When organic waste is treated through anaerobic digestion, energy is extracted in the form of biogas. What remains is digestate, a nutrient-rich material that can be applied to agricultural land. Digestate plays an important role as a fertiliser, returning nutrients such as nitrogen, phosphorus, and potassium back to the soil. However, during the digestion process, a significant share of the easily degradable carbon is converted into biogas during the digestion process, reducing the amount of organic matter that remains available for the soil (European Biogas Association).
This is where the difference between digestate and compost becomes relevant. Compost is a stable soil improver that can contribute organic matter and nutrients while supporting soil structure, water retention and microbial activity. Digestate is generally valued for its readily available nutrients, although it can also contribute stable organic matter. Their precise properties depend on the original feedstock and how each material is processed. These are all factors that are increasingly important in maintaining productive and healthy soils (as highlighted in guidance from Zero Waste Europe and the European Compost Network.
In systems where organic waste is primarily directed towards energy recovery, a shift in how organic matter is returned to the soil can occur. Energy is recovered and nutrients are returned, but the organic matter needed to maintain soil quality over time is reduced. Research into long-term fertilisation strategies shows that the effects of digestate, compost and other organic amendments depend on their composition, how they are combined and the conditions of the soil. (Frontiers in Microbiology, 2024).
This is why the conversation should not be framed as a choice between biogas and composting. They are not competing solutions, but different parts of the same system. Biogas contributes to energy and nutrient recovery, while compost supports soil health and long-term fertility. Both can play valuable roles, depending on the organic waste stream, treatment system and needs of the soil.
Ultimately, the value of organic waste is not only determined by how much is collected, but also by what it becomes.
That depends on how it is sorted, how clean the stream is, and how it is processed. Because once contamination is introduced, the ability to return materials safely to soil can be reduced.
Organic waste sits at the intersection of multiple systems: waste management, energy production, and agriculture. Understanding its role requires looking beyond individual outputs and considering the balance between them. In the end, it is not just about recovering value but about where that value goes, and what it supports.
