Bacteria break down organic matter in a sealed tank without oxygen, releasing a gas that is roughly half to two thirds methane. Removing the carbon dioxide and other impurities leaves almost pure methane, which is then compressed to vehicle fuel pressure. The solid left behind is digestate, a nutrient rich manure.
What is happening inside the tank
Anaerobic means without oxygen. Seal organic material in a tank, keep the air out, keep it warm, and a community of bacteria will take the material apart in four stages that run continuously and overlap.
First, hydrolysis. Large molecules, the cellulose, proteins and fats, are broken into sugars, amino acids and fatty acids that are small enough for bacteria to absorb. With fibrous material this is the slowest step and it sets the pace of everything after it.
Second, acidogenesis. Those smaller molecules are fermented into volatile fatty acids, along with some hydrogen and carbon dioxide.
Third, acetogenesis. The fatty acids are converted into acetic acid, hydrogen and more carbon dioxide.
Fourth, methanogenesis. A separate group of organisms, the methanogens, turn acetic acid and hydrogen into methane. These are the slowest growing and the most sensitive members of the community, and keeping them comfortable is most of what running a digester involves.
Why the tank is kept steady rather than pushed
The four groups work at different speeds. If the first two stages run faster than the last one, acid builds up, the pH falls, and the methanogens slow down further. That feeds back on itself: more acid, lower pH, fewer methanogens.
The practical consequence is that a digester rewards consistency over intensity. A steady daily feed of similar material at a steady temperature produces more gas over a year than a tank that is loaded heavily and then starved.
Two temperature bands are normally used. Mesophilic, around body temperature, is slower but stable and tolerant. Thermophilic, considerably hotter, is faster and produces more gas per day, and it is less forgiving of a change in feed. Most plants working with mixed farm residue choose stability.
What upgrading removes
Raw biogas is roughly half to two thirds methane. Most of the rest is carbon dioxide, with small amounts of water vapour, hydrogen sulphide and, where the feedstock includes municipal waste, siloxanes.
Each of those is removed for a specific reason. Carbon dioxide carries no energy, so leaving it in means compressing and transporting a gas that is partly ballast. Water and hydrogen sulphide together form an acid that corrodes tanks, pipes and engines. Siloxanes turn into an abrasive deposit inside an engine.
Once the gas is mostly methane it is compressed hard, to the pressure used for vehicle fuel. That is the point at which it becomes a product that can be moved by road and sold into an existing filling network rather than used where it was made.
Why the feedstock decides everything
The digester is the straightforward part. Feedstock logistics is where these projects are won or lost.
Residue is seasonal, bulky and low in value per tonne, which means transport cost rises quickly with distance. Moisture content varies with the weather. Dung availability changes with herd movement. A plant sized for what is available in the weeks after a harvest will run below capacity for the rest of the year.
This is one reason the energy work and the restoration work sit on the same land here. A restoration block planted in tiers produces a predictable quantity of green material on a known cycle, within a known distance of the plant, which is a very different proposition from buying residue on the open market.
What comes out besides gas
Digestate is the material left when the methane has been taken out. The carbon is gone, and the nitrogen, phosphorus and potassium are not. It goes back to the field as organic manure.
That matters for two reasons. It closes a loop, returning nutrients to the land the residue came from. And it shows up as a reduction in bought fertiliser, which is a cost saving on the farm rather than a carbon claim that needs verifying.
A plant with a plan for its digestate has two products. A plant without one has a product and a disposal problem.
Where this fits
Compressed biogas is one of several income lines on an integrated site. Its particular advantage is that its feedstock is material that is already there and would otherwise be burned or left, and that its buyer exists because of an existing fuel network rather than because of goodwill.
See also compressed biogas, digestate and feedstock in the glossary.
Related questions
- What can go into a digester?
- Wet organic material that bacteria can break down: cattle dung, food and market waste, pressmud from sugar mills, and green crop residue. Dry woody material such as bark and hard stalks resists digestion and is better suited to pyrolysis or to burning. Most plants run on a blend, because a blend is easier to keep steady than a single input.
- What is the difference between biogas and compressed biogas?
- Biogas is the raw output of the digester, roughly half to two thirds methane with most of the rest carbon dioxide. Compressed biogas is that same gas after the carbon dioxide, water, hydrogen sulphide and siloxanes have been removed and the remaining methane has been compressed. Raw biogas can run a boiler on site. Only the upgraded, compressed gas can be sold as vehicle fuel.
- What happens to the solid waste?
- It leaves as digestate, which holds the nitrogen, phosphorus and potassium that were in the feedstock. It is applied to fields as organic manure, so the nutrients return to the land the residue came from. A plant that has nowhere to put its digestate has a disposal problem rather than a product.
- Why do biogas plants fail?
- Usually on feedstock rather than on the digester. The biology needs a steady daily supply of similar material at a similar moisture content. Collecting, storing and transporting bulky low value residue reliably through the year is the hard part, and a plant sized for peak season availability will sit idle for months.