Idaho Gets Bang for Its Buck With Dixie Drain Project

The Dixie Drain project in Idaho is an example of innovative approaches to reducing phosphorus loadings on streams at affordable cost.
Idaho Gets Bang for Its Buck With Dixie Drain Project
Shawn Wilson

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Water-quality professionals are confirming what fishermen and boaters on U.S. waterways already know: Nutrient pollution is increasingly damaging the water environment.

In its National Water Program Guidance report in 2014, the U.S. EPA called nutrient pollution one of the country’s most serious and pervasive water-quality problems in the U.S. But while the threat is confirmed, questions remain about the cost of nutrient control and the impact of point versus nonpoint sources.

While many municipal treatment plants have invested heavily to limit phosphorus in their discharges, new total maximum daily loadings (TMDLs) may call for even more expenditures.

It’s a huge bill. For example, a 2012 consultant’s report to the Wisconsin Department of Natural Resources predicted the cost of effective phosphorus control at all municipal treatment plants in the state could range from $860 million to $925 million. Other studies and reports suggest that nonpoint sources are the bigger problem.

The Idaho city of Boise has taken a novel approach to the issue. As new phosphorus removal requirements loomed for the lower Boise River to prevent further nutrient pollution of the Snake River, the city faced a choice: continue to invest in phosphorus removal technology at its two wastewater treatment plants or seek alternative, less-costly solutions.

Working with stakeholders that include the state and federal regulatory agencies, Boise came up with a plan to remove phosphorus from runoff by constructing a treatment facility at one of the agricultural drains downriver from the city. Shawn Wilson, project manager for the Dixie Drain facility, explained the approach in an interview with Treatment Plant Operator.

TPO: What is the Dixie Drain and where does that name comes from?

Wilson: We have lots of agriculture and irrigation in this area. In the early 1900s, the federal government built canals and drainage areas that flow into the Boise River. The early settlers gave the name Dixie to this particular drainage area. The drain also pulls groundwater.

TPO: What precipitated the current Dixie Drain project?

Wilson: A TMDL for phosphorus is in effect for the lower stretches of the Boise River. Our West Boise and Lander Street Water Renewal Facilities, with a total capacity of 39 mgd, were converted to enhanced biological phosphorus removal about two years ago, and we have achieved 93 percent removal at the point sources. But adding more treatment to achieve 98 percent removal would have been very costly and provided a diminished return on investment.

TPO: How did the Dixie Drain idea come about?

Wilson: About 80 percent of the wastewater discharged from our facilities is diverted for agriculture. Our Environmental Division in Public Works decided the TMDL might be better met by reducing nonpoint source phosphorus draining to the river from the fields. It was a watershed approach, based on the understanding that all the drainage canals flowed back into the river. The concept was first discussed in 2009. Meetings were held with parties including the EPA and Idaho Department of Environmental Quality as well as our local politicians and nonprofit groups. It took everybody’s support to get the concept approved and implemented. We realized it would make more sense to treat the high-load water downstream. It was a common-sense approach with a better environmental return on our investment.

TPO: Tell us about the facility. How does it remove phosphorus?

Wilson: The Dixie Drain is a few miles downstream from Boise. It’s about a quarter-mile from the river and covers 49 acres. It’s similar to a water treatment plant, using decades-old technology. That’s what I like about it — the simplicity. It consists of sedimentation followed by an addition of polyaluminum chloride and flocculation. The floc settles to the bottom of a large, lined settling pond. Then, it is dredged and removed to drying beds. Settling time is about three hours. The capacity is 135 mgd, and it will ultimately remove 140 pounds of phosphorus a day, or about 10 tons a year. Two operators staff the facility. Its SCADA system is connected to the SCADA systems at our water renewal facilities, allowing for remote operation.

TPO: How much did the facility cost?

Wilson: The total cost was $21 million, paid for through user rates. The capital cost would have been much higher for ratepayers had we made the mechanical upgrades at our facilities.

TPO: What about the operating costs?

Wilson: We don’t have much data yet. It varies because the primary cost is for chemicals, and that will change when we bring the facility fully into compliance. Right now, we’re removing about 25 pounds a day of phosphorus at a chemical cost of roughly $150,000 a year. That will increase when we get to 2022 and our phosphorus removal requirements increase to meet the offset targets.

TPO: Aren’t the capital costs similar to what Boise would have had to spend at its treatment plants?

Wilson: The life cycle costs are similar and the capital costs are much lower for the Dixie Drain Facility, but the environmental benefits of removing phosphorus at Dixie Drain far outweigh those of adding more phosphorus removal technology at our water renewal facilities. We’re getting more phosphorus out of the river for the same amount of dollars.

TPO: What have been the results?

Wilson: It’s in the second year of operation, and it’s working well. The facility is required to remove at least 1 1/2 pounds of phosphorus for every pound of phosphorus not removed at the treatment plants. So far, we’ve met all our requirements for pounds of phosphorus removed.

TPO: What lessons have you learned from the first couple of years of operation?

Wilson: 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 that it may have more than the design capacity of 140 pounds per day. Also, 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.

TPO: How has vegetation affected the operation?

Wilson: This is an agricultural drain, so in the spring, we get small clippings as the fieldwork starts. As the year goes on, we get big chunks of milfoil. Then during harvest, we see another flush of small clippings and agricultural products. The material blinds our screens. We are installing an automated traveling rake to clean the screens.

TPO: How is groundwater drawn into the drain?

Wilson: It occurs naturally. The groundwater table gets pretty shallow here, and groundwater infiltrates into the drain. It’s really high in phosphorus — as much as 500 micrograms per liter.

TPO: Why was this particular drain selected for treatment?

Wilson: Fundamentally, it was chosen because it has pretty regular flows and a high concentration of phosphorus. It’s a major contributor of phosphorus to the lower Boise River. It was a perfect candidate to knock out a big chunk of phosphorus.


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