Banner photo courtesy of Mack Concrete Industries.
By Shari Held
Knox Borough, Pa., is a quaint community of approximately 1,000 residents tucked away in the northwestern quadrant of the state. Its original cast-in-place wastewater treatment plant was built in the 1930s and had only been upgraded twice – once in the ‘50s and again in the ‘70s.
The plant’s tanks showed evidence of spalling so severe that the reinforcement in the walls was exposed. The plant didn’t have the hydraulic or organic capacity to handle its customer base. It needed to be expanded or replaced quickly to keep Knox Borough in compliance with the Pennsylvania Department of Environmental Protection.
A winning solution
Replacing the wastewater treatment plant was the less expensive and more practical option. One of the first decisions to be made was what building material to use. The capacity of the new plant was 500,000 GPD, making it too large for steel, which poses issues with durability. Steel plants have a lifespan of 25-to-30 years, while concrete plants can last more than 50 years.
Martin English, P.E., an engineer with the EADS Group of Clarion, Pa., considered using cast-in-place concrete. Ultimately, though, he chose post-tensioned precast.
“Preventing leakage was our number one priority,” English said. “The post-tensioning support available with precast concrete makes it a very advantageous product for both strength and durability in environmental structures.”
The design for the new extended aeration plant called for two 45-foot (outside diameter) circular clarifier tanks and a 153-foot-by-76-foot rectangular tank subdivided into two aeration chambers, two digester chambers, a flow-splitter chamber and a return-activated sludge chamber. Precast caps on the wall tops create a walkway around the tanks.
Mack Industries’ headquarters plant in Valley City, Ohio, produced the precast for the treatment plant, built it, and installed the tanks and equipment. The company, which had worked with English before, began consulting on the project more than two years prior to receiving official approval to begin.
“We are unique in that our salespeople are working with the engineer in the early stages of a project,” said Betsy Mack Nespeca, president, Mack Industries. “The key to a customized solution is working hand-in-hand with the engineer.”
Mount Braddock, Pa.-based Global Heavy Corporation served as the general contractor for the project.
Challenges along the way
Timing was a big challenge. English was concerned about meeting DEP-imposed deadlines.
“We had an implementation schedule that had to be met from start to finish,” English said. “It definitely helped knowing we could meet our schedule for installing the tanks.”
On-site construction began in February 2016 under frigid conditions. Sub-freezing temperatures can impact the grouting and sealing process. Fortunately, precast panels can be set in the ground until there’s a deep frost, unlike cast-in-place concrete.
Because the original plant remained operational during construction, workers had to contend with space constraints. And making a safe conversion from the old system to the new one involved some tricky logistics.
Fabricating for a fast installation
The cast-in-place, steel-reinforced base (approximately 130 feet by 70 feet) was produced with keyways running through it. Mack Industries fabricated 130 precast wall panels, each measuring slightly more than 17 feet high and weighing about 10 tons. The 32 precast caps, which measured 4 feet, 6 inches wide by 12 inches thick, were various lengths, depending on the chamber size.
“Due to our large storage yard, we can produce precast components ahead of time,” said Jim Thompson, general manager of the Valley City plant. “With the number of slabs, panels and caps required, we had several months of actual production time, but everything was delivered and set within days.”
Once the base was prepared, the wall panels were set in the keyways to form the tank and the chambers. The caps were placed in position and grouted and the rails were installed.
“The time savings came with the installation of the wall panels,” said Tom Setzer, the Mack Industries sales representative for the project.
At that point, the plant was post-tensioned to form the watertight tank and seepage tests were conducted. The plant was fully operational in September 2016 and is currently running at slightly more than half capacity.
Now that the plant is operational, the results are receiving positive feedback.
“We’re so glad we used precast,” English said. “It provides a nice, finished product, and we don’t have to worry about those tanks failing.
Shari Held is an Indianapolis, Ind.-based freelance writer who has covered the construction industry for more than 10 years.