With tighter phosphorus removal regulations on the horizon, Boise, Idaho, took a proactive approach with its innovative Dixie Drain project


Boise, Idaho, is proving there’s more than one way to skin the phosphorus cat.

Facing ever-increasing phosphorus removal requirements, the city could have incorporated additional phosphorus removal equipment at its two wastewater treatment plants. Instead, Boise designed and built a unique facility down-river that removes phosphorus from agricultural runoff.

Called the “Dixie Drain,” the facility employs a simple process train of chemical addition, flocculation and sedimentation. It removes phosphorus at a rate that more than offsets what could have been achieved with additional point source treatment upstream.

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“It’s working well,” Shawn Wilson, project manager, says of Dixie Drain, which is in its second year of operation.

“We’ve met all our requirements for pounds of phosphorus removed,” he says, adding that the Dixie Drain facility is required to remove at least a pound and a half of phosphorus for every pound of phosphorus not removed at the treatment plants.

Plant improvements

With total maximum daily loads for phosphorus in force on the Boise River, the city had already converted its West and Lander Road treatment plants (total capacity 39 mgd) to biological nutrient removal, able to meet a 93-percent phosphorus removal requirement.

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When new regulations called for a 98-percent removal rate, the city worked with numerous parties, including the EPA, the Idaho DEQ, local politicians, and nonprofit environmental groups to come up with the Dixie Drain alternative.

About 80 percent of the water treated at the city’s wastewater plants is used for agriculture irrigation downstream before running off into the Boise River with a high phosphorus content. “Further modifications to the city’s existing facilities (to meet the 98 percent removal requirement) would have been extraordinarily expensive and provided a diminished return on investment,” the city stated in a press release.

Wilson calls it a common-sense solution. “We took a watershed approach and realized it would make more sense to treat the high-load water downstream,” he says.

Related: Wastewater treatment plant starts nutrient trading to improve discharge

How it works

Dixie Drain is one of many agricultural drainage points along the river, constructed in the early 1900s by federal agencies. It is located between Notus and Parma, Idaho, a few miles downstream of Boise. The new phosphorus removal facility covers approximately 49 acres and consists of a sedimentation basin followed by chemical addition (poly-aluminum chloride) and flocculation.

The floc settles to the bottom of a large lined settling pond, and then is dredged and removed to drying beds. Settling time in the pond is about three hours. Flow capacity is 135 mgd, and the facility will ultimately be able to remove 140 pounds of phosphorus a day, or about 10 tons a year.

The actuated gate is by Obermeyer, the in-line mixers are Enviropax and the dredge is a Mud Cat from Liquid Waste Technology. The facility uses MWI axial flow pumps.

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Wilson says it’s a simple, straightforward process. “That’s what I like about it,” he says. “It’s decades-old technology, the same as is used in drinking water plants across the country.”

Two operators staff the facility, which has a SCADA system connected to the SCADA systems at the city’s wastewater plants. The total cost was $21 million, paid for through user rates. It took eight years from project inception to completion and the start of operations in 2016.

Wilson says the dried floc contains a small amount of aluminum and has further phosphorus removal capacity. “We’re pilot testing it and trying to find a market for it,” he says.

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The project has generated wide interest within the wastewater treatment profession, as cities and utilities seek cost-effective ways to meet phosphorus control regulations in the years ahead. Wilson adds that the facility has generated much local interest as well, with tours and presentations.

“We’ve learned a few things ourselves,” Wilson says of Dixie Drain.  “We’ve encountered more vegetation than we expected, and we’re still learning how much phosphorus can be removed. The facility performed above our expectations in the first year of operation so we are hopeful it may have more than the design capacity of 140 pounds per day.”

The most important lesson, however, may be that there are less expensive and more sensible ways to remove phosphorus from our waterways.

For more information, check out Boise Public Works’ video on the project below.

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