North Florida dairy with 6,100 mature cows and 5,500 replacements installed anaerobic digestion system that is yielding multiple benefits.
Dairy farming in Florida has financial and logistical challenges, including hot, humid summers; high electrical costs; access to fresh water; odor; greenhouse emissions and sandy soil issues. Alliance Dairies in Trenton, Florida has found sustainable ways to overcome all of these challenges.
Ron St. John and William “Sandy” McArthur started the dairy in 1990 with 2,000 cows. They now operate the largest free-stall dairy in Florida, with 140 employees tending to 6,100 mature cows and 5,500 replacements on 4,000 acres. Much of the 450,000 to 500,000 gallons of milk produced daily at Alliance Dairy is sold in Publix Supermarkets in Florida.
Each milking cow produces more than 100 pounds of waste per day. The dairy’s two milking parlors operate 24/7, and Alliance’s annual electric bill was averaging $1.2 million. Alliance grows its own feed crops on 4,000 acres. Florida’s climate allows for triple cropping each year. The sandy soil requires fertilization to grow these crops, but its porous nature increases the likelihood of nutrient runoff from fertilizers.
Investing In AD
Four years ago, Alliance installed a DVO-designed Two-Stage Mixed Plug Flow anaerobic digester. St. John, McArthur, and St. John’s daughter, Jan Henderson, who is CEO of Alliance Dairies, applied for and received a $2.1 million federal TARP Green Energy grant to offset the $8.5 million cost. “We knew of DVO in Wisconsin,” says Charlie Smith, Alliance’s Office Manager. “It had around 100 digesters in place around the country so we weren’t the first.”
The digester was specifically designed for Alliance’s freestall barn flush system. “We use a lot of water because we flush our barns,” explains Smith. “This required us to build a larger digester, because the water makes the manure less concentrated, which reduces the ability to create methane gas. Other farms with less access to water scrape the manure out of their barns, which leaves the manure more concentrated.”
The digester vessel is 464 feet long, 165 feet wide, and 16 feet deep (14 feet of manure slurry topped by two feet of biogas). The digester is constructed of steel-reinforced concrete. Its top consists of prestressed concrete sections. The concrete walls are sealed with a tar-based material, insulated with two inches of polyurethane and sealed with a rubber coating.
Construction began in April 2012. DVO was the design engineer and Martin Machinery of Missouri sold the dairy the motors and generator. Testing to satisfy the Green Energy grant requirements was completed, and the digester system began producing electricity in January 2013.
Alliance Dairy uses sand bedding for the lactating herd. “The milk cows like it because they can move around in the sand and make it conform to their body shape,” notes Smith. “Clean dry sand is the gold standard when it comes to dairy cow bedding.”
The barns are flushed and then scraped with a rubber tire scraper down to a sand lane where more than 80 percent of the sand is reclaimed. At the end of the sand lane, the manure-laden flush liquid goes one of two ways — either to the methane digester or to a drum composter system.
The flush liquid headed to the digester first goes to a mixing pit where screen separators drop solids from the flush into the side of the pit that feeds the digester. Liquids flow to the other side of the pit and then back to the flush tanks to be reused to flush the barns.
Retention time in the digester is 21 days. Methane gas powers a 1-megawatt genset located in a building next to the digester. Biogas is conditioned to remove hydrogen sulfide prior to entering the engine. “We struggled with the original Spanish motor provided by Martin Machinery for two years,” says Smith. “It threw a rod twice. We replaced it with a CAT motor in January 2015. The oil in the motor needs to be changed every 400 hours as we are not producing a clean gas.” Alliance does all its own maintenance, which is a full-time job for one employee.