Concrete Forms and Secures

North Texas Plant Takes Water Treatment Indoors

By Rob Patterson

At first glance, the form of the $33 million Tom Harpool Regional Water Treatment Plant in Aubrey doesn't match its function. With the processes and controls enclosed in two steel-frame and roof structures faced with spilt-face CMU, it could easily be mistaken for an attractive warehouse facility. That was the goal of the Upper Trinity Regional Water District.

Roanoake-based Oscar Renda Contracting began construction in December 2004 on the project, which is aiming for a September completion.

Since a housing development is adjacent to the 86-acre site - formerly a horse ranch - minimizing the plant's visual and operational impact on its neighbors was a major element in its design. "Upper Trinity builds nice facilities, and they were especially concerned with that because it is so close to residents," said Jim Gallovich, project manager for Carollo Engineers PC of Dallas, designers of the plant and the developers of the water district's master plan.

"What is unique about this plant is that with most conventional water treatment plants, the processes are outdoors in concrete basins," said Paul Mendoza, project manager for Oscar Renda. The Harpool plant encloses its innovative membrane-filtration process inside a 25,000-sq.-ft. building. Another two-story, 8,000-sq.-ft. structure houses administrative functions and the chemical feed and control equipment.

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"There was 600,000 cu. yds. of excavation and dirt moving to get the site to the proper contours that had to be done before we started construction," Mendoza said. The excavated material was used on site to create a berm to hide the plant from nearby homes. A pre-existing sand pit on site reduced the excavation needed to create two clay-lined ponds about an acre each in size to hold the backwash from the plant.

Excavation for the concrete filtration basins went 12 ft. below grade, and a four-million gallon pre-stressed concrete well required a 24-ft. dig. "The bottom of the excavation was about five ft. below the groundwater table," Mendoza said. "So we built our own small de-watering system around it while we were building it up.

"To prepare the concrete tank site, we had to put in eight ins. of flex base below the tank that had to be shaped to within .25 in., all by hand," Mendoza said. "We had a dozen guys with shovels and a grade checker doing the work."

The four-million gallon tank, which is 195 gallons in diameter, used 5,000 cu. yds. of concrete. The 72-in. inlet and outlet ports on the tank also had to be surrounded by 100 cu. yds. of concrete to ensure they were not affected by movement or foundation problems.

Another 7,000 cu. yds. was poured for the 13 intricately formed filtration basins that are in the basement of the membrane building. With the addition of the control building, pump stations and junction structures, a total of 17,000 cu. yds. of concrete was poured on the job.

The majority of the concrete work being at or below grade was a boon for the project. "We poured about 90 percent of the concrete with a crane and bucket," Mendoza said. "We didn't have to use pump trucks, and that saved us money."

Although the well and basins required 880 tons of rebar, the contractor was not affected by recent steel shortage or price hikes. "We had our steel locked in price-wise, and hopefully our supplier didn't get hurt," Mendoza said. "The concrete shortage hasn't really impacted us either, though we did have to schedule deliveries a week in advance."

A design change from the usual scheme for membrane filtration systems did present a challenge. "Typically the pipes come up on top of the tanks," Gallovich said. "We're running them through the sides and created these pipe galleries. This is one of the first facilities that Xenon, the membrane manufacturer, had performed this on, so we couldn't cut and paste things."

The plant was also on a fast track to provide 20 million gallons a day to meet the needs of the water district's member municipalities and water supply corporations in the booming region north of Dallas. "We had such a short period of time to get it online because of the drought situation," said Randy Sliger, project manager for the Upper Trinity Regional Water District.

The schedule and the site required extra coordination. "We had multiple concrete crews working simultaneously, sometimes half a mile away," Menoza said.

Among the reasons that the water district chose the membrane process was for its expandability. The master plan calls for the plant to eventually treat 260 million gallons a day.

"All the piping has provisions for future connections," Mendoza said. "The district spent quite a bit of extra money on oversized piping , additional inlets and outlets and Y fittings so that when expansion does occur, it will minimizes the shut-down time and reduce the amount of modifications needed."

Enclosing the treatment process inside a building makes the plant more secure than outdoor facilities. "It is going to be one of the first ultra-filtration membrane plants with Xenon filters in this area," Sliger said. "It takes a smaller footprint for this type of plant than a conventional plant, and that is going to be the trend of the future."