Milk for Methane, from Innovative Digesters.

 By Maura Keller.

 Dairy cows, with their four-chambered stomachs, have always been models of digestive proficiency. However, each animal produces more than a hundred pounds of manure daily. Concerns for the management of livestock waste, coupled with a desire to produce renewable energy, has resulted in an increased interest in treating and generating biogas from livestock manure – all with the help of digesters.

Located in a small Wisconsin town south of Green Bay is a company that has made a name for itself within the digester arena. In 1989, DVO, Inc. was founded by Steve Dvorak, and in 1999 he developed the company’s unique and patented digester – one that has redefined what many around the world thought a digester could do.

Dvorak grew up on a dairy and hog farm in the heart of Wisconsin, so he has a fundamental understanding of the issues surrounding manure management. After earning his degree in engineering at the University of Wisconsin-Madison, Dvorak installed his first digester at a packing house in 1985 – a European-type “tank” digester which struggled to perform with higher solids and required a lot of maintenance.

Years later Steve and his wife Melanie purchased a farm implement dealership in Wisconsin. In the late 1990’s, as their dairy customers began expressing increasing concerns about the odor generated by untreated manure, Steve recalled his rocky experience with the earlier digester designs and thought an improved digester could solve this problem for agriculture. He initially focused on developing a new digester specifically for dairy farms. “We built our first one in 2001 in Wisconsin. Since then, we’ve installed nearly 140 digesters within 21 states in the US, along with digesters in nine other countries,” Dvorak says. “We are processing manure from over 350,000 cows a day going in our digesters. We believe DVO processes more animal wastes than any other biogas company in the world.”

So how exactly do these digesters work, and what benefits do they provide dairy producers?

Quite simply, DVO Inc. takes the entire dairy’s waste stream into the digester. This includes the urine as well as the solids from the cow, the milking parlor flush and wash water, too. Nothing is bypassed. All of it is heated and mixed inside the digester for a fixed amount of time. The biogas generated is then taken off the top of the digester.

As Dvorak explains, the company’s digester design features a “first in/first out” retention time which results in much better destruction of the pathogens that can harm cattle, as well as increased odor control.

From a nutrient management perspective, a digester will offer some large benefits to the dairyman.  “In the digester the anaerobic bacteria mineralize the organic nutrients, converting those nutrients to a more readily available inorganic form. The now inorganic nutrients are more similar to starter fertilizer, allowing for faster uptake of nutrients by the plant in comparison to raw manure. Dairy farmers can now irrigate the digested slurry onto a growing crop, significantly improving their nutrient management flexibility. The separated fiber is a high-quality clean bedding for the cattle – which works so well that even dairies that formerly bedded on sand have switched to this fiber. Any excess fiber is sold to neighboring farms for their bedding needs.

The digester also reduces much of the farm’s odor. “The organic compounds that create the manure odor are still present in the digester; however, the odorous, gaseous compounds are destroyed when the biogas is treated – either by burning the biogas to generate electricity or scrubbing the biogas to generate CNG for pipeline injection. So, the smell is eliminated with the gas,” Dvorak says. The substantial odor reduction has proven to be a major benefit for farmers – and their neighbors.

Perhaps one of the most important advantages of DVO’s revolutionary design is that it generates meaningfully higher biogas production ‘per cow’ compared to traditional complete mix digesters and covered lagoons. “Because our guaranteed retention time allows us to fully biodegrade the material, we produce more biogas than our competitors,” claims Dvorak.  DVO’s digester is also simpler to operate than the older European “round tank/CSTR” designs, and requires less maintenance and energy to run. “The volume of biogas production itself didn’t use to be that important to our industry, but now with the compressed natural gas being sold to markets in California and other states, being able to squeeze as much energy as one can from manure has become tremendous advantage for dairy biogas-to-RNG developers and farmers alike,” Dvorak says. These days there are dairies that make more revenue from their digester, whether directly or indirectly, than they do from the milk they produce.

And while most of DVO’s digesters are on dairy farms, the company also has systems processing poultry, swine, beef cattle, food waste and industrial waste. The systems can work for a wide array of organic waste streams – not just dairy.

An Exciting Future
While the percentage of farms that have digesters is still small, this market is growing rapidly. More and more producers are recognizing the key benefits this technology provides to their animals, their farms, and, crucially, to their own bottom line. Increasingly, there are also independent RNG developers who desire to partner with dairies to install a digester, too.

“We started off with odor control being the key big reason for a digester being installed – primarily due to public pressure on the odor. And then we went to a stage where both odor control and nutrient management were the driving factors for big dairies. Now we are selling digesters primarily because they make money,” Dvorak says.

Size-wise, DVO can install digesters for any number of cows a dairy farmer has. “The economics are almost like anything else – the bigger you are, the better the economics,” Dvorak says. “How small depends on how well the products from the digester can be monetized. In the northeast US, farms with less than 500 cows can potentially justify a DVO digester – but most are larger than that. With DVO’s modular and expandable system there really is no upper limit. DVO has installed some of the largest digesters in the world.”

After 20 years of being in the anaerobic digester business, Dvorak continues to be excited about the industry and how it is evolving, including the latest synergies with the ethanol and other industries.

“We are also starting to see a lot more interest in nutrient recovery, and know that digesters are very good and economical building blocks for nutrient recovery,” Dvorak says. “The industry is always changing, it’s exciting and I’m very happy to be part of that. We look forward to what is coming down the road.”