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C OMPOSTING

C OMPOSTING. David T. Brown Dept. of Tourism and Environment Brock University. Composting:. Composting:. The controlled biological decomposition of organic materials. Composting.

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C OMPOSTING

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  1. COMPOSTING David T. Brown Dept. of Tourism and Environment Brock University

  2. Composting:

  3. Composting: The controlled biological decomposition of organic materials

  4. Composting • natural biological process, but for rapid composting and consistent quality, environmental conditions must be controlled • end product (compost) bears little resemblance to original wastes from which the compost was made • typically dark brown to black in colour, with crumbly texture and earthy odour

  5. Finished compost • humus-like, resembling rich topsoil • resistant to further microbial decomposition

  6. Composting • typical volume reductions in excess of 50% of the original volume of the waste; effective & useful waste diversion strategy • good compost is devoid of organisms that may be harmful to human health

  7. Uses of compost • high organic matter content => valuable soil amendment • may be used as low-grade fertilizer to supplement plant nutritional needs • may be used to condition heavy clay or mineral soils • promotes proper balance between air and water in soils • aids water infiltration, absorption, and ion exchange in soils

  8. What can be composted? • any waste material with a high organic matter content is a potential candidate • used for centuries to stabilize human and animal wastes • used more recently for: • sewage sludges • industrial wastes (e.g. food, pulp & paper) • yard and garden wastes • municipal solid wastes (up to 70% organic matter by weight)

  9. Controlling composting To achieve maximum composting for any organic material, certain environmental conditions must be maintained in the compost pile => may be classified into interdependent • biological conditions • physical conditions • chemical conditions

  10. THE BIOLOGICAL ENVIRONMENT Key organisms: • bacteria • fungi • Actinomycetes - play active role in decomposing organic matter

  11. THE BIOLOGICAL ENVIRONMENT Secondary organisms: • earthworms • insects • other soil invertebrates • play a less significant role in decomposition process compared to microorganisms • more important in mechanical breakdown of wastes (chewing, burrowing, movement, aeration)

  12. Fate of organic matter in compost • Carbon-containing compounds are consumed by microorganisms and converted to: • microbial tissues • carbon dioxide • water • humicbreakdown products • Heat is released as a result of microbial metabolic activity => temperature in pile increases

  13. Humic breakdown products resulting from one type of microbial activity may be used as a food and energy source by another generation or type of microbes • Chain of succession continues until there is little decomposable organic material remaining COMPOST

  14. COMPOST Stable end product composed of: • living and dead microbial cells and cell fragments • byproducts of microbial decomposition • undecomposed particles (organic and inorganic)

  15. Microbial successionin compost piles • A wide variety of microorganisms are naturally present in most nontoxic agricultural wastes, yard wastes, or mixed municipal wastes ==> number and type of available organisms generally not a limiting factor • Depending upon environmental conditions, certain microbial groups may predominate at certain stages in the decomposition process

  16. If preferred organic substrate is depleted or unavailable, certain microbes may be reduced in numbers, go dormant, or die off • Competition occurs between microbe groups • Dominant groups emerge based upon current conditions in the compost pile • Succession continues as long as there is adequate decomposable organic matter present

  17. THE CHEMICAL ENVIRONMENT • determined largely by the composition of the waste materials to be composted Important factors influencing the chemical environment for composting: • adequate food / energy sources for microorganisms • balanced amount of nutrients • adequate water content • adequate oxygen • acceptable pH range • lack of toxic substances that could • inhibit microbial activity

  18. Food / energy sources for compost microbes • microbes rely on organic carbon compounds to meet energy needs • Carbon in natural or synthetic organic substances varies in degradability (e.g. sugars easily metabolized by most • microbes; lignins in wood or paper degraded more slowly, by fewer groups; plastic very resistant to breakdown)

  19. Food / energy sources for compost microbes • As the more easily degradable organic compounds are decomposed, a small portion of the carbon goes into microbial cells, while a large portion is converted to CO2 and lost to the atmosphere => reduction in weight and volume of waste

  20. Food / energy sources for compost microbes • More resistant carbon compounds form the matrix for the physical structure of finished compost. • Most municipal, yard, and agricultural wastes have adequate biodegradable carbon to support microbial activity

  21. Nutrients for compost microbes • nitrogen, phosphorus, and potassium are most important nutrients • nitrogen is usually the limiting nutrient => CARBON to NITROGEN (C:N) RATIO IS CRITICAL IN DETERMINING THE RATE OF DECOMPOSITION.

  22. - C:N ratio must be established on the basis of decomposable rather than total carbon • generally, a ratio lower than 30:1 is considered ideal; higher ratios result in slower decomposition rates => adjusted by co-composting with different materials • Typical C:N ratios for waste products: Manure - 15:1 to 20:1 Yard wastes - 20:1 to 80:1 Municipal wastes - 40:1 to 100:1 Wood chips - 400:1 to 700:1

  23. As the composting process proceeds and • carbon dioxide is lost to the atmosphere, the • C:N ratio narrows => finished compost has a C:N ratio of 10:1 to 15:1

  24. Moisture in compost piles • ideal moisture: 50% to 60% by weight • most wastes do not contain enough moisture => composting process slowed down unless water is added • excess water causes problems in compost piles: leachate generation, anaerobic conditions, rotting, and obnoxious odours • loss of moisture occurs through evaporation => controlled by adjusting the size and shape of the compost pile

  25. Oxygen in compost piles • aerobic decomposition is required for odour-free, rapid composting • pile should have enough void space to allow gas exchange with the atmosphere • 5% to 15% oxygen concentration is considered adequate • piles aerated bymechanical turning, air injection

  26. pH in compost piles • pH of 6 - 8 considered ideal Level of acidity / alkalinity affects: • nutrient availability • solubility of (potentially toxic) heavy metals • overall metabolic activity of microbes

  27. pH in compost piles • pH may be adjusted upwards by the addition of lime (calcium carbonate), but most organic substances are naturally well-buffered with regard to pH change • slight tendency towards acidification as compost matures, due to production of carbonic acid

  28. THE PHYSICAL ENVIRONMENT Includes factors such as: • particle size • temperature • mixing • pile size and shape

  29. small particle size promotes rapid decomposition due to increased surface area-to-volume ratio • However: if all particles are small, they pack together and create dense, anaerobic compost • => particles should have enough surface area to promote microbial activity, but have enough air spaces to permit gas exchange with the atmosphere

  30. Co-composting • used to achieve better balance of particle sizes (e.g. small-particle sewage sludge mixed with large-particle wood chips) • Particle size reduction by grinding is occasionally done before composting; sometimes undertaken after composting to improve aesthetic appeal of finished product

  31. Temperatures in the compost pile Different microbes have different optimal temperature ranges: • psychrophiles (cool - below 20o C) • mesophiles (warm - 20o to 40oC) • thermophiles (hot - 40o to 80o C) • sub-optimal temperatures interfere with metabolic activity and reproduction of microbes

  32. as temperatures increase above the maximum threshold, cell proteins are destroyed and the microbes die • most effective temperature range for efficient composting is 55o to 75o C (thermophile range)

  33. Thermophiles: • promote rapid decomposition • destroy pathogens • Temperatures in excess of 55o C are required for at least 3 days to ensure pathogen destruction • If compost pile is large enough, internal heat will allow composting in subzero conditions

  34. COMPOSTING TECHNIQUES Small-scale home composting: • simple compost heaps • box or barrel composters • commercial composter units • digester units Commercial composting: • windrows • aerated static piles • in-vessel composting systems

  35. PROCESSING OF MUNICIPAL COMPOST • Removal of bulky items • Particle size reduction (grinders, shear shredders, hammermills) • Screening (size requirements) • Magnetic separation • Moisture addition and mixing • Composting (numerous techniques) • Postprocessing: screening, curing, storage, marketing, application

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