Main heading

Feedstock :: Shredding and the Windrow System :: Screening :: Testing

Feedstock

Green waste is delivered to each of the 16 Household Waste Recycling Centres by the general public, where they are directed to place it in one of the designated skips indicated by gantry type signing. The full skips are transported to one of the landfill sites at either Crewe, Macclesfield or Chester where the green waste is stockpiled until there is a sufficient quantity for shredding.

Shredding and the Windrow System

The shredding machine operates by means of hammers mounted on a shaft which rotate at high speed.  This breaks down the incoming waste and ‘pounds’ it into small pieces before dropping it through an ‘opening’ or screen.

The material is then formed into a 'windrow' which is triangular in section, roughly 3 metres high, 3 metres wide and about 40 to 50 metres long.  The green waste is raked to provide optimum conditions for composting.  Each windrow receives a unique number so that its progress throughout the process can be carefully sampled, monitored and tested. Composting uses air, water and living organisms to turn waste into a useful product, the most important variables in the process being oxygen, temperature and moisture. Windrow temperature is recorded daily and when 55 degrees Centigrade is reached (normally after 2 - 3 days) then maintained for three consecutive days the whole windrow is mechanically turned and mixed to encourage the aerobic (with oxygen) process. At this stage the material gets very hot, temperatures as high as 70 degrees Centigrade have been recorded. This is necessary to destroy weed seeds and unwanted bacteria.

Moisture is a vital ingredient in composting. Too little and the chemical reaction cannot take place; too much and the spaces which should be filled with air become waterlogged. Therefore the moisture content is measured, controlled and adjusted when necessary throughout the process.

Aeration is achieved by turning the windrow several times in all and after approximately eight weeks the composting process slows down. Most of the waste has then turned into soil improver at this stage and the volume of the windrow has been reduced considerably. The material is then left undisturbed for a further 3 - 4 weeks for an initial ‘maturation period’ during which the micro-biological and chemical activity is significantly reduced.

Slide Show - How Cheshire County Council Makes Compost

Screening

Finally the material is passed through a 28mm square mesh, circulating drum screen which separates out, via conveyors, the fine material into the finished soil improver. The oversize material from the screen, called ‘tailings’, consists mainly of wood. This material is re-shredded and placed into the next composting windrow.

Testing

The specification and the PAS100 British Standard require the soil improver to be regularly tested for various requirements, these include:

1. An acidity test, referred to as the pH value should lie between 6.5 and 8. Our soil improver is averaging between 7.5 and 8, in other words slightly alkaline.
2. NPK values are checked. NPK stands for Nitrogen, Phosphorous and Potassium and is widely used in the horticultural trade to indicate the growing quality of soil or compost. Tests have shown our product to contain above the minimum quantities of Potassium, Phosphorous and Nitrogen necessary to produce an effective soil improver.
3. Moisture content should be less than 45% by weight.
4. 95% of the soil improver must pass through a 28 mm sieve.
5. A test to demonstrate it does not contain any active weed seeds.

Waste Homepage

Waste Management Service, Backford Hall, Chester CH1 6PX Map
General Enquiries Tel: 01244 973000 Fax: 01244 973746
waste@cheshire.gov.uk