Composting

Composting is the natural biological breakdown of organic waste by microbes into CO₂, water, heat and humus, a rich, dark material that improves soil. It can even break down some toxic, long-lasting compounds in soil and water.

Sorted and ground waste is mixed with water and sewage sludge. A moisture content of 45-65% is ideal. Composting can be aerobic or anaerobic; aerobic heaps must be turned or aerated to get enough air.

It goes through three phases:

• Mesophilic (warm), lasting a couple of days: microbes break down the easily soluble compounds and give off heat, so the heap warms up.
• Thermophilic (hot): above 40 °C, heat-loving microbes take over. At 55 °C and above, many pathogens die. The heap is kept below 65 °C by aeration, since hotter still would kill the useful microbes. Proteins, fats, cellulose and hemicellulose are broken down.
• Cooling and maturation (curing), lasting months, as the heap cools and the humus forms.

The workers in a compost heap:

• Bacteria make up 80-90% of the microbes; Thermus thrives at the hottest stage.
• Actinomycetes, filamentous bacteria, break down tough materials (lignin, cellulose, chitin, proteins) in wood and cardboard.
• Fungi break down complex polymers, mostly in the cooler outer layer, in all phases.
• Protozoa feed on the bacteria and fungi; mites, ants, earthworms and other worms join in.

Earthworms are a sign of fertile soil: composting turns our waste back into soil fertility.