What happens to our food waste?

Do you know what happens to the food waste that you put out in your caddy each week for collection? Biffa takes it to Severn Trent’s Green Power’s Anaerobic Digestion facility, east of the A4074 in Crowmarsh, where about 700 tonnes of food waste is treated each week to produce electricity and fertiliser for farmland. The site takes all the food waste from South Oxfordshire and Vale of White Horse District Councils (DC). It also takes commercial food waste from commercial waste collectors who collect from hotels, bakeries, and other businesses. Severn Trent Green Power also operate another Anaerobic Digestion facility at Cassington, which processes the food waste from West Oxfordshire DC, Cherwell DC and Oxford City.

On arrival at the site, the bin lorries tip the waste into a large bunker. From here, it goes into a ‘hammer mill’ where it is macerated to less than 12mm in particle size. Liquid waste is added to create a liquid which is pumped into a holding tank.

Any bags are ripped apart and removed by this process, even the biodegradable ones. This waste is taken to Grundon in Ewelme. They take it to a plant for energy recovery by incineration. To reduce single use plastic waste, it is fine to reuse bags we may already have in the home – an empty bread bag for example. Alternatively, you can line your food waste caddy with newspaper, or simply not use a liner at all.

Every couple of hours, some of the macerated food waste (which is called ‘soup’) is pumped into one of the five ‘anaerobic digestors’. You can see these green dome-shaped buildings as you pass 8 along the road. They are made of thick concrete, which is then insulated. On top of that is a green cladding. The domed roofs are made of what looks like thick rubber, and these contain the gases produced in the process. Each tank contains around 4,500 cubic metres of food soup, which is kept heated to body temperature, mimicking the human stomach. The soup is gently stirred, and as the food waste begins to break down, it gives of methane, which is captured in the domed roof of each tank. These tanks are heated using excess heat from the engines. This is then injected to one of two gas engines on site, which produce electricity, some 2.4 MW, enough to power 4,800 homes. (This amount of electricity is about 1,000 kettles being run continuously). Some of the electricity produced is used on the site and the rest is exported to the

National Grid. The waste takes 90 days to break down and passes through five digestors during this time. At the end of the process, the food soup, which is now called ‘digestate’ undergoes a final screen to remove any residual large solids and is then pasteurised at 71 degrees Centigrade for one hour using waste heat from the engines. This, along with many other processes on site, are requirements from the Animal By-Products Regulations, which were brought into being after the 2001 foot and mouth outbreak, and under which the site must operate. It ensures that the liquid fertiliser produced is fit to be applied to farmland. This happens within a seven-mile radius of the site. It is a more sustainable source of fertiliser for farms than artificial fertilisers produced from oil. So it’s really worth putting any food waste into your caddy, although it’s even more sustainable to reduce your food waste as much as possible.

I visited the site earlier this month. Several of the Green Power staff showed us around and explained the process. It’s a bit smelly in the building where the waste arrives but they do have a bio filter system to deal with this and to make sure it is not smelly outside. They also make you disinfect your footwear – another requirement of the Animal by-Products Regulations 2003. I organised the visit online through the Vale of White Horse Waste Team who organise several free trips, which last an hour, during the year: https://www.whitehorsedc.gov.uk/vale-of-white-horse-district-council/recycling-rubbish-and-waste/what-happens-to-your-waste-new/join-a-free-waste-facility-tour/. Beware there is a very long waiting list because it’s a working site and can only host a limited number of visitors per year.

More information, including a video, is available on https://www.stgreenpower.co.uk/food-

waste-recycling/. It’s well worth a visit on a nice day!

Jane Kinniburgh