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, while digestate remains as a rich by-product.

Digestate comes from two main steps in the anaerobic digestion process: acidogenesis and methanogenesis. Each of these steps gives digestate different qualities, depending on what was used to start with and how the process worked. Because of this, digestate can vary quite a bit in its features and uses.

Even though it might sound complicated, digestate is very useful. It can be used as a nutrient-rich soil conditioner or fertilizer, helping to improve soil health and support plant growth. This makes it an important part of recycling organic waste and creating sustainable ways to handle materials that would otherwise end up in landfills.

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 final stage of breaking down materials without oxygen. During this stage, special tiny organisms called methanogenic archaea create methane gas from substances made earlier in the process. These substances are mostly acetate and hydrogen.

By the end of this stage, most of the organic material has broken down, leaving behind something called methanogenic digestate, which looks like a sludge. This sludge is rich in helpful nutrients like ammoniums and potassium. The methane gas made during this process is often captured and used as a fuel.

Whole digestate

When the solid part of digestate from acidogenesis and the liquid part from methanogenesis are mixed together, they form what is called whole digestate. This mixture looks like thick mud. The liquid part makes up most of the volume and holds important nutrients like nitrogen and potassium. The solid part has more phosphorus and is thicker.

Mixing these parts together makes many 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 things like its pH, nutrients, and how much solid material it has. These qualities change depending on what was used to make the digestate and the type of system used. Digestate often has a lot of ammonia, which is a big part of its nitrogen. For some types of food waste, it can have even more. Digestate also usually has more phosphorus and potassium than compost, in a ratio of about 1:3. This makes digestate useful for improving soil for growing certain crops.

Uses

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

Digestate can also be used in making certain types of plastic and has been shown to help plants grow well in special farming methods. It can even help plants fight off diseases by making the soil healthier.

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. These tiny organisms can break down certain parts of plants better than others.

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, which can create a leftover material. This leftover might contain things like small pieces of plastic, glass, or metal, depending on how well the original materials were prepared before processing.

Standards for digestate

The quality of digestate, the material left after natural decomposition, can be checked in three ways: chemical, biological, and physical. Chemically, we look at things like metals and other harmful substances, as well as important nutrients such as nitrogen, phosphorus, and potassium. Sometimes, waste can carry germs that might spread diseases, so this needs careful handling.

Physically, the compost should look good and not smell bad. While this doesn’t affect health, things like plastic or metal mixed in can make people unhappy about using it. Even when the compost is perfect, some people still worry because it comes from waste. 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