Wednesday, 21 March 2012

Food waste is a waste, even if we make it into biogas

 I read recently about a second installation which will be injecting biogas generated from food waste into the gas grid for general use. (Food Waste Biogas for Grid Injection Contract in UK) This is excellent news, as it increases flexibility in how we can use biogas. In this case as in others, the gas is generated by anaerobic digestion of organic waste. That means it is processed by bacteria in large tanks, a little like huge compost bins except temperature controlled and without air. The bacteria produce gas which is a mix of methane and carbon dioxide plus some smelly stuff so it has to be cleaned up before it can be injected into the grid. There is also a lot of sludge left over which can be spread on agricultural land as fertiliser.

Sadly, however, there is no way that recycling food waste for energy will contribute more than a fraction of our energy usage and it doesn't make it OK to waste food because it took a lot more energy to grow and deliver that food than we can recover from recycling it.


According to WRAP we waste around 10.5 million tonnes of food per year in this country, of which about three quarters is thrown out by households and the rest comes from earlier in the supply chain [1]. This food is of all types: meat and dairy, salad, fruit, bread and potatoes -- all sorts.

If absolutely all that waste was processed by anaerobic digestion, you would get around 25 TWh of gas [2]. In practice many people will put that waste into compost for the garden.

In 2010 the UK used a total of 1254 TWh of gas, of which 389 TWh was used by households [3]. This food waste could contribute 2% of our overall gas consumption or 6% of our domestic gas. (If we reduced our gas usage, then food waste could supply a greater proportion.)

Growing and delivering our food takes about 11% of our overall energy consumption, or 300 TWh [4]. I don't have a figure for how much we eat in total, so let me guess we eat 1kg food per day each. That comes to 22 million tonnes of food per year. Add 10.5 million tonnes wasted - that means a third is wasted which required 100 TWh to produce. The energy from biogas from digesting it would return only a quarter of this. These numbers are illustrated in the chart below.


If you are interested in reducing food waste,  here are some tips from a group I am involved in: Cambridge Sustainable Food Alliance. or visit Love Food Hate Waste. You may also like to contribute to the Green Food Project online discussion (DEFRA).


[1] For 2008/2009 WRAP reports 11.25 million tonnes (excluding packaging) but the November 2011 update suggests this was reduced somewhat.
[2] 0.17 kg methane per kg food waste, which has an energy value (as heat) of 8500 kJ (www.anaerobic-digestion.com)).
[3] Digest of UK Energy Statistics 2011 (DECC)
[4] Energy and Carbon Emissions: the way we live today.  by Nicola Terry

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