Massive filters installed at Wal-Mart site

A fleet of heavy metal is rolling across the Wal-Mart Supercenter site under construction at Hutton Ranch Plaza these days.

But one of the heaviest loads any of the earth-moving and construction equipment will haul came along early in May - a 12-ton concrete base slab topped by a stack of 10-foot-diameter concrete barrels.

Taken together and multiplied by two, for the other half of the system, it became the stormwater drain filter that will collect water, oil, grease and other debris running off Wal-Mart's parking lot and roof and let only the clean water escape.

"Wal-Mart takes a pretty big one," Glacier Precast Concrete owner Tom Anderson said of the drain filter designed by a New York company, Environment 21 Stormwater Treatment Systems. "Only two forms in Montana can make this filter."

Glacier Precast fabricated one of those forms, from which it poured the Wal-Mart filter. The other form is in Billings or Helena, Anderson said.

It's a state-of-the-art system when it comes to ease of maintenance combined with protection of the environment, he said, one that will trap virtually all pollutants in the holding tank with no overflow, even when the area gets a real gully-washer.

LHC Inc. of Kalispell is the general contractor hired by developer Phil Harris to prepare the pad for the new Wal-Mart store. Crews began excavation in April to relocate water utility lines and install all other utility lines.

But long before that, the city of Kalispell and Wal-Mart's Washington-based engineering firm worked to finalize design details. Initial plans for handling stormwater runoff caught the eye of Susie Turner, senior civil engineer for the city's Public Works Department.

"We've been implementing water quality treatment since we were approved for our permit in fall 2006," Turner said.

That permit has a big name: State General Permit for Storm Water Discharge Associated with Small Municipal Separate Storm Sewer System. Kalispell is the only city in the area with such a permit.

But it has a simple purpose: To let developments discharge stormwater to state waterways as long as they don't pollute. The city writes the rules to be sure each development complies with state water quality standards.

So far Silverbrook Estates, Spring Prairie Phase 3, the new Department of Natural Resources and Conservation building and the new 911 emergency response center have fallen under the requirement since the permit came into play in 2006.

Runoff from the Wal-Mart site flows off to a swale and down a channel into a pond along the banks of the Stillwater River, only a quarter mile or so from store to Stillwater. So when Turner noticed the plans didn't have a stormwater drain filter, she sent them back for tweaking.

"Stormwater quality designs are based off of pollutant removal efficiency," Turner explained. "For example, the water quality treatment facilities are required to remove 80 percent of the total suspended solids at a specified flow rate."

That's engineer lingo meaning it doesn't need to be perfect, but it does need to be close.

The engineering firm let LHC know that a filter was required, then got in touch with Glacier Precast, which was subcontracted under LHC.

"We proposed using Environment 21," Anderson said.

Its vortex design lets water gush into a 120-inch diameter catchment basin where silt and the heavier solids settle out from the swirl. Oil and grease float to the top and flow along with the water into a second, 96-inch diameter, basin. Clear water escapes through an outlet pipe at the bottom while oil and grease are trapped at the top. Its design ensures treatment even in high-flow water events, and simplifies clean-out by requiring only that it be pumped regularly.

"We said we could save money and use the Environment 21," a double bonus, Anderson said, because it would be fabricated locally and offer high-level water quality protection.

"The engineering firm drew it up, the city OK'd it and now it's at Wal-Mart."

Glacier Precast crews used the Environment 21 designs to form and pour the stormwater drain filter at their plant south of Kalispell.

They used a couple of their own cranes to load it onto a truck bed to haul it to the Hutton Ranch site, but it took two massive cranes from Harmon Crane and Rigging to set it in place just to get enough length to reach across the excavated area.

Capacity of the new drain filter is 27.7 cubic feet per second from its 30-inch-diameter inlet and outlet pipes.

The 15-foot-tall barrels are designed to handle runoff from Wal-Mart's 8.42 acres of impermeable surface.

Anderson figures it's a win for everyone involved.

"These can be manufactured locally," he said, noting he employs a crew of about 16 people, "and Kalispell wants to protect the water."

Reporter Nancy Kimball can be reached at 758-4483 or by e-mail at nkimball@dailyinterlake.com