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25-year-old digester receives an upgrade

PD Editor Karen Lee Published on 19 March 2012

When it looked like electricity prices were about to skyrocket in 1985, the Hurst family of Lititz, Pennsylvania, decided to manage that risk by installing an anaerobic digester.

0512pd lee digester 1 full

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They consulted with the Waybrights of Mason Dixon Farms in Gettysburg, Pennsylvania, to build a continuous, plug-flow digester.

At the time they were milking 285 cows at Oregon Dairy Farm LLC, located in an urban area. Therefore, the odor mitigation provided by digesters was another main reason they embarked on the project.

Click here or on the image at right to view it at full size in a new window.

The electric rates did not skyrocket so payback took a little longer, about eight to nine years, George Hurst estimates.

In the next 25 years, the farm grew in size to have 450 milking cows, which generated more manure than the existing digester had been built to handle. Hurst contacted engineer Stan Weeks in New York to work on the expansion project.

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In addition to the existing plug-flow system, they added an above-ground vertical silo for additional storage. Here the manure and liquid food waste is constantly agitated and heated before entering the digester.

The farm also replaced its old generator with a newer, larger model to convert the additional methane into more electricity. The new system is sized to hold 200 cows’ manure more than the farm’s current numbers.

Only the milking herd contributes to the digester. Heifers and dry cows are housed on pen-pack at another location. Their manure is mixed with separated solids from the digester into a compost system. The farm sells some of its compost to the general public.

Besides the farm, the Hurst family owns and operates Oregon Dairy, a supermarket, restaurant, gift shop and bakery. Electricity generated by the dairy powers this endeavor as well as the farm.

In his years of operating a digester, Hurst says he’s learned to stay constant with the temperature and product fed into it to not disrupt the bacteria. More recently, he’s found that continued agitation results in more methane produced and captured because it reduces the crusting on top.

His son, Chad, manages the digester and does a 10-minute to 15-minute walk-through twice a day. He is constantly testing the oil in the generator to know when it needs to be changed and has found that keeping up with maintenance yields better results.

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Hurst says he is pleased with the new expansion thus far. “We’re exceeding our expectations from it,” he notes. PD

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