A visit to the Barfoots anaerobic digester
Barfoots are a sweetcorn grower and processor based on the South Coast close to the port of Portsmouth. They take in sweetcorn cobs from their own fields in the summer and ship in more from other countries the rest of the year. They had a lot of organic waste to deal with and opted for an AD to use it to generate electricity to power their factory.
The plant has been running for over 2 years now and is a good example of a closed loop system.
The factory waste travels about 500m to the location of the AD. It is initially stored outside in silage bays where the process begins.
When the silage has fermented a little it is moved to a container where the digesters are automatically fed from at regular intervals.
In this picture you can see Chris Cooper of Barfoots explaining the AD process to us and in the background is a view of the AD tanks.
Barfoots have two primary digesters and one secondary tank.
The tanks are concrete base and walls, sealed and has a plastic skirt to catch any leaks. The grey roof is made of a double skin sheeting. Methane fills the lower section and air is pumped into the upper section to keep the methane pressure constant and inflate the roof to deflect rain.
This picture shows the flares that are lit when the pressure gets to much and there is a release of methane. It doesn't happen very much.
Methane is a 20 times worse greenhouse gas than Carbon dioxide so in the event of a methane leak, it is far better to burn it than leave it as methane.
This is a picture of one of the two generators that convert the methane into electricity. Any excess electricity is exported to the national grid.
One generator runs permanently and the other is brought on line at times of peak electricity demand when the spot market price is high.
Finally the residue of the the digester called the digestate is stored in a lagoon. The digestate is a liquid with fine suspended particles and has high nutrient value as a fertiliser. Barfoots link into an exisiting irrigation system to spray it onto the field without the need for further transportation.
When we visited, the only smell we noticed was a slight silage smell. There didn't appear to be any smells coming from the digesters. The Lagoon used to whiff a little until a neighbour complained and Barfoots now float a layer of plastic pellets with a special coating on top which neutralises the smell.
It is a very good model for an AD. Small scale and entirely fed locally by their own factory and the digestate spread on their own fields.