What is anaerobic digestion? This is the magical part where organics become energy. A 70/30 blend of food scraps and yard trimmings are mixed and loaded into an air-tight chamber where naturally occurring microorganisms are introduced via a liquid “percolate”. Over a 21 day process, these organisms break down the biodegradable materials to produce methane gas with a minor amount of carbon dioxide. These biogases are then filtered and cleaned to power a combined heat and power engine that produces electricity. Waste heat is captured and reused to maintain a constant 130 degree percolate temperature.
The MRWMD dry anaerobic digestion (AD) project, commissioned in 2013, was the first of its kind in the U.S. This 5000 ton per year project is a public-private partnership between MRWMD and Zero Waste Energy (ZWE). The four shop fabricated steel digesters can accept 5,000 tons per year of food scraps (up to 70 percent) mixed with yard waste. Energy from the project supplies 10 percent of the electricity need for the neighboring Monterey Regional Water Pollution Control Agency. The “digestate” that comes out of the digesters then undergoes a complete windrow composting process by Keith Day Company to complete the decomposition process.
The AD pilot project is an important next step for our community in utilizing “waste as a resource,” while maximizing the opportunity to realize value from the materials we receive and helping businesses meet the regulatory requirements of AB 1826.”
— Tim Flanagan, MRWMD general manager
Anaerobic Digestion – Step by Step