COMPOSTING
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

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
Finished compost


humus-like, resembling rich topsoil
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

good compost is devoid of organisms that
may be harmful to human health
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
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)
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
THE BIOLOGICAL ENVIRONMENT
Key organisms:
bacteria
 fungi
 Actinomycetes

- play active role in decomposing organic
matter
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)
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 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


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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:

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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.

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- 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

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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



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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


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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:

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particle size
temperature
mixing
pile size and shape

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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
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COMPOSTING TECHNIQUES
Small-scale home composting:

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simple compost heaps
box or barrel composters
commercial composter units
digester units
Commercial composting:
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windrows
aerated static piles
in-vessel composting systems
PROCESSING OF MUNICIPAL
COMPOST
1.
2.
3.
4.
5.
6.
7.
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