A series of biological processes in which microorganisms break down biodegradable material in the absence of oxygen resulting in two end products: biogas and digestate. There are many different AD technologies, including wet and dry versions, the latter being generally better suited for food waste mixed with yard waste.
More than 2,000 sites in the U.S. utilize ADs, primarily in agricultural, wastewater, and urban settings, but only 40 to 50 are dedicated to processing food scraps today. The primary physical byproduct of AD is a digestate. The liquid fraction of digestate can be applied to fields seasonally as a biofertilizer, and the solid fraction can be composted. Biogas, the primary economic byproduct, consists of roughly 60% methane and can be: • minimally treated and used to generate heat on-site, offsetting natural gas, • treated to remove contaminants and fed into a natural gas pipeline, • treated and converted to compressed natural gas (CNG) as a renewable vehicle fuel, or • converted to electricity and heat with a combined heat and power (CHP) system. Capital costs for a larger AD facility (50,000 tons per year) are expected to be over $20 million. Similar to Centralized Composting, the combination of high disposal fees, high compost values, and high electricity prices make the Northeast the most favorable area for Centralized AD economically. The Northwest has very cheap electricity, but if the gas is used to power vehicles rather than create electricity, it can be profitable. Areas like Las Vegas and Tampa, Fla., show profit potential, but low disposal fees and high collection costs make it unlikely that projects will be developed without additional policy support.
Composting is the process of transforming organic waste into humus, a critical component of healthy, fertile soil. In rural areas, this can be accomplished by periodically turning large piles, or windrows, of organic waste over themselves using specialized equipment. In more urban areas, Aerated Static Pile (ASP) composting is generally preferred, where piles can be covered and mechanically aerated in order to minimize the site’s footprint and odors.
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