23-Chem-B10 Life Cycle Assessment (LCA) · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-Chem-B10, Life Cycle Assessment (LCA) — December 2013. 3 hours, Closed-Book Exam (Casio/Sharp approved calculator and one double-sided aid sheet permitted). Question 1 is mandatory; any three (3) of the remaining four (Questions 2–5) constitute a complete 100-mark paper, and only the first four questions as they appear in the answer book are marked. All five questions (and, in Question 5, all five sub-parts) are solved below for completeness.
Reference texts: Baumann & Tillman, The Hitch Hiker's Guide to LCA; Graedel & Allenby, Industrial Ecology and Sustainable Engineering; Kemp, Pinch Analysis and Process Integration, 2nd ed.; Mackay, Multimedia Environmental Models: The Fugacity Approach, 2nd ed.; Davis & Cornwell, Introduction to Environmental Engineering.
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.
Topic (iv) is selected: reusable cloth diapers vs. single-use disposable diapers.
Both products deliver the same service to a caregiver: absorbent, hygienic infant diapering. The comparison is drawn cradle-to-grave for both systems, and the functional unit is chosen as "diapering one infant for one year," rather than "one diaper," since a disposable is used once and discarded while a cloth diaper is laundered and reused roughly 50–100 times before being retired — comparing "per diaper" would badly bias the result toward disposables in exactly the way discussed in Question 1(a). The targeted outcome is a relative comparison of energy use, water consumption, and solid-waste generation per functional unit, with qualitative treatment of toxicity/ecotoxicity concerns specific to each system.
For the disposable-diaper system, the dominant life-cycle burdens are upstream: wood-pulp and superabsorbent-polymer (sodium polyacrylate) production consume significant energy and water and generate pulping-process air/water emissions (chlorine-based bleaching historically produced dioxins/furans as a byproduct, now largely eliminated by elemental-chlorine-free bleaching but still a process to scrutinize); the plastic backsheet/tape components draw on petrochemical feedstocks with associated upstream GHG burden. The use phase is minimal for disposables (no washing energy), but the end-of-life phase is dominant and essentially irreversible: each diaper becomes solid waste, and since it contains both cellulose and non-biodegradable plastic/SAP components, landfilling is the near-universal fate; landfilled diapers are also frequently cited as a source of untreated fecal-pathogen loading to landfill leachate, an inventory flow narrow single-metric energy comparisons often omit.
For the reusable-cloth-diaper system, upstream burdens (cotton or hemp cultivation — cotton is water- and pesticide-intensive; and manufacture of the fabric/inserts) are incurred once per diaper but amortized over its many use-cycles. The use phase dominates the cloth system's total burden: each wash-dry cycle consumes hot water, detergent, and either electricity (dryer) or clothesline drying, repeated 50–100+ times per diaper's service life. This is the single most sensitive parameter in the comparison — a caregiver using a high-efficiency front-loading washer and line-drying can make the cloth system's per-use energy burden far lower than a household using an old top-loader and electric dryer on every load, potentially reversing which system has the lower total-energy footprint. End-of-life for cloth is comparatively benign (natural-fibre components are largely compostable/biodegradable, versus the disposable's persistent SAP/plastic landfill fraction), but water consumption over the diaper's service life is typically higher for the cloth system once laundering is included.
Impact-category assignment: global warming potential (energy-derived GHGs, upstream for disposables and use-phase for cloth), water depletion/eutrophication (cotton irrigation and pesticide/fertilizer runoff for cloth; pulp/SAP process water and downstream leachate for disposables), solid-waste/land-use (landfill volume, essentially unique to the disposable system), and human/ecotoxicity (bleaching byproducts and SAP for disposables; pesticide residues for conventional cotton). Engineering judgment required: the relative ranking is genuinely region-and-behaviour dependent — a jurisdiction with a clean electricity grid and efficient laundry appliances favours cloth on GHG grounds, while a water-stressed region may weight the cloth system's laundering and cotton-cultivation water burden more heavily. No single "winner" should be declared without stating these dependencies explicitly.
Multiple independent LCA studies on this exact comparison (UK Environment Agency, 2005 and 2008 updates, being the most frequently cited) have found the two systems' global-warming-potential results to be within the margin of uncertainty of each other once realistic laundering behaviour is assumed, with the deciding factor being how the diapers are laundered (load size, temperature, dryer use) rather than an intrinsic superiority of either material system. The correct engineering conclusion is therefore process-oriented, not a declared "winner": both systems have legitimate environmental trade-offs, the ranking is sensitive to user behaviour and regional infrastructure (grid mix, water stress, landfill vs. incineration practice), and a defensible LCA report presents this sensitivity explicitly rather than asserting a single unconditional recommendation.
