BOISE — From the road, Twenty Mile South Farm in southern Ada County looks like any other farm. To the city of Boise, it’s so much more.
On the sprawling 4,220-acre farm, city employees tend fields of corn, alfalfa and winter wheat to sell to local dairies, all fertilized with the thousands of tons of biosolids that come out of treated sewage from Boise, Garden City and Eagle. For nearly 25 years, Boise’s Public Works Department has run the farm as an alternative to putting the city’s solid waste into a landfill, allowing the city to recycle the materials and reduce sewer rates by selling the crops.
Steve Burgos, Boise’s director of public works, said the farm has always been an integral part of the city’s wastewater treatment strategy. Instead of looking at all of the materials that need to be removed from treated sewage as trash to be thrown away, the city has been studying how to use it for other purposes and reduce the city’s impact on the environment.
“We are not in the business of waste,” Burgos said. “Yes, we have these used products, but we’re renewing them all in many ways. We’re taking those things and doing other things with them for some beneficial use for the community.”
When sewage enters either the wastewater treatment facility in West Boise or the Lander Street wastewater plant, the solids are separated, and all of the water is removed through a belt press system. Then the solids are “digested,” which means they are cooked at a certain temperature for a period of time in a large storage container to remove bacteria. The material is then trucked out to the farm where it is stored until it’s time to fertilize the fields.
The city’s two wastewater treatment facilities produce a total of 4,000 dry tons of biosolids a year. Even though the city spreads the material on the fields only roughly 20 days a year, right before planting and harvesting, all of the material is used every year.
Currently, only about 1,600 acres of the farm are being tended by 14 full-time employees. The city has the rest of the land available to expand its operations as the city grows. According to city officials, it’s the largest contiguous farm in Ada County.
Biosolids Program Manager Ben Nydegger said the city selected crops that absorb nutrients well from the soil, which can then be harvested and sold in bulk as cattle feed for local dairies.
The material has high levels of nitrogen and phosphorous, but the farmers supplement the soil with potassium to give the crops the necessary balance to grow.
“We’re not in the cattle-feeding business as a city,” Bydegger said. “We want to sell everything we produce to offset our water renewal fund.”
Farm crop sales come in at $2.8 million to $3.5 million annually, Burgos said. Although this is a small portion of the overall $300 million wastewater fund budget, he said it is an important piece of the revenue, and if the city had a bad growing season, it would affect rates.
Boise operating its own farm instead of using a contractor to spread biosolids is unusual, according to Burgos. Denver’s Metro Wastewater Reclamation District uses a similar method to dispose of its 80 tons of dry biosolids per day, but Burgos said it’s managed by a contractor.
In addition to the city’s work to reuse its biosolids, the Twenty Mile South Farm has long been a place where the city focuses on its environmental impact. In the fall of 2016, the city opened a new “net zero energy” administration building and shop on the farm property. This building, the first of its kind built by the city, produces as much or more electricity than it uses.
The building reached this goal with a combination of 198 solar panels on the roof, ultra energy-efficient construction and materials, and the use of ground source geothermal energy for heating and cooling. The project cost $3.2 million, but the city said the long-term costs of the building are much lower than traditional projects.
Burgos said Public Works looked at this building as an early steppingstone toward the city’s internal energy goals, which now include a goal of all new city buildings to be net zero energy by 2030 and a 50 percent energy reduction in existing buildings by the same year. Earlier this year, the city adopted a citywide goal of having all residences and businesses powered by renewable energy by 2035.
“We learned a ton,” Burgos said, about the process of designing the building. “It was a big experiment to see what’s possible, knowing we were going to be focusing on energy in the future.”