Open Windrow Composting
Windrow composting is very similar to making a compost heap in your own garden. It can only treat garden waste and similar material and not any food or kitchen waste or organic material that might have come into contact with food. The main difference between our method at Waterbeach and home composting is that we have to deal with much more waste, which has the major benefit that our ‘compost heaps’ get much hotter than home compost heaps and thus the organic material rots down into a usable soil conditioner much more quickly and thoroughly.
Currently we treat around 12,000 tonnes of organic waste input, but after the process is complete about 7,000 tonnes of soil conditioner is produced. Most of the waste we treat at our windrow facility is garden waste delivered by the public to the 9 household recycling centres across Cambridgeshire and brought here in large skips, but we also receive deliveries from private businesses such as tree surgeons, landscape gardeners, grounds maintenance contractors and also some council parks departments.
The material we receive is relatively woody, but varies a little seasonally; we get more grass cuttings in spring and early summer, more leaves and prunings in autumn and lots of old Christmas trees in early January. However the composition of the final compost varies very little. When the material arrives it is first shredded (chopped into small pieces) by a large machine with revolving blades that can deal with tree trucks up to 0.3m thick. This ensures that there is a large surface area of organic matter for the bacteria and fungi that do most of the composting process to live and grow on. Apart form water, we do not add anything to the material and all the composting ‘bugs’ occur quite naturally in the organic waste.
A mechanical loading shovel then piles this shredded waste into windrows, which are shaped like a giant Toblerone bar (triangular prism to be precise); this ensures that not too much rain gets into the compost heap and blocks air pockets, plus the sloping sides give a large surface area for air to get into the heap. The windrows are typically 5m along the base, 2.5m high and 20m long. As the bacteria break down the organic material they generate heat and the hotter it gets the faster they work. The inside of a windrow gets up to 55°C, but as they work faster, the bacteria soon use up the oxygen inside the heap.
To ensure that oxygen is present, the windrow is turned using a large self-propelled compost turner, so that material on the inside comes to the outside and material that was on the outside gets into the middle, heats up and composts more quickly. In cold weather the material gets so hot, steam can clearly be seen rising from the windrows. The windrows continue to be turned for about 12 weeks, after which the compost process is virtually complete. The final material is screened (put through a giant rotating circular sieve called a trommel screen) to pull out any large woody items that haven’t composted properly and these are put through the process again.
The final soil conditioner that comes through the screen is now ready for use and the vast majority goes to local farmers and landscape contractors, who can come to our Waterbeach Site and collect it throughout the year. This compost meets the national accreditation standard PAS 100 and to ensure it keeps to this standard, samples of compost have to be independently tested every 3 months. The tests look at the nutrient content of the compost and ensure it is clear of harmful bacteria such as salmonella.