing regions.

(c) Classification, characterization and evaluation

Working from the raw-material-acquisition inventory built in (b): Classification sorts each inventory flow into the impact category (or categories) it contributes to — crude-oil extraction and cracking energy classify under global warming and fossil resource depletion; cotton irrigation water classifies under water depletion/scarcity; and agricultural pesticide/fertilizer runoff classifies under eutrophication and potential freshwater ecotoxicity. Characterization converts each classified flow into a common unit within its category using established equivalency factors — fossil-fuel combustion CO₂ and any associated methane leakage are converted to a single "kg CO₂-equivalent" score using each gas's 100-year Global Warming Potential, while irrigation water withdrawal is weighted by a regional water-scarcity factor (withdrawal in a water-stressed growing region counts far more heavily than the same volume drawn in a water-abundant one). Evaluation (normalization and weighting) then combines the several characterized category scores into an overall comparative judgement, e.g. normalizing each category against a regional per-capita reference load and weighting global warming vs. water scarcity vs. eutrophication, so that a single relative ranking of "plastic bag" vs. "cotton tote" per 1000 grocery trips can be reported.

Assumptions to state explicitly: the specific cotton-growing region assumed (and therefore its water-scarcity weighting factor, which is the single most sensitive parameter in this comparison — irrigated cotton from a water-stressed region scores very differently from rain-fed cotton); an assumed bag reuse count and expected service life for the tote; a 100-year GWP time horizon; and an assumed end-of-life fate for each bag (landfill vs. litter vs. energy recovery), since litter carries additional marine/wildlife-ecotoxicity impacts that a pure landfill-fate assumption would omit entirely.

(d) Benchmark compounds and their environmental concern