Sustainability is a core mission for the University of Wisconsin – Oshkosh. To demonstrate this objective, the university commissioned the building of three anaerobic digesters in the state. The first was a dry fermentation digester in the city of Oshkosh. The second was a small, plug-flow system at a dairy with 130 head of cattle near Oshkosh, and the third was a large farm, complete-mix digester system at Rosendale Dairy in Pickett, Wisconsin.
The university has an agreement with Rosendale Dairy to take some of the manure produced from the farm’s 8,500 cows. Once the manure is processed through a sand separation system, it is pumped through an underground 10-inch pipe to the digester facility located just south of the dairy.
Click here to download a PDF of the digester diagram. (PDF, 1.4MB)
In the agreement, everything but the water belongs to the university. A second pipe runs along the first to return the liquid back to the farm, primarily to be used in the sand separation system.
“We wanted to bring value to the farm and partner with them to solve any issues they had with odor and management of solids,” says Greg Kleinheinz, professor and chairman for sustainable technology at the University of Wisconsin – Oshkosh.
Doug Renk, an engineer on the project, explains it all started with a power purchase agreement that was set at 1.5 megawatts, and the entire project was reverse-engineered from there.
“We could make twice that and more,” Renk says of the electrical output, given the amount of manure available on site and the capabilities of anaerobic digestion. However, they found a way to maximize the amount of manure digested while staying within the power agreement.
The method used to optimize this system was to first separate the manure and then mix it back together at an ideal level of solids of 14.5 percent.
Once the manure arrives at the digester facility, it is processed through rotary screen separators. The liquid returns to the dairy while the solids are sent via conveyor to two walking floors – which are used to reintroduce the press cake into the mix pump.
“We factored in the value of the manure solids and liquid and then calculated the output of the digester with the right recipe of ingredients,” Renk says. “We just had to find a way to separate the manure and put it back together.”
The walking floors are what allow this system to custom-mix the material based upon the flow rate.
“We were able to optimize what’s been built and have no need for any additional material,” Renk adds.
The higher amount of solids produces more biogas and eliminates any need for the addition of co-substrates.
The facility processes 1.2 million gallons of water in a day – but only uses 55,000 gallons.
The excess liquid returns to the dairy, where it flows through DAF buffer tanks and centrifuges to remove as many remaining fine solids as possible. It mixes with parlor washwater and storm water and is recycled to the sand separation system or discharged to the dairy’s storage lagoons.
Back at the digester, the 14.5 percent solids mixture enters the first of a two-stage digester. The first stage retains material for six to seven days, and the second digester holds it for 13 days, for an overall retention time of 19 to 20 days. By the end of the digestion process, the solids count is reduced to 9.5 percent.
Once digestion is complete, some of the material is sent back to the walking floors while the remainder is separated. These liquids are also returned to the dairy. For now, the solids also return to the dairy and are land-applied for fertilizer.
“The biggest issue we face with this project is what do we do with the solids,” Kleinheinz says. Prior to construction, the university had a partner lined up to utilize the solids. However, he reports, the partner’s ideas weren’t fully vetted and the relationship dissolved.
Knowing the post-digested solids are a valuable stand-alone product, the university is researching options for the fiber material. This is currently being studied at a laboratory on campus.
Renk mentions this research includes nutrient recovery technologies to remove some of the nutrients from the post-digested solids before they go to compost.
There is a second phase planned for this project which includes expanding the use of the heat generated by the engine. Currently the heat is used to warm the digesters. In the future, it could be used for drying fibers, running a chiller and heating an educational center.
The on-site education center will allow students to visit the site and study samples in a laboratory there. Plus, it can be used for research and development as well as training industry partners.
As a public entity, Kleinheinz says the university wants to help solve the problem of making digesters pay for themselves by understanding the electrical output and creating viable options for the solids output.
“We want to share information and the lessons we’ve learned,” Kleinheinz says.
By partnering with dairies, the university is not only able to become more sustainable, but it can also provide educational opportunities for its students in a real-world setting. PD
Karen Lee
Editor
Progressive Dairyman
