Fundamentals of On-Farm Composting

1
Fundamentals of On-Farm Composting

• Dr. Tom Glanville
• Agricultural Biosystems Engineering
• Iowa State University

2
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.

3
The Ideal Composting Process
4
Fundamental Parameters Affecting the Composting
Process

• Moisture Content
• CarbonNitrogen Ratio
• Oxygen Content
• Temperature
• Particle Size

5
Moisture Content

• lt 40 ...too dry
• Insufficient moisture to transport nutrients
  through bacterial cell wall
• gt65...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

6
Carbon Nitrogen Ratio

• Too High (too little N) slow decomposition
• Too Low (excess N) ammonia released (toxic to
  microorganisms, mildly odorous)
• Recommended CN ratio 201 - 301

7
Carbon Nitrogen Ratios for Manures

• Typical CN ranges

Source NRAES -54
8
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

9
Oxygen

• Aerobic decay products are less odorous than
  anaerobic products
• Aerobic decay releases much more heat than
  anaerobic decay
• helps to maintain thermophilic temperatures

10
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

11
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

12
Temperature

• Temperatures exceeding 60C 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

13
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

14
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

15
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
  doesnt 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