Holdfast Bay compostable bag trial: food waste diversion from 9% to 37%
Prepared for the City of Holdfast Bay and Green Industries SA. Pilot ran March 2018 to March 2019. Final report 28 May 2020.
Summary published
Councils that hand out a kitchen caddy for food scraps run into the same problem. Residents need something to line it with, and buying bags to put rubbish in is a hard sell.
Holdfast Bay tried solving it somewhere else entirely. Two Foodland stores, at Brighton and Glenelg South, swapped their plastic fruit and vegetable bags for compostable ones sized to fit the existing dispensers. Shoppers carried produce home in a bag they could then use to line the kitchen organics basket.
Rawtec measured what happened to the bins.
Food waste diversion rate, March to November
More food waste in the green bin, per household per week
Of surveyed customers wanted the bags to continue
Dwellings that received a promotional flyer
What the bin audits showed
Diversion went from 9% to 37% in eight months
At the March baseline, 9.0% of household food waste was reaching the green organics bin. By May it was 20.1% and by November 36.9%. Food waste in the general waste bin fell from 1.99 to 1.00 kg per household per week over the same period, while correctly presented food waste in the green organics bin rose from 0.20 to 0.60 kg.
The bags were reused at home rather than binned
Supermarket bags turned up in general waste bins early on and dropped to almost zero by the late trial, while the number found in organics bins rose across successive audits. Nearly 250 kitchen organics baskets were handed out in the first months of the pilot, against roughly 60 in the same period the year before.
The surveys overstated how many people were using them
73% of Glenelg South and 44% of Brighton customers said they used the kitchen organics basket. Visual inspection found a compostable bag in about 50% of bins. Rawtec reported the gap rather than the survey figure on its own.
Cost per bag is the sticking point
Plastic produce bags cost between $0.01 and $0.025 each. The compostable bags used in the pilot cost $0.054. Holdfast Bay already spends $60,600 a year supplying 6,000 rolls of bags, reaching an estimated 40% of households, so a full supermarket rollout would cost more in total than the current model, while reaching more people.
What the extra diversion is worth
Rawtec estimated a further 308 tonnes a year could be diverted from landfill, cutting greenhouse gas emissions by 154 tonnes of CO2 equivalent and supporting an estimated 0.15 full-time jobs. The disposal saving was put at about $33,900, revised to $43,116 at 2020 landfill levy rates.
One drawback worth knowing
Fresh produce does not last as long in a compostable bag as in a plastic barrier bag, because more oxygen gets through. The report notes noticeable deterioration only began after two weeks.
What the report recommended
Order well ahead so the two supermarkets are never without bags. A gap in supply undoes the habit the pilot built.
Share the result and approach the rest of the sector rather than treating it as a council-specific scheme.
Local manufacturing was flagged as the most likely route to bringing the cost per bag down.
Compostable checkout bags were suggested as the next trial, to cut more plastic from the same shop.
The full report covers the audit methodology, all three customer surveys and the cost modelling in detail.
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