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 Compost Pile Carbon Nitrogen Inorganics Water Microorganisms O2O2 Water Heat CO 2 O rganic Matter Inorganics Microorganisms

4 Fundamental Parameters Affecting the Composting Process Moisture Content Carbon:Nitrogen Ratio Oxygen Content Temperature Particle Size

5 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

6 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

7 Carbon : Nitrogen Ratios for Manures Typical C:N ranges Source: NRAES -54

8 Oxygen Composting consumes relatively large amounts of oxygen –1 to 4 lbs. of O 2 per lb. of biodegradable volatile solids –Minimum desirable O 2 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 O 2, 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 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 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

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 CO 2 & NH 3 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 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