Composting is an aerobic biological treatment process, meaning it takes place in the presence of oxygen, in contrast to anaerobic digestion, which occurs without oxygen. Under controlled environmental conditions, composting converts biodegradable organic waste into compost, a nutrient-rich organic soil amendment (humus; that improves soil structure, fertility, and biological activity), CO2 and heat (70°C). Composting does not produce any energy that can be recovered, all heat is consumed in the biological process.
A wide range of organic materials can be treated through composting, including:
- Crop residues (e.g., straw, stalks, leaves),
- Garden and green waste (pruned branches, grass clippings, wilted plants),
- Food waste (fruit and vegetable peelings, coffee grounds, kitchen leftovers), animal manure, often mixed with bedding materials such as straw, agri-food by-products, including grape marc, fruit and vegetable pulp, and most unsold or rejected produce.
By encouraging the natural decomposition of organic matter, composting provides a sustainable and circular waste management solution. It significantly diverts organic waste from landfills, thereby reducing greenhouse gas (methane) emissions and supporting climate change mitigation efforts. Unlike anaerobic treatment processes, composting primarily emits carbon dioxide (CO₂) and produces negligible amounts of methane (CH₄), making it a climate-friendly option for organic waste management.
composting phases.
Composting is an aerobic biological process in which organic waste is decomposed by successive microbial communities. Bacteria dominate at high temperatures, fungi degrade more resistant materials (especially at the heap edges), and actinomycetes act later on complex compounds such as cellulose and lignin. Macro-organisms like earthworms contribute during maturation.
Fermentation Phase (Active Phase).
This aerobic stage includes three sub-phases:
- Mesophilic phase (≈ up to 30°C): Bacteria and fungi degrade simple organic compounds, generating heat.
- Thermophilic phase (up to 70–80°C): Pathogens and most weed seeds are destroyed. Proper aeration is essential to avoid anaerobic conditions, odors, and leachate formation.
- Cooling phase (≈ 40°C): As easily degradable matter declines, temperature drops and microorganisms begin breaking down more complex compounds (cellulose, lignin), initiating humification.
Maturation Phase.
This phase lasts several months and leads to the formation of stable, humus-rich compost through continued microbial and earthworm activity. The process relies entirely on biological reactions.
An optimal initial C/N ratio is around 30. If it exceeds 50, degradation slows significantly. At the end of composting, the C/N ratio typically decreases to about 15. Carbon-rich materials include leaves and wood, while grass clippings and vegetable waste are nitrogen-rich.
composting in an MSW context.
Composting is not classified as a Waste-to-Energy (WtE) technology, as it does not produce energy, biogas or electricity; its main output is compost, a humus and valuable soil amendment. Nevertheless, composting remains a key element of generizon’s integrated approach to city waste management, serving as an essential treatment method for non-contaminated green waste, such as city tree trimmings. The logistics for managing this type of waste are relatively straightforward, making it easy to avoid landfilling and the long-term methane emissions associated with cellulose organic matter. This aligns with the primary objectives of Moroccan tenders for landfill and waste management.
By improving the collection, handling, and separation of green waste to prevent contamination, the resulting compost is indeed a most valuable product as it provides a nutrient-rich resource for agriculture and urban green spaces fully circular and sustainable.
In contrast, composting the Organic Fraction of Municipal Solid Waste (OFMSW) does not make sense: the resulting humus remains contaminated and is legally unsuitable for agricultural use, there is no product.
