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

Step 1: Source separated food scraps collected from local food generating businesses averaging 3 – 5 tons a load are hauled to the MRWMD AD site and tipped onto a bed of mulch to absorb any liquid. The organics are visually inspected ad contaminants are removed. If organics are contained within compostable plastic bags the bags need to be manually ripped open so contents will be exposed inside the digester. Materials are blended in a ratio of 70% food with 30% yard waste mulch and are then loaded into a receiving building until one of four digesters are ready. To minimize odors, the receiving bay and digesters all operate under negative aeration during loading and unloading so odors can be scrubbed through a biofilter.
Step 2: Each digester holds approximately 65 tons of organic material. Once a digester is empty, and enough material is stockpiled in the receiving building, then the digester is loaded. When the digester door closes, the system operates aerobically (with oxygen) until temperatures reach 125 to 130 degrees F.  At this temperature, the system switches to an anaerobic (oxygen-less) environment and a microorganism-rich percolate liquid application is sprayed onto the material.
Step 3: Biogas (primarily methane) released from the organic material is stored in a roof-mounted inflatable bladder which is the reservoir to supply a combined heat and power engine (2G Cenergy 100 kW). This engine uses the methane as fuel to generate electricity. The biogas methane content is approximately 63.5 percent and the system has about 90 percent maximum electricity potential.
Step 4: The “digestate” that is removed from the digester after a 21 day residency has lost approximately 30% of its volume and retains moisture from the percolate application cycles.  This material is then moved to an adjacent windrow composting area where it will undergo a complete composting process over 60-90 days. The compost is then screened to remove residual contaminants such as paper, plastic and other inert materials. The final screened food scrap compost is then ready to market to nearby farms and vineyards.
Additional Eco-Benefits: Heat from the engine is captured and used to maintain a constant and optimal temperature in the percolate tank. At the end of the 21 day cycle the percolate is screened for solids and flows back to the underground storage reservoir so that it can be reused.
To learn more about anaerobic digestion at MRWMD, check out the November 2013 article published in BioCycle Magazine!