Introduction
Composting is of great importance as it is considered to be a suitable technology for environmentally sound waste treatment (Bujang and Lopez-Real, Reference BUJANG and LOPEZ-REAL1993; Lau et al., Reference LAU, LO, LIAO and YU1992; Liao et al., Reference LIAO, VIZCARRA, CHEN and LO1993; Maynard, Reference MAYNARD1994; Georgacakis et al., Reference GEORGACAKIS, TSAVDARIS, BAKOULI and SYMEONIDIS1996). It is an aerobic biological process in which naturally occurring microbes convert biodegradable organic matter into humus like product (Lau et al., Reference LAU, LO, LIAO and YU1992; Liao et al., Reference LIAO, VIZCARRA, CHEN and LO1993; Sims and Wolf, 1994; Georgacakis et al., Reference GEORGACAKIS, TSAVDARIS, BAKOULI and SYMEONIDIS1996). Composting is used to recycle organic matter back into the soil to maintain its fertility (Anonymus, 1996; Maynard, Reference MAYNARD1994; Golueke and Diaz, Reference GOLUEKE and DIAZ1996). Poultry waste includes a mixture of excreta (manure), bedding material, waste feed, dead birds, broken eggs and feathers, and may be derived from large houses, cages and conveyer belts or water-flushing systems. These waste components contain a high nutritional value and are used as a chemically organic fertiliser, and thus recycle nutrients like nitrogen, phosphorous and potassium (Kelleher et al., Reference KELLEHER, LEAHY, HENIHAN, O'DWYER, SUTTON and LEAHY2002). However, over-application of this material can results in eutrophication of water courses, spread of pathogens, production of phytotoxic substances, air pollution and emission of green-house gases. Eutrophication from manure and other waste products has been considered as the main cause of impaired surface water resources (US EPA, 1996). Bitzer and Sims (Reference BITZER and SIMS1988) reported that over-application of poultry litter in crops can result in nitrate (NO2) contamination of ground-water. High levels of NO3 in drinking water can cause methaemoglobinaemia (blue baby syndrome), cancer and respiratory illness in humans (Stevenson, Reference STEVENSON1986).
Characterisation of poultry waste
Poultry wastes include poultry litter, manure and dead birds. The composition of these wastes mainly consists predominately of water and carbon (C) with smaller amounts of nitrogen (N), phosphorus (P), traces of chlorine (Cl), calcium (Ca) magnesium (Mg), sodium (Na) manganese (Mn), iron (Fe), copper (Cu), zinc (Zn) and arsenic (As). Due to the presence of high levels of protein and amino acids, poultry manure contains significant amount of organic nitrogen (60-80%) such as urea. Within the nitrogen compounds contained in fresh manure, 40-90 % is converted to ammonia within a year (Kelleher et al., Reference KELLEHER, LEAHY, HENIHAN, O'DWYER, SUTTON and LEAHY2002). Ammonia exists as either gas (NH3) or in an ionised state NH4, which is water-soluble. NH4 can be transformed by soil microorganisms to nitrate (a process known as nitrification).When poultry manure is degraded anaerobically by microbes, the concentration of endogenous nitrogen rises and such excess ammonia can inhibit the destruction of organic compound resulting volatile fatty acid production (Krylova et al., Reference KRYLOVA, KHABIBOULLINE, NAUMOVA and NAGLE1997).
Composting processes
Due to the considerable physical, chemical, and biological changes during the composting process, the natural degradation of biomass components does not occur in a steady state, rather in unsteady conditions. Though there is no obvious or distinct delineation between the two phases or stages of the composting process, some researchers (Diaz et al., Reference DIAZ, SAVAGE, EGGERS’ and GOLUEKE1993; Haug, Reference HAUG1993; Manser and Keeling, Reference MANSER and KEELING1996; Glanville and Trampel, Reference GLANVILLE and TRAMPEL1997; Keener and Elwell, Reference KEENER and ELWELL2000; Kube, Reference KUBE2002) have divided the composting into two major phases. Haug (Reference HAUG1993) indicated that the first phase (also called the ‘developing’ or ‘heating’ phase) is characterised by high oxygen-uptake rates, thermophilic temperatures, and high reductions in bio-degradable volatile solids (BVS). This phase has a high potential for developing significant odour
Bollen et al. (Reference BOLLEN, VOLKER and WIJNEN1989) and Keener and Elwell (Reference KEENER and ELWELL2000) categorised the first phase of the composting process into three sub-phases: initial, high rate, and stabilisation. In the initial sub-phase which lasts one to three days, the temperature increases from ambient to as high as 43°C (110°F), and mesophilic microorganisms degrade sugars, starches, and proteins. In the second sub-phase (high rate), the temperature increases from 43°C (110°F) to nearly 71°C (160°F), and thermophilic microorganisms degrade fats, hemicelluloses, cellulose, and some lignin. Finally, in the third sub-phase (stabilisation), the temperature drops and remains above 40°C (105°F). During this final sub-phase, further degradation of specific celluloses (probably shorter chains), hemicelluloses, and lignin occurs, and mesophilic microorganisms recolonise (Bollen et al., Reference BOLLEN, VOLKER and WIJNEN1989). The high temperatures in the first two sub-phases (initial and high rate) of composting are a function of the amount and degree of uniformity in aeration, moisture content, and composition of required materials (Bollen et al., Reference BOLLEN, VOLKER and WIJNEN1989). During equivalent phases in the composting cycle, the temperature of a pile in which carcasses are composted will be lower than that where organic plant residues are composted, unless physical and chemical conditions are optimised to provide microbiological uniformity and adequate aeration. Additionally, the compost pile must be large or have insulating material to maintain high temperatures, as described by Keener and Elwell (Reference KEENER and ELWELL2000).
The second main phase of composting is known as the ‘maturation’ or ‘curing’ phase. In this phase, aeration is not a determining factor for proper composting therefore it is possible to use a low-oxygen composting system. A series of retarding reactions, such as the breakdown of lignins, occurs during this stage and requires longer time periods to complete. According to Bollen et al. (Reference BOLLEN, VOLKER and WIJNEN1989), the maturation phase could be as long as five months at temperatures below 40°C (105°F).
Effective factors
temperature
Harper et al. (Reference HARPER, ESTIENNE and COLLINE2002), Keener and Elwell (Reference KEENER and ELWELL2000) and Langston et al. (Reference LANGSTON, CARMAN, VAN DEVENDER and BOLES2002) indicated that the rate of the decomposition process at thermophilic temperatures ranging from 40°C to 71°C is much faster than that of a mesophilic range of 10°C to 40°C. Since weed seeds are usually destroyed at 62°C (144°F); these microorganisms are destroyed when exposed to temperatures of 60°C (140°F) to 70°C (158°F) for more than two to three hours (Jimenez et al., Reference JIMENEZ, DOMINQUEZ, CASTILLON and ACEBAL1995; Busto et al., Reference BUSTO, ORTEGA and PEREZ1997; Kube, Reference KUBE2002). Hoitink and Keener (Reference HOITINK and KEENER1993) confirmed that fungus effectively assimilates complex carbon sources such as lignin or cellulose that are not available to most bacteria; however, fungal activity is greatly restricted above 55°C (131°F). It was further observed that at high temperatures of 60-70°C (140-158°F), many carbon-digesting enzymes will be inactive, nitrogenous compounds are lost, and unpleasant nitrogen gases produced, therefore the most desirable temperatures during the first phase of carcass composting range between 55°C to 60°C.
porosity
The air-filled porosity affects the availability of oxygen, temperature, microbial activity, composting time, bulk and packed densities. The porosity should be around 35% to facilitate the air penetration inside the pile and maintain optimum microbial growth (Keener et al., Reference KEENER, ELWELL and MONNIN2001; Harper et al., Reference HARPER, ESTIENNE and COLLINE2002; Looper, Reference LOOPER2002). Looper (Reference LOOPER2002) indicated that the optimum particle size of composting material for proper aeration ranged from 3.1 to 12.7 mm (1/8 to 1/2 in).
oxygen and aeration
When the availability of air is too low in the compost pile, aerobic biodegradation to decompose organic material decreases, nitrogen loss by denitrification increases and the temperature reduces (Henry and White, Reference HENRY and WHITE1993). Chaw (Reference CHAW2001) used a mechanical forced-aeration system to compost sheep offal mixed with carbon sources in a ratio of 1:1 (coarse wood chips and sawdust with offal) to prevent this situation developing.
carbon to nitrogen ratio
Addition of carbon sources to carcasses provides suitable conditions for successful composting. It provides proper aeration, speeds the escape of potentially toxic gases like ammonia, reduces the accessibility of composted material to insects and rodent and provides additional energy for microbial (specifically fungal) activity (Rynk, Reference RYNK1992; Haug, Reference HAUG1993; Sander et al., Reference SANDER, WARBINGTON and MYERS2002). Acceptable C:N ratios generally range from 25:1 to 40:1, and may reach as high as 50:1 (Murphy and Carr, Reference MURPHY and CARR1991; Glanville and Trampel, Reference GLANVILLE and TRAMPEL1997; Keener and Elwell, Reference KEENER and ELWELL2000; Franco, Reference FRANCO2002; Bagley et al., Reference BAGLEY, KIRK and FARRELL-POE1999). Reduction of the C:N ratio during the composting process is a good indication of digestion of carbon sources by microorganisms and production of CO2 and heat. Mukhtar et al. (Reference MUKHTAR, AUVERMANN, HEFLIN and BORIACK2003) reported that the C: N ratio of the finished product was nearly one half of the C:N ratio of the mixed raw materials at the beginning of the first phase.
pH
Alkaline (pH>7) or acidic (pH<7) environments are not well suited to carcass composting. A large amount of free carbon mixed with the nitrogenous materials of carcasses not only helps nitrogen immobilisation and prevents its loss by ammonification, but also maintains the carcass pile at neutral (7.0) or slightly lower pH (Henry and White, Reference HENRY and WHITE1993). Since the biochemical reactions release CO2 (a weak acid) and NH3 (a weak base), the composting process can buffer itself within the neutral range as composting proceeds (Haug, Reference HAUG1993). A proper C:N ratio keeps pH in the range of 6.5 to 7.2, which is optimum for composting (Carr et al., Reference CARR, BROIDE, JOHN, MALCONE, PALMER and and ZIMMERMANN1998).
Raw materials and energy requirements
moisture
Water, as a medium, transports nutrients to beneficial microorganisms, facilitating production of required enzymes in the compost process. The required moisture content (wet basis by mass) for carcasses depends on the material characteristics, but typically range from 40-60% (Murphy and Carr, Reference MURPHY and CARR1991; Keener et al., Reference KEENER, ELWELL and MONNIN2001; Franco, Reference FRANCO2002). As previously noted, excess water (>60%) removes oxygen from the small pores of the compost pile and inhibits its aerobic activity. Additionally, it creates conditions that favour odour production in the pile and restrict its temperature rise (Murphy and Carr, Reference MURPHY and CARR1991; Kube, Reference KUBE2002; Looper, Reference LOOPER2002). Saturated composting piles quickly exclude the oxygen needed to degrade the more odorous compounds and support thermophilic microbes. However, turning the compost over and adding more dry absorbent materials usually solves the problem. As a rule of thumb, if the compost mixture feels moist without water dripping from a handful when squeezed, the moisture is adequate (Looper, Reference LOOPER2002).
carbon source
Raw materials that provide supplemental carbon to microorganisms absorb excess moisture from the carcass, distribute moisture content throughout the compost bulk, maintain porosity (low bulk weight) and modify the C:N ratio of the pile (Fulhage, Reference FULHAGE1997). Mixtures of sawdust and straw may be used to construct compost piles (Keener and Elwell, Reference KEENER and ELWELL2000). A 50:50 mixture by volume of separated solids from manure and a carbon source can be used as a base material for carcass composting (Looper, Reference LOOPER2002). Mukhtar et al. (Reference MUKHTAR, AUVERMANN, HEFLIN and BORIACK2003) successfully used spent horse bedding, a mixture of horse manure and pinewood shavings for composting cow and horse carcasses.
heat energy
The heat energy required for inactivation of microbes is a function of both temperature and length of exposure. The inactivation energy, obtained from time/temperature relationship equation of the Arrhenius Model, is between 50 and 100 kcal/g-mol (200 and 400 BTU/g-mol) for many spores and vegetative cells (Haug, Reference HAUG1993). Based on this theory, the heat inactivation of enteric pathogens was calculated by considering the conditions common to composting, and concluded that average temperatures of 55 to 60°C (131 to 140°F) for a day or two provides this energy and should be sufficient to reduce pathogenic viruses, bacteria, protozoa (including cysts), and helminth ova (parasites such as a tapeworm, liver fluke, Ascaris or leeches) to an acceptably low level. However, the endospores produced by spore-forming bacteria (e.g., Bacillus anthracis) would not be inactivated under these conditions.
bulking agents
Bulking agents are a source of nutrients for composting. Because of the fact that they have bigger particle sizes, they are able to maintain adequate air spaces (around 25-35% porosity) within the compost pile and do so by preventing congestive packing of materials. Haug (Reference HAUG1993) suggested that bulking agents should have a three-dimensional matrix of solid particles capable of self-support by particle-to-particle contact. In order to achieve high porosity and void volumes in co-compositing materials, the particles should have three visible dimensions rather than being flat. Haug (Reference HAUG1993) reported that the high organic content of sludge cake increases the energy of the compost mixture, as it is capable of occupying part of the void volume between particles. Commonly used bulking agents for organic composting are wood chips sized between 2.5-5 cm (1-2 inch), refused feed pellets, shredded tires, peanut shells, and tree trimmings, but these have not been used in carcass composting. Hay and straw also work well as bulking agents. Dried hay contains more carbon (i.e. provides a higher C:N ratio) than greener or legume hay (Morse, Reference MORSE1996). Crop residues such as wheat straw or corn stalks can be used as co-composting materials for carcass composting, but may require shredding or some other form of particle size reduction.
In choosing a bulking agent, the two important factors are availability and cost. The ratio of bulking agent to carcasses should result in a bulk density of final compost mixture that does not exceed 600 kg/m3 (37.5 lb. /ft3). As a general rule, the weight of compost mixture in a 19 litre (5 gal) bucket should not be more than 11.4 kg (25 lb); otherwise, the compost mixture will be too compact and lack adequate aeration (Haug, Reference HAUG1993).
Disease agent considerations
During the first phase of composting, high thermophilic temperatures result in the inactivation of pathogenic bacteria; a function of both temperature and length of exposure (Haug, Reference HAUG1993). High temperature is required for proper product sterilisation, however although heat is generated during carcass composting which causes some microbial destruction, this is not sufficient to completely sterilise the end product, and some potential exists for survival and growth of pathogens. The levels of pathogenic bacteria remaining in the end product depend on the heating processes of the first and second phases, and on cross contamination or recontamination of the end product. Clumping of solids, which can isolate material from the temperature effects, non-uniform temperature distribution which can allow pathogens to survive in colder regions and re-introduction of pathogens after the high temperature phase (Haug, Reference HAUG1993) can all affect degradation of pathogens.
In order to avoid these conditions throughout the composting process it is important to have uniform airflow and temperature. Keener and Elwell (Reference KEENER and ELWELL2000) reported that because carcass compost is an inconsistent mixture, pathogens survival may be sporadic within the non-uniform composition of material. Keener indicated that the preparation process (e.g. grinding and mixing of carcasses with co-composting materials) as well as modifications to the composting system (e.g. aeration) will provide more chemical and physical consistency and better conditions in controlling temperature and inactivation of pathogenic bacteria. Aerating the compost pile by periodic turning reduces the probability of microbes escaping the high temperature zone. In spite of non-uniform temperatures, Glanville and Trampel (Reference GLANVILLE and TRAMPEL1997) reported that when the temperature is in the middle of the pile reaches 65°C, within one to two days pathogenic bacterial activity is reduced. If such heat temperatures are achieved, salmonella and total coliform populations can normally be reduced to levels below 1 and 10 MPN/g dry solid. However, spore-forming bacteria would not be inactivated under these conditions (Haug, Reference HAUG1993).
Trouble shooting
Where liquids leach out of the pile, a well absorbing carbon source material should be spread around the pile. If the pile appears damp or wet and is marked by a strong offensive odour and a brown gooey appearance, it should be transferred onto a fresh layer of bulking agent in a new location (Keener and Elwell, Reference KEENER and ELWELL2000). During the first phase, if the moisture content is low (less than 40%) and the internal pile temperature is high (more than 65°C) , the compost pile coverage or its cap should be raked back and water should be added at several locations. Conversely, if the internal pile temperature is very low (less than 55°C), the compost pile may have been too moist (wet) and/or lacked oxygen, resulting in anaerobic rather than aerobic conditions. Samples should be collected and the moisture content determined by a hand squeeze moisture test. If the compost temperature does not rise to expected levels within 1-2 weeks of the pile being covered and capped, the initial pile formulation should be evaluated for C:N ratio (Haug, Reference HAUG1993). Alternatively, cattle, chicken, or horse manure can be added to the compost pile. In cold climates or winter, compost piles should be protected from the elements prior to loading. Carcasses should be stored in a barn, shed, or other covered space to protect them from freezing temperatures if they cannot be immediately loaded into the pile as frozen bodies may not compost until thawed. Bulking agents and other compost ingredients should also be kept dry to prevent freezing into unusable clumps (Sander et al., Reference SANDER, WARBINGTON and MYERS2002).
Carcass composting systems
The major goals of carcass composting are to prevent the transmission and diffusion of infection, minimise chances for infectious materials to contaminate important elements of the environment (air, water, soil, vegetation, etc.) and to transform carcasses to beneficial end products (Mescher et al., Reference MESCHER, WOLFE, FOSTER and STOWELL1997). Frequently, carcasses are encrusted into the pile with no mixing occurring until after the high rate sub-phase of composting has occurred and the carcasses have fully putrefied. Although most carcass composting schemes work well with small amounts of mortalities (Mescher et al., Reference MESCHER, WOLFE, FOSTER and STOWELL1997), their initial cost and management requirements fluctuate widely. These are listed below.
windrow composting
In this system, a pile is constructed on compacted soil with low liquid permeability or on concrete pads. During, for example, disease emergencies, a windrow-system that remains unturned for 90-120 days from initiation of composting can be used for cattle carcasses with an average weight of 454 kg or 1000 lb (Glanville, Reference GLANVILLE2002).Windrow systems for carcass composting piles are generally located in open spaces and are not protected from weather, rain, or wind. According to Henry and White (Reference HENRY and WHITE1993) aligning the uncovered stacks of carcass piles north to south and maintaining windrow with moderate side slopes maximises solar warming and avoids accumulation of precipitation.
bin compositing
Bin composting can be used for small and medium sized mortalities (including poultry). In this system, carcasses and co-composting materials are confined within a container built by wooden and slatted walls, which usually have a roof. This system is sometimes referred as containerised compositing. Temporary bins, which are constructed from large hay bales (with or without a roof) are structurally adequate to confine the compost pile material and can be used for large amounts or sizes of carcasses (Fulhage, Reference FULHAGE1997; Looper, Reference LOOPER2002; Mukhtar et al., Reference MUKHTAR, AUVERMANN, HEFLIN and BORIACK2003).
Applications for poultry compost
Composting poultry carcasses has been shown from various trials in the USA as economically viable and a suitable environmentally-friendly option for the disposal of dead birds (Donald and Blake, Reference DONALD and BLAKE1996). The aim of this various research was to produce a hygienic material for use as fertiliser on arable land and further work has been conducted to see whether the technology could be used in the UK climate. Product testing was required because composts can often be detrimental to seed germination and early plant growth (Keeling et al., Reference KEELING, PATON and MULLET1994) due to high electrical conductivity of the composted materials. Laying hen carcasses (125) were composted in a wooden bin measuring 1.5×1.5×1.5 m (USDA, 1994). The phyto-toxicity and fertiliser values of the compost were evaluated in perennial ryegrass (Loliumperenne) and spring wheat (Triticum aestivum L.) in glasshouse trials. Experiments were carried out by mixing soil with the compost at up to 300 t/ha (eq.) in 20×14×5 cm trays. Seeds were sown at typical field rates, fourteen hours of day-light and a minimum temperature of 5°C was maintained. Germination rates and dry weights of shoots were determined at various times after sowing. It was concluded that the USDA method of composting poultry carcasses was applicable in the UK and the product may be used safely on grass and wheat at up to 50 t/ha, although the seasonal contribution of the compost to nitrate leaching over all application rates needs to be evaluated.
Conclusions
Composting poultry carcasses and waste, if done properly, can result in a beneficial end product that can be utilised as fertiliser or composting material. However, certain implementations are required to achieve this in the best way. In order to minimise the environmental impacts, composting carcasses should begin within 24-48 hours of death. For a carcass compost pile, a carbon/nitrogen (C: N) ratio of 30-35:1, moisture content of 40-60% (wet basis by mass) and proper air movement (particle size of 3.1-12.7 mm and 35% air-filled porosity) providing thermophilic temperatures of 55-60°C for more than two weeks is recommended. In-bin composting allows the pile to be protected from predators, pests, and runoff and it is efficient and suitable for small-scale carcass composting (less than 50,000 lb of mortalities/week). In an emergency (e.g. foot and mouth disease, avian influenza) situations, windrows are preferred for composting large amounts of intact carcasses.
