Cows make a lot of milk, but they also produce a lot of waste. Unfortunately, disposing of it or treating it like municipal wastewater isn't easy or easily affordable—especially for small farms. Many big facilities have anaerobic digesters, but their challenge is finding faster, more efficient ways to convert all the waste from their herds.
To help with these difficulties, Universal Sanitary Equipment Manufacturing Co. in Tomah, Wis., designs and builds turnkey pump stations, control systems, tanks, pressure vessels and support equipment for farms, industry and municipalities. USEMCO started out in the wastewater business and later moved into the controls and clean water business.
Tank Design Cuts Retention Time
More recently, USEMCO began seeking a way to give smaller farms some of the same wastewater and waste-processing capabilities as their larger counterparts, including using manure to produce methane and electricity. About eight years ago, it developed plans for a small digester system for farms with 500 cows or less and targeted it at farms with just 150 to 200 cows. Rezin estimates there are about 13,000 of these small farms in Wisconsin alone.
"We developed two prototypes, first 100 gallons and then 1,000 gallons, and followed them with a 30,000-gallon version," says Patrick Rezin, PE, USEMCO's president. "We had to find a way to remove solids and keep the system unplugged, and we realized that we had to flush solids away from the bottom of our tanks or skim them from the top. So we came up with a combination of valves, standpipe and vents, which could be opened and closed on the opposite side of the tank, and this system can be automatically monitored and controlled."
In fact, USEMCO already has one of its patent-pending Bi-gester systems up and running at Wayne Peters' Peters Farms Inc. in Chaseburg, Wis., which is one of six founders of the 1,834-member Organic Valley Cooperative. The 30,000-gallon tank was installed two years ago, and takes in about 50 gallons of waste every 15 minutes, or about 7,000 gallons per day from the farm's 200 Holstein cows (Figure 1).
After four or five days, the anaerobic digester begins to produce biogas, mostly methane, which goes to low-pressure storage and a 100-kW generator. It heats water for the digester and preheats water for washing the barn's parlor where the cows live. The generator also heats the parlor in winter, which saves $5,000 per year, and sends electricity back to the local grid (Figure 2). Rezin estimates that Peters' single Bi-gester saves about $30,000 per year in electricity, but it provides several other benefits. "First, heavy solids and liquids from the digester are easier to turn into fertilizer in the farm's lagoon, which saves the owner another $50,000 per year in fertilizer costs," explains Rezin. "Also, light solid fiber from the tank can be run through a screw press to produce excellent bedding for the cows that's almost as good as sand, and this saves another $30,000 per year." In all, the digester system will have a six- to seven-year payback.
Rezin adds that USEMCO's initial one-tank system is scalable and can be expanded to two, four or even six 30,000-gallon tanks for larger farms and herds, or as an existing farm's needs change. "Our tanks are also more portable, and so they're also useful when financing isn't available for a big, in-ground tank," adds Rezin. "Also, one of Bi-gester's most important advantages is that its 4.5-day retention time is much shorter that the 20 days required by other digesters."