Located 34 miles downstream from the City of Boise’s two wastewater treatment plants, the Dixie Drain facility removes 140 pounds of phosphorus per day (10 tons annually) from groundwater and surface water in the lower Boise River watershed.
Brown & Caldwell Located 34 miles downstream from the City of Boise’s two wastewater treatment plants, the Dixie Drain facility removes 140 pounds of phosphorus per day (10 tons annually) from groundwater and surface water in the lower Boise River watershed.

Every year, roughly 80% of the Boise River is used to irrigate peas, barley, hops, hay, sugar beets, plums, and prunes as well as the potatoes for which Idaho's famous. When rains wash the water back into the river, it's full of soil-enhancement chemicals that kill off plants that feed the fish and wildlife that make the state a recreational paradise.

About a decade ago, U.S. EPA Region 10 announced it would start requiring municipalities to remove more of one of those chemicals: phosphorus. Boise, Idaho's capital city and the state's largest city, is one of them. Its next National Pollutant Discharge Elimination System (NPDES) permit would require a 98% reduction. Adding enhanced biological phosphorus removal (EBPR) at one of its two treatment plants would get the city to 93%. As permit renewal time loomed, public works proposed a solution for removing the remaining 5% that would benefit the entire watershed. It's one of those stunningly simple ideas that's extremely difficult to execute because our nation's regulatory paradigm focuses on point rather than non-point polluters. Instead of spending millions more to further upgrade the treatment plants, the department proposed removing phosphorus closer to where it's produced: a manmade basin called the Dixie Drain that was dug in the early 1900s to store and convey agricultural runoff to the river.

Before the state Department of Environmental Quality (DEQ) even signed off on the plan, which required building a treatment plant in another county, Boise's city council approved buying 47 acres near the basin. Then the negotiating began. Because the public thinks there's something shady about water-quality trading, the city had to agree to remove 1.5 pounds of phosphorus at the drain for every pound not removed by its wastewater treatment plants 34 miles upstream.

The $21 million facility treats 130 million gallons daily (mgd) via a fairly conventional process: chemical addition, flocculation, and sedimentation. Water coming in is temporarily diverted and screened for vegetation. Low-head pumps dose it with a polyaluminium coagulant to form floc that settles to the bottom. A floating dredge sucks up and deposits the sediment on adjacent land, where it dries. Public Works Director Steve Burgos told "Building a Greener Idaho" cleaned water is returned to the drain within six hours to eight hours, and that the city's experimenting with chemicals to optimize flocculation.

This non-point phosphorus-removal offset is written into the city’s NPDES permit and is the first in the U.S. It's a commonsense way to meet total maximum daily load limits (how much pollutant a water body can receive without violating U.S. Clean Water Act standards) for any resource-strapped community, of which there are plenty nationwide. They're surrounded by farmland and losing population. Until farmers are required to change their practices, which won't happen soon because the agricultural lobby is so powerful, these communities will bear the entire financial burden of cleaning up pollution they didn't cause. Their stories are painful.

For every pound of phosphorus uncollected at an upstream facility, 1.5 pounds are removed from the downstream facility, a more cost-effective solution with much greater watershed benefits.
Brown & Caldwell For every pound of phosphorus uncollected at an upstream facility, 1.5 pounds are removed from the downstream facility, a more cost-effective solution with much greater watershed benefits.

“This innovative solution eliminated the need to implement more capital-intensive modifications and at the same time remove 50% more phosphorus that would have been otherwise untouched,” Burgos says. “The project is setting a national precedent for addressing water quality issues more effectively and affordably.” Let's hope so.

“What began on the back of an envelope over lunch blossomed into an outstanding local, state, and federal team effort,” said EPA Region 10 Administrator Dennis McLerran in August 2016, when the city broke ground on the facility. In addition to the city, U.S. EPA and Idaho DEQ, stakeholders included the Idaho Conservation League and elected officials. Brown and Caldwell conceptualized, piloted, and designed the facility. Since then, the project has earned numerous awards including engineering’s “Academy Awards”: the American Council of Engineering Companies Grand Award.

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