The Logansport installation uses new PWI™ assemblies, including porous plates. Installed cost for first four dual-cell filters was $114,000, with additional $60,000 for air-assisted backwash option
Click here to enlarge imageWater management at Logansport Municipal Utilities' (LMU) 9 mgd surface water treatment plant solved erosion-related blowouts of Wheeler bottoms in its gravity filters with PWI™ assemblies —- a new system of plastic inserts, including porous plates, manufactured and installed by Roberts Services Inc., a subsidiary of The Roberts Filter Group of Darby, PA.
Installed cost for the first four dual-cell filters was $114,000, with an additional $60,000 for an air-assisted backwash option. Similar costs are estimated for the remaining four cells. Complete demolition and rebuilding of the filtration system would have cost over $700,000. Meanwhile, failures of a previous porous plate solution, installed in 1998, were projected to cost about $25,000-50,000 per occurrence after warranty expiration.
"Our first noted occurrence of the Wheeler bottom problem in our plant, which was built in 1954, was in 1997," said Paul Hartman, LMU's water and wastewater manager. "Porcelain thimbles and concrete hopper bottoms had eroded away in at least two cells, allowing filtration media to be lost into the plenum. Several of these holes in a cell effectively blew the whole gravity filter.
"We had to remove all the gravel, sand, and anthracite remaining above, and clean out the plenums. This revealed that the concrete was badly spalled and often gone —- it could now no longer hold either the Wheeler bottom's porcelain balls, which had apparently been spinning to help cause the damage, or hold the filter media."
The Logansport plant runs at a 3 1/2 mgd annual average, with highest flow at 5 1/2 mgd. Raw water from the Eel River is first oxidized with potassium permanganate, and flocculated with liquid alum before entering clarifiers. It then enters the filtration system of four dual-cell gravity filters. Chlorine gas is applied at the raw water intake and at the entrance and exit of the filtration system. Fluoride is added at the point of entry to the distribution system.
The filters had been originally designed with standard Wheeler bottoms — concrete pyramidal hoppers, porcelain balls, and porcelain thimbles (exit orifices). Filter media originally consisted typically of four 3 inch layers of gravel; one layer of sand, 1 - 1 1/2 feet; and one layer of anthracite, also 1 - 1 1/2 feet.
Hartman reported his first option selected to solve the problem was to install a porous plate across the top of his Wheeler bottoms, using a combination of sealant and bolts. That solution did not prove satisfactory and had to be repaired a number of times.
Hartman said he entertained the notion of ripping out the Wheeler bottoms and installing brand new underdrain systems, but then discovered that Roberts Services offered a Wheeler bottom rehabilitation service.