Building a biodigester as a teaching tool

Chris Sohlberg, Zsolt Pasztor, Matt Dunaway Chris Sohlberg, left, and Matt Dunaway, right, point out features to Zsolt Pasztor. (Marc Studer photos)

By Douglas Esser
Farmer Frog, a nonprofit educational farm in Woodinville, has added a biodigester to its teaching tools thanks to a capstone project by two University of Washington Bothell mechanical engineering students.

A biodigester is a specially equipped tank that turns plant and animal waste into a gas. Think of it as a mechanical stomach. The methane it produces can be burned to generate heat or electricity. Remaining liquid and solids may be used as fertilizer.

What’s different about the UW Bothell project is the way it was designed by Professor Pierre Mourad’s students Matthew Dunaway and Christopher Sohlberg. They built it to be especially easy to operate and duplicate. It’s a teaching tool for renewable energy.

Chris Sohlberg, Matt DunawayThey started with a 1,000 liter (264 gallon) tank commonly used for industrial purposes and known as an intermediate bulk container. It was parked on campus during spring quarter near the Truly house where Dunaway, the project lead, and Sohlberg, mechanical systems, could install airtight pipes and valves. They also painted it black to prevent sunlight from interfering with the bacterial decomposition.

An important part of the project was writing detailed instructions on how to build another one. All of the components are easily available and most could be purchased at a local hardware store. Operating instructions are simple.

“All they have to do is insert the waste. Everything else takes care of itself. They’ll have valves that they open and close manually," Dunaway said. "But it’s minimal work.”

Matt Dunaway, Chris Sohlberg, Zsolt PasztorThe biodigester will be an important demonstration tool for Farmer Frog classes, interns and others, said Zsofia Pasztor, founder and executive director. They plan to burn its methane to warm water in the winter for the aquaponics system, growing plants and fish. Eventually, they plan to duplicate the biodigester for other locations.

Last year, Farmer Frog also worked with a UW Bothell team on a solar-power project. Pasztor said the engineering students are flexible and focused on the work, “Amazing people, professional and caring.”

Operating with a $1,000 budget, Dunaway and Sohlberg, had the experience of building a real project, under design constraints and working with the customer. They delivered their product to Zsolt Pasztor of Farmer Frog, who picked it up in a truck June 7, the week before Dunaway and Sohlberg graduated.

Zsolt Pasztor says Farmer Frog welcomes the opportunity to work with more UW Bothell students who may add sensors to the biodigester to improve its operating efficiency.

“That’s the point – teach others,” he said. “Great!”