Lesson 10:Composting

Goals:

• Describe the most commonly used composting processes.
• Explain basic concepts associated with aerobic and anaerobic degradation.
• Explain the environmental factors affecting degradation processes.
• Explain the operational techniques used to control composting processes.

• Text: Chapter 8 (pp. 336-359)
• MSW Management article: Composting: Waste Not, Want Not, by Daniel P. Duffy
• MSW Management article: Composting Technology An Overview by S. Kumar, S. Zafar, S. Zafar

Definition:
Biological decomposition of organic constituents of wastes under controlled conditions (approximately 70% of MSW is decomposable).

General Information

• In 1995 - at least 15 MSW composting operation in US
  • Ten others have closed
  • Dade Co., FL (closed) processed 800 tons/day using windrows
  • Baltimore, MD facility processes 700 tons/day using a tunnel digester
• Yard Waste composting is the largest use of the composting process
  • In 1995 - over 3,300 yard waste composting operation in US

Classified based on O₂ use

• Aerobic
  • High temperature
  • Absence of odors
  • Rapid decomposition
• Anaerobic
  • Low temperature
  • Odorous intermediate products
  • Slow decomposition
  • Minimum attention required

Environmental conditions and microbial process

• Only degradable and non-hazardous materials
• Mixing or mechanical aeration required in aerobic operations
• Temp - rises to >60° C
  • If too high, >65° C destroys organisms
  • Must be >50° C to destroy pathogens
  • 50-60° C ideal
  • Controlled through mixing and aeration
• Nutrients - often Nitrogen is limiting
• Moisture Content
  • A moisture content of 50 to 70% is ideal
  • Too low inhibits
  • Too high - anaerobic condtions develop
• Particle size >1”
  • Small to increase surface area
  • Large to avoid anaerobic conditions

    Shreding Equiptment for Pre-Processing Compost Materials

     

Composting Techniques

• Windrows
  • 2-6 months for decomposition
  • Triangular piles; 7’ high and 14-16’ wide
  • Aeration
    • Turning by front end loader
  • Specific turning equipment
   

  Compost Windrows Turned Using a Scarab

   

• Aerated Static Pile
  • 6-12 weeks for decomposition
  • Mechanically aerated piles
    • Diffuser underneath pile, air either blown or sucked through pile
  • 10-12’ high

 

Aerated Static Pile

 

• In-Vessel
  • <1 week composting time
  • Plug flow
  • MSW placed in a chamber or vessel
  • Mixing mechanism
  • Aeration
  • Sequenced reactions (aerobic/anaerobic)
  • Controlled moisture content
  • Proprietary processes - expensive

Pre- and Post-Processing

• Pre-processing
  • Size reduction, screening, magnetic separation
• Post-processing
  • Screening to ensure degradation
  • Curing to allow for continued microbial action at a slower rate

• Quiz for Lesson Eleven: Composting