It stinks, but food waste is feeding our hunger for energy

From the plate to the power station. Flickr/tomylees

The amount of scrap food thrown away worldwide is staggering. WRAP, a government-funded non-profit set up to encourage recycling and clamp down on waste, reports that in the UK we discard more than 7.2m tonnes of unused food and drink every year. About the same amount is generated from restaurants and the food processing industry. The waste of just one year could fill Wembley Stadium more than 15 times.

In most developed countries economic well-being is inextricably linked with poor use of resources, including food. But developing countries also waste enormous quantities of food, usually due to poor preservation techniques such as lack of refrigeration. While we can try to reduce the quantity of food wasted, it will probably always happen – the question is what to do with it.

Landfill? No thanks

Food waste typically contains carbohydrates such as potatoes and rice, proteins from meat and pulses, lipids such as animal fat and cooking oils, and water from washing and cooking.

Carbohydrates, proteins and lipids are carbon-based, organic compounds that are largely biodegradable. Dumped in the open or in landfill, they degrade to produce horrible smells and toxic liquids and gases. Gases may include flammable methane and carbon-dioxide, both potent greenhouse gases. Liquid chemicals produced from wastes can become solvents, dissolving any toxic metals they encounter – for example, in dumped electronics or industrial waste in landfills – and carrying them down through the soil to pollute the groundwater.

An EU landfill directive requires that waste is diverted from landfill and sets stringent controls. So the Department for the Environment, Farming and Rural Affairs has to encourage other means of waste disposal than just burying it in a big hole in the ground.

Making a resource out of rubbish

Rather than a problem, waste should be regarded as a high-quality, renewable resource. Rich in oils and starches formed from hydrocarbons, there is ample opportunity to convert the contents of our bins into renewable energy to power homes.

How can we turn waste into energy? One way is to use bacteria and microbes, the sort of chemical transformation that occurs naturally in anaerobic digestion or fermentation. The bugs feed on the food and break it down into methane gas or ethanol – two excellent fuels – and in doing so reduce the enormous volume of our hypothetical Wembley-sized food waste mountain to the size of a school football pitch. The leftover sludge can be treated further and used as agricultural manure.

The UK has 46 anaerobic digestion plants to do just this, and produce heat and power, ranging from small 150-tonne plants to Orchard House Foods in Northampton which processes 150,000 tonnes per year. At 1.8 million tonnes per year, these plants digest 12% of the total annual food waste generated in the UK and generate 54MW of power.

But the plants need a lot of space, and the bugs won’t eat just anything. They’re not partial to fatty foods, which means a considerable amount of food waste needs different treatment. They’re also somewhat flatulent, with odours often wafting near treatment plants.

The heat is on

But a new area of interest for the waste-to-energy business is in using heat to break down food waste into fuel and chemicals, known as thermal processing. It’s much quicker than microbes while reducing the size of the waste as efficiently. Most usefully thermal processing is not picky and tackles all types of food – fatty or otherwise – with ease.

Straightforward incineration produces large amounts of heat that can be used to generate electricity or as residential heating. There are 20 British power stations burning biomass for electricity, of which those burning food residues turn 1.3m tonnes of waste a year into nearly 120MW of electrical power. Now a power plant under construction in East London by energy company 2OC will burn waste cooking fats and oils dug from London’s sewers to generate 20MW – enough to power 39,000 homes for a year. Landfill losses are being turned into green energy gains.

Other methods such as pyrolysis, gasification and hydrothermal treatment use heat but without combustion. The result is liquid bio-oil and synthetic gas that can be refined into petrol or diesel replacements, and a solid bio-char that can be used to improve soils.

With the amount of food waste we create showing no sign of slowing, the waste-to-energy sector has found its feet. Climate change, depleted fossil fuels, and plentiful raw material are all factors in its favour.

Thermal processing might involve up-front expense to build the plants, but once running is a cleaner, quicker producer of more power per tonne of waste than the alternatives. Alchemists still can’t spin lead into gold, but in an energy-hungry world turning trash into electricity might just be the next best thing.

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