Aerobic Composting
Presentation 7:
The Composting Toolkit
Funded by the Indiana
Department of
Environmental
Management Recycling
Grants Program
Developed by the Indiana
Rural Community
Assistance Program RCAP
So Let’s Talk About Composting . . .
Compost
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NOUN OR VERB?
BOTH!
The verb: “Composting” is the process by
which microorganisms break down waste and
turn it into a valuable product.
The noun: This end product is referred to
simply as “compost.”
Types of Composting:
Aerobic or Anaerobic?
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We are talking is AEROBIC composting.
That is, composting in the presence of OXYGEN.
Microorganisms decompose organics aerobically
(with oxygen) under carefully controlled conditions
to produce finished compost.
Anaerobic decomposition is what occurs in a landfill
with Carbon Dioxide, Methane, and Leachate being
three major products.
Compost Happens
Aerobic Composting
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Composting is part science and part art. There
is no one “correct” recipe for aerobic
composting.
You will be introduced in this unit to the basics
of aerobic composting and “optimal”
conditions, but you need to always keep in
mind that flexibility is the key to successful
composting.
Aerobic Composting:
Key Components
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Air
Water
Compostable Materials
Carbon to Nitrogen Ratio
Surface Area
Temperature
pH
Other Variables
Aerobic Composting:
Optimal Composting Conditions
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Oxygen: >5%
Moisture Content: 40-60%
Carbon to Nitrogen Ratio: 30 to 1
Shredded Materials Over Chipped Materials
To Maximize Surface Area
Temperature: 90-140 Degrees F
pH: 6-8
Other Variables
Aerobic Composting
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Air:
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Optimal composting
when oxygen is greater
than 5%
The air we breath is 21%
oxygen
Right picture is testing
for oxygen level within
the compost pile
Aerobic Composting
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Air:
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The microorganisms that perform the composting process require
oxygen to survive. Because the compost pile provides a plentiful food
source for these microorganisms they quickly multiply.
This is beneficial to the composting process, but there can become too
many microorganisms for oxygen supplies within the pile. Oxygen
statically flowing into the pile will be far less than the oxygen being
consumed by the microorganisms.
Thus oxygen levels will begin to fall within the pile from the 21%
oxygen level in the surrounding atmosphere. Below 5% oxygen level
the microorganisms will begin to be impaired
Regular turning is required of a windrow compost pile restores oxygen
to the compost pile and helps to ensure that oxygen levels do not fall
below 5%
Aerobic Composting
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Water
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An ideal moisture content is between 40-60%.
Composting microorganisms need a wet
environment.
>60% is too wet, water will take up too much pore
space required for proper air diffusion.
<40% is too dry, decomposition rates will
progressively slow the drier the pile.
Aerobic Composting
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Water:
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Here in Indiana it is unusual to have a pile that is
too dry, you are more likely to have a pile that is
too wet. A pile that is too wet can be turned on an
increased schedule until moisture contents fall
back into desired ranges.
In a case where the pile is too dry water may be
added or the pile may be reshaped to allow more
water to enter the pile during the next rainstorm.
Aerobic Composting
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Water:
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Shaping the compost pile according to moisture
content needs is clearly an example of the “art” of
composting
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A concave shape helps to trap water whereas a
peak shape helps to shed water.
Aerobic Composting
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Carbon to Nitrogen Ratio
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Also commonly called the C:N ratio or the brown to green
ratio
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This is because organics high in carbon tend to be brown (wood
and old leaves being two examples)
Organics high in nitrogen tend to be green (grass clippings and
food waste being two examples)
An optimal carbon to nitrogen ratio is 30:1
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Too much carbon slows the composting process
Too much nitrogen “supercharges” the composting process and
runs the risk of going anaerobic or off-gassing nitrogen
compounds—both causing negative odors
Aerobic Composting
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Common Carbon to Nitrogen Ratios:
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High Nitrogen Materials:
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Activated Sewage Sludge: 6:1
Humus: 10:1
Vegetable Food Waste: 15:1
Digested Sewage Sludge: 16:1
Grass Clippings: 19:1
Cow Manure: 20:1
Horse Manure: 25:1
Green Leaves: <30:1
Aerobic Composting
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Common Carbon to Nitrogen Ratios:
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High Carbon Materials:
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Fruit Food Wastes: 35:1
Typical Brown Leaves: 40-80:1
Foliage: 40-80:1
Corn Stalks: 60:1
Pine Needles: 60-110:1
Straw: 80:1
Bark: 100-130:1
Paper: 150-200:1
Wood and Sawdust: 300-700:1
Aerobic Composting
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Steaming compost
during turning is a sure
sign that temperatures
are being maintained.
An indicator that the
oxygen levels, moisture
content, and the C:N
ratio are in balance.
Aerobic Composting
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Surface Area:
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Greater surface area accelerates decomposition.
There are two primary ways to increase surface
area:
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Smaller particles size
Shredding materials over chipping materials
Aerobic Composting
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Temperature:
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Exothermic microorganisms decompose waste and
in the process give off heat
An optimal temperature range is 90-140 degrees F
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Below 90 degrees the composting rate slows
substantially
Above 140 degrees the microorganisms have difficulty
functioning and the pile runs the risk of spontaneous
combustion
Aerobic Composting
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Temperature:
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A compost pile’s temperature is a balance of the
heat produced by the respiration of the
microorganisms offset by the heat lost through
surface cooling
In the winter larger pile sizes may be required to
offset heat losses
In the summer more frequent turning and smaller
pile sizes may be required to offset excess heat
Aerobic Composting
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Temperature
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One other important function of maintaining high
temperatures in a compost pile is weed seed
destruction and pathogen destruction.
Example: Salmonella destruction requires:
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20 minutes at 140 degrees F
Or
1 hour at 131 degrees F
Aerobic Composting
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Temperature: Testing temperature within the
compost pile
Aerobic Composting
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pH
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Decomposition occurs most efficiently between pH
6 and pH 8.
During the initial stages of decomposition, organic
acids are formed that, under normal conditions, are
immediately consumed by microorganisms.
Without enough oxygen in the pile, these acids will
not be converted as quickly.
Aerobic Composting
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Other Variables:
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There may be other variables of concern as a
composting operation is refined to make products
of a specified composition. These variables, which
are beyond the scope of this manual, include mass
balance, rate of respiration of the microorganisms,
mixing and agitation of the pile, nutrient balance,
inoculation, wetting agents, cation exchange ratio,
pathogen destruction, wind velocity, and specific
elemental composition.
Windrow Composting
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Most common method of yard waste
composting in the United States
Generally a “row” of material is composted,
usually about 10-25 feet wide, 6 feet high, and
as long as 150 feet.
Windrow composting has been shown to be
highly cost effective.
Windrow
Windrow
Windrow Turner
Windrows – Optimum
Composition
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Shredded and screened organics
Moisture content: 40-60%
Balanced 30:1 Carbon to Nitrogen ratio
Frequent turning: Frequent turning keeps the
pile aerobic and reduces odor
Volume and Weight Reduction
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With optimal composting a volume reduction
of up to 75% can be achieved and a weight
reduction of up to 50% can be achieved.
For high water content materials this volume
and weight reduction can be as high as 90%.
Single Biggest Problem: ODOR
Reasons
#1 Reason is Lack of Oxygen
Also . . . Out of balance C:N Ratio
And . . . Excessive wetness
Odor control is discussed in more depth in the
“Ongoing Operations” section.
Other Problems
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Pathogens destruction
Litter and contaminants (plastic bags, heavy
metals, pesticides)
End product quality
Windrow Composting
Trouble Shooting
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Symptom: Pile has a bad odor
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Symptom: Center of pile is dry
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Problem: Not enough water
Solution: Moisten material while turning pile
Symptom: Compost is damp and warm
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Problem: Not enough oxygen
Solution: Turn pile daily until odor is gone
Problem: Pile too small or needs more nitrogen
Solution: Collect more material and mix the pile. If still not heating
add more nitrogen rich materials.
Symptom: The pile will not heat up
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Problem: Lack of nitrogen
Solution: Mix in a nitrogen source such as grass clippings, fresh
manure, or fertilizer
Windrow Composting - Overall
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Capital costs  generally low
Operating costs  generally low
Land requirements  high
Control of Air  limited unless forced aeration is used
Operational control  turning frequency, amendment, or
compost recycle addition
Sensitivity to cold or wet weather  sensitive unless in
housing
Control of odors  depends on feedstock, potential largearea source
Potential operating problems  susceptible to adverse
weather
Windrow Composting - Conclusion
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Most commonly used composting technique in
Indiana and Nationally
Uses aerobic microorganisms to breakdown
woody, yard waste, and food waste products
into a useful “compost” end product
When compared against the capital costs of
other MSW management techniques, it is very
cost effective
QUESTIONS?
Thank You!
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Additional Questions, Comments, or You
Think You Have A Potential Pilot Community
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Please Feel Free to Contact Me:
Mark W. Davis
Technical Assistance Provider
Rural Community Assistance Program
Office: 1-800-382-9895
Wireless: (812) 320-0720
E-mail: mdavis@incap.org