Imagine if food waste, livestock slurry, residues from the agri-food industry and sewage sludge were not “a problem to be managed”, but rather a feedstock capable of generating renewable energy and returning nutrients to the soil. That is what biomethanation aims to do: to turn organic waste into biogas and a usable digested material, preventing it from ending up in landfill and closing the loop.
Below you will see exactly what it is, how it works step by step, and why it is regarded as a highly useful part of the circular economy.
What is biomethanation?
Biomethanation (also called anaerobic digestion or biogas production) is a biological process in the absence of oxygen in which micro‑organisms break down organic matter and transform it into:
- Biogas: a mixture of gases composed mainly of methane (CH₄) and carbon dioxide (CO₂).
- Digestate (digested material): the stabilised waste from the process, which can be reused (for example, as a soil amendment or fertiliser, depending on its treatment and applicable regulations).
In other words: biomethanation speeds up and controls natural decomposition, but does so under optimal conditions to recover energy (in the form of biogas) and manage organic waste more effectively.
How does biomethanation work?
Although it may vary according to the type of waste and plant technology, the overall process revolves around four key ideas: prepare the waste, digest it in the absence of oxygen, make use of the gas, and recycle the digestate.
1) Receive and pre‑treat the waste
Organic waste usually has to undergo preliminary operations such as the removal of non‑biodegradable and other unwanted materials (plastics, sand), shredding/homogenisation, and moisture adjustment. Substrate type greatly influences both the yield and composition of the biogas.
In anaerobic digestion, the process is also referred to as either wet digestion or dry digestion, depending on the dry matter content (for instance, some solutions define typical ranges to distinguish between them).
2) Anaerobic digestion (the “heart” of biomethanation)
Inside the digester, different microbial groups act in sequence until methane is produced. At an explanatory level, this process is typically described in stages:
- Hydrolysis: large molecules (fats, proteins, carbohydrates) are broken down into simpler compounds.
- Acidogenesis and acetogenesis: volatile fatty acids and other precursors are generated.
- Methanogenesis: methanogenic bacteria transform these compounds into methane and CO₂.
For the process to remain stable, variables such as temperature, moisture content, and pH are all controlled, keeping them close to optimal ranges.
3) What is biogas used for? (direct use or upgrading)
Biogas can be used as is to produce heat and/or electricity. However, when a renewable gas equivalent to natural gas is required, a biogas cleaning or upgrading process is carried out, removing CO₂ and impurities in order to increase the concentration of methane.
As a rough guide, biogas usually contains a high proportion of methane (for example, ranges such as 50–75% CH₄ are often reported), as well as CO₂ and traces of other substances. Upgrading produces biomethane, which has properties very similar to natural gas but comes from non‑fossil sources.
4) Managing and using the digestate
Digestate is the other main product of the process. If managed properly, it allows nutrients and organic matter to be recovered, helping to return them to the soil (always in accordance with applicable treatments and regulatory requirements based on their origin and use).
Advantages and benefits of biomethanation
Biomethanation is often described as a “doubly useful” technology because it addresses both waste and energy at the same time.
It reduces environmental impacts and emissions
- It prevents part of the organic fraction from reaching landfill, where it would otherwise degrade and release gases in an uncontrollable manner.
- In biomethanation plants, biogas production and capture are controlled so that methane can be recovered and used for energy.
It produces renewable, manageable energy
Unlike other renewables that depend on the sun or the wind, biogas/biomethane can be produced on a continuous basis (depending on substrate availability and operation), helping to provide dispatchable energy that can be used in multiple applications. The step to biomethane through upgrading is particularly relevant when a high‑quality gas is required.
It improves the management of organic waste
Biomethanation is well suited for treating agricultural and livestock residues, agri-industrial waste, the organic fraction of urban waste, and sludge, among others, provided it is implemented with proper collection and pre-treatment. At institutional level, it is described as part of the biological waste treatment processes for waste.
It restores value to the system: nutrients and organic matter
Once appropriately treated, digestate can enable nutrient recirculation instead of relying exclusively on external inputs, which connects directly with the logic of the circular economy.
Biomethanation and the circular economy
The circular economy seeks to reduce waste, keep materials in use, and regenerate systems. Biomethanation adds value on all three fronts:
1) It turns “waste” into a “resource”
The organic fraction ceases to be an unavoidable cost and is transformed into energy (biogas/biomethane) and into a by‑product (digestate) with potential for reuse.
2) It closes local loops (energy and nutrients)
In many regions, organic waste is produced near the places where energy is required and nutrients are needed to enrich agricultural soils. This proximity-based approach reduces transport, supports local value chains, and connects sectors (urban, agricultural, food industry).
3) It reduces dependence on fossil and external resources
Biomethane is regarded as a renewable gas that delivers performance equivalent to natural gas but without a fossil origin. Moreover, by recovering nutrients via digestate, it also reinforces the idea of full utilisation of the waste.
4) It drives more efficient management models
In Spain, proper management of the organic fraction is a key topic in the waste management debate. Biomethanation is repeatedly highlighted as a tool to improve organic waste management and move towards a more circular, low‑carbon model.