Composting

Composting is a natural biological process in which biodegradable organic matter, such as food scraps, garden trimmings or manure, is transformed into compost: a stable, nutrient-rich material with fertilising properties. It is one of the oldest and most sustainable techniques for managing organic waste, used in agriculture for centuries, and today central to the circular economy and to policies aimed at diverting waste from landfill.

Composting not only reduces the amount of waste sent to landfill, avoiding methane emissions and leachate, but also produces a useful product for agriculture, gardening and the restoration of degraded soils.

Legal basis

The Law 7/2022 on waste and contaminated soil for a circular economy recognises composting as a form of organic recovery of biowaste. At European level, the Waste Framework Directive 2008/98/EC requires the separate collection of the organic fraction and its treatment through composting or anaerobic digestion. In addition, Regulation (EU) 2019/1009 on EU fertilising products, applicable since July 2022, sets the quality criteria under which compost can be placed on the market as a fertilising product.

The composting process

Composting is an aerobic process, meaning it takes place in the presence of oxygen. Microorganisms (bacteria, fungi and actinomycetes) break down the organic matter in several phases:

1. Mesophilic phase

• Initial temperatures (20 to 40 degrees C).
• Rapid breakdown of sugars and proteins.

2. Thermophilic phase

• Temperature rises to 60 to 70 degrees C.
• Elimination of pathogens and unwanted seeds.
• Breakdown of more resistant materials such as cellulose.

3. Maturation phase

• Gradual fall in temperature.
• Formation of stable humus.
• Improvement of the physical and chemical properties of the compost.

Key factors

• C/N ratio (carbon to nitrogen): optimal between about 25 and 30 to 1.
• Moisture: between 50 and 60%.
• Aeration: needed to maintain aerobic conditions.
• Particle size: shredding favours degradation.

Types of composting

1. Home composting

• Carried out in single-family homes or communities.
• Individual composters for food scraps and garden waste.

2. Community composting

• Neighbourhood, school or municipal initiatives.
• Composters shared by several households or groups.

3. Industrial composting

• Large-capacity plants treating municipal biowaste, agricultural residues or sewage sludge.
• Systems based on turned windrows, enclosed tunnels or reactors.

4. Vermicomposting

• Use of red worms (Eisenia foetida) to break down organic matter.
• Produces a high-quality compost (worm humus).

Uses of compost

• Agriculture: an organic fertiliser that improves soil structure and fertility.
• Gardening and landscaping: a growing medium for plants and lawns.
• Environmental restoration: recovery of degraded soils, slopes and quarries.
• Erosion control: improving water-retention capacity.

Benefits of composting

Environmental

• Less waste sent to landfill.
• Lower methane emissions.
• Reduced use of chemical fertilisers.
• Improved soil biodiversity.

Economic

• Savings on landfill and incineration charges.
• Lower fertilisation costs in agriculture.
• Creation of green jobs in composting plants.

Social

• Environmental education for citizens.
• Encouragement of community participation.

Compost quality rules

In Spain, compost quality as a fertilising product is governed by Royal Decree 506/2013 on fertiliser products (where compost is classed among organic amendments), alongside Regulation (EU) 2019/1009 for EU fertilising products. Key parameters include:

• A minimum organic matter content.
• Absence of pathogens (such as Salmonella and E. coli).
• Limits on heavy metals (cadmium, lead, mercury).
• Stability and maturity of the product.

Composting and the circular economy

Composting is one of the clearest examples of the circular economy applied to waste. It turns organic residues into a useful resource, closing the loop of organic matter and reducing dependence on synthetic fertilisers. It also contributes to climate change mitigation by avoiding methane emissions from landfill and supporting carbon storage in soils. At Manglai we help companies measure their environmental footprint and prepare their sustainability reporting. Discover how Manglai can help you.