A new state-of-the-art anaerobic digester will turn London food waste into enough gas to power thousands of homes. Mike Farish reports. 

People who have a conscience about letting uneaten food go to waste will soon be able to sleep a little easier – at least if they live in the London area. That is because the city will soon be served by a state-of-the-art recycling facility just now in the process of being commissioned that will have the capacity to turn 160,000 tonnes per year of waste that would otherwise go to landfill into 14 million cubic metres of biomethane gas that can be fed into the supply grid. According to the company behind the facility, that’s enough to power 12,600 homes for a year.

The facility in question is the new ReFood plant in Dagenham out towards the eastern margin of the city, a smart looking installation of gleaming white buildings and brightly reflective metal piping which represents a total investment in both the land and the actual processing plant of some £32m. It will form the focal point for a citywide collection system of food waste and for larger scale recycling of liquid waste from food-related manufacturing. In the first case the sources of the waste food will generally comprise both restaurants and non-commercial institutions such as hospitals. In the second, likely sources might include, for example, breweries or any drinks manufacturing operation.

The mix is actually important because the process involves diluting the solid food waste with enough liquid to enable it to be pumped through the pipes and tanks of the facility and to act as a catalyst for the process itself. According to Philip Simpson, commercial director for ReFood, the process at Dagenham that converts the material into both usable gas and also fertiliser for use in agriculture is essentially that of anaerobic digestion (AD) – a well-established set of procedures and technologies.

The food waste arrives at the plant entirely conventionally in green plastic ‘wheelie bins’ of the sort found in any domestic residence. The bins are unloaded from the delivery vehicles in the central reception building in which the atmosphere is both ionised and slightly pressurised and are then taken to a station where they are automatically inverted to empty their contents into chutes that take their contents into the system. “Food waste is unloaded, de-packed and blended to create a homogeneous mix,” said Simpson. After a mechanical process to separate out any non-food material, the mixed, de-packed food waste is then heated to a temperature of 70degC to pasteurise it in order to neutralise any harmful bacteria, before being passed through a heat exchanger – which reverses the heat cycle to cool the product – and into a holding tank.

The next step in the now fully automated process sees the waste stream passed from the holding tank to one of the four digesters on the site with feeding taking place every 30 minutes.

The digestion process involves the food waste being broken down by different strains of bacteria.

In the first stage bacterial hydrolysis of the blended food waste breaks down the cell structure of organic molecules such as carbohydrates making them available for the other bacteria. Then acidogenic bacteria convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia and organic acids. Those organic acids are then further converted into acetic acid plus further ammonia, carbon dioxide and hydrogen by acetogenic bacteria. At the final stage, methanogenic microbes convert these products to methane and carbon dioxide in the process known as methanogenesis. The complete flow-through time for the process is some 30 days.

The combined methane and carbon dioxide is then scrubbed to remove any acid gases, followed by water washing to remove the carbon dioxide. The final stage sees the calorific value of the refined biomethane tested to ensure it meets the requirements of the National Grid before being compressed and exported down the pipe. The gas consumed in the pasteurisation stage represents only about five per cent of that produced by the process as a whole, so the net contribution to the grid is still considerable.

Moreover, the remaining material can be used as an organic bio-fertiliser. Indeed, the material marketed by ReFood under the name ReGrow is already produced at two already established processing facilities in Widnes and Doncaster.

Simpson said that the new Dagenham plant includes a number of upgrades in both the processing and bio-fertiliser production stages. The company is also seeking a BREEAM (Building Research Establishment Environmental Assessment Method) ‘Excellent’ rating, which would place it in the top 10 per cent of all UK new non-domestic buildings assessed that way.

Despite the benefits the process can bring in turning what would otherwise be waste into usable energy, the overall UK situation is somewhat mixed, according to Simpson. “In Scotland, Wales and Northern Ireland, significant steps have been taken to prevent food waste from being sent to landfill,” he said. “However, in England we have no such barriers to using landfill for the disposal of food waste.”

Even despite the lack of a landfill ban in England, the UK now has over 500 anaerobic digestion plants in operation. In contrast to the new Dagenham plant, though, the vast majority use crops such as maize, which are specifically grown for the production of energy. In fact, says Simpson, “it is believed that around 30 per cent of UK maize yield is now being used as a feedstock for AD plants.”

But using prime agricultural land to produce energy crops while allowing the landfilling of a far more suitable feedstock such as food waste is plainly not efficient at any level. As such, claimed Simpson, the argument for more facilities like the Dagenham plant are becoming more convincing: “The current volume of food waste being landfilled would support over 80 plants similar to Dagenham and produce enough biomethane for more than one million homes,” he said.