Canteen lunch boxes after the EPS ban: compostable, recyclable or reusable?
The canteen of a Hong Kong secondary school serves 1,200 hot lunch boxes a day, 190 school days a year. Since the local ban on expanded polystyrene food containers, the caterer must choose a replacement. Option A: bagasse (sugarcane pulp) box, HK$1.20 each, 45 g, compostable in industrial composting, but the nearest facility is across the border and the school has no collection contract; the box is grease-resistant for about 45 minutes. Option B: PP box with lid, HK$3.50, reusable for about 50 cycles if washed, needs a deposit scheme and a dishwasher (0.9 kWh per 100 boxes, plus labour at HK$60/h at 3 minutes per 100 boxes), with 1,200 boxes in circulation × 3 for turnaround. Option C: paperboard box with PLA lining, HK$1.80, industrially compostable only, and the school's general waste goes to landfill. The school aims to cut lunchtime waste by mass by 50% within a year, and the canteen budget is HK$6 per meal.
1. Using a stated functional unit, compare the three options and recommend one, showing your calculations and naming any missing data.
Read the worked answer
Functional unit: one hot meal served to one student, including container, washing or disposal, and end-of-life. Annual volume = 1,200 × 190 = 228,000 meals.
Option A (bagasse). 228,000 × HK$1.20 = HK$273,600 per year. Mass to landfill = 45 g × 228,000 = 10.26 t. Cost is within budget, but all of it goes to landfill because there is no composting contract, so the 50% waste-mass target is not met.
Option B (reusable PP). Containers needed = 1,200 × 3 = 3,600, capital 3,600 × HK$3.50 = HK$12,600. Washing energy = 2,280 × 0.9 = 2,052 kWh per year, at about HK$1.3/kWh ≈ HK$2,667. Labour = 228,000 / 100 × 3 min = 6,840 min = 114 h at HK$60 = HK$6,840. Running cost ≈ HK$9,500 plus capital. Waste: about 3,600 × 60 g = 0.22 t in circulation, with perhaps 5% annual loss (about 11 kg). This meets the 50% waste target comfortably.
Option C (paperboard/PLA). 228,000 × HK$1.80 = HK$410,400 per year, and still landfill because there is no industrial composting route. The compostable claim delivers nothing here.
Recommendation: Option B, conditional on dishwasher throughput, storage space and a return rate of at least 95%.
Missing data to state explicitly: washing water and detergent impact, transport, actual breakage and loss rate, and whether the caterer will accept the handling. The judgement tested is that 'compostable' is not the same as 'low impact' when the end-of-life infrastructure is absent, and that a higher unit price can still be the cheaper system.
2. Explain why 'compostable' may not reduce environmental impact in this school, and what questions a designer should ask before accepting a 'green' material.
Read the worked answer
Compostability is a property of the material–waste-system pair, not of the material alone. Bagasse and PLA need industrial composting at roughly 55–60 °C with controlled moisture and aeration for several weeks. A landfill is anaerobic and cool, so neither breaks down on a meaningful timescale, and PLA can generate methane. In this school, with the nearest facility across the border and no collection contract, the actual end-of-life for all three options is landfill. The label therefore does not divert any mass; worse, 'compostable' wording can contaminate the PP recycling stream if users put the wrong items in the wrong bin.
Questions to ask before accepting a green material claim:
- Where will this item physically go after use, and who pays for that route?
- What is the functional unit, and are we comparing like with like (one meal, not one kilogram of material)?
- How many trips does the reusable alternative need before it beats single-use on energy and material? For most food containers the break-even is roughly 10–20 trips, and a 50-cycle PP box is well past that.
- What is the evidence for the claims - a certified standard, a specific facility, or marketing?
Even if a composting route existed, the reusable PP system avoids manufacturing 228,000 single-use items a year, and reuse generally dominates single-use once the trip count is high. That is why the answer is not simply 'choose the compostable one'.
3. Design a four-week pilot to test your recommendation, with metrics, thresholds and risks.
Read the worked answer
Scope. One canteen, four weeks, 300 students in two classes, PP system with a HK$10 deposit and a return point at the exit.
Baseline. Weigh the lunchtime waste stream for five school days before the pilot (kg/day) and record composition, so the 50% target has a real starting point.
Metrics.
- Return rate (% of boxes issued that come back).
- Breakage and loss rate per month.
- Washing throughput (boxes per hour) against dishwasher capacity.
- Student time in the return queue.
- Cost per meal against the HK$6 budget.
- Contamination rate in the return bin (non-PP items).
Success thresholds. Return rate ≥ 95%, loss ≤ 2% per month, cost per meal ≤ HK$6, lunchtime waste mass reduced by at least 50% against baseline.
Controls. Same menu and same weekdays before and during, to avoid confounding changes.
Risks. Students taking boxes home, the caterer refusing to handle washing, and hygiene concerns. Mitigate with a signed handling protocol, a visible deposit message at the till, and about 10% spare stock.
Decision gate. If the return rate is below 90% after two weeks, add an incentive such as a HK$2 discount on return, or move the return point to a two-bin station at the exit, and re-measure. The examinable judgement is that a sustainability claim should be converted into measurable trial criteria with a decision rule, rather than asserted.