Prevention and composting are four steps apart in EU policy, and only one of them cancels the shopping. The European Commission's food use and waste hierarchy, drawn from the Waste Framework Directive, puts prevention first, then redistribution of surplus food, use as animal feed and industrial use, with waste treatment — the step that contains composting and anaerobic digestion — in fifth place, ahead only of energy recovery and disposal. The condition that changes which one matters to you this week is whether the food has already been bought: before the shop, only prevention is available; after the peel is on the board, only treatment is.

Four steps apart, and what each step decides

Prevention acts on the amount that enters the kitchen. Treatment acts on material that is already waste, and it decides what that material becomes — gas, electricity, fertiliser or compost — rather than whether it existed.

Write the two questions on separate lines and keep them there. "Could the amount match the meals people wanted?" is a purchasing question with an answer in grams and pounds. "Which local route accepts this material?" is a service question with an answer on the local waste service's own page. An improvement in the second does not appear in the first, and a bin that gets lighter proves neither on its own.

Not everything in the caddy was preventable. An unused whole ingredient, a portion nobody wanted and an inedible part raise different questions, so name the material in the notes rather than calling all of it leftovers. Across the UK in 2021 and 2022, the edible parts were 6.1 million tonnes of the 8.6 million tonnes measured after the farm gate and the inedible parts 2.5 million tonnes, 73% against 27%, in WRAP's UK food waste key facts.

What the two routes do to the material, in one English district

South Oxfordshire District Council, in England, publishes what happens to each of its bins, and the contrast is concrete. Food waste goes to an anaerobic digestion facility in Wallingford, where liners are removed, the food is mixed with water and pumped into sealed tanks heated to 37°C and stirred for around 85 days; the gas released is converted into energy for up to 4,800 homes, and what remains is sold to local farmers as fertiliser (what happens to your waste).

Garden waste from the same district goes to open windrows at Wallingford and takes eight to ten weeks to break down into compost sold to farmers. Non-recyclable waste goes to the energy recovery facility at Ardley, whose electricity the council puts at enough for over 60,000 homes across Oxfordshire, and it states that this route has diverted 95% of its residents' rubbish from landfill.

Two instructions from that same council page are worth copying exactly, because they are the kind of thing a symbol does not tell you: no food packaging in the food caddy, including compostable packaging, because the process runs with little or no oxygen and compostable material contaminates the fertiliser; and no compostable sacks in the garden bin. Check your own service before applying either, since routes differ by district — that lookup is what where does our household waste actually go is for.

An example: one 750 gram bag of carrots, counted twice

The example below shows its arithmetic. Two meals need 400 grams of carrot. A 750 gram bag is bought and 400 grams are used, leaving 750 − 400 = 350 grams.

On the prevention side, buying the 400 grams the meals actually need removes that 0.35 kilograms from the kitchen entirely. At the UK 2022 value of edible household food waste — £260 for 65 kilograms per person, so £260 ÷ 65 kg = £4.00 a kilogram — that is 0.35 kg × £4.00 = £1.40 not spent (key facts).

On the treatment side, the same 350 grams goes into the caddy, spends about 85 days in a sealed tank and comes out as energy and fertiliser. That is a better destination than the residual bin, and the £1.40 is still spent. Peelings from the 400 grams that were eaten go the same way and were never preventable; those two facts do not cancel each other.

Give each person the decision they actually hold. One writes the amount on the list before the shop; the other checks the local service's page for the exact item and copies the wording, dated. Then agree one action at whichever stage is weakest — an unclear local instruction is resolved by a single lookup, while a repeated surplus is resolved on the list, as in one shopping list for changing plans.

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