Transcript No Slide Title

Fundamentals of On-Farm
Composting
Dr. Tom Glanville
Agricultural & Biosystems Engineering
Iowa State University
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Elements of Successful
Composting
• Composting is a natural decay process
carried out primarily by bacteria.
• If conditions within the compost are
optimal, bacteria will thrive and decay or
organic matter will be rapid and thorough.
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The “Ideal”
Composting Process
Water
Heat
Carbon
Nitrogen
Inorganics
Water
Microorganisms
CO2
Organic Matter
Compost
Pile
Inorganics
Microorganisms
O2
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Fundamental Parameters Affecting the
Composting Process
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Moisture Content
Carbon:Nitrogen Ratio
Oxygen Content
Temperature
Particle Size
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Moisture Content
• < 40% ...too dry
– Insufficient moisture to transport nutrients
through bacterial cell wall
• >65%...too wet
– Pore spaces in compost matrix that normally
let oxygen in and toxic gases out, become
blocked by excess water
– Can lead to anaerobic conditions and odors
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Carbon : Nitrogen Ratio
• Too High (too little N) slow decomposition
• Too Low (excess N) ammonia released (toxic to
microorganisms, mildly odorous)
• Recommended C:N ratio 20:1 - 30:1
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Carbon : Nitrogen Ratios
for Manures
• Typical C:N ranges
Broiler Litter
12 - 15
Cattle
11 - 30
Horse
22 - 50
Laying Hens
3 - 10
Swine
9 - 19
Turkey Litter
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Source: NRAES -54
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Oxygen
• Composting consumes relatively large amounts of oxygen
– 1 to 4 lbs. of O2 per lb. of biodegradable volatile solids
– Minimum desirable O2 concentration in pile = 5% to avoid
anaerobic conditions and high odor potential
– frequent turning or mechanical ventilation necessary to achieve
aerobic conditions throughout the pile at all times
– static pile composting typical of on-farm operations generally do
not achieve this level of O2, but the outer layers of the pile are kept
sufficiently aerobic to maintain acceptable odor releases
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Oxygen
• Aerobic decay products are less odorous
than anaerobic products
• Aerobic decay releases much more heat
than anaerobic decay
– helps to maintain thermophilic temperatures
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Temperature
• Too high OR too low....slows
decomposition
• Biochemical reaction rates approximately
double with each 10 degree C increase in
temperature
• Extremely high temperatures Do Not
guarantee high degradation rates
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Temperature
• Recommended operating temperature 40-65
degrees C (“thermophilic range”)
• Temperatures above 55 degrees C kill human and
animal pathogens
• Temperatures above 63 degrees C kill weed seeds
• Excessive temperatures call for cooling via higher
aeration rates
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Temperature
• Temperatures exceeding 60°C cause
microorganism to die or go dormant
– leads to premature “shutdown”
– incomplete decay
– compost that will continue to decay and may
produce odors upon cooling
• Aeration sometimes necessary to limit
maximum temperatures
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Particle Size
• Rate of decay depends on microbial “access” (i.e.
available surface area to attack) to the organic
material to be decomposed
• Small particles
– High surface area : volume ratio
– Rapid degradation
• Large particles
– Low surface area : volume ratio
– Slow degradation
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Particle Size
• Mixtures of extremely small particles have low
porosity….poor gas transport through the
composting material
– Impedes movement of oxygen in, and CO2 & NH3
out
• Recommended particle size range 1/8 - 2 inch
diameter
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Summary
• 5 key parameters affect the performance of composting
operations
• Understanding the key parameters is beneficial in
operating and troubleshooting
• Most on-farm composting operations do not attempt to
optimize all operating parameters
– the acceptable range of values for each parameter is relatively
broad
– for the most part, failure to achieve optimal conditions slows
the composting process, but doesn’t stop it
– excessive moisture is the most critical concern…. composting
operations should be protected from excessive rainfall during
wet seasons to avoid unacceptable odor releases
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