Digestate

Digestate is the material left behind after a process called anaerobic digestion. This process breaks down biodegradable materials, like food scraps or plant matter, in an environment with very little oxygen. During this process, two important things are made: digestate and biogas. Biogas is a type of gas that can be used as energy. Digestate is a rich by-product.

Digestate comes from two main steps in the anaerobic digestion process: acidogenesis and methanogenesis. These steps give digestate different qualities.

Digestate is very useful. It can be used as a soil conditioner or fertilizer. This helps improve soil health and supports plant growth. It is an important part of recycling organic waste.

Digestate feedstock sources

Anaerobic digestion can use many different materials to create digestate. Some common materials include:

• Sewage sludges: Liquid and treated sewage.
• Animal wastes: Fats, blood, food remains, and manure from poultry, fish, and livestock.
• Energy crops: Corn, maize, and other plants, either whole or just the leftover parts.
• Municipal wastes: Food scraps and kitchen waste.
• Agricultural wastes: Fruit scraps, plant stems, and bagasse from sugarcane.
• Industrial wastes: Waste from food processing, dairies, and breweries.

The type of material used changes the digestate's properties. Sewage sludge and animal manure are often used together with other materials to make better digestate for farming and to produce more biogas.

Acidogenic digestate

During this stage, special bacteria change water-soluble chemicals into short-chain organic acids, alcohols, and gases like carbon dioxide and hydrogen. These bacteria work best in a pH range from 4.0 to 8.5, so the pH level must be watched closely.

Because this is an early step in the process, most of the organic material hasn't broken down yet. The digestate left over is often fibrous and contains plant material like lignin and cellulose. It also holds a lot of moisture and may have minerals such as phosphorus, along with bits of bacteria.

Methanogenic digestate

Methanogenesis is the last step in breaking down materials without oxygen. In this step, tiny organisms called methanogenic archaea make methane gas from substances created earlier. These substances are mostly acetate and hydrogen.

By the end of this step, most of the organic material has broken down. What is left is called methanogenic digestate, which looks like a sludge. This sludge is full of helpful nutrients like ammoniums and potassium. The methane gas made during this process is often caught and used as fuel.

Whole digestate

When the solid and liquid parts of digestate are mixed together, they form what is called whole digestate. This mixture looks like thick mud. The liquid part holds important nutrients like nitrogen and potassium. The solid part has more phosphorus.

Mixing these parts together makes nutrients available, which can help plants grow. Some digesters have only one chamber, so the two parts mix by themselves naturally.

Digestate characteristics

When we use digestate for farming, we look at its pH, nutrients, and solid material. These qualities change based on what was used to make the digestate and the type of system used. Digestate often has a lot of ammonia, which is important for nitrogen. For some food waste, it can have even more. Digestate usually has more phosphorus and potassium than compost, in a ratio of about 1:3. This makes digestate useful for improving soil for growing crops.

Uses

Digestate is mainly used to help soil stay healthy. It keeps the soil moist and adds useful organic material. It also gives plants the nutrients they need to grow and can help protect soil from washing away.

Digestate can also be used to make certain types of plastic. It helps plants grow well in special farming methods and can make soil healthier, which helps plants fight off diseases.

Digestate and compost

Digestate is not exactly the same as compost, but they share some similar features. Compost is made through a process called aerobic digestion, which uses tiny living things like fungi and bacteria to break down materials.

Special treatments, like ultrasonication, can help make digestate easier to use by releasing more useful materials into the water. In some plants, digestate is further processed in the same way as compost. This can create a leftover material that might contain small pieces of plastic, glass, or metal.

Standards for digestate

Digestate is the material left after natural decomposition. We check its quality in three ways: chemical, biological, and physical.

Chemically, we look at metals and other harmful substances, as well as important nutrients such as nitrogen, phosphorus, and potassium. We also make sure there are no germs that could cause disease.

Physically, the compost should look good and not smell bad. Things like plastic or metal mixed in can make people unhappy about using it. Checking the materials before they are processed helps make sure the final product is good quality.

In the UK, a special rule called PAS110 decides what counts as good digestate from certain types of waste. If a biogas plant follows this rule, its digestate can be called a “bio-fertiliser” and sold.

Publicly Available Specification biogas