Thanks for all your brilliant questions about municipal compost – and thanks to Wendy and Tom at LondonWaste’s EcoPark for answering them! For more info on municipal compost and how it can be used in containers, see my other post: Muncipal Compost: is it a good thing.
At the London Waste Eco Park, the compost is made from about 5% food waste, the rest is green waste (grass clippings, prunings etc). It varies a bit seasonally (less grass clippings in winter!) and from site to site around the country (eg some sites do not use food waste). Is it suitable for organic gardens? I spoke to the Soil Association about this and the answer is yes, probably. The official line is: ‘The Soil Association recognises that the use of green waste compost is compatible with the basic principles of sustainability and can be used as part of an on-farm fertility and soil health programme.’ However, if you want to use municipal compost to gain organic accreditation you’ll need permission from the Soil Association first. Most composts that have been tested to BI PAS100 will pass the Soil Association’s verification process – but the two tests are slightly different so its not 100% guaranteed. Some municipal composts in the UK have been verified independently by the Soil Association – check with your local producer.
I have experienced the same issue, too, Simon. Here’s what they do to minimize it at the Eco Park (it may vary in other places). The food and green waste is first inspected visually. Any loads that are badly contaminated are diverted to landfill. Then, after composting, the material is screened and anything larger than 20 mm is removed. This takes out all the larger impurities (eg plastic bags), although anything smaller that 20 mm will not be removed. I tend to sieve all my municipal compost at home to remove these smaller bits.
See above for the answer to the first part of your question, Vanessa. In answer to the second part: yes, as part of BI PAS 100 Standard (which this and many other municipal composts meet), every 5000 m3 of the compost is tested for both chemical residues and heavy metals.
Nigel, the pH does vary slightly. Usually it is 7.5, but seasonal variations in the green waste collected do affect it. For example, in January, the composting of Christmas trees can make it more acidic – perhaps as low as pH 6.
Yes, Elaine, the compost is independently tested for herbicidies – this is done by sowing tomato seeds in samples of the municipal compost and comparing to tomato seeds in a control compost. The germination rate is compared and the seedling growth is monitored. If the tomato leaves curl, this is an indicator of herbicides. You should be aware, though, that it is impossible to detect herbicides in the green wastes coming into the site. The tests are run once on every 500o m3 of compost, not every bag . So the risk of herbicides being present cannot be removed completely. But as the London Eco Park has passed every herbicide test done to date, this risk is presumably pretty small.
Julieanne, you are doing the right thing! Common weeds like couch grass and bindweed will be killed by the high temperatures in the process. The exception is the pernicious and dangerous weeds like Japanese Knotweed which should not be put out for council collection.