What is a biogas plant?
A biogas plant is a facility where biogas is produced by fermenting organic material. The Anaerobic Digestion (AD) process is the biological breakdown of organic materials by bacteria in a gas tight tank without oxygen – hence the term ‘anaerobic.’ The products of this process are renewable natural gases that can be injected directly into our local gas network, CO2 for use in food and drinks, as well as organic fertilisers and soil conditioners that can replace imported chemicals.
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What is digestate?
Simply put, digestate is the material that remains after the AD process has taken place. It is made up of a mixture of solid and liquid materials.
AD draws carbon, hydrogen, and oxygen from the feedstock and into biogas. However, the essential plant nutrients (nitrogen, phosphorus and potassium) remain in the digestate. AD is therefore a remarkable process, as it produces renewable energy from organic material without reducing its potential fertilising properties. About 85% of what is fed into the plant is left as digestate, which means all the atmospheric carbon dioxide that has been absorbed by the crops in growing is locked into the digestate and put back into the ground.
The essential plant nutrients are mineralised during the digestion process which means they’re more easily absorbed and therefore more useful to the plants grown with it.
The undigested organic fibres in the digestate help to retain moisture in the soils, improve structure, and eventually break down aerobically to release more nutrients and protect the soil from erosion.
What do we do with the digestate?
1. Pasteurisation & Hygenisation
The AD process is great at destroying weed seeds and bacteria; however, the digestate is also treated to 70°C for at least 1 hour to ensure that any weed seeds that have come in with the crops and bedding are destroyed. This gives local farmers confidence that the digestate product generated by AD is entirely safe to use.
2. Separation
The pasteurised digestate is ‘squeezed’ to separate it into liquid and solid fractions. The solid fraction is used as soil conditioner and fertiliser, whilst the liquid fraction is recirculated to the front of the process and mixed with incoming material. Where there is more liquid than required at the front end, this is stored on site in a buffer store and then pumped to offsite connection points in or near fields where it can be put to use as a fertiliser.
What do we feed the plant with?
1. Crops
The crops that the AD plant processes will vary year to year, but will typically consist of grass, whole-crop cereals and maize. The AD plant will rotate the crops it uses and guarantees to:
- Ensure the most sustainable cropping rotation at any point
- Allow land to be used for food production, as well as allow it to rest for periods
- Displace previously used chemical fertilisers with organic digestate fertiliser
The AD plant will grow and harvest crops throughout the year to spread the production of feedback throughout the year and avoid ‘monocropping’ – the process of growing a single crop year after year on the same land. The following will be taken into consideration and may be of interest to local residents:
- Grass will be harvested in May, and then again in July and September (depending on growth)
- Maize is generally harvested in October
- Other whole crops are generally harvested in July and August
2. Straw
Baled cereal straws, peas, linseed and oilseed rape straws are chopped up in the hopper before being fed to the plant. Straws are high in ‘Dry Matter’ and contain significant locked up energy potential; however, they are ‘slow release’ and therefore require longer in the digester to release the large amount of gas bound up in them.
There is a significant benefit to digesting straw prior to spreading it on land. Unprocessed straw can reduce nitrogen levels in the soil, through a process called ‘Nitrogen Lock.’ By breaking down the straw in the digester, the nitrogen and other beneficial nutrients become more readily available to the plants that are grown on them.
3. Manures – Pig, Cattle and Poultry
South Norfolk is heavily populated with, and nationally renowned for, its pig and poultry farms. There are a number of substantial pig and poultry farming units within 5km of the Deal Farm site, and Deal Farm itself includes large pig and poultry units that already produce the feedstock that will be processed by the AD plant.
Latest guidance from the Environment Agency and the Government’s Department for Environment, Food & Rural Affairs (DEFRA) conclude that the storage and spreading to land of untreated slurries and manures are a major contributor to greenhouse gas emissions and ammonia. By delivering them to be processed in the AD plant, the biological degradation process captures all the methane and CO2 that would otherwise have been released into the atmosphere.
