Case study: biogas harvest for US farm


Farmers around the world are showing increased interest in Anaerobic Digesters, as a result of higher energy and fertiliser prices, the growing costs of complying with waste disposal legislation in many countries and the continuing need to diversify in order to maintain farm incomes. One dairy farm in the USA is using the technology to produce electricity, heat and bedding for animals.

Deere Ridge Dairy – or Gordondale Farms – is a dairy operation in Nelsonville, Wisconsin, USA. (In eastern Portage County). The farm has some 850 Holstein cows.

Some 30,000 gallons of manure, bedding and milking parlor wastes are generated per day and 'scrape-collected' at two hour intervals.

The farm uses digested solids for bedding and its former manure storage system was a pit.

Harvesting the biogas

Deere Ridge Dairy installed the first farm-scale digester – designed by GHD Inc. – in 2001.

It is a below-grade, U-shaped mixed plug-flow digester, with a fixed concrete cover. It uses biogas induced mixing and return of activated sludge. The digester has two distinct digestion phases within the main chamber. Figure 1 shows the digester and the adjacent equipment building.

The digester has a design operating temperature of 100ºF, and a target influent solids content of 8%–9%. The design hydraulic residence time is 22 days. Solids separation after digestion is undertaken with a Fan brand screw press.

Outputs and uses

Biogas from their digester is treated with a water trap. It is sold to local utility Alliant Energy and run through its on-site Caterpillar 140 kW (net) engine generator set. Alliant Energy is responsible for the operation and maintenance of the energy generation equipment. Figure 2 shows the monthly kW hours of electricity generated – from January 2007 through to June 2008.

The heat captured from the engine (via water jacket and exhaust) is used to heat the digester and milking parlor, and for the facility's water heating. Deere Ridge does not have a backup boiler for digester heat on site in the event the engine should be down for repairs or maintenance. However, the farm does have the option of having GHD bring in a boiler if needed.

All digested solids are used for bedding on the farm. Figure 3 shows the cows with digested solids bedding.

History and comments

This installation originally came about because the farm owners were building a new dairy facility, and were aware of the benefits of anaerobic digestion. Alliant Energy was also interested in a pilot project using biogas.

The two parties talked with GHD, Inc. and agreed to have the first GHD digester installed on their farms. To reduce the financial risk for the farm, Alliant Energy agreed to supply, operate and maintain the engine generator set.

The digester has been operating as it is supposed to and the farm operators feel it is a good fit for the farm. Gale Gordon said he is surprised more digesters have not been built given the obvious advantages.

For eP) is concentrated mostly in the solids. After digestion, screw press solids separators take out about half the solids, and settling can remove most of the rest. The concentrated P in the lighter-weight solids can give the farm more flexibility in land application over greater distances, and on fields that can use it. This added control helps farmers work within their nutrient management plans. He feels very strongly that digester designs should be as simple as possible.

NB: This publication is the property of the Public Service Commission of Wisconsin and was funded in whole or in part by the Wisconsin Focus on Energy program (see http://www.focusonenergy.com).

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Bioenergy  •  Energy efficiency