The process for composting begins with collecting, processing, and storing feedstock materials, followed by mixing and pile construction. The compostable materials must be screened for non-biodegradable contaminants and then chipped or shredded into uniform particles that will decompose quickly. A balanced compost mixture should contain approximately 25 to 30 parts carbon-rich materials and 1 part nitrogen-rich materials. The mixed feedstock materials are then formed into piles to decompose.

Pile sizes are typically 3 to 4 meters high, 6 to 8 meters wide, and 30 to 40 meters in length. Perforated pipes are laid under each compost pile. Each pipe in the aeration system is connected to blowers that blow air through the pipes. The use of negative air allows air to be biofiltered to eliminate unpleasant odors. The aeration pipes are typically covered with a layer of wood chips that facilitates the distribution of air, providing uniform aeration. The compost mix is placed on top of the wood chip layer forming the piles. The piles are then covered with a layer of material, usually recycled cured compost, for insulation. Pile temperature is controlled by increasing aeration to vent the excess heat. However, if positive pressure is used to vent the heat by blowing air up through the pile, it may be difficult to meet the temperature requirements near the aeration pipes.

Oxygen, temperature, and moisture are key environmental parameters that must be maintained within a specific range to provide optimum conditions for the microorganisms. Enough oxygen must penetrate the pile to maintain aerobic decomposition. If anaerobic decomposition occurs, odors will result. The temperature generated from decomposition must be high enough to kill pathogens and weed seeds but not the microorganisms, or approximately 140-150 degrees F. Composting materials should be moist but not wet. Moisture may be added after turning, since turning releases moisture.

Following the composting period (typically 21 days for biosolids), the piles are broken down and reconstructed into curing piles for additional aging and drying of the material. Curing compost stabilizes it to prevent odors or other nuisances from developing while the material is stored. After curing, the compost can be screened to improve the quality of the final compost product.

States and/or localities may have additional regulations regarding composting that should be followed. The use of a front-end loader will increase fuels on site which may increase a facility's need to comply with SARA (40 CFR 355, 370) and EO 13148 reporting requirements as well as spill plan requirements under 40 CFR 112. In addition, the fans used to aerate the piles will increase electricity consumption. Under EO 13123, federal facilities are required to reduce energy consumption.

The compliance benefits listed here are only meant to be used as general guidelines and are not meant to be strictly interpreted. Actual compliance benefits will vary depending on the factors involved, e.g., the amount of workload involved.

Consult the base safety office and your local industrial health specialist to determine the proper personal protection equipment and the necessary training prior to using this screening and shredding equipment.

Consult your local industrial health specialist, your local health and safety personnel, and the appropriate MSDS prior to implementing this technology.

• Complete composting of yard waste will reduce the amount of waste disposed by an average of 12%, while the addition of food waste will divert another 11.2% (U.S. EPA, 2000).
• The aerated static pile method requires less land than other methods.
• The aerated static pile can produce compost in about three weeks.
• A negative air system with a biofilter can contain odors.
• Indoor operation reduces access to scavengers.
• Controlled conditions can produce a high-quality, marketable compost.

• Higher capital cost, and requires more resources to operate than outdoor methods.
• Requires maintenance to keep perforated pipes from clogging. Clogged pipes will reduce the efficiently of the systems.

Assumptions:

• Process 25,000 tons/yr of wood and yard waste.
• Produce 15,000 tons/yr of finished compost.
• Capital costs: $3 million
• Solid waste disposal costs: $13/ton
• Operating costs (labor & maintenance): $25/ton
• Avoided topsoil purchases: $50/ton
• Hauling to landfill: $5/ton
• Hauling to composting facility: $2/ton

Economic Analysis Summary:

• Annual Savings for Composting Facility: $525,000
  
• Capital Cost for Diversion Equipment/Process: $3 million
  
• Payback Period for Investment in Equipment/Process: < 6 years

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Valley Compost & Topsoil
P.O. Box 1013
Buellton,  CA   83427
Phone: (805) 965-6617 
FAX: (805) 733-7319
Contact: Mr. Don Landry

Composting -- June 2002, Air Force Center for Environmental Excellence/Environmental Quality (AFCEE/EQ)