Life cycles and recycling: equal service, real reuse and limits
Introduced| English | Meaning |
|---|---|
| life cycle assessment/laɪf ˈsaɪkl əˈsesmənt/ | An assessment of a product’s environmental impacts across the stated raw-material, manufacture, use and end-of-life stages. |
When does a reusable bag save resources?
- A reusable bag can cost more energy to make than one disposable bag. Compare the same shopping service over actual repeated uses before recommending it.
- This lesson covers life cycle assessment 生命周期评估 and reducing, reusing and recycling materials. Supplied numerical examples are fictional teaching data, not universal environmental ratings.
Set the full life-cycle boundary
- A life cycle assessment includes extraction/processing of raw materials, manufacture/packaging, use/operation and disposal after useful life. Transport and distribution belong at every relevant stage.
- Use the same delivered service and lifetime for both routes. A disposal-only comparison may answer a disposal question, but cannot establish the best whole-life product.
Which comparison can support a full-life-cycle recommendation?
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A consistent service, lifetime and system boundary are needed. Include relevant raw materials, manufacture, use, disposal and transport at the stages; one selectively chosen stage cannot decide the full life cycle.
Name the assessment across raw materials, manufacture, use and disposal.
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Life cycle assessment compares the relevant stages for a common service. Transport/distribution can matter throughout; it is not a disposal-only rating.
Separate measurements from judgements
- Suitable energy, water, resource and waste data can be quantified, with measurement uncertainty and assumptions. Check the actual service, local energy supply and use frequency behind the numbers.
- Combining different pollutant effects into one score requires value judgements. An LCA is not purely objective. Keep categories separate when their priorities differ, rather than calling a low-energy route best in every respect.
An LCA is purely objective because every pollution effect can be combined without judgement.
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Some quantities can be measured with uncertainty, but weighting different effects involves value judgements. State assumptions and keep categories distinct when priorities differ.
Model the actual number of tasks
- A fictional single-use paper bag needs 12 energy units per shopping task. A reusable plastic bag needs 30 units to make and 1 per task to clean. Totals after $n$ tasks are $12n$ and $30+n$.
- Equality gives $12n=30+n$, so $11n=30$ and $n=2.73$. At three complete tasks, paper needs 36 units and plastic 33; the reusable route first uses less energy at the third task under these assumptions.
Distinguish reducing, reusing and recycling
- Reducing material use can avoid extraction and processing. Reusing a whole glass bottle after collection and washing avoids remaking it. Recycling crushes and remelts glass into a new product.
- Metals can be melted and recast/reformed. Some scrap steel can be added to iron from a blast furnace, reducing ore extraction. Final properties determine how much sorting and separation are needed; a mixed stream is not automatically suitable for every product.
Match the term with its precise meaning.
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Use the chemical structure, process or comparison boundary to distinguish the terms.
Include the burdens of the recovery route
- Collection, transport, sorting, washing and remelting still need resources. Compare those burdens with avoided mining, quarrying and processing for the same material and quality.
- A recyclable material has a possible processing route. Actual recycling also needs collection, suitable facilities and sufficiently clean material; do not treat the label as proof of its eventual disposal route.
Calculate a saving with a boundary
- Fictional equal-batch energy is 50 MJ for new production and 8 MJ for recycling. Saving is $50-8=42$ MJ, or $42/50\times100=84\%$ of the original route's energy.
- Check whether collection and transport are included before using that percentage. A lower energy total does not automatically mean lower water demand, fewer toxic emissions or less litter.
Which two limits belong with an 84% energy-saving claim from supplied recycling data?
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The 42 MJ saving relative to 50 MJ is 84% for the stated energy boundary. Other environmental categories and actual collection need their own evidence.
Transfer to a different reuse model
- Independent case: disposable energy is 15 units per task; reusable energy is 40 initially plus 5 per task. Equality is $15n=40+5n$, so $n=4$. Reuse uses less energy from the fifth complete task, not the fourth.
- If washing water is much greater for reuse, report the trade-off and local priorities. Missing transport or actual reuse information is a limitation to investigate, not a number to invent.
Use a consistent service and all relevant stages. Distinguish break-even equality from the first strict saving, and possible recyclability from actual collection and processing.
Disposable energy is 15 units per task; reusable energy is 40 initially plus 5 per task. At which first whole task does reuse use strictly less energy?
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15n = 40 + 5n gives 10n = 40, so equality is n = 4. Strictly less reusable energy starts at task 5: 65 units versus 75 units. Do not report equality at 4 as a strict saving.