Award-winning on-site wastewater treatment and dispersal system boasts some unique features.
By Shari Held
The City of Afton, Minn., may be small – less than 3,000 residents – but it’s a big tourist attraction. Located on a bay near where the St. Croix River merges with the Mississippi River, The Village, Afton’s commercial district, retains the flavor of a mid-19th century river-town settlement. It also boasts Afton Alps, the largest ski and snowboard resort in the Twin Cities area.
Unfortunately, during the hot summer months the odor of raw sewage was ever present. And when the levee overflowed, sewage washed into the streets. The city’s reputation as a destination location was threatened, and the nearby St. Croix River faced ongoing pollution.
That was the situation seven years ago. It took a lot of effort to convince residents that investing in an on-site wastewater treatment and dispersal system was the best solution. The project also experienced several other delays, including one to investigate possible underground Native American relics. But today, Afton boasts nine subsurface precast tanks – five of which are 38,000-gallon tanks and four are 10,000-gallon tanks – making it Minnesota’s largest on-site decentralized wastewater treatment and dispersal system. Afton’s sewage issues are now underground and out of sight, just where they should be, and precast played a major role.
When the job went out for bid, both fiberglass and precast concrete products were options for the tanks.
“We found precast concrete was a better price, easier to install and more readily accepted by the installation contractors,” said Peter Miller, executive vice president and project manager for Wenck, a Maple Plain, Minn.-based environmental engineering firm.
Tony Birrittieri, general manager for Petersen Onsite, based in Fredonia, Wis, also prefers precast.
“Financially, fiberglass tanks are just not feasible because they have to be customized,” he said.
Cast-in-place concrete was never an option.
“With all the custom openings, inlets and outlets, it’s very difficult to maintain the quality with cast in place,” Birrittieri said.
Fabricating for strength and precision
It’s not every day a precast concrete product manufacturer gets the opportunity to fabricate five 38,000-gallon and four 10,000-gallon precast tanks for one job. The larger tanks measure 14 feet wide and 40 feet long. The smaller ones are 12 feet wide and 20 feet long.
To create the clamshell-style tanks, Wieser Concrete Products, based in Maiden Rock, Wis., added a ConShield Technologies additive to its standard 7,000-psi concrete mix to help prevent corrosion. The tanks rely on a combination of post-tensioned cables as well as traditional steel reinforcement.
Most of the tanks use a Bio-Microbics FAST (Fixed Activate Sludge Treatment) aeration system, which is pre-assembled and installed in the tank bases at the precast plant.
“Then they just need to be hooked up at the site,” said Wieser Concrete Products General Manager Andy Winkler. “It saves a lot of installation time.”
As a last step, the tank elements were coated with a waterproof sealant.
“Wieser Concrete not only customized the tanks the way we needed them to be customized, but also did it affordably and increased the quality control as well,” Birrittieri said.
Also included in the project was a control building to house the pumps, control panel and a small room to store denitrification chemicals. The building’s 10-inch-thick precast concrete walls were fabricated with three inches of foam insulation sandwiched in the middle. The 16-foot-wide-by-8-foot-tall front and back walls weigh nearly five tons apiece, while the side walls measure 14 feet 4 inches wide by 8 feet tall and weigh 8,955 pounds each. A 12-inch-thick, two-piece, insulated precast roof tops the building. Wieser Concrete also fabricated two front overhang panels and two interior walls.
All of the elements for the control building were complete within two weeks.
Challenging but speedy installation
Transporting the 38,000-gallon tank elements was challenging simply because of their sheer size and weight – the heaviest piece weighed 39 tons. Delivery trucks had to take special routes that could accommodate the large loads, and site conditions complicated access to the job site. With only one way in and out, coordination was crucial to keep deliveries in order. Despite this, all of the precast tanks were shipped in one day.
“We literally had to drive the trucks in, turn them around, unhook the trailers and use the crane to spin the trailer around so they wouldn’t have to back all the way out,” Winkler said.
Because of the sandy soil conditions, the crane’s reach needed to extend further than what would typically be required to maintain a safe distance from the excavation. Wieser Concrete solved this issue by using a 550-ton rough-terrain crane with eight loads of counterweights for stability.
“It’s very expensive, but we calculated the difference between moving a smaller crane multiple times or having one crane and leaving it in one location,” Winkler said. “The larger crane won out.”
Workers set all the tanks in one day. Then they connected the internal components and applied joint wrap at the mid-seam to seal the tanks. They also set the walls and roof of the control building the first day and installed the hardware the following day. The entire project installation was complete in two days.
“I don’t know how it could have gone any better,” Winkler said. “It was absolutely perfect.”
How the system works
The treatment process begins when collected wastewater is pumped into the subterranean tanks to settle and equalize. It’s then recirculated through a large gravel filter before undergoing a denitrification process. Finally, the treated wastewater is transferred to a dose tank and allowed to drain into the ground. The system has a 55,000-gallon daily treatment capacity.
Replacing treated wastewater into the underlying aquifer replenishes the groundwater, keeping it local. It’s a more environmentally-friendly method than pumping wastewater into the river, where it is lost.
“The design of this system, with a large gravel filter and denitrification behind it, is unique,” Birrittieri said. “And the quality of the wastewater leaving the system is exceptional.”
Teamwork yields success
Two on-site pre-planning meetings helped the project go smoothly. And working together on previous projects was an undeniable bonus. Earlier this year, Wenck won a design award for the project from the Minnesota chapter of the American Council of Engineering Companies.
“All-in-all, it took seven years from planning, design and construction,” Miller said. “But in the end, we are very happy. The product is outstanding.”
Shari Held is an Indianapolis, Ind.-based freelance writer who has covered the construction industry for more than 10 years.