Dear tiki: if landfill sites that have waste food in them - like potato peelings - make methane, don't compost heaps do the same thing? puzzled, tara

Dear Tara: It's all to do with the bacteria. Bacteria in compost heaps are aerobic - which means they thrive in airy conditions when there's plenty of oxygen. The aerobic microbes break down the waste food and make useful compost which gardeners and farmers use to improve their soils. Sometimes very large compost heaps are made by companies who deal with garden 'waste' by the truckload. But although these are large, the bacteria still get plenty of air because machines regularly turn them over.
    In landfills, organic waste (like food, babies' diapers, paper and so on) just gets compacted together with everything else, rammed down by big machines. This means that any air left in the stuff quickly gets used up by the aerobic bacteria which then die. The waste then becomes anaerobic, which means there's no oxygen. Some bacteria - anaerobic bacteria - can live in these conditions because they don't need oxygen. The decay process is slow and smelly and one of their waste products is methane gas. In well managed landfills, this gas is collected and burned to make heat and electricity (on a small scale). If it isn't burned, the methane escapes into the atmosphere where it becomes a powerful greenhouse gas.
So all in all, it's better to make compost out of waste food.