Life-Cycle Assessment & the Circular Economy

Judging products by their whole life. Learn life-cycle thinking, ISO 14040/44 phases (goal and scope, inventory, impact assessment, interpretation), functional units and system boundaries (cradle-to-gate/grave/cradle), allocation, impact categories (climate, acidification, eutrophication, water, toxicity), break-even analysis, the waste hierarchy and circular-economy strategies (reduce, reuse, repair, remanufacture, recycle), EPR and design for environment — with Python comparing reusable and single-use bottles.

Is a paper bag better than a plastic one? An electric car better than petrol? It depends on the whole life cycle — raw materials, manufacturing, transport, use and end of life — and on which impacts you count. Life-cycle assessment (LCA) answers such questions quantitatively and avoids 'burden shifting' from one stage or impact to another.

The circular economy uses those insights to keep products and materials in use at their highest value for as long as possible, designing out waste.

An analogy

LCA is like judging a restaurant meal by the whole supply chain — farm, transport, kitchen energy, washing up and food waste — not just by the plate in front of you.

Life-cycle thinking

Every product has impacts at every stage: extraction, processing, manufacturing, distribution, use and end of life. Cradle-to-gate stops at the factory gate; cradle-to-grave includes use and disposal; cradle-to-cradle loops materials back. Comparing products fairly requires the same function (functional unit, e.g. 'carrying 1000 litres of drinks').

The four ISO 14040/44 phases

An LCA study:

• Goal & scope — purpose, functional unit, system boundary, assumptions
• Inventory (LCI) — all inputs (energy, materials, water) and outputs (emissions, waste) per FU
• Impact assessment (LCIA) — classify and characterise into categories (ReCiPe, EF 3.0)
• Interpretation — hotspots, sensitivity, uncertainty, conclusions; critical review for public comparisons

Circular strategies

The waste hierarchy: prevent > reuse > recycle > recover energy > dispose. R-strategies in order of value retention: refuse, rethink, reduce, reuse, repair, refurbish, remanufacture, repurpose, recycle, recover. Design for disassembly, modularity, durability and mono-materials enables them. Extended producer responsibility (EPR) — e.g. India's rules for plastics, e-waste and batteries — makes producers responsible for end of life.

Methodological choices

Allocation between co-products (mass, economic, system expansion), recycling methods (cut-off vs avoided burden), attributional vs consequential LCA, dynamic LCA for timing of emissions, and data quality/uncertainty (Monte Carlo) can change conclusions — so they must be transparent. Tools: SimaPro, GaBi/Sphera, openLCA with ecoinvent data. Environmental product declarations (EPDs) publish verified results.

Formulas

• Impact score: Impact = Σ (inventory flowᵢ × characterisation factorᵢ)
• Reusable vs single-use: Reusable total = production + N × per-use impact
• Break-even uses: N* = production_reusable / (single-use impact − per-use impact)
• Recycling credit (avoided burden): credit = recycled mass × (virgin − recycled production impact)
• Material circularity (simplified): MCI ≈ 1 − LFI·F(X)

Worked example: Break-even

Given: Reusable cup 1.2 kg CO₂e, wash 0.02; disposable cup 0.06

Find: N*.

• 1.2/(0.06 − 0.02)

Answer: 30 uses

Worked example: Recycling credit

Given: 1 t aluminium recycled; virgin 12, recycled 0.6 t CO₂e/t

Find: Avoided emissions.

• 12 − 0.6

Answer: 11.4 t CO₂e per tonne

Worked example: Use-phase dominance

Given: Fridge: 300 kg CO₂e manufacture, 250 kWh/yr × 0.7 × 12 yr

Find: Share of use phase.

• Use = 2100 kg
• 2100/2400

Answer: 87.5% — efficiency matters most

Reusable steel bottle vs single-use PET

# Simplified cradle-to-grave LCA: reusable steel bottle vs single-use PET bottles
uses = 500
pet_bottle = 0.083           # kg CO2e per single-use 0.5 L PET bottle (production + EoL)
steel_bottle = 3.0           # kg CO2e to make a steel bottle
wash = 0.005                 # kg CO2e per wash
steel_total = steel_bottle + wash * uses
pet_total = pet_bottle * uses
print(f"{uses} uses: steel {steel_total:.1f} kg CO2e vs PET {pet_total:.1f} kg CO2e")
breakeven = steel_bottle / (pet_bottle - wash)
print(f"break-even after {breakeven:.0f} uses")
# 500 uses: steel 5.5 kg CO2e vs PET 41.5 kg CO2e
# break-even after 38 uses

The reusable bottle has a higher 'up-front' impact but wins after about 38 uses and is 7.5× better after 500. The same break-even logic applies to shopping bags, cups and packaging — reusables only win if actually reused enough. (Factors here are illustrative; real LCAs use databases like ecoinvent.)

Common mistakes

• Wrong: Comparing products with different functions. — Right: Define a clear functional unit.
• Wrong: Looking only at climate impact. — Right: Check water, toxicity, land use to avoid burden shifting.
• Wrong: Assuming recyclable = recycled. — Right: Use real collection and recycling rates.

Applications

• Eco-design: Finding hotspots early in product design.
• EPDs for construction: Low-carbon cement, steel and timber choices.
• Packaging decisions: Reusable vs recyclable vs compostable.
• Policy: EPR schemes and green public procurement.

Key takeaways

• Judge products over their whole life.
• Goal/scope → LCI → LCIA → interpretation.
• Break-even logic for reusables.
• Circular strategies keep value highest longest.

Going further

• Consequential LCA: Modelling market-mediated effects of decisions.
• Dynamic LCA: Time-dependent characterisation of emissions.
• Material flow analysis: Tracking resources through economies (Sankey diagrams).