Foster Brothers Farms
| Location |
Middlebury, Vermont |
| Project Type |
Farm Scale |
| Animal Type |
Dairy |
| Population Feeding Digester |
380 |
| Baseline System |
Storage Tank or Pond or Pit |
| Digester Type |
Horizontal Plug Flow |
| System Designer |
Hadley and Bennett |
| Biogas Generation |
37500 ft3/day |
| Biogas Use |
Electricity |
| Generating Capacity |
125 kW |
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Photo: Manure Manager
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Foster Brothers Dairy began operating an anaerobic digester in 1982. Each day the farm produces about 7,000 gallons of manure. At four designated collection points, the manure is pushed over the lip of the barn floor into a wheeled manure spreader. The spreader transports the manure to the plug flow digester.
The digester is a below-grade concrete tank with a flexible cover. It is maintained at a temperature of about 95ºF and has a hydraulic retention time of 29 days. Recovered biogas is transferred from the digester into another building where it is scrubbed and then fed into a 125 kW Caterpillar 3304 engine-generator set. The engine operates on methane and diesel gas, using a 75 percent ratio of methane to diesel under optimum operating conditions. The system generates approximately 360,000 kWh of electricity each year. Recovered heat from the engine-generator set is used to heat the digester.
Originally, the generator was fueled by methane gas only and the generated electricity was sold to a local utility. However, when the price of electricity from the grid exceeded the amount the local utility paid the farm, the Foster family decided to disconnect their farm from the grid and use the electricity to power the farm and three adjoining residences.
Foster Brother Farm’s digester project includes the following benefits:
- Odor reduction
- Electricity production
- Revenue from the sale of soil and compost
A Fan separator is used to separate digester effluent into solid and liquid fractions. All liquid is piped underground into a storage lagoon and later applied to the cropland. In 1992, Foster Brothers Farm began composting the separated solids and separated solids from other farms to establish an organic soil, compost, and growing mix business. The farm launched a sister company, Vermont Natural Agricultural Products (VNAP) to market and sell its compost in the Northeast. VNAP sold about 750,000 bags of soil mix and approximately 10,000 cubic yards of bulk compost in 2005, grossing an estimated $1.75 million.
Some of this technology’s advantages, like odor control and greenhouse gas reductions, are a little hard to put numbers on, but I think people should consider taking a look at it. In any case, we definitely achieved our original goals.
—Robert Foster, Owner, Foster Brothers Farm
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