Learn about AD

Anaerobic digestion (AD) is a safe biological process in which food waste and other organic materials are broken down by micro-organisms in the absence of light and oxygen, to produce renewable energy and a nutrient-rich biofertiliser.

In summary, Anaerobic Digestion is based on a very simple, natural process. If managed correctly, with well a constructed and reliable plant, it is a consistent and sustainable source of renewable energy.

AD turns the problem of food waste into 2 valuable resources:

  • Bio-fertilisers that help our farmers enrich the UK's soils without the use of petro-chemical fertilisers, which are costly to our environment, and;
  • Green electricity to power our local homes and businesses, generated from the very plate scrapings they put into their food waste bins only weeks before.

Our video "The journey from food waste to renewable power" explains the process, or you can read about it in detail below.

The AD process is sometimes referred to as the "Concrete Cow", this is because in very simple terms we are replicating many aspects of the digestion system of a Cow.

The process, shown in the diagram above, follows these simple steps:

  • Food waste is fed into the bunker where it is stored before we clean and prepare it to be fed into the beginning of the digestion process.
  • Next we de-package this food and remove any non-organic contamination that we don't want to feed through to our digester tanks. Like a Cow's mouth our de-packaging technology shreds the materials, to make the organic fraction as readily available as possible to aid the digestion process, and separates the packaging and any other contamination away from the food.
  • We add a little liquid (usually waste liquids sourced from food and drink manufacturers) to help wash the food away from the packaging and to make it into a pump-able consistency. The clean, packaging free substrate that is left is sometimes known as "soup". This is the bit our "mechanical Cow" is interested in eating.
  • We feed this soup into our digester tanks; our mechanical Cow's stomach.
  • Heat, generated by our engines, gently warms the material within the digesters. We also gently stir this material to prevent any inedible crusts or sediments forming inside the tanks.
  • From here we let Biology take over. Just as within a Cow's stomach (or your own for that matter!) a culture of bugs and healthy bacterial develop and physically digest the materials we feed into our tanks.
  • Whilst these micro-organisms eat our food they produce a bio-gas. This gas is rich in Methane.
  • We capture the bio-gas in the roof of our digester tanks and push this through a Combined Heat and Power Gas engine. This is the clever bit that real cows aren't able to do.
  • Our engines burn the gas and use the energy to turn a generator, thus creating renewable electricity.
  • The heat the engine produces during the process is used to heat various tanks and pipework within the AD plant.
  • We use a small amount of the electricity we produce to power the AD plant, thus ensuring we are self-sufficient in terms of energy use. The remainder of what we produce is supplied onto the National Grid to power local homes and businesses.
  • In total Severn Trent Green Power produces enough renewable electricity to power approximately 115,000 local homes.
  • Power produced by AD is particularly useful to energy companies as it is not intermittent like some of the other renewable power sources; our power production is not dependent on weather conditions as is the case for Solar and Wind generated power. The industry terms this steady, continuous supply of power as "baseload energy supply".
  • Once we have harvested all of the gas we can from our food substrate, we are left with a rich organic bio-fertiliser known as Digestate.
  • We filter and then pasteurise the Digestate to ensure that it doesn't contain any harmful bacteria or anything else that we wouldn't be happy to put back into the UK's soils. All of our AD plants are PAS 110 accredited which means we have passed and maintain high and stringent standards with regards to our Digestate quality.
  • We supply Digestate to local farm land.
  • Our Farmers really value the digestate that we supply to them as it adds valuable nutrients to their depleted soils, which in turn helps them grow more food. Learn more about our Digestate and other bio-fertiliers here.

Operating one of our AD plants has the net carbon benefit equivalent of taking 71,000 cars off the road.

Each year, our West London AD plant recycles:


It produces:


Operating this facility gives the equivalent:


That's a lot of carbon savings!

Being able to supply a high quality bio-fertiliser to our farmers, joining the closed loop circle of food waste back to the growth and production of more food for our country, is of utmost importance to us.

Our ongoing achievement of the PAS110 industry recognised standard and quality protocall proves that the materials we produce are consistently high quality and meet all legal/moral criteria required to treat this as a new "product" rather than a "waste".

Learn more about our Bio-fertiliser standards and what we do to ensure we meet them here.

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