Lesson 7: 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.

Introduction

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

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

• Only degradable and non-hazardous materials
• Mixing or mechanical aeration required in aerobic operations
• Temp - rises due to heat release during exothermic degradation of organic matter
• If too high, >70° C destroys organisms
• Must be >~55° C to destroy pathogens (see Table 8-3)
• 50-60° C ideal
• Controlled through mixing and aeration
• Nutrients - often Nitrogen is limiting
• C/N ratio critical prameter, > 80:1 nitrogen limiting, optimum is ~ 25:1
• pH - optimum 7-8
• Moisture Content
• A moisture content of 50 to 70% is ideal
• Too low inhibits
• Too high - anaerobic conditions develop
• Particle size >1"
• Small to increase surface area
• Large to avoid anaerobic conditions

Shreding Equiptment for Pre-Processing Compost Materials

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

• lack of markets for finished product
• small volume reduction
• odor problems during processing