Introduction

The composting process can be defined as: the controlled biological decomposition and stabilisation of organic substrates, under conditions that are predominantly aerobic and that allow the development of thermophilic temperatures as a result of biologically produced heat. It results in a final product that has been sanitised and stabilised, is high in humic substances and can be beneficially applied to land, which is typically referred to as 'compost' (modified from Haug1).

Farmers have practised composting for centuries, returning waste organic residues to the land in order to maintain soil fertility and organic matter. It is only relatively recently that modern society has begun to recognise the important role composting has to play in managing the ever increasing quantities of waste it produces. Composting is now employed as a treatment process for a wide range of organic substrates such as municipal solid wastes, sewage sludges, and agricultural and industrial by-products.2 Actively composting materials, or finished composts, have been shown to degrade a wide range of organic pollutants, and are thus used in the bioremediation of contaminated soils.3

There are a number of ways in which composting can be carried out, ranging from small-scale composting of garden wastes by householders, through medium-scale composting by community groups and farmers, to large-scale composting at specially designed sites, although the range of technologies at the latter can vary considerably.

Composting Practices

The activities carried out at a composting site, irrespective of its size, are aimed at amending or controlling feedstock structure, composition, temperature or oxygen content.

1 R.T. Haug, The Practical Handbook of Compost Engineering, Lewis, Boca Raton, Florida, 1993.

2 E. J. Gilbert, D. S. Riggle and F. D. Holland, Large-scale Composting - A Practical Manual for the UK, The Composting Association, Wellingborough, UK, 2001.

3 K.T. Semple, B.J. Reid and T.R. Fermor, Environ. Pollut., 2001, 112, 269-283.

Issues in Environmental Science and Technology, No. 18

Environmental and Health Impact of Solid Waste Management Activities

© The Royal Society of Chemistry, 2002

Most materials received at a composting facility require processing prior to composting. In practice, there are four main activities, namely: shredding, to reduce particle size and increase the surface area to volume ratio; mixing different feedstocks together to improve homogeneity and adjust the carbon to nitrogen (C:N) ratio and/or moisture content; adding water where particularly dry materials are received; and removing contaminants.

The degree of process control employed during the active composting phase varies depending upon the size and location of the site, the nature of the feedstocks and the intended end uses of the composted materials. In practice there are four principal approaches that can be adopted for composting wastes on a large-scale.

Windrow systems. Open-air turned windrows are the simplest system, and they are widely used to compost green wastes (sometimes called botanical residues or yard trimmings). Feedstocks are laid out in long piles called 'windrows', usually shaped as an elongated triangular prism, although the exact shape varies according to the material and equipment used. The term originates from the farming practice of piling hay in rows so that it will dry out in the wind.

These windrows are 'turned' to blend the composting mix, introduce fresh air, and release trapped heat, moisture and stale air. In practice, the technique often involves breaking up the windrows by lifting the composting materials into the air and allowing them to drop back down.

At smaller facilities, a tractor with a front-end loader or grab is often used to scoop up portions of the composting materials. Each bucket-load is carried to another area and emptied onto the ground, re-forming a new windrow. At larger sites, which process greater quantities of material, specialised windrow turning machines are often employed.

Aerated static piles. Some mechanised systems may dispense with turning, and either blow (positive aeration) or suck (negative aeration) air through the composting materials. These 'forced aeration' systems often rely on a perforated pipe running through the pile or a trough underneath the composting materials, which is attached to an air compressor. However, some operators may combine both approaches, using a forced aeration system with some element of physical turning.

The rate of aeration may be linked to oxygen concentration and/or temperature via a negative feedback system, or fans may be switched on and off for defined periods of time. The exhaust gases from negative pressure systems may be passed through a biofilter or scrubber before discharge to the environment, whereas in positive pressure systems the piles may be covered with layers of mature compost to act as an in situ biofilter. These techniques have been widely employed in the USA to compost sewage sludges.

In-vessel systems. Unlike windrows and aerated static piles, in-vessel systems contain the composting feedstocks in vessels that are usually enclosed, which affords a much greater level of process and emission control. Wide ranges of systems are marketed, each with their own unique benefits and applicability to different feedstocks and situations. In broad terms there are six different types of in-vessel system, although many may be classified into more than one category:

• Containers - These are generally small-scale systems designed for decentralised use, especially food processing and catering wastes. Most systems operate as batch units, supplying air through perforations in the floor of the container.

• Tunnels - These tend to be larger and more sophisticated than containers, and have been adapted from the mushroom composting industry. They operate in similar ways to containers although some element of mechanical agitation may be employed. Both batch and continuous systems have been developed.

• Agitated bays - These consist of rows of rectangular beds separated by low walls on each side along which turning and shredding machines run. The machines mix the compost and deposit it further along the bay in a continuous flow; forced aeration may also be provided via flooring ducts.

• Rotating drums - These are large rotating cylinders (generally 3-4 m in diameter, and anywhere up to 50 m long) that are slightly inclined from the horizontal. Feedstocks are introduced in the top end and mixed and fragmented as they move towards the outlet. Water and nutrients may be added and forced aeration provided.

• Silos or tower systems - These are vertical units that operate on a continuous basis. Feedstocks are loaded into the top of the unit and are composted as they pass down through the unit. The resultant composts are harvested at the bottom of the vessel using augers.

• Enclosed halls - These compost material on the floor of the hall and are usually contained in one long bed. The whole composting process tends to occur in the same hall; large bucket wheels are used to turn and move the material through the system.

Vermicomposting. Vermicomposting is the process of using selected species of earthworms to help compost organic wastes, and stems from the established business of vermiculture (the breeding of earthworms, mainly for the fishing bait market). It is usually carried out in long troughs in which the temperature is kept below 35 °C. With traditional composting, the compost piles are mixed and aerated mechanically, whilst with vermicomposting it is the earthworms that fragment, mix and help aerate the waste.

Post processing. Following composting, most facilities grade the composts into different particle size fractions to create products for varying end uses, and to remove contaminants or partially composted fragments. This process is termed 'screening' and often involves the use of purpose-built machines, of which there are three common types, namely oscillating or shaker screens, rotary trommel screens, and disc or star screens.

Screening compost is the most common method of adding value. Most of the compost sold as soil improvers in the UK is screened to a diameter of 10 mm or less, most mulches to a diameter of 11-25 mm, although different markets and intended uses require different particle sizes. Blending composts with other

Table 1 Key stages of the composting process

Stage

Key features

Stage characteristics

Approximate duration

High rate composting

Stabilisation

Micro-organisms consume forms of carbon they can easily break down, e.g. sugars and starches

Maturation (Curing)

Micro-organisms consume forms of carbon they can break down fairly readily, e.g. cellulose

Amount of available carbon is much reduced and microbial consumption slowed down.

Re-colonisation by soil microbes

High rate of biological activity characterised by high oxygen demand and of heat generation rates

Tendency for pH to initially drop below the optimum of 6-8, then rise above 8 as composting proceeds

Biological activity starts to decline. Oxygen demand gradually decreases. Declining heat generation Tendency for pH to remain above 8

Reduced biological activity. Medium to low oxygen demand Little heat generation; temperature should be below 50 °C Oxidisation of ammonium to nitrate ions Tendency for pH to fall towards neutral

4-40 days depending upon system type

20-60 days depending on system type

Variable duration depending upon test method used and intended end use

Source: Gilbert et al.2

materials (such as coir and/or artificial plant nutrients) is often carried out to create high-value products for specialist uses in horticulture and turf management.

The Composting Process

In very simple terms there are three key stages to the composting process (Table 1), although they are by no means mutually exclusive, and are dependent upon the feedstocks and processing conditions employed.

The biochemistry and microbiology of composting remain poorly understood to date. Despite extensive research over the past twenty years into engineering aspects and the benefits of using composts, composting is still essentially considered a 'black box' process. This stems, in part, from the inherent complexity of the composting process, which is heterogeneous in nature and is directly influenced by factors such as feedstock composition and structure, temperature, pH, moisture, oxygen and ammonia concentrations.4 In many cases indirect methods, such as calorimetric analyses for example, have been used to measure microbial metabolic activity.5

Composting relies upon the inter-related activities of a diverse range of micro-organisms to convert organic waste substrates into a stabilised material ('compost'), which is high in humic substances ('humus') and contains useful plant nutrients. In most feedstocks, the principal source of carbon and energy is derived from lignocelluloses.6 Cellulase activities in composting materials have been widely studied and correlated to decreases in cellulose content.7 The degradation of recalcitrant lignins in composting systems has been less well characterised, although thermophilic microfungi, and to a lesser extent actinomycetes, are thought to play key roles.8

Humification (the process of forming humus) is complex and thought to involve a number of degradative and condensation reactions involving lignins, carbohydrates and nitrogenous compounds.6,8 Nuclear magnetic resonance spectroscopy, gas chromatography-mass spectrometry and Fourier transform infrared spectroscopy have all been used to track changes in feedstock composition and the formation of humic substances (for example González-Vila, et al.9).

The composting process can be split into three key stages based on changes in temperature:4

• Phase 1 is characterised by an increase in temperature from ambient as a result of microbial metabolic activity and has been termed the 'high rate' composting phase. During this phase simple carbohydrates and proteins are readily degraded, firstly by mesophiles, which are then succeeded by thermotolerant and thermophilic species as the temperature rises above 45 °C.

• Phase 2 has been termed the 'stabilisation' phase and is characterised by the attainment of thermophilic temperatures ( > 50 °C), which selects for thermophilic bacteria.10 However, this may be an over simplistic assumption, as the survival of isolates typically characterised as mesophiles has been suggested by Droffner et al.11 The thermophilic stage plays a key role in the thermal destruction of pathogenic micro-organisms, weed seeds and propagules, although antagonisms such as competition and the formation of secondary metabolites may be significant.12

The thermophilic composting phase has received the greatest attention to date,

4 F. C. Miller, in Microbiology of Solid Waste, ed. A. C. Palmisano and M. A. Barlaz, CRC Press, Boca Raton, Florida, 1996.

5 P. Weppen, Biomass Bioenerg., 2001, 21, 289-299.

6 J. M. Lynch, in Science and Engineering of Composting: Design, Environmental, Microbiological and Utilization Aspects, ed. H. A. J. Hoitink and H. M. Keener, Renaissance Publications, Worthington, Ohio, 1993, pp. 24-35.

7 F.J. Stutzenberger, A.J. Kaufman and R.D. Lossin, Can. J. Microbiol., 1969, 16, 553-560.

8 M. Tuomela, M. Vikman, A. Hatakka and M. Itàvaara, Bioresource Technol., 2000, 72,169-183.

9 F.J. González-Vila, G. Almendros and F. Madrid, Sci. Total Environ., 1999, 236, 215-229.

10 P.F. Strom, Appl. Environ. Microbiol, 1985, 50, 899-905.

11 M.L. Droffner, W.F. Brinton, Jr. and E. Evans, Biomass Bioenergy, 1995, 8, 191-195.

12 J. Sidhu, R. A. Gibbs, G.E. Ho and I. Unkovich, Water Res., 2001, 35, 913-920.

especially composts produced for the cultivation of mushrooms on a commercial scale.13 Thermophilic actinomycetes,14 Bacillus species13'15'16 and Thermus species17 have all be shown to dominate, whilst thermotolerant fungi from the genera Aspergillus and Penicillium have been widely reported.4'18

However, species diversity is thought to decrease at high temperatures,10'15 whilst Gram-positive bacteria have been shown to predominate.15'19

• Phase 3 is the 'maturation' phase and is typically characterised by a reduction in temperature towards ambient as a result of decreases in metabolic activity following oxidation of readily biodegradable substrates. Mesophilic actinomycetes and fungi begin to predominate during this stage, and are thought to be responsible for degrading and converting lignins, which occurs optimally at these lower temperatures.8

In recent years a number of approaches have been adopted to monitor the changes in microbial communities during the composting process. Dynamic changes have been tracked using carbon source utilisation and phospholipid fatty acid analyses.20"22 Advances in nucleic acid techniques are now beginning to shed light on the roles of microbial communities during the composting process.23 Many of these techniques have been applied in soil microbiological studies, where the phenomenon of viable but not culturable states is known to exist.24 Consequently genera previously not identified in composts using classical plate culture techniques are now being identified (see for example, refs. 17 and 19). Blanc et a/.23 described the characterisation of Operational Taxonomic Units based on endonuclease restriction profiles of cloned 16S ribosomal DNA recombinants isolated from hot composts, and measured changes in population diversity in young and old composts. Similarly, Peters et al.25 demonstrated changes in microbial communities at different stages of the composting process using PCR amplification of small-subunit ribosomal RNA genes.

Organic Gardeners Composting

Organic Gardeners Composting

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