CHEM2522 · Sustainable Chemical Manufacture
Microplastics & Life-Cycle Analysis
Week 13 closes the unit by widening the lens: the sources and environmental fate of microplastics (fragments, tyre wear, synthetic microfibres, nurdles, microbeads, paint), and life-cycle analysis (LCA) as the rigorous framework for judging whether a manufacturing route is genuinely sustainable. The KeepCup case shows how a reusable product's footprint can be dominated by the use stage (washing), so a fair comparison must count the whole life cycle. Exam questions ask you to reason about microplastic sources and to interpret or set up a simple LCA comparison.
What this chapter covers
- 01Microplastic sources: synthetic microfibres (~35%), tyre wear (~28%), fragments (~22%), nurdles, microbeads, paint
- 02Environmental fate and harms: persistence, entry into the food chain, sorbed toxins
- 03Terrestrial vs marine biodegradability - degradation is environment-dependent, not a fixed property
- 04Life-cycle analysis (LCA): assessing impact across raw materials, manufacture, use and end-of-life
- 05Impact categories: greenhouse-gas / climate, energy, water use
- 06The KeepCup lesson: the use stage (washing) can dominate a reusable product's footprint
- 07Break-even thinking: reusable items must be used enough times to beat single-use alternatives
- 08Caveats when comparing polyolefins with biopolymers; renewable energy's leverage on process GHG
A simple LCA break-even: reusable vs disposable cup
- +1Set up the totals over N uses: reusable = embodied + per-use washing = 350 + 15N (g CO2e); disposable = 25N (g CO2e), a new cup each time.
- +1Break-even when the totals are equal: 350 + 15N = 25N.
- +1Solve: 350 = 25N - 15N = 10N, so N = 35 uses. Below 35 uses the disposable actually has the lower footprint; at 35 they are equal; beyond 35 the reusable wins.
- +1Interpretation: a reusable is not automatically greener - it carries a high embodied footprint that is only paid off through enough uses, and its use-stage impact (washing, here 15 g each time) matters, exactly the KeepCup lesson that the use stage can dominate. Only a whole-life-cycle comparison, not just manufacture, gives the honest answer.
Key terms
- Microplastics
- Plastic particles smaller than ~5 mm, from primary sources (nurdles, microbeads) and secondary fragmentation of larger plastics; major contributors include synthetic microfibres, tyre wear and general fragments.
- Environmental fate
- What happens to a material after release - how it disperses, persists, degrades and enters ecosystems and the food chain; for microplastics, persistence and toxin sorption are the key concerns.
- Life-cycle analysis (LCA)
- A systematic accounting of a product's environmental impacts across its whole life - raw materials, manufacture, use and end-of-life - used to compare options fairly rather than on a single stage.
- Impact categories
- The dimensions an LCA quantifies, such as climate change (greenhouse-gas emissions, CO2e), cumulative energy demand and water use; a route can look good on one and poor on another.
- Use-stage dominance
- The finding (e.g. the KeepCup LCA) that for a reusable product the use phase - here repeated washing - can contribute the largest share of the total footprint, so it cannot be ignored.
- Break-even (uses)
- The number of uses at which a higher-embodied reusable product's cumulative footprint equals that of a single-use alternative; below it the disposable is lower-impact, above it the reusable is.
Microplastics & Life-Cycle Analysis FAQ
Where do microplastics actually come from?
Mostly from everyday wear and fragmentation rather than deliberate microbeads. The largest contributors are synthetic microfibres shed when washing textiles (~35%), tyre wear from driving (~28%) and general fragmentation of larger plastic litter (~22%), with smaller inputs from nurdles (pre-production pellets), cosmetic microbeads and paint. Once released they are persistent, disperse widely and can enter the food chain, which is why source-level design (durable textiles, better tyres, less unnecessary plastic) matters more than clean-up alone.
What does a life-cycle analysis actually compare?
It compares the total environmental impact of options across their whole life - raw-material extraction, manufacture, use and end-of-life - usually in categories like greenhouse-gas emissions, energy and water. The point is that judging by one stage is misleading: a reusable cup has a high manufacturing footprint but low per-use impact, while a disposable has the reverse, so only the full life cycle (and the number of uses) tells you which is greener. The KeepCup study is the unit's worked example of this.
Why can a reusable product be worse than a disposable one?
Because a reusable carries a large embodied footprint from manufacture that only pays off if it is used enough times, and its use stage (for cups, washing with hot water and detergent) adds impact every time. If you use it only a handful of times, or wash it inefficiently, you may never reach the break-even point where it beats single-use. That is why LCA insists on counting real usage patterns, not just assuming 'reusable = green'.
Can Sia help me with microplastics and LCA questions?
Yes. Sia can help you organise microplastic sources and their fate, set up and solve a break-even LCA comparison, and reason about which impact category dominates and why. It explains the method and checks your reasoning step by step; it does not do graded assessment for you, and University of Sydney academic-integrity rules apply.
Exam move
Treat Week 13 as the unit's synthesis: it asks you to judge sustainability rigorously rather than by slogan. Know the main microplastic sources and their rough ranking (microfibres, tyres, fragments) and be able to discuss fate and harm and the fact that biodegradability depends on the environment. For LCA, learn the four life stages and the common impact categories, and internalise the KeepCup lesson that the use stage can dominate a reusable's footprint. Practise the break-even calculation (embodied difference divided by per-use difference) because it is the most likely quantitative item, and rehearse structuring a balanced 'which option is more sustainable, and under what assumptions' answer. This chapter rewards clear, caveated reasoning that ties back to the green-metrics thinking from Week 1, closing the loop of the whole unit.
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