Although carbon to nitrogen ratios and moisture content must be considered, the composition of feed materials is less critical for in-vessel systems than it is for windrow or aerated static pile systems. This flexibility allows for different mixes to be composted, based on availability of feed materials. 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. Feedstock materials are mixed using a pugmill, front-end loader, conveyor or paddle-blade mixer to distribute the carbon and nitrogen evenly.

Oxygen and temperature regulation is critical to maintain optimum conditions for microbial action. The temperature must be high enough to kill pathogens and weed seeds but not so high as to kill the microorganisms. The method of ensuring the maintenance of an air supply to the compost mix depends on the particular in-vessel system selected. Generally, air is supplied by blowers, and flows up through the compost. Air can be supplied via piping networks or through damper arrangements beneath the compost. Blowers can operate automatically based on measured temperature set points or can be set at regular intervals. Pile temperature is controlled by cycling the aeration system on and off. Good odor control for this system is achieved by collecting and treating the process air and building air.

States and/or localities may stipulate additional composting regulations. The equipment used for composting may increase fuels on site which may increase a facility’s need to comply with SARA (40 CFR 355 and EO 12856) reporting requirements and SPCC (40 CFR 112) issues.

The compliance benefits listed here are only meant to be used as a general guideline 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 your local industrial health specialist and your base safety office prior to implementing this technology.

• Compost reduces the amount of waste to be disposed. Yard waste composting can reduce waste generation by an average of 15.9% and composting of food waste can divert another 6.7% (EPA, 1994).
• In-vessel composting provides a faster method of composting than windrow composting or aerated static pile composting.
• In-vessel composting requires fewer operators than other composting methods.
• In-vessel materials handling systems are more efficient in design than aerated static pile and windrow composting systems.
• Some in-vessel composting technologies often have considerable operational flexibility regarding feedstock composition.

• Higher capital cost.
• Requires operator training.
• Requires maintenance.

Assumptions:

• Process: 4,000 ton/yr. of yard waste, food waste, etc.
• Produce: 10,000-20,000 lbs./day of finished compost, or, an average of 2,738 tons/yr.Capital Costs: $2,800,000 (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): $125,000/yr.
• Avoided topsoil purchases: $25/ton

Economic Analysis Summary

Annual Savings for Diversion Method over Disposal: $255,450
Capital Cost for Diversion Equipment/Process: $2,800,000
Payback Period for Investment in Equipment/Process: < 11 years

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