C OMPOSTING David T. Brown Dept. of Tourism and Environment Brock University.
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Transcript of C OMPOSTING David T. Brown Dept. of Tourism and Environment Brock University.
CCOMPOSTINGOMPOSTING
David T. BrownDavid T. BrownDept. of Tourism and EnvironmentDept. of Tourism and Environment
Brock UniversityBrock University
Composting: Composting:
The controlled biological The controlled biological decomposition of organic decomposition of organic
materialsmaterials
CompostingComposting
natural biological process, but for rapid natural biological process, but for rapid composting and consistent quality, composting and consistent quality, environmental conditions must be environmental conditions must be controlledcontrolled
end product (compost) bears little end product (compost) bears little resemblance to original wastes from which resemblance to original wastes from which the compost was madethe compost was made
typically dark brown to black in colour, typically dark brown to black in colour, with crumbly texture and earthy odourwith crumbly texture and earthy odour
Finished compostFinished compost
humus-like, resembling rich topsoilhumus-like, resembling rich topsoil resistant to further microbial decompositionresistant to further microbial decomposition
Composting Composting
typical volume reductions in excess of typical volume reductions in excess of 50% of the original volume of the waste; 50% of the original volume of the waste; effective & useful waste diversion strategy effective & useful waste diversion strategy
good compost is devoid of organisms that good compost is devoid of organisms that may be harmful to human healthmay be harmful to human health
Uses of compostUses of compost high organic matter content => valuable soil high organic matter content => valuable soil
amendmentamendment
may be used as low-grade fertilizer to supplement may be used as low-grade fertilizer to supplement plant nutritional needsplant nutritional needs
may be used to condition heavy clay or mineral soilsmay be used to condition heavy clay or mineral soils
promotes proper balance between air and water in promotes proper balance between air and water in soilssoils
aids water infiltration, absorption, and ion exchange aids water infiltration, absorption, and ion exchange in soilsin soils
What can be composted?What can be composted?
any waste material with a high organic matter any waste material with a high organic matter content is a potential candidatecontent is a potential candidate
used for centuries to stabilize human and animal used for centuries to stabilize human and animal wasteswastes
used more recently for:used more recently for:
sewage sludgessewage sludges
industrial wastes (e.g. food, pulp & paper)industrial wastes (e.g. food, pulp & paper)
yard and garden wastesyard and garden wastes
municipal solid wastes (up to 70% organic matter by weight)municipal solid wastes (up to 70% organic matter by weight)
Controlling compostingControlling composting
To achieve maximum composting for any To achieve maximum composting for any organic material, certain environmental organic material, certain environmental conditions must be maintained in the conditions must be maintained in the compost pile compost pile
=> may be classified into interdependent=> may be classified into interdependent biological conditionsbiological conditions physical conditionsphysical conditions chemical conditionschemical conditions
THE BIOLOGICAL ENVIRONMENTTHE BIOLOGICAL ENVIRONMENT
Key organisms:Key organisms:
bacteriabacteria fungifungi ActinomycetesActinomycetes
- play active role in decomposing organic - play active role in decomposing organic mattermatter
THE BIOLOGICAL ENVIRONMENTTHE BIOLOGICAL ENVIRONMENTSecondary organisms:Secondary organisms:
earthwormsearthworms insectsinsects other soil invertebratesother soil invertebrates
play a less significant role in play a less significant role in decomposition process compared decomposition process compared to microorganismsto microorganisms
more important in mechanical more important in mechanical breakdown of wastes (chewing, breakdown of wastes (chewing, burrowing, movement, aeration)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
COMPOST
Stable end product composed of: living and dead microbial cells and
cell fragments byproducts of microbial
decomposition undecomposed particles (organic and
inorganic)
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
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 6 - 8 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 decompositiondestroy 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
1. Removal of bulky items
2. Particle size reduction (grinders, shear shredders, hammermills)
3. Screening (size requirements)
4. Magnetic separation
5. Moisture addition and mixing
6. Composting (numerous techniques)
7. Postprocessing: screening, curing, storage, marketing, application