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 and polute underlying soils and groundwater, and can flow as runoff into 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. With most home composting systems the lechate drains into the soil under the bin but a number of systems allow it to be collected.  There can be problems with using it (see below) but provided it is an odourless dark brown liquid that is odourless it can be used. If it has a  acidic or  fruity odour and is yellowish liquid it should not be used as a fertiliser on plants.

Leachate from wormeries is normally easily collected as 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. As for leachate from conventional bins it is unsuitable for use if smelly. 

Some advice angainst the use of lechate as 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 or 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.

Making compost without the addition of manure reduces the risk of contamination by faecal coliforms but plant organisms capable of causing human disease are still a possibility. The amount of leachate produced will be increased where food waste with a high moisture content is composted.  The risks are higher with commercial composting where larger quantities of waste are involved which is why environmental regulations and enforcement are necessary.  In 2012 the risks associated from leachate escape was illustrated when E.coli and salmonella were found at a food composting firm in Wales by the Environment Agency Wales (EAW) where leachate was seeping from a building used for food waste deliveries. Leachate from home composting is more likely to contain pathogens if:

• contaminated food waste has been added to the bin or
• rats, birds or other creatures have gained access or
• manure has been used particularly in a cold composting system.

 An additional means of reducing the risks of leachate contamination is to use potable water to soak and maintain the moisture level  during the composting process rather than water from a pond or stream.  If this is not possible, rainwater can be collected from outbuildings, sheds etc in covered water butts. If pond or stream water must be used the area round the pond should be checked to ensure that it is not being contaminated by seepage from stored manure, manure applied to adjacent land or farm livestock.   

In addition to containing microorganisms, the leachate will contain dissolved chemicals and larger organic and suspended inorganic particulates such as colloids. The leachate will also contain nutrients beneficial to plants e.g. nitrogen and potassium, fulvic acid and humic acid.

The dissolved and particulate organic matter gives the leachate its characteristic yellow to dark-brown in colour. During the initial decomposition the leachate contains oxidized functional groups while in the later stages, as the compost matures and the plant lignin in the material decomposes, more phenolic groups are present. In addition to organic matter, and nutrients any contaminants present in the original feedstock may be found in the leachate.  The highest concentrations of organic matter, nutrients, and contaminants are found in the initial leachate and the concentrations present will decrease as the material is diluted by rain water if it is permitted to run through the compost.

While fresh, odour free and light brown in colour the leachate may be spread on the plot or garden and used as a liquid fertiliser around plants.. The normal dilution rate is 1:10 (1-part leachate to 10 parts water) but for sensitive plants a ratio of 1:20 is suggested.

 Lechate differs from Compost tea which is specially brewed to increase the numbers of microbes in the liquid to be applied to the plants and soil. For information on Compost teas click the link Compost Teas

Leachate can be collected from tumbler bins by installing a drip tray, or other forms of containment, beneath the drum. The tray should be fitted with a drainage tube so that the leachate can be collected in a bucket. Drip trays should be emptied regularly so that they do not overflow. Such leachate can be added to the compost bin both to increase the moisture content in dry weather and as an activator.

Rainfall can add to the leachate increasing run off from the bin increasing the risk of contaminating groundwater or any water courses adjacent to the composting area. While the composting material needs to stay moist to decompose properly it should not be allowed to become waterlogged or produce significant amounts of runoff. Covering the bin or heap with a tarpaulin or fitting a lid not only reduces this risk but helps prevent the organic material becoming waterlogged. Hot composting systems using a New Zealand bin or a modern composter such as the Hotbin tend to produce less leachate as a considerable amount of the water evaporates

If the compost or heap bin is mounted directly on the soil moving it every two or three years allows the area fertilised by the leachate to be utilised for growing. Alternatively, the bin could be mounted on a waterproof membrane which is drained into a bucket set into the ground.

If a permanent composting area is being used the heaps or bins can be put on a concrete bed or slabs which slope to a gutter which drains into a bucket set into the ground to collect the leachate. It is advisable to use a lidded bucket or have a wooden cover over it to reduce the egress of rain water which would dilute the leachate.