Smooth Solution

A polyurethane spray coating helps a treatment plant restore an older clarifier, reduce surface maintenance and extend service life

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During a product installation demonstration for the rehabilitation of some aging brick manholes and a concrete lift station, Mack McDonald saw what he thought might be the solution to save a deteriorating clarifier and reduce the cost of maintaining it.

The clarifier was one of two older 70-foot-diameter units at the Lancaster (S.C.) Wastewater Treatment Plant, says McDonald, public utility director. Put into service in the early 1950s, the treatment plant had seen many upgrades and changes. Today the 2.2 mgd (average) plant serves 6,000 customers in the city, 8,000 customers in the surrounding county, and three large industrial facilities.

The clarifiers were installed in 1950 and originally were equipped with all metal weirs and components. In 2000, the city replaced the rotating arm, motors and sweep arm in both clarifiers and replaced the metal weirs with fiberglass weirs.

“We knew the substrate had some wear, but after we replaced the internal components, it looked so good that, in comparison, the deterioration on the concrete to which the fiberglass weir was connected started to really show its age even worse,” says McDonald. Plant staff found the solution in a spray-applied polyurethane coating that secured and smoothed the surface, reduced maintenance, and extended the clarifier’s life significantly.

Maintenance headache

The concrete in the old clarifier was heavily pitted and had numerous voids, creating a major maintenance concern. “Anybody who knows anything about wastewater treatment knows that the solids coming in create a real good environment for algae growth,” says McDonald. “With those voids in the concrete and the deterioration, algae would get hooked in those spots, and it became difficult to keep the concrete washed down and clean.”

To keep the algae and debris in check, crews had to wash down the clarifier twice a week, spending three hours each time. They used a fire hose because it took high pressure to free the buildup from the pitted surface. Each cleaning dislodged more loose material from the surface, making it even rougher.

While investigating solutions, McDonald looked at brushes attached to the swing arm that could scrub the surface during normal rotation, and at a spray wash process, but neither offered a long-range solution. Then he remembered that the city had used spray-applied polyurethane linings from Sprayroq Inc. to rehabilitate manholes and lift stations, and he thought the material might also solve the clarifier problem.

Osborn Contract Services, the local Sprayroq Certified Partner, suggested using SprayWall structural polyurethane coating.

If the polyurethane application was to be successful, the clarifier surface had to be completely dry and clean. That meant taking the clarifier out of service and letting it dry for at least three days. Once the clarifier was taken down, Osborn’s crews began preparation.

They thoroughly cleaned the structure using a 4,000 psi pressure washer, taking care to remove all loose material and creating a solid substrate for the surface build-back. Once the surface was clean, the crew applied FX-263 rapid-set repair mortar from Fox Industries Inc. as the initial build-back material and to fill the large voids. Crews then applied up to one-half inch of Fox Industries FX-262 micro silicate mortar to profile the floor and interior walls and prevent future leakage.

Maintaining the original dimensions of the trough/weir plate system was critical during the rehabilitation process. Crews took great care to maintain design tolerances. Walls were restored to maintain weir overflow rates and assure proper fall into the trough. Also, clearances for the sedimentation paddles were checked and maintained at various locations during the process. To accomplish this, they used Fox Industries FX-472, a high-build, fast-setting epoxy, on the trough walls. With this material, they could profile the walls precisely and eliminate any potential for epoxy buildup at critical transition points.

Once surface preparation was complete, the crews were ready to apply the lining. Using a plural-component gun, they sprayed on the SprayWall material at a thickness of 125 mils on the floor, interior wall and exterior wall of the trough. This thickness provided instant leak protection and the necessary structural strength and corrosion resistance.

Smooth as silk

The entire process took 10 days, and the structure was able to return to full service immediately after spray application. The change was obvious right away. The structure no longer leaks, is much easier to clean, and can be cleaned less often, says McDonald. Lancaster plans to use the same process to rehabilitate and protect other treatment structures as part of its five-year capital improvement plan.

“This clarifier was a great place to test this technology, because we have another structure in a similar condition right beside it for comparison,” says McDonald. “We saved the concrete and got a slick finish for easy wash down, and we also extended the life of this clarifier for many more decades.”


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