Turn organic residues into biogas your site can use or sell.

Anaerobic digestion is a biological process where microbes break down organic material without oxygen, producing biogas (mostly methane and CO₂) and a nutrient-rich digestate.

Most assume the digester tank is the hard part. In practice, the project lives or dies on steady feedstock, stable biology and a buyer for every output, including the digestate.

Quick answer

Anaerobic digestion (AD) is a sealed biological process where bacteria and archaea convert organic matter such as manure, food waste or wastewater solids into biogas and digestate. In Australia it suits sites with a reliable daily residue stream, space for storage and a use for heat, power, gas or nutrients on or near site.

Four biological stages inside the digester

Each stage depends on the one before. When one group of microbes is stressed, the whole chain slows and gas output falls.

Breaking down complex material

Enzymes split carbohydrates, proteins and fats into sugars, amino acids and fatty acids. Fibrous material like straw or wood is slow here, which is why lignin-rich inputs often suit thermal routes instead.

Often the rate-limiting step for fibrous feedstock
Particle size and pre-treatment matter
Fats give high yields but need careful dosing

Fermenting into volatile acids

Fast-growing bacteria turn the products of hydrolysis into volatile fatty acids, alcohols, hydrogen and CO₂. Overfeeding easily rich food waste can make acids build up faster than later stages can use them.

Fastest stage in the chain
Acid build-up lowers pH
Feeding rate control prevents souring

Converting acids into acetate

Acetogenic bacteria convert longer-chain acids into acetate, hydrogen and CO₂. They work only when hydrogen is kept low by the methane-forming microbes in the next stage.

Depends on low hydrogen levels
Sensitive to sudden load changes
Links acid production to methane production

Producing methane

Methanogenic archaea turn acetate, hydrogen and CO₂ into methane. They grow slowly and are the most sensitive to temperature swings, ammonia, sulphide and pH, so they set the safe operating pace.

Slowest-growing, most sensitive microbes
Raw biogas is typically around 50 to 65% methane
Stable temperature is essential

What keeps a digester stable

Temperature regime

Most farm and food digesters run mesophilic (around 35 to 40°C). Thermophilic (around 50 to 55°C) can be faster and help sanitise but is less forgiving.

Retention time

Material must stay long enough for slow methanogens to keep up. Typical wet digesters hold material for weeks, not days; the right figure depends on feedstock and design.

Feedstock balance

Carbon to nitrogen ratio, solids content and consistency matter. Co-digesting manure with food or processing residues often stabilises both.

Inhibitors

Ammonia from high-protein inputs, sulphur compounds, salts, disinfectants and antibiotics can all suppress gas production. Test before you commit volumes.

Monitoring

Track pH, alkalinity, volatile fatty acids, gas volume and methane content. Rising acids are an early warning before output drops.

Outputs and offtake

Every cubic metre of gas and tonne of digestate needs a destination: on-site heat and power, upgrading, flaring as backup, and lawful nutrient use.

Digestate and nutrient recovery Biomethane Feedstocks hub Feedstock calculator

Frequently asked questions

What is anaerobic digestion used for in Australia?

In Australia, anaerobic digestion treats dairy and piggery manure, food and beverage processing residues, abattoir waste and sewage sludge. The biogas is mostly used for on-site heat and electricity, with a growing number of projects upgrading it to biomethane for gas networks or transport.

What feedstocks work best in a digester?

Wet, readily degradable materials work best: manure, food waste, processing residues, fats and wastewater solids. Woody, lignin-rich material breaks down poorly and usually suits thermal conversion instead. Mixing feedstocks often improves gas yield and stability.

How much biogas will my site produce?

It depends on the volatile solids in your feedstock, its biodegradability and the digester design. Use Shelby's feedstock calculator for a first screen, then confirm with laboratory biochemical methane potential (BMP) tests before sizing any plant.

What happens to the material after digestion?

The remaining material is called digestate. It keeps most of the nutrients from the feedstock and can be used as a fertiliser or soil conditioner where state rules allow. It must be stored and applied lawfully, which is why digestate planning belongs in the project from day one.

Is anaerobic digestion the same as composting?

No. Composting is aerobic, uses oxygen and releases carbon mainly as CO₂ and heat. Anaerobic digestion is sealed, captures methane as a usable fuel and leaves a wetter digestate. Some sites compost the solid fraction of digestate afterwards.

Does Shelby say whether my project is viable?

No. Shelby screens opportunities, shows what evidence is missing and connects you with specialists. Feasibility needs feedstock testing, engineering design, approvals and confirmed buyers.