 Chad Kruger and Lynne Carpenter-Boggs with
prototype for a small-size anaerobic
digester. (Photo by Tim Marsh, WSU Today) |
Washington State University researchers Chad Kruger and Lynne Carpenter-Boggs know that compostable waste, whether at home or in the fields, is an opportunity to save resources and money in other areas around the house and the farm.
Kruger, the interim director of WSU’s Center for Sustaining Agriculture and Natural Resources, has developed a small-size anaerobic digester in collaboration with Shulin Chen, WSU’s Biological Systems Engineering program. According to Kruger, a digester could theoretically be installed under your kitchen sink just like an appliance. You would scrape table scraps into the disposal as usual, but instead of being shipped out to the city’s wastewater treatment plant, the waste would be converted into energy to power your hot water heater or stove.
The other output would be high-quality organic fertilizer that could go back into your garden.
For an investment of around $500, you’d be saving energy, reducing waste and creating a nutrient-rich soil amendment.
“This is one of the most efficient ways you can capture energy on a small scale,” said Kruger.
In large-scale anaerobic digesters, such as those being used in dairy farms, “wasted heat” is recycled to the reactor. But in small-scale systems that isn’t economically feasible. Possible solutions, Kruger said, include capturing ambient heat from a greenhouse or selecting a microbial mix that performs at lower temperatures.
Two of the small-scale anaerobic digester prototypes are being beta tested by local small-scale farmers, one north of Spokane and one near Okanogan. These farmers are looking for a renewable alternative to liquid propane as a source of energy. Once the researchers get feedback from the farmers the next step would be commercialization.
Carpenter-Boggs is the research leader for Biologically Intensive Agriculture and Organic Farming at WSU’s Center for Sustaining Agriculture and Natural Resources. She specializes in helping farmers compost unusual agricultural waste and has been working with Washington mint growers to help them recycle their waste in profitable ways.
Not so long ago, mint farmers in Washington had two choices for disposing of the mountain of cuttings that remain after the oil has been extracted. They could pile it on an unused plot of land and let it rot or they could spread it back onto fields. Unfortunately, piled mint tends to spontaneously combust and land-applied mint leads to nitrogen deficiency in the next crop.
Now there is a third option: they can sell the mint leftovers.
With a value of about $50 million annually, the Pacific Northwest mint crop supplies about 25 percent of the U.S. market and is a major export crop.
Since each pound of mint oil leaves behind 500 pounds of mint hay, what to do with the waste was a considerable problem.
“There was a small fortune in nutrients in that waste,” Carpenter-Boggs said, so she, along with her colleague Steven Verhey of the Cascadia Carbon Institute and WSU graduate student Noah Bonds, figured out a formula for converting the waste into a nutrient-rich soil amendment, helping farmers to recycle and save resources.