This fall, Willow Pond in Roseville will be the site of a new project designed to remove phosphorus from stormwater, helping to improve water quality downstream in Bennett Lake.

The contractor constructing the project, Peterson Companies, plans to start work the second week of September and will be onsite for about two months, with the possibility of additional finishing work to be completed by spring of 2019.

The construction process includes digging a spent lime treatment basin, running new storm sewer and electrical lines, and installing an innovative control system to regulate stormwater flow through the basin.

Residents and park users can also expect to see:

• Limited tree and brush removal around the construction site.
• Temporary closure of the walking trail along Willow Pond.
• Slight, intermittent lowering of the water level in Willow Pond.

Cleaner water through spent lime

The filtration basin will be filled with spent lime, a clay-like material repurposed from municipal drinking water treatment.

Once completed, the system will work like this:

1) Polluted stormwater from Willow Pond flows into the filtration basin through a new pipe running beneath Willow Park trail.
2) Phosphorus in the water binds to the spent lime material in the basin. Having soaked through the spent lime, treated water is expected to contain up to 70 percent less phosphorus.
3) Treated water flows through another new pipe to the existing storm sewer system and heads downstream (eastward) to Bennett Lake, which is impaired for excess phosphorus.

A matter of timing

What makes this project unique is the high-tech control system used to determine how much stormwater is treated at any given time. According to Charlie Hinds, water resources engineer at Barr Engineering, the goal is to optimize efficiency of the spent lime filter. “We want to experiment with how long water should react in the basin, and how long the spent lime material needs to dry out before it can effectively remove phosphorus again.”

For an emerging technology like spent lime treatment, the Willow Pond project is an opportunity to fine-tune this method of removing phosphorus from stormwater. Monitoring data collected here and at the recently completed Frost and Kennard Spent Lime Filter near Wakefield Lake in Maplewood will help us evaluate the effectiveness of spent lime under various conditions. Most of all, it will send cleaner water downstream.

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