Composting Processes
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.
Diagram showing UK waste processing streams
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.
Size
Open windrow composting is large-scale and needs lots of space for storing and processing waste.
Process
- Feedstock is shredded, mixed, and formed into windrows (long lines) along a non-permeable surface.
- Windrows are turned on a regular basis to regulate their oxygen, heat, and moisture content.
- The final step involves the screening of the compost and removal of contaminants such as plastics or metals, as well as grading the compost for various end uses. Oversized materials are also removed and fed back into the composter until they have sufficiently composted.
Processing time
It takes approximately 16 weeks on average, depending on the maturity requirements of the final product.
Types of waste processed
Windrow composting is only suitable for garden waste. It cannot be used to process organic materials, which include catering and animal wastes. Under Animal By-Products Regulations (ABPR), any organic waste, including catering and animal waste, must be processed using in-vessel composting or anearobic digestion (AD).
Suitability for processing biodegradable or compostable plastics
It is not yet known whether biodegradable and compostable plastics can degrade in open windrow composting.
Suits households
- Where local authority garden or green waste collection is provided.
Advantages
- Open windrow composting can process large volumes of green or garden waste.
Maintenance
- It is a relatively simple process requiring a large area.
- Open windrow composting requires regular turning.
Availability
You will need to check whether your local council offers open windrow composting as part of its green or garden waste collection services.
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.
Size
IVC involves large-scale composters that can be broadly categorised into six types: containers, silos, agitated bays, tunnels, rotating drums, and enclosed halls.
Process
- Stage 1 (1–3 weeks): The local authority food waste collection arrives at an enclosed reception area. It is then shredded and placed into a bay/tunnel where naturally occurring micro-organisms begin to break down the food waste, releasing nutrients and, in doing so, increasing the temperature within the composter to the 60–70℃ needed to kill pathogens and weed seeds, and meet the regulations for processing Animal Bi-Product (ABP) material.
- Stage 2 (1–3 weeks): The composting material is transferred to a second bay/tunnel where the composting process continues. Processing in three stages ensures that all parts of the composting mass reaches the required temperature. The oxygen level, moisture, and temperature are carefully monitored and controlled during both composting stages to ensure the material is fully sanitised.
- Stage 3 (10–14 weeks): Once the sanitisation process is complete, the compost is left to mature in either an enclosure or open windrow to fully mature.
Processing time
The whole process takes approximately 12–20 weeks.
Types of waste processed
IVC can process council food waste collections, either separately or mixed with garden waste, as well as commercial and industrial sources.
Suitability for processing biodegradable or compostable plastics
It is not known yet whether IVC is suitable for processing biodegradable or compostable plastics.
Suits households
- Where local authority food or garden waste collection is provided.
Advantages
- IVC can process a large volume of mixed organic waste.
- The processing of all animal by-products, including municipal kitchen waste, must comply with the ABPR (Animal By-Products Regulations, introduced in 2003).
Maintenance
- IVC involves several stages, and necessitates the use of large-scale facilities.
- The careful separation of stages is required to prevent cross-contamination of ‘clean areas’ (where the product is processed and stored) from ‘dirty areas’.
- Under the UK treatment standards for in-vessel composting ‘catering waste’ either includes or excludes meat: ‘Meat-excluded’ catering waste requires a one-stage barrier system to treat organic waste, plus 18 days storage; ‘meat-included’ catering waste requires a longer two-stage system.
Availability
You will need to check whether your local council offers IVC as a food waste collection service.
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.
Size
AD composters vary in size; industrial-scale composters require significant machinery and storage space.
Process
- Organic waste arrives at an anearobic digestor facility and is screened for contaminants. A wide range of organic waste can be processed, including food and green waste, animal slurries, and sewage.
- The organic waste it then treated to ensure a smooth fluid consistency in preparation for transfer to a digestion unit. Digestion units run at either mesophilic (around 20–45℃) or thermophilic (around 50–60℃) temperature ranges depending on the type of microorganisms present.
- Once in the digestion unit, the organic waste undergoes several processes of decomposition, including hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The resulting by-products are methane biogas, which can be supplied to the grid or converted to biofuel, and a concentrated nutrient-rich fertiliser called digestate.
Processing time
The whole process takes approximately 2–6 weeks depending on the system temperature.
Types of waste processed
A wide range of food waste, green waste, animal slurry, and sewage can be processed owing to the controlled closed-system environment of anaerobic digestion.
Suitability for processing biodegradable or compostable plastics
This is not yet known.
Suits households
- Where local authority food or green waste is provided. AD can also process animal slurry and sewage.
Advantages
- AD can process a wide range of organic material.
- Biogas and digestate, two useful by-products of AD, can be harvested from the process.
Maintenance
- AD requires specialist machinery and monitoring to ensure optimum conditions for microorganisms to break down organic waste.
- AD requires a large area for machinery and the storage of organic waste, as well as the resulting by-products (biogas and digestate).
Availability
You will need to check whether your local council offers AD as part of its green or food waste collection services.
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.
Size
Ideally, the compost bin should be at least 1 metre square and 1 metre high.
Process
This is a DIY homebuilt system that handles garden and uncooked food waste such as vegetable peelings. It produces easily harvestable compost.
Processing time
This varies, but approximately 6–24 months.
Types of waste processed
It takes garden waste and some uncooked food waste.
Suitability for processing biodegradable or compostable plastics
This is not yet known.
Suits households
- Where there is a garden needing compost.
- Where there is a food digester to deal with other types of food waste.
Advantages
- This system does not need good drainage.
- It is either low cost or free.
- It is easy to setup.
Maintenance
- The compost heap needs turning occasionally.
- The compost needs to be harvested.
Availability
You can make it yourself at home.
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.
Size
There are various sizes available.
Process
This type of compost bin can handle garden waste, as well as uncooked food waste such as vegetable peelings. It produces easily harvestable compost.
Processing time
This varies, but approximately 3–24 months.
Types of waste processed
It processes garden waste and uncooked food waste such as vegetable peelings.
Suitability for processing biodegradable or compostable plastics
This is not yet known.
Suits households
- with a garden that needs compost.
- with a food digester to compost other food waste.
Advantages
- It takes garden waste and some uncooked food waste.
- Less attractive to vermin
- It is easy to install.
- It is easy to get hold of, commonly subsidised through local councils.
Maintenance
- The compost needs turning or mixing occasionally.
- Setup on surface with good drainage
- The compost must be harvested.
Availability
You can find an outdoor compost bin at any garden centre, or online. Also, check online for council-subsided offers.
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.
Size
These come in various sizes, from around 100 to 300 litres.
Process
This bin handles garden and cooked/uncooked food waste (including meat, fish, and dairy). It produces easily harvestable compost.
Processing time
This varies, but approximately 1–3 months.
Types of waste processed
It takes cooked and uncooked food waste.
Suitability for processing biodegradable or compostable plastics
This is not yet known.
Suits households
- Of up to 5–6 people.
- With a large amount of food waste.
- With some garden waste.
- With a garden that needs compost.
- With space for a large container.
Advantages
- It takes cooked and uncooked food.
- It also takes garden waste (approximately 1/3 of its volume).
- It can be placed on any free draining surface (since it produces a lot of moisture).
- It does not attract vermin.
Maintenance
- It requires stirring with a stick, which is usually provided.
- The compost needs to be harvested.
Availability
You can find these at garden centres or online. Also check online for council-subsided offers.
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.
Size
There are various sizes available for indoor or outdoor use.
Process
Wormeries utilise worms to break down organic food waste matter; tiger, blue nose, or red worms are commonly used. Wormeries produce compost and liquid fertiliser.
Processing time
It usually takes 10–12 weeks to produce liquid fertiliser, and up to 12 months for compost.
Types of waste processed
Wormeries process food waste (excluding meat, fish, and large amounts of citrus fruits).
Suitability for processing biodegradable or compostable plastics
This is not yet known.
Suits households
- With a use for liquid plant feed.
- With limited outdoor space.
Advantages
- It takes some cooked and uncooked food, although it is best to avoid large amounts of meat, dairy, or fatty waste.
- It is small, compact, and flexible.
- It is easy to set up, and can be used indoors or outdoors.
- It does not smell or attract vermin.
- It produces high-quality compost and liquid fertiliser.
Maintenance
- It does not take garden waste.
- It requires some attention.
- It must be kept cool.
- The compost or liquid fertiliser needs to be harvested.
Availability
You can find wormeries at garden centres or online. You can also check online for council-subsided offers.
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.
Size
Bokashi bins are usually 18 litres in volume, and are sold in pairs to allow for continuous composting.
Process
The bokashi bin is a small indoor composter, which is ideal for use in the kitchen, as it can take all cooked food (including meat, fish, and dairy). Two units allow for continual composting.
Processing time
It takes approximately 10–14 days.
Types of waste processed
It takes cooked and uncooked food waste including meat, dairy, bones, and oily food waste.
Suitability for processing biodegradable or compostable plastics
This is not yet known.
Suits households
- With a small-to-medium amount of food waste.
- With a use for compost.
- With limited garden space.
- With a need for indoor compost production, but lacking in the facilities to compost outside.
Advantages
- It is a fast process, small, and compact.
- It does not smell or attract vermin.
- It does not require installation.
- It produces ‘pre-compost’ and liquid fertiliser.
Maintenance
- It requires activating bacterial bran (an associated cost).
- It does not take garden waste.
- After two weeks, its contents need to be dug into a traditional garden compost heap or bin, or added to a wormery. It is important to check the pH level of a wormery as bokashi is slightly acidic; the pH can be easily regulated by adding alkaline lime.
Availability
Bokashi bins are available at garden centres or online. You can also check online for council-subsided offers.
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.
Size
The underground chamber is approximately 80 litres in volume.
Process
It handles cooked/uncooked food waste (including meat, fish, and dairy). The solar-heated cone circulates warm air and breaks down food waste in a digestion chamber below the ground.
Processing time
It takes approximately 12–24 months.
Types of waste processed
It takes cooked and uncooked food. It does not take garden waste.
Suitability for processing biodegradable or compostable plastics
This is not yet known.
Suits households
- Of up to 4 people.
- With a fair amount of food waste.
- With an existing garden compost bin or heap.
- With a sunny garden.
- With space for installation.
Advantages
- There is minimal residue.
- It does not attract vermin.
- The residue feeds the surrounding soil.
Maintenance
- It is best in free draining soil and sunny conditions.
- It requires some effort to install (a hole needs digging).
- There is no compost to harvest.
- It uses a bacterial accelerator powder to speed up decomposition.
Availability
It is available at garden centres or online. You can also check online for council-subsided offers.
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.
Size
Trenches are approximately 30cm deep, and can be as wide as necessary.
Process
This is suitable for kitchen scraps, old garden plants, and other garden waste.
Processing time
This varies, but approximately 6–12 months.
Types of waste processed
It takes food and garden waste.
Suitability for processing biodegradable or compostable plastics
This is not yet known.
Suits households
- With access to a vegetable plot, or spare ground
Advantages
- It is low maintenance.
- The residue feeds the surrounding soil.
Maintenance
- It requires some effort to dig the trench or hole.
- The addition of lime helps to maintain the pH level of the soil.
Availability
You can dig-it-yourself (DIY).
If you compost and want to take part in a home composting experiment, you’ll need:
- a selection of identical compostable or biodegradable items (see list below)
- a net bag and clip to securely close it (see guidance below)
- a plastic milk bottle cap (or similar) and permanent marker pen to identify your items
If you do not use a composter but you're interested in finding out more, please see our Composting Processes page for more information.
1. Choose your experiment items
Select the type and quantity of compostable or biodegradable plastic items from our list that you would like to test. Please only select items that display the following manufacturer information:
- 'compostable' (only)
- 'home biodegradable'
- 'home compostable'
- 'suitable for home composting'
- TUV OK Compost 'HOME' certification mark
- Din Certco 'HOME COMPOSTABLE' certification mark
NOTE: If testing multiple items together please ensure they are an identical type and brand (one type and brand of item per net bag).
2. Prepare your experiment equipment
To make locating your experiment items in your compost easier, please compost your items in a plastic net bag (the kind of net bag used to hold fruit in a supermarket would be ideal). Use a permanent marker pen to identify your items on a plastic milk bottle cap (or similar) and add to your net bag. To test fruit and veg stickers please stick each label on a plastic milk bottle cap first then add to your net bag. Use a permanent marker pen to identify them. You will have the option to upload photographs of your composter and experiment equipment to help us analyse your results. See our Photography Guidelines for more information.
NOTE: Please ensure you use a perforated net bag. This is so a sufficient flow of oxygen, organic waste and microorganisms can come into contact with your experiment items.
3. Create a login account
Create a login account and share a few extra details about what composter you will be using for the experiment, its location in the UK for our Live Composting Map, and your composting method. Please set the duration of your experiment according to how long it usually takes you to make compost. There is the option to share photos of your composter and experiment items and equipment, if you wish. Please indicate on your form if you would like to share your images on the Big Compost Experiment website. See our Photography Guidelines for more information.
4. Start your experiment
Securely close the net bag containing your experiment items and identifying markers and add to your composter. Carry on composting as usual. We'll send you an email reminder when your experiment is due to finish and it's time to look for your net bag and items.
5. Submit your results
When your experiment timer has finished, please look for the net bag and items in your compost. You can use a trowel, spade or household sieve to locate it. Under 18s must be accompanied by a responsible adult. Check the contents of your net bag for any traces of your items. If there are any traces please compare them with our 'Scale of Degradation' in the results form in your login account and share any other useful observations about your experiment. There is the option to share a photo of your results. Please indicate on your form if you would like to share your image(s) on the Big Compost Experiment website. See our Photography Guidelines for more information.
NOTE: Once you have completed your experiment, please dispose of any item remains in your general waste collection. Check locally for recylability of other experiment equipment and rinse before recycling.
How to take a photograph of your experiment equipment
Pre- and post-composted items and equipment
- Place your biodegradable or compostable items on a clean surface alongside any secondary packaging (if applicable) and net bag. A contrasting surface is preferable (i.e. one on which the items will stand out).
- Orientate items and packaging (if applicable) in order to capture any compostable labelling or certification marks (if displayed). If testing multiple items together please ensure they are an identical type and brand (one type/brand of item per net bag).
- Position your camera approximately 1 metre above the objects. Position items clearly within the viewfinder of your camera or camera phone, so that all edges of items are visible. Please ensure no recognisable features (people, faces, house number, etc.) are visible in the image.
- Take a photograph (while keeping your hand steady!)
- Upload photographs via your login account, with the option to display them on our Gallery page.