By Karen Lee, Editor, Progressive Dairyman

The anaerobic digester system at Dallmann East River Dairy in Brillion, Wisconsin, does more than digest waste from the farm. A nutrient- recovery system after the digester not only helps to clean the biogas but also removes excess nitrates and converts them into a saleable fertilizer.

Set up as a research and development site for DVO Inc., it is one of four nutrient-recovery systems operating in the country. There are two on dairies in Washington and one on a poultry farm in Ohio.

Steve Dvorak, president of DVO Inc., says portions of this technology were developed in partnership with a post-doctoral program at Washington State University.

On this Wisconsin dairy, the process begins when manure is scraped from four freestall barns into a flume that transfers the manure to a reception pit, where it is agitated prior to entering the two-stage mixed plug-flow anaerobic digester.

It is a two-stage system because of the two forms of bacteria at work, Dvorak says. The first set of bacteria is the acid-formers that break down the long-chain carbon molecules. The methanogenic bacteria then use the smaller carbons to create methane and carbon dioxide.

“We try to create the optimum environment to encourage the bacteria to eat more and double their population as fast as they can,” Dvorak says, noting the digester is kept at 101°F, which is similar to the temperature of a cow’s digestive system.

Biogas from the digester consists of methane, carbon dioxide and hydrogen sulfide. By removing all of these compounds, Dvorak reports a 97 percent reduction in odor in the remaining materials.

A separator removes the liquid and sends the solids to a stack for bedding. Dallmann East River Dairy uses the solids to bed its cows on this farm and its heifers at another location. Excess solids are sold to neighbors for bedding.

Considering the solids hold a significant amount of nitrogen, phosphorus and potassium, the farm is able to reduce its nutrient-loading of the land by finding additional uses beyond field-application.

The remaining liquid portion is made up of water, ammonium, some phosphorus and some potassium.

In the nutrient-recovery system, the liquids are sent through an aeration tank, which has diffusers on the bottom that bring in air from the outside. With aggressive aeration, gas retained within the liquid is released. The liquid is stripped of some carbon dioxide, all of the hydrogen sulfide and a lot of ammonia, Dvorak says.

The nutrient-filled air is then sent through a stripping tower filled with a plastic medium. It is rinsed with sulfuric acid, which pulls the hydrogen sulfide and ammonia into solution and produces ammonium sulfate, a popular commercial fertilizer.

“We’ve eliminated the last vestiges of the smell and produced a fertilizer,” Dvorak says. “We’ve taken 70 to 80 percent of the nitrogen out of the liquid. If the farmer had a nitrate problem before, they can now spread the liquid at the same volume without having struggles.”

The commercial fertilizer generated by this system is at 38 to 40 percent with a nutrient value of 8-0-0-9. If it were dried, its nutrient value would be 21-0-0-24. This can be used by the farm or sold to farmers or a nearby cooperative.

Dallmann East River Dairy generates 200 to 250 gallons per day of the ammonium sulfate fertilizer. Once it completes construction and fills a fifth freestall barn on the in Ohio is able to generate 2,000 gallons per day, due to the higher nitrogen content of the manure and the size of the operation.

Another advantage, particularly for poultry, is that stripped liquid from the first aeration tank can be run back into the digester to dilute down manure with higher solids content without the concerns of added nitrates that can kill bacteria in a digester.

On this farm, a second aeration tank pumps in the biogas from the digester and uses liquid from the first tank, which is deficient in carbon dioxide and hydrogen sulfide, to pull those molecules from the biogas, leaving pure methane that can be used as compressed natural gas (CNG).

“We need to get to CNG. There’s going to be a future in CNG in agriculture,” Dvorak says, noting farms and trucking companies have interest in running CNG in their trucks.

“It is a cleaner energy,” he adds. By the end of 2013, there will be 35 CNG reload stations in Wisconsin.
For now, the clean biogas at this dairy runs through a 600-kilowatt engine, and all electricity is sold off the farm.
Waste heat from the engine is used to heat the digester and other areas on the farm, as well as fed to hot-water heaters.

Liquid from the second aeration tank is fed back to the first aeration tank, where it is peeled off and sent to a dissolved air-flotation (DAF) tank.

This tank was recently installed to process the liquid just prior to the lagoon. Here the remaining solids
in the liquid float to the top, are skimmed off and sent to the solids pile for bedding. The liquid is now down to 0.1 total suspended solids and sent to a clay-lined storage lagoon.

From the lagoon, the inorganic liquid is irrigated onto growing crops to give them water with a little fertilizer value.

As a result of the entire system, 85 percent of phosphorus is now contained in the solids.

According to Dvorak, the post- digestion nutrient recovery system was developed as a way to help the company broaden its base and enter the poultry market, but it certainly has its benefits for dairy farms, too.