Anaerobic digestion

Anaerobic digestion is a biological process in which microorganisms break down organic matter in the absence of oxygen, producing biogas (mainly methane and carbon dioxide) and a solid-liquid by-product called digestate, which can be used as a fertiliser.

It is one of the most important technologies in modern waste management, because it turns biowaste, manure, sewage sludge and other biodegradable waste into renewable energy and products that are useful for agriculture. Within the circular economy and the energy transition, anaerobic digestion plays a strategic role by cutting greenhouse gas emissions, recovering value from waste and generating clean energy.

Legal framework

In Spain, Law 7/2022 on waste and contaminated land for a circular economy recognises anaerobic digestion as a form of organic recovery of biodegradable waste.

At EU level, the use of biogas contributes to climate and energy objectives set out in instruments such as the Renewable Energy Directive (Directive (EU) 2018/2001, known as RED II) and the land-use and forestry rules of Regulation (EU) 2018/841 (LULUCF), while Regulation (EU) 2019/1009 governs digestate when it is placed on the market as an EU fertilising product.

The anaerobic digestion process

Anaerobic digestion has four main phases:

1. Hydrolysis

Complex organic polymers (carbohydrates, proteins, fats) are broken down into simpler molecules (sugars, amino acids, fatty acids).

2. Acidogenesis

The resulting compounds are converted into volatile fatty acids, alcohols, hydrogen and CO2.

3. Acetogenesis

The volatile fatty acids are transformed into acetic acid, hydrogen and carbon dioxide.

4. Methanogenesis

Methanogenic microorganisms produce methane and CO2, generating the biogas.

Types of anaerobic digesters

By temperature

• Mesophilic (35-40 C): more stable and with lower energy demand.
• Thermophilic (50-60 C): faster digestion and more effective pathogen removal.

By design

• Continuous stirred-tank reactors (CSTR).
• Plug-flow digesters.
• Covered lagoons for manure.
• Dry digesters for material with low water content.

Feedstocks for anaerobic digestion

1. Urban biowaste: food scraps, pruning and garden waste.
2. Sewage sludge: by-products of wastewater treatment plants.
3. Livestock manure and slurry: a large potential in countries with significant livestock sectors.
4. Agri-food waste: by-products of food, brewing or dairy industries.
5. Energy crops (controversial): maize and sorghum, questioned for competing with food production.

Regulatory context

• Directive 2008/98/EC (Waste Framework Directive).
• Directive (EU) 2018/851, on the separate collection of biowaste.
• Directive (EU) 2018/2001 (RED II), on renewable energy.
• Law 7/2022 on waste and contaminated land.
• The Spanish Biogas Roadmap (2022).

Benefits of anaerobic digestion

Environmental

• Reduced methane emissions from landfills and livestock operations.
• Better management of manure and slurry.
• Substitution of fossil fuels with renewable biogas.

Economic

• Savings on chemical fertilisers thanks to digestate.
• Income from selling electricity, heat or biomethane.
• Creation of green jobs in rural areas.

Social

• A contribution to energy security.
• Development of local economies linked to the agri-food sector.
• Reduced odours and nuisance in livestock areas.

Anaerobic digestion and the circular economy

Anaerobic digestion is a pillar of the circular economy because it turns waste into energy and fertiliser. It closes the loop of organic matter, replaces fossil fuels and synthetic fertilisers, and supports the energy self-sufficiency of municipalities and businesses.

At Manglai we help companies measure their carbon footprint and prepare their sustainability reporting. Discover how Manglai can help you.