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Fundamentals of On-Farm Composting

Dr. Tom GlanvilleAgricultural & Biosystems

EngineeringIowa 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

CompostPile

CarbonNitrogenInorganicsWaterMicroorganisms

O2

Water

HeatCO2

Organic MatterInorganicsMicroorganisms

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Fundamental Parameters Affecting the Composting Process

• 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 16

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