The process of composting begins with collecting, receiving, processing, and storing feed stock materials. These steps are then followed by mixing and pile construction. The compostable materials must be screened or hand picked for non-biodegradable materials, and then chipped, ground, or shredded into uniform particles that will decompose quickly. The high-carbon, dry wood and paper waste should be mixed in equal proportion with high-nitrogen, high moisture grass clippings and food waste to provide balanced nutrition for the organisms of decomposition. Feedstock materials are mixed using a pugmill, front-end loader, or paddle-blade mixer to distribute the carbon and nitrogen evenly. The material is then formed into piles to decompose.

Oxygen levels and temperatures must be maintained within a specific range to provide optimum conditions for the microorganisms. The temperature must be high enough to kill pathogens and weed seeds but not so high as to kill the microorganisms. The decomposition produces heat, and the organic material itself provides insulation. Oxygen is a primary requirement for aerobic decomposition. A well-aerated and properly mixed compost pile should not produce unpleasant odors.

Compost is formed into long piles called windrows that are typically 1.5 to three meters high, three to six meters wide, and up to 100 meters or more in length. Windrows can be placed directly on soil or paved area. The land requirement for a windrow composting facility depends on the volume of material processed. Generally, all of the materials handling and pile building can be accomplished with a front-end loader. The windrows can be aerated mechanically by turning with a front end loader for smaller operations or using a windrow turner.

Following the composting period, the windrows 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, depending on the requirements of the compost buyer or consumer.

States and/or localities may have additional regulations applicable to composting that should be followed. The equipment used for composting may increase fuel use 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, composting increases water use at a facility. Under EO 13123, federal facilities are required to reduce water 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.

• Compost reduces the amount of waste to be disposed. Complete recovery of yard waste will reduce the amount of waste generated by an average of 12% while the addition of food waste will divert another 11.2% (EPA, 2000).
• Easy to implement and operate.
• Handles a large volume of material.
• Low capital costs.
• Less equipment and maintenance needed than other composting methods.
• Uses few resources.

• Requires a lot of land for composting.
• Attracts scavengers.
• Often produces odors.
• Requires large buffer zones due to odor and vectors.
• May require a permit depending on size.
• May require processing of rainwater runoff.
• Compost can become anaerobic in rainy conditions.

Assumptions:

• Process 3000 tons/yr. of yard waste, food waste, etc.
• Produce 1,500 tons/yr. of finished compost.
• Capital costs: $600,000 (does not include land costs)
• Solid waste disposal costs: $28/ton
• Cost to pickup and haul waste to landfill: $50/ton
• Operating costs (labor and maintenance): $165,000/yr.
• Avoided topsoil purchases: $25/ton

Economic Analysis Summary:

• Annual Savings for Diversion Method over Disposal: $106,500
  
• Capital Cost for Diversion Equipment/Process: $600,000
  
• Payback Period for Investment in Equipment/Process: 5.6 years

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