Chemistry · Lesson

Lifecycle assessment, circularity and sustainability trade-offs

Evaluate lifecycle and circularity claims through equal service, allocation assumptions, physical recovery and scenario sensitivity.

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By the end, you can

Compare burdens per equivalent delivered service under stated lifetimes.
Distinguish allocation from a physical inventory change.
Close a recovery chain before applying a separate displacement assumption.
Compare scenario sensitivity while retaining distinct impact units.

A lifecycle comparison starts with equivalent service

Define the service delivered, duration, performance requirement and system boundary before comparing routes or materials. Production, transport, use, maintenance and end-of-life can contribute differently. A per-kilogram result may not compare equivalent service.

A reference flow converts the functional requirement into the amount of material or energy the system needs. Different lifetimes or efficiencies can therefore change the required amounts even when the function is the same.

A comparison needs the same function and an explicit method

A functional unit states the service compared. In the original lifetime example, each use has equal stipulated quality, but the options provide different numbers of uses. Dividing lifetime totals by their own assumed service counts changes the result. Durability evidence matters because a shorter life can reverse the per-use comparison.

When one process produces several outputs, allocating a shared burden is a method choice. Mass and economic partitions use different denominators. Changing a price assumption changes the economic attribution while physical output and total burden stay fixed. An allocation number is not an observed change in chemistry.

A fuller assessment should report goal, system boundary, inventory, characterization method, uncertainty and assumptions about cofunctions. A substitution credit needs an explicit counterfactual describing what production is displaced; it should not silently appear as another physical stream.

An inventory and an impact indicator are different steps

A lifecycle inventory records defined resource inputs and output streams. An impact calculation applies a stated method to those flows. Several indicators can describe different consequences, and they need not rank choices identically.

A climate-equivalent quantity additionally requires the specified characterization factors and time-horizon convention. It is not the mass of one physical mixture or a complete measure of environmental harm. State the method rather than assuming all impact indicators share one basis.

Recovering material does not settle its replacement value

Collection, recovery and qualification are successive physical stages. Count their rejected streams as well as the returned material. Multiplying the fractions gives a qualified-mass share; it does not by itself establish equal function or virgin replacement. The separate assigned displacement factor in the case below makes that distinction visible.

A scenario can change a climate-category comparison while leaving other quantities unchanged. In the invented grid-factor case, one option uses less electricity but has more fixed other climate burden, creating a crossing. The water-volume inventory remains separate from climate burden and from characterized water-scarcity impact.

Circularity and sustainability claims should retain those assumptions and unresolved categories. A mass recovery percentage or one favorable impact score cannot demonstrate every environmental outcome, practical quality or net benefit. Use sensitivity to identify which evidence would materially change the conclusion.

One invented climate comparison changes with the electricity factor

Assigned burden / kg CO₂eq

Assigned g / kg CO₂eq kWh⁻¹

Assigned option A
Assigned option B
— Assigned A climate inventory

Underlying data

Assigned g / kg CO₂eq kWh⁻¹	Assigned burden / kg CO₂eq
0.22	1.912 · Assigned option A
0.45	2.9699999999999998 · Assigned option A
0.75	4.35 · Assigned option A
0.22	2.238 · Assigned option B
0.45	2.9050000000000002 · Assigned option B
0.75	3.775 · Assigned option B

Original equal-function inventory model A=0.900+4.60g and B=1.60+2.90g. Circles/squares mark g=0.220,0.450,0.750; dashed curve evaluates A. Water 8.00/22.0 L remains a separate inventory. No actual grid factor or complete sustainability ranking is supplied.

Circular use still has losses and additional requirements

Reuse, repair and material recovery can reduce some new inputs, but collection, recovery yield and suitable quality constrain the actual loops. Retained material can circulate repeatedly while losses require replacement.

Energy, transport, auxiliary materials and discarded streams belong to a complete account of the chosen loop. Avoid counting the same internal circulation as new recovered output every time it crosses a unit boundary.

A conclusion needs its assumptions and sensitivity

Allocation among coproducts, energy supply, lifetime and recovery assumptions can affect a comparison. Test how the result changes when uncertain inputs vary and report which conditions support the conclusion.

The original lifecycle balances below show boundary and sensitivity reasoning. They are neither environmental certifications nor purchasing, investment or legal-compliance advice.

Worked examples

Compare the same delivered function

Invent two options delivering the same quality per use. A has manufacturing burden 9.00 kg CO₂eq, energy 0.120 kWh/use, assigned life 90 uses and end-of-life burden 0.800 kg CO₂eq. B has 16.0 kg, 0.0550 kWh/use, 220 uses and 1.20 kg. Assign electricity factor 0.450 kg CO₂eq/kWh on a common characterization convention and horizon.

A lifetime total is 9.00+0.120×90×0.450+0.800=14.66 kg CO₂eq. Per equivalent use it is 14.66/90=0.162888889 kg CO₂eq.
B lifetime total is 16.0+0.0550×220×0.450+1.20=22.645 kg CO₂eq. Per equivalent use it is 22.645/220=0.102931818 kg CO₂eq.
B has a larger lifetime total but a smaller burden per equivalent use under the assigned lifetime. Equal function and survival over that life are assumptions. Other impacts, maintenance, replacement timing and actual durability are not supplied.

On the stated per-use basis, B is lower: 0.102931818 versus 0.162888889 kg CO₂eq per equal use.

Allocation changes an attribution, not the inventory

Assign one joint-output burden 174 kg CO₂eq, desired product 24.0 kg and coproduct 46.0 kg. Invent prices 3.20 and 0.800 consistent arbitrary units/kg. Compare mass and value allocation.

Mass share for product is 24/70=0.342857143. Assigned product burden is 174×24/70=59.6571429 kg CO₂eq, or 2.48571429 kg CO₂eq/kg product; the remaining 114.342857 kg is assigned to the coproduct.
Assigned values are 76.8 and 36.8 units. Product value share is 76.8/113.6=0.676056338. Product burden is 117.633803 kg CO₂eq, or 4.90140845 per kg product; coproduct burden 56.3661972 kg.
Each method’s two shares sum to one and allocated burdens sum to 174 kg. The physical outputs are unchanged. Neither chosen partition is established as universally causal or objectively preferable; avoiding allocation or a defined system expansion can address other questions.

Mass and invented-value shares differ; total burden and physical outputs stay fixed.

Returned mass and assumed displacement are different accounts

Start with 625 kg discarded hypothetical material. Assign collection 0.820, recovery of collected material 0.870 and qualifying share of recovered material 0.900. Separately stipulate 0.700 kg virgin displacement per kg qualifying return.

Collected mass is 512.5 kg. Recovered mass is 445.875 kg; qualifying return is 401.2875 kg.
Uncollected mass 112.5 kg, processing reject 66.625 kg and quality reject 44.5875 kg close the physical ledger: 112.5+66.625+44.5875+401.2875=625 kg.
Assigned avoided virgin mass is 401.2875×0.700=280.90125 kg. This is a counterfactual functional result, not another physical output to add to 625 kg.
Energy, transport, reprocessing burden and the displaced-production account are absent. Collection does not prove full quality, functional substitution, hazard destruction or a net environmental benefit.

Physical qualifying return 401.2875 kg; separately assumed virgin displacement 280.90125 kg.

Sensitivity can reverse one impact comparison

For the same assigned function, A uses 4.60 kWh with other climate burden 0.900 kg CO₂eq and water inventory 8.00 L. B uses 2.90 kWh with other burden 1.60 kg and water 22.0 L. Let invented grid factor g vary under one common climate convention.

Climate totals are A=0.900+4.60g and B=1.60+2.90g kg CO₂eq. At g=0.220 they are 1.912/2.238, favoring A in this category. At g=0.750 they are 4.350/3.775, favoring B.
Equality occurs when −0.700+1.70g=0, so g=7/17=0.411764706 kg CO₂eq/kWh. This is a mathematical crossing in an invented inventory model, not a current grid datum or physical safety threshold.
Water volumes remain 8.00/22.0 L. They are inventory quantities without location, scarcity or quality characterization. They cannot be added directly to kg CO₂eq. More evidence and explicit value choices are needed for an overall assessment.

Climate ranking changes at the assigned crossing 7/17; the separate water inventory does not disappear.

Try it yourself

Halve only option B’s assigned lifetime to 110 equivalent uses. Recompute its lifetime climate total and per-use intensity, and compare with unchanged A.
Show the answer and reasoning
B total 19.9225 kg CO₂eq over 110 uses, intensity 0.181113636 per use; unchanged A 0.162888889 is now lower.
The fixed manufacturing/end-of-life burden is spread over fewer equivalent uses, increasing intensity even while the lifetime total falls.
Double only the assigned coproduct price to 1.6 units/kg. Which allocation result changes, and which physical/mass-based quantities stay fixed?
Show the answer and reasoning
Value share 24/47=0.510638298; product burden 88.8510638 kg CO₂eq, or 3.70212766/kg; physical masses, total 174 kg burden and mass share 24/70 stay fixed.
The changed invented coproduct value changes only that allocation method; both assigned burdens still sum to the same total.
Change only the assigned displacement factor to 0.45 kg virgin/kg qualifying return. Recompute avoided virgin mass and state which physical flows change.
Show the answer and reasoning
Assumed avoided virgin mass 180.579375 kg; qualifying 401.2875 kg and all physical flows stay fixed.
The functional displacement factor changes the counterfactual result rather than the closed mass recovery chain.
Set the assigned grid factor to 0.45 kgCO2eq/kWh while retaining all other invented inputs. Compare climate totals and water inventories.
Show the answer and reasoning
A 2.970 and B 2.905 kg CO₂eq per equal function; B is slightly lower in climate burden but water stays 8.00 versus 22.0 L.
Evaluate both unchanged linear inventories at g=0.450; keep the different impact/inventory units separate.

What to carry forward

Compare the same delivered function and disclose lifetime assumptions.
Allocation partitions a burden without changing the physical inventory.
Material return and counterfactual virgin displacement need separate ledgers.
Sensitivity in one category does not remove other impact dimensions.

References

Teaching resources and reference material supporting this lesson’s explanations and examples.