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About Composting

Food waste in the UK is currently managed through a range of systems. Larger scale municipal methods include: council food waste collection and composting using ‘in-vessel’ composting, ‘open windrow’ composting, or ‘anaerobic digestion’. A variety of popular smaller scale domestic composting methods have been developed also. Each method has it advantages and disadvantages depending on where you want to compost, how much space you have available, the amount of compost you would like to make, cost of buying or making the composter, and the time you have available to maintain or harvest your compost.

Industrial-scale composting

Open windrow composting

Open windrow composting is used for processing garden waste (such as grass cuttings, pruning, and leaves) in either an open air environment, or within large covered areas where the material can break down in the presence of oxygen.

In-vessel composting

Industrial in-vessel composting (IVC) describes a large-scale process of composting food waste using vessels or containers. This system ensures that composting takes place in an enclosed and controlled environment to ensure that any pathogens present in the waste material are destroyed. This technique is commonly used to convert organic waste, including food and garden waste mixtures, to a suitable state for use as a soil conditioner.

Anaerobic digestion

Anaerobic digestion (AD) is the process whereby organic matter, including food waste, animal waste, and sewage sludge, is broken down in the presence of microorganisms to produce biogas and digestate fertiliser. This process happens in the absence of oxygen in a sealed, oxygen-free tank called an anaerobic digestor.

Domestic-scale composting

Outdoor compost heap

You can make a compost heap yourself using found or cheap materials such as wooden pallets, or corrugated sheet metal. This method is better suited to garden or allotment spaces where there is a generous amount of space to compost in, and where a large amount of compost is useful. Small gaps in the sides of the compost bin help aerate the decomposing organic matter, but also allow some heat to escape. The compost bin should be at least 1 metre square and 1 metre high, and ideally more than one will help with ‘turning’ your compost, a useful process of digging and mixing up the organic matter to help aerate it and speed up decomposition.

Outdoor compost bin

Outdoor compost bins are convenient if you only have a small outdoor space to compost in. These are usually made of plastic; the plastic lid and container help to retain the heat and humidity in your compost to speed up decomposition. Plastic bins are commonly made from black or dark green plastic; darker colours help to absorb heat from the sun to speed up decomposition, while blocking out direct sunlight and stopping the regrowth of unwanted weeds. Ideally, this type of bin should be placed on grass or earth for drainage and temperature stability.

Outdoor hot compost bin

Outdoor hot compost bins allow decomposition to happen at a much faster rate by increasing the temperature of the decomposing matter. As these bins are usually insulated with a tight-fitting lid, decomposition can be decreased from around 6 months to 1–3 months. The resulting compost is finer than when produced with other methods. These bins are similar in size to council wheelie bins and are suitable for hard standing surfaces.

Wormeries

Wormeries, also know as ‘vermicomposters’, are perfect for small-scale indoor or outdoor composting of household kitchen waste. The worms aerate and break down decomposing organic matter quickly and produce a nutrient-rich liquid fertilister called ‘leachate’ or ‘worm tea’. Not all waste can be composted in a wormery, as certain strongly flavoured foods such as citrus, ginger, or chilli harms the worms. Instead, this type of foodwaste should be put in a compost bin or council food waste collection service. Compost worms can also be added to most other types of large open compost bins, except for hot compost bins.

Bokashi bin

‘Bokashi’ is a Japanese word meaning ‘fermented organic matter’. The practice of bokashi relies on the fermentation process, using special bacteria to break down the organic waste into a ‘pre-compost’. This pre-compost material can then be more easily, and widely decomposed in regular compost bins or wormeries, for example. One advantage of bokashi composting is that a wide range of organic waste can be fermented including meat, dairy, bones, and oily or cooked food waste, which is not suited to other compost methods. It takes about two weeks to ferment organic waste and is perfect for small indoor or outdoor spaces.

Green cone digester

A digester is a type of composter that is partially buried into the ground. This system encourages naturally occuring soil organisms to inhabit the organic waste and help with decomposition, while preventing unwanted pests or vermin from getting inside. A ‘green cone’ digester, so named because of its appearance, is a common type of digester that comprises an upper cone-shaped sleeve sitting over the top of an open bottom dug into the ground. It is usually made of dark-coloured plastic, as the exposed top absorbs the sunlight and ambient heat, helping to warm the organic waste inside and speed up decomposition. A wide range of organic waste can be composted using this method, including vegetable peelings, raw and cooked meat fish, bones, dairy products, and other organic kitchen waste.

Trench composting

Trench composting is suitable for those with a vegetable plot, or spare ground. This method is anaerobic, so it does not produce the same material as traditional composting, but makes an ideal growing medium for a wide range of plants. Trench composting is a relatively simple process, without the need to worry about moisture or pH levels, or aerating.

A pit or trench needs to be dug, with the removed soil kept to one side. Garden or kitchen waste should be added to the pit or hole in layers, ideally adding a sprinkle of lime to the top of each layer. Once the hole is full, the removed soil should be mounded on top. In the spring, this mound can be used to grow marrow or plants from the squash family, sweet corn, or runner beans. The following spring, the mound will be suitable for growing deep rooters such as brassicas, peas and beans, potatoes, onions, and other root vegetables.