How does composting work?

Composting process SUEZ
1. Collection
Organic waste – which is any natural and biodegradable material – is collected from people’s homes, recycling centres, and businesses such as restaurants and hotels. The waste is taken to a composting facility.

2. Sorting
Organic waste is treated in two different ways, depending on whether or not it contains food waste. Enclosed ‘in-vessel’ composting is used where organic waste includes food. Waste without food in it – such as grass and other garden clippings – is called green waste and suitable for open windrow composting.

3. In-vessel composting
In-vessel composting destroys the pathogens in decaying food as part of the composting process.

The organic waste is shredded, water is added and it is all mixed together. The mixture then goes into ‘barrier one’ composting where it has to achieve a set temperature of 60°C for two consecutive days.

Then the mixture is turned and moved to ‘barrier two’ composting, where it has to achieve a temperature of 60°C for a further two consecutive days.

The processed organic material can then be formed into long triangular-shaped heaps known as windrows for maturation. It is turned every week for 6 to 10 weeks until it becomes compost.

4. Open windrow composting
Green waste can be turned into compost in the open air using the windrows method (long, triangular-shaped mounds).

The green waste is checked for quality before being shredded and formed into windrows. These are turned every week for 14 weeks as the organic materials break down to form compost.

5. Screening and grading
The compost (from either process) is screened through a trommel (large barrel) where any remaining contaminants (like bits of plastic) are removed and the compost is graded into the required size. Different grades of compost are required for different uses, e.g. 10mm for horticulture, 25mm for agriculture.

6. Compost
Now the compost can be supplied to its end market – usually restoration, landscaping, garden centres and agriculture.