The word 'compost' conjures up images of an untidy heap of assorted kitchen and vegetable remains at the far end of the garden. A more scientific definition of compost would be the product of natural degradation of botanical and putrescible waste by the action of bacteria, fungi, insects and animals in the presence of an adequate air supply. The biological decomposition processes break down complex organic substances into carbon dioxide, water and a residue: compost. The final product is relatively stable and can be used as a soil improver, a mulch or a component in a growing medium.

Composting occurs when there is a plentiful supply of oxygen, moisture and warmth. Compost must be regularly aerated, by turning the heap or by injecting air into the composting material, for the process to be successful. Under properly controlled conditions the temperature of composting refuse can rise to levels which are sufficient to kill pests, weed seeds and pathogenic bacteria.

Anaerobic fermentation occurs when the oxygen supply is restricted. Complex hydrocarbons are broken down into reduced intermediate by-products; both methane - which has a potential value as a fuel - and carbon dioxide are released. Anaerobic decomposition - called fermentation or digestion - occurs all the time in landfill sites containing household refuse.

Three main classes of micro-organisms are involved in the decomposition of refuse: bacteria, fungi and actinomycetes. Bacteria and fungi predominate as the organic material begins to decompose. If sufficient air is available, the rate of metabolic activity of these micro-organisms is so great that temperatures can rise to 70 °C or more. At this stage, only heat-tolerant (thermophilic) bacteria and actinomycetes can continue to decompose the waste. Gradually, as the substrate is consumed, the rate of decomposition slows, the compost cools, and the fungi and non-thermophilic bacteria become active again. All micro-organisms need water to function; if the moisture content of composting waste falls below 40%, microbial activity slows. If the moisture content is too high, however, air spaces within the composting material fill with water, creating anaerobic conditions which can cause unpleasant odours.

Mature compost is a valuable substance. It can act as:

• a soil conditioner - improving soil structure, especially for heavy clay soils. Compost also retains moisture and so helps to improve light sandy soils. It reduces soil erosion, and helps to bind nutrients, preventing them from being washed out of the soil

• a soil fertiliser - encouraging a vigorous root system

• a mulch - if applied around plants it will smother small weeds and prevent the surface soil from drying out

• a peat substitute - for use in potting mixtures

Household refuse contains a high proportion of organic materials which are suitable for separation and composting. The compostable fraction of domestic waste includes food scraps, animal wastes, soft plant materials, paper and card. However, not all materials of biological origin decompose fully during composting. Less readily decomposed materials include wood, bone and industrially 'altered' organic materials such as leather. Surveys show that MSW often contains as much as 50% organic material.

A typical aerobic composting system for mixed waste has several stages: Pre-processing. To modify the incoming material, making it more suitable for composting. Materials recovery takes place at this stage, and the waste is then pulverised and/or screened to separate the organic fraction.

Decomposition. The methods used for the large-scale production of compost fall into three main categories. Windrow composting uses mechanical turning to aerate periodically the composting waste, which is placed in elongated heaps around two metres high. Blowers may be used to force air through the windrows for more efficient aeration and heat removal.

In forced aeration (or static pile) systems, the waste is carefully piled over a ventilated floor area or a perforated pipe, through which air can be passed. The ventilation may be vacuum-induced, with air drawn inwards through the waste. This allows any odours produced to be contained and, if necessary, treated. Alternatively air may be blown outwards through the composting waste; this method is often preferred as it allows the heat from the most rapidly decomposing waste at the centre or base of the pile to be transmitted to the outer, cooler regions. Forced aeration systems generally have higher capital costs, and lower operating costs, than windrow systems. Alternatively, a variety of enclosed reactor systems may be used, which are designed to allow close control over temperature, moisture and aeration and mixing rates, but which also require a high investment of capital. These technologically more advanced systems are typically applied to more complex mixtures of wastes.

Maturation. Even when the compost has been stabilised using one of the above methods, it may not be ready for use. During decomposition, complex organic molecules are converted by stages into simpler compounds. Some of the intermediate breakdown products in this process can be toxic to crops. The compost may need to pass through a final maturation or curing stage while these plant toxins are further decomposed before it can be used. This may take weeks or months.

The amount of compost which a mixed municipal waste composting plant can produce will, of course, vary according to the quantities of waste received, but will also depend on the composition of the waste and the quality required of the final product. Most organic waste has a high moisture content, and there is a substantial drying effect during composting; there is also a considerable loss in dry weight, due to the transformation of organic carbon to gaseous carbon dioxide. The final stages of processing the composted material also have an effect on the yield of a composting plant. For high quality compost, it is usually necessary to incorporate a final sorting stage to remove uncomposted particles and inert contaminants such as wood, glass or plastic. Such final screening stages may not always be necessary, for example in cases where the compost is used for landscaping, land reclamation or landfill cover.

Compost from mixed domestic wastes is not easily marketed. Cases of failure to find markets have been well documented in Germany and other European countries. However, compost produced by plants in the Middle East has been in such demand that in some cases would-be customers have had to join a two year waiting list. Composts produced in Japan (from sewage sludge, agricultural and municipal wastes) and the former USSR are used almost exclusively by farmers. Increasingly, countries are favouring the separate collection of putrescible household wastes. In the Netherlands, separate collection systems have been mandatory since 1994. Austria, Germany and Switzerland all have similar requirements now in place.

Compost must compete with other soil conditioners, and municipal authorities have been reluctant to invest in plants without an assured market for the product. Markets have been hampered by poor quality, badly stabilised compost, with visible contaminants such as pieces of glass and plastic. However, even when no ready market is available, composting can provide an effective means of reducing the polluting potential of household refuse. Composting stabilises degradable materials and reduces their volume, conserving landfill space and decreasing the risk of pollution from landfill gas and leachate. Another objection to municipal compost is that it may contain heavy metals (and other toxic substances) because of its origin as mixed refuse. It is difficult to separate out the main sources of heavy metals before the refuse is composted, and research is currently under way to improve the process. Most of the heavy metals present are found in the smallest particles of the household waste, so effective screening can remove a high proportion. However, complete removal of these contaminants is rarely possible. Some of the most difficult metal-rich contaminants to remove are household dust (which can have a high lead content), staples, wine bottle caps and batteries. To produce high quality composts, it is far better to ensure that contaminants do not mix with the compostable waste, by source separation for central composting, or by composting at home.

If the individual householder composts organic refuse at home, not only is it removed from the waste stream, but the householder has complete control over the type of refuse put on the compost heap, and can therefore include only biodegradable material. Where it is not possible for householders to compost their own refuse, the local authority can operate a service in which the organic fraction is collected separately from other rubbish. In this case, the resulting compost will be of much more consistent quality, and many of the problems (such as contamination with heavy metals) are minimised.

If organic waste is not separately collected, the production of compost from MSW is perhaps best carried out as part of an integrated waste management system. Such a plant has been operating in Duisberg, Germany, for 30 years. Mixed household waste from an area serving 95000 residents is first passed beneath a magnet to remove ferrous metals, then along conveyor belts where glass bottles, non-ferrous metals and hard plastic items are picked out by hand. The rest of the refuse is then fed into a rotating reactor vessel, in which temperatures and moisture are carefully controlled. The waste remains in this reactor for 36 hours, and is then screened. The large particles (paper, card, textiles and plastic) are incinerated, and the smaller fraction is separated into two different grades of compost which are matured for a further three weeks, aerated with the air which is expelled from the enclosed reactors. The compost is used mainly in vineyards, with most of the rest used for landscaping and land reclamation. In Germany, composting activity has increased considerably in recent years. In 1985, twelve facilities produced around 0.25 million tonnes pa. By 1993, there were around 100 plants in operation, yielding 1.1 million tonnes pa. Almost 100 other projects were either under construction or had received planning approval.

In the 1980s, French composting facilities would traditionally process mixed household solid waste. As a result, the product was of low quality, tainted by heavy metals and containing sharp glass and plastic film. As farmers demanded higher quality material for land application, so end-markets for mixed waste compost declined. Early in the 1990s a few municipalities (Bapaume, Niort, Lille and Le Creusot) introduced selective kerbside collection for compostable waste.

Of these, Bapaume has now developed a cost-effective system that works well in a rural or semi-urban setting. The Bapaume experiment, which collected green wastes, kitchen wastes, disposable nappies and cartons) demonstrated a range of benefits:

• bio-bins collected 40% of the waste stream

• more than 95% of the bio-bin contents were compostable

• less than 1 % of bio-bins were refused by collectors

Trials were scaled up into a commercial composting plant using the OTVD-SILODA process. The unit, operational in 1997, will process up to 14000 tonnes pa as more municipalities join the scheme. A survey in France in 1993 suggested that there were 73 MSW composting units (no source separation), 30 green waste composting plants, 16 farmyard manure composting facilities, ten mixed organics composting plants and five experimental source-separated plants.

In America, the total number of garden waste composting facilities reached more than 3300 in 1995. Some two-thirds of these involve the composting of leaves collected in the Autumn. Around half of US states have programmes to support home composting. More than twenty states have banned garden wastes from direct landfill disposal. In 1993, 18 pilot programmes were underway to evaluate composting of compostable (non-garden) wastes. These were mainly windrow systems with capacities up to 100 tonnes per day. Most took source-separated material from residential, commercial or industrial sectors.

In Japan, composting plants have been built in many cities, to reduce the waste load to incineration facilities. By the mid-1990s some 29 composting units were operating in Japan. In less prosperous nations, there is often a higher level of organic material in the waste stream. Brazil, which produces some 90 000 tonnes MSW each day with an organic content of up to 60%, now has 74 composting plants, 19% of which are fully mechanised. Around 30 municipalities in Brazil employ some form of source-separation for organic waste, further improving the quality of the final product.

The demand for compost made from source-separated materials should increase as experience in its manufacture and use grows. It is already used for spreading on areas of salty land to restore fertility, and as the world's topsoil is already being lost at the rate of three tonnes per acre every year it seems likely that compost will eventually be in demand for restoring the structure of farm soils everywhere.

A report from the European Commission15 identified a number of effective centralised and home composting schemes in Spain, France, Ireland, Portugal, Italy and the UK. A major influence in the development of these schemes was the European Union's 1999 landfill directive, which sets high standards for waste disposal. This instrument includes provisions to reduce landfilling of biodegradable wastes to reduce environmental damage from the release of landfill gases and leachate. Some Member States have already introduced limits for the amount of biodegradable waste permitted to go to landfill.

15 European Commission, Success stories on composting and separate collection, ISBN 92 828 9295 6,

Catalogue No. KH-27-00-726, Environment DG Information Centre, BU-9 0/11, 200 rue de la

Loi, B-1049 Brussels, Belgium, 2001.

The Commission's study concluded that successful diversion of biodegradable waste from landfill depends on separation at source. These wastes include kitchen waste (e.g. fruit and vegetable peelings), garden waste and (often) card and newspaper. Most successful schemes provide free containers of bags for collection. Frequency of collection varies from daily to once a week or alternate weeks. Organic waste collection must be frequent enough to prevent waste accumulating to levels unacceptable to the householder, especially in warmer climates.

The existence of end markets is extremely important; revenue from sales of compost or soil conditioners can fund the scheme. Use of the end product is fundamental in achieving the full environmental benefit of composting.

Many schemes share collection vehicles with schemes to collect dry recyclables. The scheme in Niort, France for example, achieved cost savings where the unit cost is half that of landfilling - additionally, a state landfill tax is avoided for all materials composted. Cost avoidance is an important benefit of composting. Most of the schemes surveyed in the Commission study had received additional financial assistance from local or national governments.

Good publicity and information was the most important overriding factor. Administering the schemes involved extensively the local municipalities or governments. Detailed planning and design are important when developing collection systems and treatment plants. Key success factors in the programmes were:

• clear achievable objectives

• right mix of waste types to targets

• effective and convenient to householders

• establishing a market for end product

• good financial management and planning

• good publicity and information

With the growing international trend of introducing a policy to ban landfilling of untreated organic wastes, composting is set to benefit from an increased enthusiasm for landfill diversion.

Organic Gardeners Composting

Organic Gardeners Composting

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