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In-Vessel Composting ProcessThe mixed organic waste is delivered by trucks into an enclosed, purpose built tipping barn to help keep vermin and other nuisance animals like seagulls away from the food waste. The waste is shredded by a large machine with revolving blades; the ABPR require that all waste containing food must be shredded within 24 hrs and to a size of less than 40 cms. 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. The only thing we add to the shredded organic waste is water; this ensures the waste has the correct moisture content for good composting to help speed the process up. The nature of the waste we receive varies during the year, with more garden waste, compared to food waste and cardboard in the spring and summer and vice-versa in the winter. A mechanical shovel then takes the shredded waste into a series of 20 clamps. The clamps are made from 3 walls of concrete, 8m wide, 15m long and 2.5m high, with a removable metal door at the front (one of the 8m sides) and a retractable plastic roof. When being loaded with fresh organic waste, the metal door is removed and the roof rolled back. The clamps hold up to 200 tonnes of waste and when full the door is replaced (using the loading shovel) and the roof is rolled over the top of the compost. This ensures that no animals can get to the food waste and it helps keep the heat in the compost.  Under the ABPR the organic waste must achieve at least 60°C for at least 2 days. We therefore put 2m long temperature probes through the roof into the compost to measure its temperature and because the composting bacteria produce lots of heat as they grow and multiply, the temperature in the in-vessel composting system we use regularly reaches over 65°C for more than 4 days. However, as this is happening the bacteria are rapidly using up any oxygen trapped in the waste and as the material is inside a clamp it cannot be turned as with windrow composting. Each clamp therefore has a large fan at the back that constantly blows in air to keep the compost aerated, and this also helps to control the temperature of the compost, because it can get too hot and kill off the composting bacteria.After this period of composting, the material must be moved by loading shovel into a second clamp, where again it must reach at least 60°C for at least 2 days. This is to ensure that any potential disease organisms in the waste are killed off. After these 2 periods in the clamps, it is now deemed safe to remove the compost from the clamps and it is placed on a large concrete pad in windrows and treated in almost the same way as the organic waste in our open windrow system. The windrows are turned regularly for a 6-week period to ensure they stay aerated and after this the material is screened (put through a giant rotating circular sieve called a trommel screen) to pull out any contaminants, mainly plastics. We assume that some residents are putting food waste in plastic bags before they put it in their kerbside organic waste collection and this leads to some material mixed with plastics being unsuitable for use by farmers or the public. The final soil conditioner that comes through the screen is now ready for use and the majority goes to local farmers, who can come to our Waterbeach site and collect it throughout the year, but we also make this soil conditioner available for the public to collect free of charge from a number of our sites. Under the ABPR the whole composting process is regularly inspected and the material is sampled weekly after the second vessel treatment to ensure that it is clear of harmful bacteria such as salmonella, plus we also have it tested for nutrient content. We are working towards PAS 100 for this product. We currently have 20 clamps (10 for the first stage of 60°C composting and 10 for the second stage) with room to expand to a total of 30 clamps, to take account of the expected 100,000 increase in the population of Cambridgeshire during the life of the PFI waste contract between Cambridgeshire County Council and AmeyCespa (East). |
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