C OMPOSTING David T. Brown Dept. of Tourism and Environment Brock University

Composting:

Composting: The controlled biological decomposition of organic materials

Composting natural biological process, but for rapid composting and consistent quality, environmental conditions must be controlled 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 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 typically dark brown to black in colour, with crumbly texture and earthy odour

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

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

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

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

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 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 biological conditions physical conditions physical conditions chemical conditions chemical conditions

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

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

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

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

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

Microbial succession in 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

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

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

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)

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

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

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.

- 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

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

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

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

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

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

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

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

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

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

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)

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

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

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