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Last edited 17 Jan 2023
In simple terms a compostable item or material is one that is capable of being broken into its natural elements in a suitable environment (a composter or compost bin). The process should take around 90 days to break down into natural elements which cause no harm to the environment, normally producing only CO2, water, inorganic compounds and biomass, such as common biomass materials like garden leaves or peelings.
The specific definition of compostable has had an increasingly important role for a number of reasons. In particular the gradual move away from oil based plastics towards bio-plastics, which may be biodegradable though not necessarily compostable but also through increased awareness of the impacts of waste and pollution, both plastic pollution but also around organic food waste. There are differences in how items might be composted, in terms of the biochemical process, in terms of scale and in terms of the mechanics, some of which are described here, increasingly items might be labelled as being suitable for home composting, or commercial composting.
 Biochemical processing
Anerobic composting makes use of the biochemical process anaerobic digestion during which complex organic matter is decomposed in absence of oxygen, by various types of anaerobic microorganisms. It is the process that occurs in standard composting piles left in situ, it takes longer than aerobic digestion but doesn't require turning (to allow access t air) it can be a smelly process letting off hydrogen sulphide and/or methane gases.
Aerobic composting makes use of the biochemical process aerobic digestion involving microorganisms that require the presence of oxygen, usually air. Many organisms require the presence of oxygen to survive and grow, such as plants, animals, and many microorganisms. For aerobic digestion to take place the matter needs to be rotated or turned regularly giving it access to oxygen, this reduces the odour given off as less or no hydrogen sulphide or methane is produced, it is also said to be more efficient a removing some nutrients with rain water as it is raised off the ground it is also much quicker taking as little as one month, depending on the sizes of the original organic material used.
Vermicomposting is also an aerobic process, but one that relies specifically on earthworms (that require oxygen so are aerobic), alongside microorganisms to bio-oxidize and stabilize organic materials. This natural process of biochemical degradation occurring in the environment is recreated in for example wormeries, whereby the earthworms help in the process of breaking down the waste and converting the organic material into compost. This process can help speed up digestion when compared to anaerobic digestion, though slower than aerobic digstion carried out correctly, taking around 3 months.
 Non-commercial composting
 Open air composting
Open air composting, sometimes called hot or warm composting but also called cold composting (!?) because the material creates its own heat in the process but is also done outside in the cold. It is probably one of the most commonly used methods of composting, using open piles of organic material stored in sectioned bays, or standard green compost containers outside. The bays or containers are filled and emptied on a rotational basis with a mix of green a brown organic waste, over time it breaks down primarily through anaerobic digestion at the centre or base where air cannot reach, emitting heat in the process, and creating compost.
 Direct or in-ground composting
Direct or in-ground composting is exactly as the name suggests, it is the oldest and possibly simplest approach, where by a hole or trench is dug in the ground, organic material laid inside and then covered. As with open composting it takes time, it helps for the waste to be in small pieces, can be prone to wildlife digging it up and should be left fallow for a while. It is though already in the ground, so once ready and then tilled can be used directly for planting.
 Tumbler or vessel composting
Tumbler or vessel composting is a common form of aerobic digestion, where organic material is placed in a horizontal or vertical drum with a small holes on its surface. The organic material is kept damp and turned or mixed regularly, this ensures that it has access to oxygen, helping the process of breaking down the material to its natural parts forming compost. It is normally kept off the ground on a stand to allow water to drain and to allow turning.
 Worm farm composting
As described in vermicular aerobic digestion or vermicomposting, worm farm composting is the same, a container with worms and the organic material. It can also be used as an education tool in schools, with a glass or clear front to allow children to see the worms in action.
 Effective Micro Organism (EMO) or composting
Effective Micro Organism (EMO) composting is a form of anaerobic digestion that makes use of specific bacteria to facilitate the break down and de-composition of organic matter. It is also called Bokashi and originally would have been carried out directly in the ground in ancient Japan and Korea, burying food scraps, allowing microbes in the fertile soil to break down the waste, creating fertile soil. Today Bokashi uses specific microorganisms and is a form or fermentation, it is a method that is suitable for smaller spaces as well as inside. Some of the types of EMOs used include yeasts (Saccharomyces), bacteria that produce lactic acids (Lactobacillus) and (phototrophic) purple non-sulfur bacteria (Rhodopseudomonas). The process produces a leachate by-product which needs to be drained regularly and once complete the compost should be neutralized by being buried for a few weeks before being used on plants as it is very acidic.
 Combination or compot composting
Combination or compot composting is most likely the composting approach most people use, even though they may think they may refer to it as open composting. It is the simplest and least controolled form where or organic matter is placed in a container and left for sometime, naturally it becomes in effect a combination of open-air composting, direct composting, vermicomposting, and EMO composting through the natural additioves and processes that occur. .
In general commercial composting methods also know as Waste Treatment Technologies (WTT) are very similar as the non-commercial methods described, just that they are scaled up, have slight variations because of their nature and are given different names or references, some of which are highlighted below.
Mechanical composting, although not so common might be considered a commercial approach because it is used in commercial buildings such as hotels or hospitals where space and time management are key, like those above though it can be carried out on a relatively small rather than industrial scales. Mechanical Composting uses electricity to create heat required and to rotate or aerate the organic matter, speeding the process up dramatically, producing semi-composted waste with in as little as 24 hours. It can be used as an in-house alternative to often costly organic waste collection services, producing a semi composted material that has reduced bulk and weight.
Windrow systems are possibly one of the most common commercial compost approaches, similar to open composting described above organic waste is piled into long rows and aerated periodically by watering and turning, with some variations, the rows may be 2-3 meters tall and 4-6 metres feet wide.Some of the variations are given below.
- Enclosed windrow; forced aeration, with an automatic turning process, humidified with industrial and process water.
- Trapezoidal windrow in the open air with pre-fermentation anaerobic period, turning with humidification no additional aeration.
- Triangular windrow, covered and a simple form of composting, no forced aeration, turning and humidification regularly.
- Triangular windrow, not covered with a flexible composting time, no additional aeration, turning and humidification by industrial and process water.
Similar to windrow systems but not turned, the waste is mixed in large piles, loosely layered along with bulking agents (woodchips, branches, shredded newspaper, etc) to allow air to filter through the pile. Then aeration pipes are normally installed at the base of the pile to force air through and out of the centre of the pile encouraging aerobic digestion.
Waste is placed in a drum, silo, or concrete-lined trench where environmental conditions are controlled by the waste management facilities. Variation include rotting boxes which are closed units, where air is forced in though no turning, humidification carried out in the process.
Tunnel systems allow for specific control and adjustment of process parameters. It takes place in completely closed off rotting tunnels, purified exhaust air is emitted to the atmosphere after a two-step process of an acidic wash and biofilter. The purification of the exhaust air minimises greenhouse gas emissions as well as odour emissions of waste treatment. A final composting process migh also be carried out in windrows.
- In its chemical composition it should limit content levels of volatile matter, heavy metals and fluorine.
- With 6 months 90% of the original material should biodegrade and convert into CO2, water and minerals.
- The 90% that biodegrades should disintegrate into particles able to pass through 2mm gauge sieve.
- The material should be free from toxic and other substances that slow down or prevent composting.
UNE-EN 13432 is used to assess what can and can’t go to the organic (compostable) container. All of the criteria above must be met for a material to be considered compostable.
The ASTM defines compostable as anything that undergoes degradation by biological processes during composting to yield CO2, water, inorganic compounds and biomass at a rate consistent with other compostable materials and leaves no visible, distinguishable or toxic residue.
‘…a natural process which converts organic waste into an earth-like mass by means of bacteria and micro-organisms. The composting process is also supported by larvae, wood lice, beetles, worms and other such creatures.’
- Located in a dedicated position.
- Easily accessible to all users.
- Integrated within the design of the home achieving reduced visual impact.
- Storage locations are durable, low maintenance and cleanable.
- Managing odour and noise issues.
- Addressing health and safety issues (including fire and vermin).
- Anaerobic digestion.
- Circular economy.
- Managing packaging waste streams.
- Our waste, our resources: a strategy for England.
- Site waste management plan.
- The Courtauld Commitment 2030.
- Types of fuel.
- Waste and Resources Action Programme WRAP.
- Waste management - explained.
- Waste management process.
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