Dixie Drain: Improving The Boise River

A groundbreaking approach to water treatment technologies

A critical component of river health in Boise, Idaho is its nutrient makeup. One such nutrient is phosphorus which, at normal levels, is a vital part of Boise River water quality. However, high amounts of phosphorus entering the river can produce algal blooms and caused detrimental effects on the river’s ecosystem and overall water quality. Because of this, federal regulations will soon require the City of Boise to remove 98 percent of the phosphorus from the water that is leaving its treatment facilities along the river.

Removing phosphorus upstream in Boise at their water renewal facilities and downstream at Dixie Drain, an extra 50 percent of phosphorus is removed from the river.

The City of Boise is making improvements at its water treatment facilities in order to remove 93 percent of phosphorus and protect the upper stretches of the river. The remaining five percent would require expensive modifications, and miss an opportunity to remove greater amounts of phosphorus that would otherwise be untouched. However, the city has collaborated with Brown and Caldwell’s team of engineers, scientists, consultants, and constructors to conceptualize, pilot, design, and construct the Dixie Drain Phosphorus Removal Facility.

Completed in July 2016, the new facility was built near the confluence of the Boise River and the Dixie Drain, an agricultural and groundwater drain that discharges into the Boise River 34 miles downstream of the city’s West Boise Water Renewal Facility. The Dixie Drain Phosphorus Removal Facility collects ground and surface water from this location and treats it to remove phosphorus. This innovative approach removes an extra 50 percent of phosphorus, a total of 140 pounds per day, resulting in a more cost-effective solution, a significantly greater environmental benefit to the Boise and Snake rivers, and a model for other areas facing similar water quality concerns.

The Boise River is an iconic and essential element of the City of Boise’s celebrated livability, providing untold economic, aesthetic, wildlife, and recreational benefits.

This was the first time this approach has been used to offset a specific point source phosphorus load reduction. It was also the first time is was used to meet a National Pollutant Discharge Elimination System (NPDES) wastewater permit requirement, with ongoing system inflow and outflow monitoring to prove load reduction.

“One of the most complex elements of this project was gaining stakeholder support and involvement early in the project,” says David Bergdolt, vice president at Brown and Caldwell. “The Dixie Drain Phosphorus Removal Facility would not be operational today without the diligent work of a group of stakeholders supporting the project.”

The City of Boise, and the Brown and Caldwell team, engaged in an innovative and collaborative design process, using the Construction Manager/General Contractor (CM/GC) mechanism to get early input on constructibility and sequencing to meet the project’s aggressive schedule. The city also worked closely with the U.S. Environmental Protection Agency (EPA) and the Idaho Department of Environmental Quality (IDEQ) to permit the project. The facility was completed at $1.2 million under the construction budget.

This was the first time this approach has been used to offset a specific point source phosphorus load reduction.

Significant efforts were made to educate the public on the project and its benefits, including a commissioning ceremony attended by media, community partners, elected officials, and key stakeholders. Promotional videos, as well as a dedicated website with updates and educational resources were created to engage the community. “These efforts resulted in a strong, positive reception by the public,” Bergdolt says.

The city and Brown and Caldwell worked closely with McAlvain Construction from the 60 percent design mark through facility startup and commissioning. This collaborative approach steered the project toward success through several challenges, including creating separate design packages to meet schedule constraints, value engineering to meet the project budget, and managing design changes during construction.

According to Bergdolt, during construction, the high groundwater table at the project site posed a risk to construction excavation efforts and erosion while the facility was being built. To mitigate this risk, the team completed multiple geotechnical studies to understand subsurface conditions and craft a solution that would overcome this adverse condition. An underdrain system for site dewatering was designed and implemented, which improved constructability, kept the work on schedule, mitigated environmental risk, and reinforced project safety.

Face with the requirement to remove 98 percent of phosphorus that enters their water renewal facilities, the City of Boise implemented the Dixie Drain Phosphorus Removal Facility.

One of the greatest challenges to this project was its large scale. At a design flow rate of 200 cubic feet per second, the Dixie Drain facility can treat about four times as much water as the city’s primary water renewal facility. This type of treatment had not previously been used to meet regulatory requirements for such a large amount of water. Early in the project planning phase, the Brown and Caldwell team tackled this challenge by conducting multiple rounds of jar testing and pilot studies to make sure that the process would work at the designed flow rate. During workshops, Brown and Caldwell brought in team members with national expertise to help the city navigate project complexities and make informed decisions.

Since the facility began operation, nearly 10,000 pounds of phosphorus have been prevented from entering the Boise River. “Without this project, there would have been significantly less phosphorus removed from the water — too much phosphorus can cause hypoxic conditions, which damages aquatic habitats,” explained Bergdolt. By improving the water quality flowing in the Boise and Snake rivers, the area’s natural resources and community assets are significantly improved as a direct result of this project.

“We hope that the innovative yet pragmatic approach used for the Dixie Drain Phosphorus Removal Facility project will serve as a model to follow for other communities facing stringent total maximum daily load (TMDL) requirements and limited funds,” says Bergdolt. “More importantly, we hope that this project will serve as an example of what could happen when a community comes together and dedicates themselves to the pursuit of lasting and innovative solutions to environmental issues.”

Carina Calhoun is the Editor-at-Large for American Infrastructure magazine. She may be reached at ccalhoun@penpubinc.com

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