Anaerobic Digestion and Composting

Anaerobic digestion (AD) is the decomposition of organic material (biomass) by anaerobic bacteria in the absence of oxygen normally inside a sealed tank or digester. Anaerobic Composting works best with wet materials e.g. animal slurries and manure, or high moisture content nitrogen-rich materials such as catering and food waste.

It is an old technique used by Chinese farmers to produce compost using   water-filled pits to create anaerobic conditions to ferment straw, greens and animal manures. In the UK we have our own “traditional” version of an anaerobic composting technique although it is seldom described as such. That is Trench or pit composting, normally used when growing runner beans of marrows, where the waste is buried in a trench or pit preventing exposure to the air and oxygen during the composting process. This is a form of dry anaerobic digestion as unlike the techniques described below no additional water or liquids are added. The decomposition process depends on the water content in the feedstock. 

Today Anaerobic Digestion it is used industrially for the treatment of biodegradable industrial or domestic waste, including sewage sludge, providing an environmentally friendly alternative to landfill.  AD produces biogas, consisting mainly of methane (45-85%) and carbon dioxide (15-45%) which can be used as a fuel. The sludge and solid component produced can be used as a soil conditioner while the liquid portion can be used as a “compost tea” liquid fertiliser

AD is used to treat food waste, including cooked food, collected from the kerbside by many councils in the UK.    While domestic anaerobic digesters are available they are more suitable for hotter climates in temperate area such as the UK a small domestic unit would need supplementary heating in cold weather. Domestic AD plants may be feasible for use in isolated houses “off the grid” to treat kitchen and garden waste and human faeces

Larger AD systems can maintain the temperature for effective Digestion temperature.  As an alternative to large systems community schemes, processing food waste at a local level, have advantages over individual household units or large plants that require the transport of waste over long distances.

Small Anaerobic Digesters are appropriate for use on farms where large quantities of manure are produced e.g. dairy and pig farms as an alternative to collecting the manure in open lagoons.

Home Anaerobic Composting

Having considered different ways of making compost elsewhere on this website and stressing that composting is an aerobic process it is necessary to consider home anaerobic composting. This is not a new process, but it appears to be increasingly discussed on the internet particularly in the US. Some methods such as bag composting require the minimum of equipment and can be undertaken where space is limited. However, as the process involved is anaerobic rather than aerobic it may be more correctly described as anaerobic digestion rather than composting although the term fermentation is commonly used to describe the process when using the Bokashi system.

Fresh or immature anaerobic compost may be acidic, so it is best to store for about a month exposed to air to neutralize its ph.  If one of the slower composting techniques which take about a year have been used the compost should be safe to use with a minimum storage period.  Anaerobic fermentation is a cool process not producing the heat generated during aerobic composting where the material is regularly turned to aerate the contents. As a consequence, fermentation will not reach the temperatures necessary to kill potential   pathogens and weed seeds present in the feedstock.  However, the evidence suggests that most pathogens will not survive a yearlong fermentation process, where immature compost is produced in a shorter time it may contain harmful pathogens so is best stored for at least a month before use.

Catering waste including meat can be treated by anaerobic composting and while a limited amount of meat can be composted by the bag or sealed container methods there are regulations governing the use of Animal By-products and they may attract vermin to a heap.  As with aerobic composting oily, fatty foods, such as mayonnaise and vegetable oils should be avoided.



Anaerobic & Aerobic bacteria

Before looking at methods of anaerobic digestion I have included a basic definition of the two types of bacterial respiration.

Aerobic Bacteria

These are bacteria which require oxygen for growth, respiration and reproduction. Aerobic species can detoxify oxygen for use in their metabolism. Aeration of the compost bin is designed to expose all the material being composted to oxygen to help aerobic bacteria to grow and function.   Turning the compost in aerobic systems is more labour intensive than anaerobic digestion, although this is only the case where hot composting is being used as cold systems are often not aerated. The oxidation process that takes place in the compost heap releases some of the nitrogen, and carbon dioxide, from the organic material which is evaporates and of course leachate may be lost to the soil under the bin neither of which occur in a sealed container.  

Anaerobic Bacteria

Anaerobes are bacteria that can survive without oxygen for growth. They will decompose organic material but more slowly than aerobes and the process produces an unpleasant smell due to the production, amongst other things, of hydrogen sulphide and amines. The process also produces methane a greenhouse gas.  It also produces methane, a greenhouse gas.   I would not recommend it as the main composting method at home where a significant amount of garden or food waste is produced unless the methane is being harvested using an Anaerobic Digester for use as a fuel.  

For anaerobic composting a closed container, or a very restricted air supply is necessary to exclude the oxygen.  Some home anaerobic composting methods are closed batch systems using airtight containers while others allow limited air access during the opening of the container to add new organic matter.  Another system involves excluding air by covering the material to be composted with loose sand or water.

Bag or Sack Anaerobic Composting

 Composters on the internet recommend bag or sack composting where the householder does not have a garden or space for a compost bin as the sacks can be stored in a small space or in a shed or garage.  It is also offered as a way of composting kitchen waste during the winter but a disadvantage of this is that it will entail saving the waste in a bin, or other leak and smell proof container, until there is enough to fill the bag in one go when combined with the browns. It is also argued that anaerobic composting in a bag or sack require less work than conventional composting as the contents do not need to be aerated by turning at regular intervals, but it will smell.

The simplest method is to put the kitchen waste in a refuse bag, soaking the material and sealing the bag closed.  Some of the demonstrations of bag composting on You-Tube show conventional kitchen waste bags being used to contain the compost but I would recommend thicker bags, such as old compost bags or builders refuse bags. As a safety measure double bagging is recommended to avoid the full bag splitting when being handled.  I would suggest the one bag is put inside the other before filling with organic material rather than have to lift a full bag to put it inside the other

 Do not use Compostable or Biodegradable bags the Compostable bags may decompose before the compost and biodegradable bags are unlikely to be strong enough.   

 As it helps the process to turn the bags occasionally during the composting process it is important to choose a size that is relatively easy to handle. I00-150 litre bags are a good size, but a smaller 70 litre bags are easier to handle and can be used if the larger bags are too heavy to turn.  If only one or two bags of compost are being made I would suggest putting them in an old plastic dust bin which can be laid on its side and rolled to mix the decomposing material. It is important to ensure that there are no holes in the bags to prevent air entering the mix as the objective is to achieve anaerobic conditions.

Mix for anaerobic bag composting

"Composting in a Bag" is usually treated as a batch composting system with the materials being set aside separately until the bag can be fully filled. As mentioned above nitrogen rich wet material is ideal and while it is necessary to include carbon rich Browns to provide a roughly equal quantity of Greens and Browns chose the Browns to avoid adding too much very dry material such as such as leaves, sawdust and woodchip.

Some recommend the addition of commercial compost activator, active compost from a compost heap, or garden soil to speed the start fermentation process by introducing microbes to the bag.

This can be added in two ways:  

  1. If using the above method of alternative layers of Greens and Browns active compost, commercial compost activator or soil can be sprinkled over every layer of the Greens (or kitchen waste).  ensure that composting micro-organisms   are distributed throughout the material in the bag. 
  2. Rather than using a mix of equal parts Green and Browns use compost or soil as a third layer using 1/3 compost or soil, 1/3 Greens and 1/3 Browns.

With either method I recommend adding a layer of active compost to the bottom of the bag to act as an activator and to help absorb the liquid which will drain to the bottom of the bag.

The dry spent compost from pots or planters, can be used if available, and is best added at an early stage to help soak up some of the liquid produced from the decomposing food waste.

 Continue with alternate layers of greens, activator, browns and spent compost till the bag is full. When the bag is full the contents are soaked with water and the surplus air squeezed out of the bag. The bag should be tied, or taped, to prevent the access of air.  

Ideally the bag should be turned every couple of weeks to mix the materials. Turning is made easier if the bags are put into a plastic dust bin which can be put on its side and rolled with minimum effort. The bags should be left in a sunny spot during the summer and preferably in a heated or frost-free shed of garage during the winter. In colder spots the fermentation will take longer.

The time to produce compost varies with the mix and conditions in a sunny spot immature in eight weeks to six months in colder positions the fermentation will take longer.  When the process is complete the material should look and smell like normal compost however it should be stored   under aerobic conditions for a least a month before use on plants to allow it to lose its acidity.



Bucket or Bin anaerobic composting

This simple method of anaerobic composting and uses a plastic bucket complete with lid, or smallish plastic bin with an airtight lid.  The container can either be buried, so that the leachate drains away or mounted above the soil and the leachate harvested.  This involves more effort but provides a liquid plant food.

If the bin is to be buried the site where it is to be placed is dug and either the top two or three inches of soil is removed or deeper to allow half of the bucket to be buried. In some methods the container bottom is cut off while others leave the container base in place and drill drainage holes through it. This does keep the container more rigid and is recommended for sandy soil with good drainage. In poorly draining soils cutting off the bottom of the container is recommended and in Leicestershire clay digging out an area of soil where the bucket is to be situated and replacing it with a soil mixed with pebbles will help. If on heavy clay the answer is to either build up or mount the bucket above the ground.  To build a mound on which to mount the bucket dig out the clay soil as in the previous example and then build a mound of soil and pebbles mounting the bucket on top. This can be held in place by a wooden garden log roll boards or stones once the bucket is positioned.

The bucket is put into position and the soils is replaced to bury the bottom of the bucket both inside and outside and compacted to provide a relatively airtight seal and slow the rate at which leachate produced from the feedstock drains from the container preventing the contents drying out. It also gives worms and other soil microbes a means of access when the conditions in the bucket are favourable to them  

The above-ground digester needs to have drainage holes drilled in the bottom of the container and be stood on bricks. The height of the bricks will depend on the container to be used to catch the leachate. There must be enough room to slide the receiving vessel from under the digester without spilling the liquid. The receiving vessel should be just smaller than the base of the bin or bucket so to prevent it filling with rainwater.

 Filling the Bin

The container is filled with the organic waste e.g. Kitchen waste, grass clippings, shredded paper etc.  Compress to remove any extra air and the lid fitted in place. Leave it for eight weeks for immature compost, that will need storing for under aerobic conditions for a couple of months, or for a year for produce digested ready to use compost.

  The container is left sealed and undisturbed for about a year. In practice several buckets will be required to provide a means of dealing with waste throughout the year.  If a top up system batch is being used and the feedstock is not added all in one go only add fresh material on as few occasions as possible saving it to add one a week or fortnightly as every time the bin is opened air is allowed in.


Covered Static Compost Heap of Bin

This variation of anaerobic fermentation relies on the use of water to exclude oxygen from the material.  This technique can be used for a compost heap or a conventional wooden New Zealand bin.

Soaking and covering a heap

  This method was described in Rodale’s Complete Book of Composting using a heap five feet wide and three feet high. A base was prepared directly on recently dug bare soil which was soaked with water.  The compost heap was made in the same way as a conventional aerobic heap and was soaked so that water replaced air in any spaces in the mix. The heap is covered by a plastic sheet. Black plastic is preferable if clear plastic is used a layer of black building paper (made from Kraft paper saturated with a waterproofing of asphalt) should be spread on the heap between the material to be composted and the plastic sheet to exclude light. Soil should be placed round the edges to make a seal. The objective is to maintain a moisture level of 70%+ rather than the normal 40-60% so if testing the content by squeezing compost in the hand water should run freely rather than the compost having the wrung-out sponge effect. The heap is left undisturbed for two of three months when the immature compost should be ready

Anaerobic Composting using a New Zealand bin

A New Zealand bin can be used to retain the compost using a variation of the above method provided the bin has air tight sides, such as solid boards lined with plastic with a thick plastic cover to retain moisture and exclude air.  This can be held in place by a boarded lid which fits inside the wooden sides of the bin and rests on the plastic sheet cover.

As the bin is likely to be smaller than the heap described above the moisture content will need to be monitored regularly and additional water added if it starts to dry out, so the cover(s) will need to be removed exposing the surface to air but once watered the any air spaces between the material will be filled with water recreating anaerobic conditions. With this type of composting the presence of wet slimy composting and unpleasant smells are good signs indicating anaerobic decomposition and that it will be advisable to wear gloved when moisture testing by the squeeze method.


Submersion or Underwater Composting

This method has the advantage of limiting the smell because the reactions take place under water.

The materials to be composted are put in a suitably sized container, covered with water, and allowed to decompose. Submersion composting usually takes longer than other methods as the water acts as a cooling agent slowing the metabolism of the composting microbes.

As far as home composting is concerned probably the main disadvantages of wet anaerobic composting is that potential pathogens might survive the process


A Bokashi bin makes a good container for subversive composting as it will allow the leachate to be drained off regularly and without mess.