Dec 28, 2000

Underdrain Plate for Potable Water Filtering Provides for Retention of Media

Products at Work

Tetra Technologies, Inc., has developed an underdrain plate called Savage Plate for water filtering applications. It offers retention of finer filtering media while expanding filtration performance and increasing filtration efficacy.

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.

Successful application of the 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," Softcheck 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 haven't 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 this plate has been installed over a Wheeler-type bottom, plant superintendent Bob Spencer said he had seen no problems in the filter's first three weeks of operation, but that he would be allowing four to six months for complete evaluation.

"It'll take me a month of washing just to get the new filtering media in the way I want it, and then I'll be gathering data on filtering effluent," he said.

Spencer also pointed out he had been losing gravel and sand media-from an original depth of 24< down to 17<.

"Instead of reconstructing the whole drain, our engineers came up with the alternative of changing from a dual filtering media to a mixed media of garnet, sand and anthracite," he explained. "We also took the spheres out of the cones in the Wheeler bottom and then overlaid it with Tetra's Savage Plate. We'll 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 we're losing any. The plate is to deliver the water uniformity, and we'll be watching to see if enough water is coming through to break everything up and carry it out to the wash troughs and if there's enough flow to raise it up and wash it out."

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