How A Methane Digester Works

In 2009, a methane digester at Fiscalini Farms came online. (See the full story on Fiscalini Farms here.) Two circular aboveground tanks stand 26 feet tall, 86 feet wide and hold about 860,000 gallons of effluent.

Here’s how manure is converted to energy:

Once manure has been flushed from freestall barns, water has to be separated out. A sludge thickener concentrates the solid manure particles, pumping them into digester tanks. The separated water is used for reflushing.

The digester tanks are made of 14-inch-thick concrete walls and thick floors, which contain heating coils. Once inside the digester, the thick slurry is stirred by an agitator while being heated to 101 degrees. “We’re trying to create the same environment that the cow has in her stomach, or one compartment of her stomach, known as the rumen,” Fiscalini says. “By doing that, the gas inside that is produced is methane.”

Once produced, the methane rises to the top of the tank, and is captured in an expandable rubber bladder. Methane is then piped from the tanks, cooled and dried. Blowers move it through a pipeline to the combined heat and power unit (CHP), designed to solely burn biogas, which then powers Fiscalini Farms.

Additionally, the byproduct from the methane is separated into liquid and solid. The liquid fertilizes all the farm’s crops. The solid is used as livestock bedding.

And, says Fiscalini, “we also use hot water from the engine, so the exhaust of the engine is about 600° or 700°, and through a heat exchanger system we actually create our own hot water.

Therefore, he says, “We use less propane, have less propane trucks coming out to the dairy to fill up our tanks, and we can be a little more sustainable in that regard as well.”

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