Resource intensity and the functional unit

A lightweight container can look better than a heavier one when compared item for item, but that comparison does not answer the service question. A durable container may serve many cycles. Define the functional unit before estimating impact: deliver 1,000 litres of water to users at equivalent safety and quality over a specified period. The unit is neither a thousand containers nor a tonne of plastic, since those are inputs that differ between options. Fix the temporal and physical boundary and list included stages: container production, washing, transport, loss and end of life. If a stage is excluded because data are absent, disclose it and test its plausible effect rather than calling the comparison complete.

Separate material intensity into new mass per unit of service, and water or energy intensity into consumption per unit of service. Do not convert washing water into emissions without a supported factor or claim confirmed savings from mass alone. For a screening comparison, display a vector: grams of new material per litre, wash water per litre delivered, and transport trips per 1,000 litres. Reuse may perform better on material yet worse on water or transport; the decision owner must identify impact priorities and safety limits. Check service equivalence first. If one option cannot maintain hygiene or the required delivery rate, a favourable calculation does not make it an acceptable option. The quality requirement is a gate, rather than another point to average into a score.

A 90% return rate does not automatically deliver 20 actual uses. If 10% is lost independently and consistently after each cycle, expected uses before loss follow a geometric series, 1 + 0.9 + 0.9 squared and so on, approaching ten over a long horizon. That is a conditional estimate, not a measured lifespan. Some returns may be late, physical damage may end use before loss, and incentives may change return behaviour. Distinguish ‘returned in this batch’ from ‘lifetime services’. Build entry cohorts: number of containers beginning service, actual cycles achieved, and reasons for exit. Without a cohort record, 20 cycles is a design target. Obtain measured washing-water inputs and any actual reuse of water, while retaining hygiene criteria; water conservation cannot be pursued by silently relaxing safety.

Test sensitivity instead of reporting only a central figure. At 12, 16 and 20 uses, durable material in exercise A is 40, 30 and 24 kilograms respectively; compare each with 60 kilograms for single use. These are scenarios, not a probability distribution without frequency evidence. Label low, central and high assumptions and explain each. If washing is constant per service, its water demand follows delivered services rather than the number of new containers, and may remain higher even in the optimistic material scenario. A long reverse-logistics route requires actual return-trip and load data. Counting only the outward distance can favour or penalize the alternative depending on the network. A short monitored pilot should precede a long purchase commitment.

In the client report, show a decision table instead of a marketing headline: functional unit, comparison boundary, calculated material mass, wash water, missing transport evidence, safety condition and reconsideration threshold. With a 300-gram durable container versus a 30-gram single-use one, the material-only break-even is ten services. At ten, material mass per service matches; other impacts do not thereby match. If the pilot produces fewer than ten services, do not recommend broad adoption on material savings grounds. Above ten, investigate water, transport and cost; no one metric settles the decision. Keep assumptions separate from observed results so new operational data can change the recommendation without rebuilding every calculation.

A fractional quotient of services divided by average life is not a purchasable container. Round up for whole-unit procurement, but do not call the rounded amount waste: the last unit may retain life. Distinguish starting inventory from material consumed over lifetime. Concurrent peak demand and washing turnaround can require many containers at once even with a long average life. Track units rejected after hygiene inspection as failed serviceable return rather than lost assets. Poor return and slow washing may create similar purchasing pressure but require different interventions.