All compost bins and heaps produce leachate. This is the liquid that seeps from the decomposing organic material and can be a significant pollutant when composting is undertaken on a commercial scale. The chemical and physical nature of the compost leachate is of importance as the leachate can migrate into underlying soils and groundwater, and can flow as runoff polluting ditches, ponds and streams.
Normally this is not a problem with domestic compost bins as relatively little leachate is produced and the liquid will be absorbed by the soil under and around the bin. It has been estimated that the total for a year using a modern basic plastic bin is in the region of twenty-four litres. The majority of commercially available wormeries have a reservoir with a drainage tap allowing the liquid, often described as worm wee to be collected. This is often referred to as worm wee and is used as a liquid fertilizer.
Food scraps and garden waste all carry bacteria, fungi and parasites and may also become contaminated during decomposition in the heap or bin by the faeces of rats, birds etc. The types and potential pathogenicity of the microorganisms present in the compost and leachate will vary with the contents being composted at any time. Manure and waste meat being high risk source ingredients. Normally such pathogens will be destroyed by competition from other materials in the bin and by the heat during hot composting. Faecal bacteria such as E. coli in compost are a potential health risk if they contaminate vegetables that will be eaten without adequate washing and cooking.
A new www.carryoncomposting.com page on leachate looks at potential problems associated with leachate, the contents of leachate and its use as a liquid feed. A revised page on Liquid Feeds is to be published soon.