11-CS-3 Engineering Management · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2017 — 11-CS-3 Sustainability, Engineering and the Environment. Closed book; approved calculator permitted. Any four questions constitute a complete paper; all questions are of equal value (25 marks each).
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 product should last as long as it is useful and then break down or be recovered. It should not persist for centuries after its service ends. Example: agricultural mulch film and food-service packaging made from a certified-compostable polymer (e.g. PLA or PHA) instead of conventional polyethylene. The film does its job for one growing season, then biodegrades in soil or an industrial composter. Persistent plastic fragments and microplastics therefore do not accumulate in fields, waterways and landfills. The converse also applies: a product that must last, such as a bridge bearing, should be designed for its full service life so it is not replaced prematurely.
Minimizing the diversity of materials lets a product be disassembled and recycled cleanly. Example: making a product housing or package from a single polymer rather than a laminate of different plastics and metals, so it can be recycled as one material stream instead of being landfilled—retaining value and preventing waste.
Designing a process to connect with existing energy and material streams prevents waste. Example: combined heat and power (cogeneration), which uses one fuel input to produce both electricity and useful heat, capturing waste heat that would otherwise be rejected; or industrial symbiosis, routing one plant's by-product or waste heat as the feedstock of a neighbour—turning waste into input and avoiding both disposal and virgin-resource use.
(i) Functional unit: protecting a defined roof area (e.g. 100 m² of roof, one typical house) from weather for a defined service period (e.g. 50 years), including every replacement needed in that period. The time element is essential. The plastic shingle carries a lifetime warranty, while cedar shingles typically last about 20–30 years, so over 50 years cedar needs about one re-roof. Comparing "one roof of each" would ignore that difference.
(ii) Four life-cycle stages: (1) raw-material acquisition; (2) manufacturing (including transport and installation); (3) use/reuse/maintenance; (4) recycle/waste management (end of life).
(iii) Material and energy uses by stage, and the likely higher-impact alternative:
| Stage | Cedar shingles | Recycled-plastic shingles | Likely higher impact |
|---|---|---|---|
| 1. Raw-material acquisition | Materials: western red cedar logs from coastal forests (often older, slow-growing stands). Energy: diesel for felling, yarding and trucking logs. | Materials: post-consumer/industrial plastic waste, plus virgin additives (UV stabilisers, pigments, fire retardant, mineral filler). Energy: collection trucks, sorting and baling. | Cedar: habitat and biodiversity loss, soil erosion and carbon release from harvesting slow-growing trees. The plastic diverts waste from landfill. |
| 2. Manufacturing (incl. transport and installation) | Materials: logs, preservative/fire-retardant treatment. Energy: sawing/splitting (electricity) and kiln drying (heat); trucking the shingles; hand nailing. | Materials: cleaned plastic, additives, water for washing. Energy: grinding, washing, drying, extrusion or injection moulding (heat and electricity); trucking; nailing. | Synthetic: melting and moulding plastic is far more energy-intensive than splitting and drying wood, and uses chemical additives. |
| 3. Use / maintenance | Materials: periodic cleaning, moss removal, preservative or fire-retardant re-treatment. Replacement shingles, and about one full re-roof in 50 years. Energy: re-manufacture and re-installation for the replacement. | Materials: essentially none (no treatment, lifetime warranty). Energy: none. Minor UV degradation over time. | Cedar: maintenance chemicals plus a second roof's worth of every stage-1 and stage-2 burden. Treated cedar is also a higher wildfire ember risk. |
| 4. End of life | Materials: untreated wood can be composted, chipped or burned for energy. Treated wood must be landfilled, where it can leach preservative. Energy: hauling. | Materials: roofing with nails, tar and mixed additives is hard to recycle again, so it is usually landfilled, where it persists and can shed microplastic. Energy: hauling, or re-grinding if recycled. | Synthetic: it does not degrade and has few recycling routes. Cedar returns to the biological cycle. |
(iv) Greatest-impact stage, each alternative separately: For cedar it is raw-material acquisition: harvesting slow-growing cedar forest carries the largest ecological burden, and the re-roof within the functional unit doubles it. For the recycled-plastic shingle it is manufacturing: the feedstock is waste that is effectively free of burden, the use stage needs almost nothing, so the energy and additives used to wash, melt and mould the plastic dominate its profile. Because that burden is incurred only once in 50 years, while cedar's stage-1 and stage-2 burdens are incurred about twice, the durable synthetic shingle will often come out ahead overall. This conclusion is sensitive to the service lives assumed in the functional unit.
Reuse and recycling: reuse employs a product again in its existing form without reprocessing; recycling reprocesses the material into new products—reuse conserves more embedded value. Natural capital: the stock of natural resources and ecosystems yielding valuable services. Externalities: costs/benefits borne by third parties and not reflected in market prices.
