|Project Type||Farm Scale|
|Population Feeding Digester||1,600|
|Baseline System||Storage Lagoon|
|Digester Type||Mixed Plug Flow|
|System Designer||DVO, Inc. (formerly GHD, Inc.)|
|Generating Capacity||110 kW|
Farm Bill Funded Project
In 2005, Emerald Dairy replaced its covered lagoon digester with a U-shaped mixed plug-flow system designed by GHD, Inc. The digester, which became operational in 2006, has gas induced mixing and return of activated sludge. The below-grade concrete structure has a fixed concrete cover and operates at about 100°F using heat from a boiler fueled by the biogas.
Biogas produced by the digester is processed to compressed natural gas (CNG). The new digester allowed the owner to arrange an innovative sales contract with 3M, Inc., which was interested in purchasing CNG as a renewable fuel. Gas cleanup and upgrading equipment was installed on the farm. First, a moisture trap and iron sponge remove hydrogen sulfide. Then, the gas is upgraded to CNG using water column technology. Finally, tube tanker trucks move the biogas to a pipeline injection point.
The dairy produces about 45,000 gallons of combined manure, bedding, and wastewater per day. Manure is scrape-collected three times per day and averages about eight percent solids. Effluent from the digester is stored in a lined lagoon before being land applied. The farm separates about 38 tons of digested solids per week from the digester effluent. The solids are used for bedding in higher proportions than at typical dairies because they use deep beds. They sell 10-20 percent of the solids to other farms.
Benefits of the Emerald Farms anaerobic digester include the following:
- Revenue from the sale of gas and solids helps recover capital investment
- Heat recovered from the boiler heats the digester
- Digester reduces odor and greenhouse gas emissions
Emerald Dairy plans to partner with two other large dairies to build a gas distribution pipeline to allow biogas to be piped from each dairy to the injection point. The plan is to move the gas upgrading equipment to the injection point so the biogas from all three farms can be processed with it.