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. The original is on the Green Industries SA website.
Summary published
Residents given a kitchen caddy for food scraps need something to line it with.
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%, and the bags were reused at home
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.
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.
Survey results and the cost per bag
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.
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, and produce shelf life
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.
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.
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.
Related reports
Other organics and kerbside work.
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