Local Energy From Food Waste

May 25th, 2011 by Katharine Mann Leave a reply »

Green Officer’s visit to Agrivert’s anaerobic digester at Cassington on Thu 3rd March 2011

The 6 month old facility is on the edge of existing gravel pits and is run by 3 operators and supporting staff. It’s one of 5 commercial sized digesters in Britain.

The 2 products of the anaerobic digester are:

  • Digestate/ liquid agricultural no odour fertiliser which is rich in nitrogen, phosphorus, potassium and sulphur which is used on their own farm and by farmers in the local area and produces enough to cover 3,000 hectares. They don’t process the digestate to turn it into a liquid and solid as this is energy intensive so the carbon emissions in producing this fertiliser are much less than other fertilisers.
  • Methane or biogas gas (4.5 million cubed units a year) used to turn turbines to generate electricity in the 2.1 MegaWatt plant. 11% of the electricity is used to power the plant and the engines, the rest is fed into the national grid to power 4,500 homes.

Reception area: 120 tonnes of food waste each week day is received from West Oxfordshire, South Oxfordshire, Vale of the White Horse and half of Oxford city and is deposited here. Each Council put all their food waste in one large vehicle to reduce the number of vehicles travelling on the A40. The doors to and from this area open and close very quickly to minimise the amount of odour which can escape into the local area.

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Unpackaging machine: removes the contaminates in the food waste eg plastics, cardboard, compostable bags. The compostable bags go to a composting site where the temperatures are higher and they break down.

Food waste is then moved into the pasteurising tanks to be sterilised at high temperatures as it contains animal by products. These tanks are heated by the excess heat from the engines. It’s cooled before being moved into the digestion tanks.

Digestion tanks: there are 5 tanks, each of which can hold 450,000 tonnes of solid food waste (more capacity for liquid waste eg from breweries, dairies and abbatoirs). They have been sunk into the ground and clad in steel to reduce visual impact. The temperature is similar to human body temperature ie 38 to 42 degrees C and this is where the anaerobic (without oxygen) digestion takes place. They each contain:

  • 4 giant stirrers, only 1 is needed but the others are back ups and will be bought into use as they break
  • flexible diaphragm layer under the cover which move as the gas levels change
  • bacteria which has been specially produced to produce a lot of gas. The bacteria, like babies, were initially fed liquids then food waste was gradually introduced. If the mix goes out of balance or there is a lack of food waste silage (like baby rice) is added. The may and barley silage is grown on Agrivert’s own 500 acre farm nearby in Yarnton and if there’s excess they sell it as animal feed.

Control room: contains a screen showing all the components in the process, how they are operating and the 5 digesters and the levels of methane in each.

Pump room: powers the pumping of the food waste between the primary and secondary digestion tanks and each time it’s broken up further. The food waste spends 50 days in the first tank then it’s pumped into the second tank for 30 days then another 50 days in the final tank so 130 days in total so it has low odour. There are taps to sample the digestate and check the balance in the tanks.

Bucket of sample digestate: this has been in the system for about 90 days so it’s not ready yet.

Digestate loading bay: 27 tonnes of digestate are pumped into the tanker in 4 minutes and this then sold to local farms and fed into a machine to spread it on the land.

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Gas engine for the 2.1 MegaWatt plant: the methane gas is fed from the digestion tanks through a condensate pipe into the gas engine to turn turbines to generate electricity, of which 8% is used in the plant and 3% to power the engines. The excess electricity goes into the National Grid and is enough to power 4,500 homes (size of Eynsham). Heat is also produced and this is used to heat the pasteurisers and the digestion tanks and may be used in the future to dry wood chips which can be used in boilers to heat Oxfordshire schools.

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