Phosphorous Removal

Sept. 11, 2018

FOR MANY RESIDENTS in Southwestern Idaho, the Boise River is an iconic and essential element of the City of Boise, providing untold economic, aesthetic, wildlife, and recreational benefits. For a body of water of its size and importance to the community, water quality protection is essential to the sustainability and livability in the Treasure Valley.

Nutrients are critical to the health of the Boise River. One such nutrient is phosphorus, which, at normal levels, is a key part of the river’s water quality. However, high amounts of phosphorus entering the river can produce algae blooms and cause detrimental effects on the river’s ecosystem and overall water quality. To reduce the impact of these effects, regulations required a 98% reduction of phosphorus being discharged from the city’s water renewal facilities into the lower Boise River.

To meet these regulations, the City of Boise was making improvements at its water renewal facilities to remove about 93% of phosphorus, which protects the upper stretches of the river. However, the remaining 5% would require expensive modifications. The city retained Brown and Caldwell to develop a ground-breaking approach to cost-effectively and sustainably meet the 98% phosphorus removal requirement, resulting in significant water quality enhancements for the Boise and Snake rivers.

Together, the city and Brown and Caldwell brought the new Dixie Drain Phosphorus Removal Facility to life. The project collects ground and surface water from agricultural operations in the lower watershed. By removing phosphorus both upstream in Boise at the water renewal facilities and downstream at the Dixie Drain Phosphorus Removal Facility, 140 pounds of phosphorus per day are removed (10 tons annually)—a reduction that has greatly improved the quality of Boise River water. This innovative approach offers greater environmental benefits to the river system while making more efficient use of city funds and setting a precedent for innovation in water quality treatment approaches.

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The city hired Brown and Caldwell to lead the planning, jar/pilot testing, design, and engineering services during construction for the nearly $19,000,000 construction project, which included the following design features and results:

  • Inlet diversion and screening: Located in the Dixie Drain channel, a set of pneumatically operated bladder gates control the water surface elevation in the channel and divert water into the screens. The screens are
    vertical wedge wire and prevent vegetation from
    entering the process.
  • Intake pump station: The intake pump station takes the diverted water from the inlet channel and pumps it to the sedimentation basin. Each of the four pumps has 150 horsepower (hp) and can convey 25 to 50 cubic feet per second (cfs) of water. This pump station controls flow through the facility from 25 to 200 cfs.
  • Sedimentation basin: The sedimentation basin receives flow from the intake pump station and removes large sediments before the coagulation process. The total basin volume is 12 acre-feet, and it is designed to store 8,000 tons of wet solids.
  • Flash mix facility: Water discharged from the sedimentation basin flows over slide gates and into the flash mix facility where the coagulant is delivered to one of four pipe-mounted flash mixers and is rapidly mixed.
  • Settling pond: After coagulation, the phosphorus-containing floc particles precipitate out in the settling pond. The total volume of this pond is 97 acre-feet, providing a minimum three-hour hydraulic retention time under maximum design conditions. Settled floc is removed from the bottom of the settling pond by a rail-mounted dredge system and discharged to the floc management area.
  • Outlet structure: The outlet structure is where the treated water is discharged from the settling pond, returns to the Dixie Drain, and subsequently to the Boise River. The outlet weir maintains the desired water surface within the settling pond, and a fence boom prevents any floating material from exiting the facility.
  • Floc management area: Floc from the settling pond is dredged and discharged to a 3.4-acre solids storage and floc drying area, which stores the phosphorus-containing floc and sediments removed by the treatment process, thereby improving water quality.
  • Operations building: The brain of the facility, this structure is where the facility is monitored and controlled by operators and it contains two large chemical tanks and four chemical feed pumps that deliver chemicals into the flash mix facility. The building contains an electrical room, lab, and space for storage and equipment.

The project team was made up of several subconsultants and the primary contractor, including:

  • J-U-B Engineers Inc.: civil/facilities design
  • Apex Manufacturing Solutions LLC: systems integrator
  • Strata Inc.: geotechnical
  • The Architects Office: architecture
  • McAlvain Construction: construction manager/general contractor (CM/GC)

The city and Brown and Caldwell team engaged in an innovative and collaborative design process using the CM/GC mechanism to get early input on constructability and sequencing to meet the project’s aggressive schedule. The city also worked closely with the US Environmental Protection Agency (EPA) and the Idaho Department of Environment Quality (IDEQ) to permit the project, and the facility phosphorus offset requirements are written directly into the city’s National Pollutant Discharge Elimination System (NPDES) permit for the West Boise Water Renewal Facility.

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Installation of the discharge pipes at the intake
pump station. The intake pump station delivers
screened water from the Dixie Drain to the
sedimentation basin, where the collected
debris settles in the pond before the treated
water is discharged to the Boise River.

Pioneering Pollutant Offset ­Project. The Dixie Drain Phosphorus Removal Facility, which began operation in July 2016, treats 130 million gallons per day (mgd) of water from 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.

This unique project is the first-of-its-kind in the US and is considered a model facility in watershed-based approaches for meeting total maximum daily load (TMDL) limits. This unique nonpoint source treatment facility uses conventional water treatment technologies, but at a much larger scale than is typical. The facility diverts water from the slough, settles out solids in a sedimentation basin, uses a coagulation process to form phosphorus-containing floc particles, and precipitates the floc from the stream in a settling pond. This innovative application of existing techniques represents the first nonpoint source project used to offset total phosphorus requirements in a wastewater NPDES permit. This innovation can be used in other regions facing similar water quality issues. In this respect, the Dixie Drain Phosphorus Removal Facility demonstrates an innovative method of solving complex engineering challenges.

New Thinking Applied to Proven Techniques. The Dixie Drain Phosphorus Removal Facility was the first nonpoint source project used to offset total phosphorus requirements in a wastewater NPDES permit. Under traditional water quality regulations, nonpoint source reductions are typically achieved through voluntary programs that provide limited water quality benefits. The project serves as a new way of thinking about how society can work together to solve nonpoint source pollution. This proves the viability of this approach for agencies and the engineering community to leverage in solving water quality issues.

Improved Water Quality Enhances Community Livability. The project promoted the City’s Lasting Environments, Innovative Enterprises, Vibrant Communities (LIV) initiative and garnered positive attention from local and regional regulators and legislators. Regarding the project, Boise Mayor Dave Bieter said, “Rather than just meeting our obligations, we ask ourselves how we can reach farther and do things better. With great partners, we are setting new standards that are making people across the nation fans of our effort to become the most livable city in the country.”

Providing Social, Economic, and Sustainability Benefits. For the same cost as upgrading the city’s existing water renewal facilities, the facility removes 50% more phosphorus from the Boise River; for every pound that is not removed at an upstream water renewal facility, 1.5 pounds are removed downstream at the Dixie Drain Phosphorus Removal Facility. The result is better water quality flowing in the Boise and Snake rivers. These natural resources and community assets are significantly improved as a direct result of this project.

“Our commitment and passion to solving environmental challenges for clients while enhancing the communities we touch is abundantly evident at the Dixie Drain Phosphorus Removal Facility project,” said Brown and Caldwell CEO Rich D’Amato. “Not only does the project enable Boise to meet federal regulations, it also provides long-term environmental benefits.”

Stakeholder Involvement to Generate Project Buy-in. One of the most complex elements of this project was gaining stakeholder support and involvement at the outset. The Dixie Drain Phosphorus Removal Facility would not be operational today without the diligent work of a group of stakeholders supporting the project, including the EPA, IDEQ, Idaho elected officials, the Idaho Conservation League, staff members from Treasure Valley cities that discharge into the Boise River, and others. With the support of Idaho senators and representatives, the city worked with the EPA to write the Dixie Drain phosphorus offset into its West Boise NPDES permit. The Idaho Conservation League endorsed the project from day one and lent assistance when the city applied for the project’s water rights. Early in 2010, the city and Brown and Caldwell partnered to conduct Dixie Drain water sampling and jar testing, and to develop an initial concept plan. This information was then summarized and presented to a wide variety of stakeholders. Through dozens of stakeholder workshops and meetings during preliminary design, project approval was finally awarded.

Navigation of Complex Project Risks. This type of treatment system had not previously been used for such a large scale of water. 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. Furthermore, during construction the high groundwater table at the project site posed a construction risk. This risk was mitigated through multiple geotechnical studies and installation of an underdrain system for dewatering, which improved constructability, kept the work on schedule, and reduced risk.

The Dixie Drain Phosphorus Removal Facility greatly enhances the water quality of the Boise and Snake rivers by removing up to 140 pounds of phosphorus per day from the water flowing downstream.

Exceeding Client Expectations. The project is a great example of the city’s constant willingness to imagine and pursue lasting and innovative solutions to environmental issues. This innovative project helped catalyze water quality improvements in the Boise River watershed, creating the opportunity for trading between municipalities and agriculture that should ultimately provide both environmental and economic benefits for all. A strong, collaborative effort between the city, Brown and Caldwell, and many stakeholders made this first-of-its-kind project come to life. The project also represented a cost-effective, win-win solution for the city—costs were comparable to what would have been spent on water renewal facility upgrades, but offered significantly more social and environmental benefits while still meeting the city’s NPDES permit requirements.

Collaborative Delivery Approach. The city elected to construct the facility under the CM/GC framework; it was the first time that this contracting mechanism was used for a public works project in Idaho. Brown and Caldwell worked closely with McAlvain Construction (the selected CM/GC) from 60% design through facility startup and commissioning. The city emphasized the importance and success of the Brown and Caldwell/McAlvain teamwork, which resulted in a project that was delivered on time and within budget, while achieving an incredibly low change order rate of less than 1%. This collaborative approach steered the project toward success through several challenges, including creating separate design packages to meet schedule constraints, post-bid value engineering to meet budget constraints, and managing design changes during construction. The facility was completed under the construction budget and met the city’s NPDES compliance schedule.

The Dixie Drain Phosphorus Removal Facility—an innovative project eight years in the making—is now up and running. The facility is a pioneering effort in the world of pollutant trading, and has a more positive impact on the Boise River than upgrading the city’s West Boise Water Renewal Facility. Rather than remove all phosphorus at its water renewal facilities, the city worked with the Brown and Caldwell team and regulators to conceptualize and implement a solution that removes nonpoint source pollution and is applied as an offset in the city’s NPDES permit.

Innovation Leads to Cost-Effective Water Quality Improvements. The new Dixie Drain Phosphorus Removal Facility uses conventional treatment technologies in a new way. This innovative approach can be used in other areas where total pollutant levels pose a significant water quality issue. The project also provided a significantly greater environmental benefit. The result is cleaner water flowing into the Boise and Snake rivers.

Public Engagement Garners Strong Project Support. The Dixie Drain Phosphorus Removal Facility would not be operational today without the diligent work of a group of stakeholders supporting the project, including the EPA, IDEQ, Idaho elected officials, the Idaho Conservation League, and multiple Treasure Valley community representatives. The city’s public outreach activities truly enhanced project success as well as overall enthusiasm for the value of engineering.

Overall, one of the greatest impacts of the Dixie Drain Phosphorus Removal Facility is the precedent it sets for meeting water quality regulations. This ground-breaking yet commonsense approach is a model to follow for other regions facing stringent TMDL requirements and limited funds.
About the Author

Don Bloomquist

Don Bloomquist is a project engineer in Brown and Caldwell’s Boise office.