What happens to your waste

Composting is a biological process in which micro-organisms convert biodegradable organic matter, such as flowers and plants, grass cuttings and hedge trimmings, into a product known as compost.

This can be done on a small scale, for example by using a compost bin at home, or on a much larger scale...

...such as the facility run by Angus Council which composts green waste collected from recycling centres and the green bin kerbside service.

It is preferable to compost biodegradable material rather than sending it to landfill because of the negative environmental impacts associated with disposing of waste this way.

Compost is a valuable resource that can be used in a number of ways.

As a soil-enricher, or a lawn fertiliser and to replenish soil levels in potted plants around the house and garden.

Angus Council's composting facility is located at Restenneth, near Forfar. In 2010 it received over 13,000 tonnes of waste for composting.

Garden Waste Collection Vehicles containing garden waste from kerbside collections and skip lorries containing green waste from Recycling Centres arrive at the composting facility and drive onto a weighbridge.

This allows us to measure the amount of material arriving for composting.

This is where the garden waste is deposited when it arrives at the facility. At this stage any visible contaminants, such as plastic bags, are removed by hand.

It is important to remove any non-biodegradable waste, as this will affect the composting process and quality of the final product.

Here is some of the contamination removed from this batch of garden waste.

To speed up the composting process the green waste is shredded. This ensures all the material is roughly the same size and allows it to decompose at the same rate.

Once the material has been shredded, it is put into piles called windrows.

Windrows are long rows of compostable waste, up to three metres high and sixty metres long, which are turned at regular intervals to encourage the breakdown of the waste.

Each windrow will go through a phase of sanitisation and then stabilisation.

This ensures that the compost material is maintained at a high enough temperature to kill any nasty pathogens, as well as ensuring the compost material is given sufficient time to decompose properly. This usually takes around 12 weeks. During this period the compost material is turned at regular intervals.

Turning compost adds oxygen to the compost pile, which is good for the microorganisms. It also ensures that all parts of the pile are subjected to the high internal heat, thereby ensuring total pathogen kill, and yielding hygienically safe, finished compost. The more we turn the compost, the more it becomes chopped and mixed, and the better it looks when finished. Finally, frequent turning can speed up the composting process.

As the windrows are turned they are moved down the slab, so the further away a windrow is, the older it is.

The composting process is very natural and requires very little additional input.

Compost is broken down by a range of living creatures, including worms, insects and microbes; which already exist in the compost material. These creatures work best when there is lots of oxygen, which is why the compost is turned frequently.

They also like a warm and moist environment, which is provided by the compost.

During the composting process, carbon dioxide and water vapour are produced as by-products. This differs from the process of anaerobic biodegradation that takes place at landfill sites, since this occurs without oxygen, and produces additional environmentally damaging greenhouse gases such as methane.

Once the green waste has broken down to form a compost material it is then screened in order to produce a product with a fine grade.

The compost material is loaded into the hopper of the screener by a shovelled loader and sieved.

This screening process also removes oversize materials, which are returned to the first stage where they are shredded and recomposted to encourage them to decompose.

Screening also separates out any physical contaminants such as plastic and glass.

Once the material has been screened it is ready for use as compost.

Angus Council plans to produce between 6000 and 7000 tonnes of compost. Although most of the compost produced is currently used for land restoration on finished landfill cells, in the near future it is hoped to distribute the compost to the parks department, gardening associations and allotments, for use in agriculture and farming and to the general public.

For more information about waste and recycling please visit www.angus.gov.uk/recycling