New Underdrain Plate Improves Performance

Tetra Technologies has introduced a new underdrain plate for potable water filtering applications that offers improved retention of finer filtering media, while expanding filtration performance and increasing filtration efficiency.

Tetra Technologies has introduced a new underdrain plate for potable water filtering applications that offers improved retention of finer filtering media, while expanding filtration performance and increasing filtration efficiency.

A patent is pending for the Savage Plate™, which is suitable for a wide variety of underdrain types, including “U” Block and Wheeler. A separate method patent is also pending for the Wheeler application. Gary T. Hunkele, Tetras manager of field service, says the new, three-layer plate offers varying benefits in treatment plants whose engineers had made filtering media changes.

“For example,” he noted, “when granular activated carbon has been introduced to improve taste and reduce odor, the new plate helps retain this medium when it typically breaks down to smaller particle sizes during its 4- or 5-year useful life. And in a Wheeler bottom underdrain type, where water is coming up at the rate of 18 feet per second, it reduces the velocity so you dont get unacceptable levels of turbulence on the other side."

E. Stuart Savage, Tetras technical marketing manager, says the new plate eliminates the need for a gravel layer in a variety of applications, allowing for increased filtering media capacity and higher filtration rates within the same filtering structure, and also eliminates the need for porcelain spheres in Wheeler bottom cones.

“With the new plate over Wheeler bottoms, an application invented by Gary Hunkele, operators no longer have to worry about the porcelain spheres in the cones getting elongated or eroded when the floors above get eroded and the filtering media trickle through,” he said “The new 1-1/4-thick plate can also replace about 12-18 inches of gravel in a variety of applications, so that room is left for installing additional media for both improved performance and higher filtering rate. This is especially important in American practice, where filtering beds are typically shallow, and a new 17-inch space can be very important for retrofitting more filtering media.”

The Savage Plate consists of three layers of black, ultraviolet-resistant, porous, high density polyethylene (HDPE) resin. The outer layers have a pore size of about 500 microns, while the inner layer openings are about 300 microns.

The resin has been listed by the National Science Foundation (NSF) for use in potable water treatment systems. In addition to benefits offered by the three-layer design, the porous HDPE material is said to offer a number of benefits over ceramic types.

The advantages include lighter weight, about 1/3 that of ceramic; availability in lengths as long as 16 feet; ability to withstand rough handling that might otherwise cause cracking, shattering, or deformation on impact; and easier installation — fewer joints are required because of longer lengths, field fitting is readily accomplished with hand saws, and bonding is achieved with installation adhesives.

Successful application of the Savage Plate on a “U” Block-type underdrain has been reported by the North Fayette County Municipal Authority in southwestern Pennsylvania.

According to Bob Softcheck, general manager, the installation is part of a continuing program to replace clay underdrains that had been breaking up and erupting in a filtration plant built in 1920.

“We had achieved success with one design, and we wanted to establish a satisfactory alternative,” he said. “We were not only changing the drain type, but also moving from sand/gravel filtering media to granular activated carbon. We got a guarantee from Tetra that their “U” Block with Savage Plate™ would perform as well as or exceed the previous design, and we have been completely satisfied. There havent been any problems with that filter, with both filter run time and turbidity requirements either meeting or exceeding our criteria.”

At the Hilltop Water Treatment plants for the Palo Pinto Municipal Water District #1 in Texas, where the Savage Plate has been installed over a Wheeler-type bottom, Plant Superintendent Bob Spencer said he had seen no problems in the filters first three weeks of operation, but that he would be allowing four to six months for complete evaluation.

“Itll take me a month of washings just to get the new filtering media in the way I want it, and then Ill be gathering data on filtering effluent,” he reported

Spencer said he had been losing gravel and sand media — from an original depth of 24 inches down to 17 inches.

“Instead of reconstructing the whole drain, our engineers came up with the alternative of changing from a dual filtering medium to a mixed media of garnet, sand, and anthracite,” he said.

“We also took the spheres out of the cones in the Wheeler bottom, and then overlaid it with Tetras Savage Plate. Well be looking at the backwash uniformity rate; seeing if the filter forms nice and even when it settles down; watching for turbidity problems; and determining media depth to see if were losing any.

“The plan is to deliver the water uniformly,” Spencer said, ” and well be watching to see if enough water is coming through to break everything up and carry it out to the wash troughs, and if theres enough flow to raise it up and wash it out.”

“In a Wheeler bottom underdrain type, where water is coming up at the rate of 18 feet per second, it reduces the velocity so you dont get unacceptable levels of turbulence on the other side."

“With the new plate over Wheeler bottoms … operators no longer have to worry about the porcelain spheres in the cones getting elongated or eroded when the floors above get eroded and the filtering media trickle through.”

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