Georgia facility upgrades its underdrain system with lateral filter system
The North Columbus (Ga.) Resource Facility recently completed a $12-million replacement of its settled water filtration by removing the existing Wheeler filters and their three-part media, as well as the facility’s 10-in. poured-concrete underdrains, which were no longer efficient.
Prior to the filter rehabilitation, the utility’s backwash procedure consisted of a hydraulic-only backwash for which the high-rate wash cycle lasted up to 20 minutes. Backwashing each dual-cell filter used 150,000 to 200,000 gal of water. The filters then operated for up to an hour in rewash or filter-to-waste operation to reach the desired output of less than 0.09 ntu.
“We originally set out to do only a filter media replacement project because 40% of the media was 36 years old and the other 60% was 26 years old,” said Water Treatment Superintendent James Stephens. “The utility decided to undertake a much more proactive program. We evaluated several types of replacements, including membrane technology, restoring conventional gravity filtration like that in place and the option of replacing the entire Wheeler ball underdrain and media with the Leopold dual lateral system and advanced controls.”
A visit to the Leopold Product Development Center convinced Stephens to recommend the Leopold system, along with other upgrades. The Wheeler filters were replaced with a state-of-the-art Leopold universal Type SL dual lateral filter system fitted with a factory-installed Integral Media Support (I.M.S.) cap of plastic beads sintered together and attached to the surface of the segmental underdrain.
The new filter system includes an air-scour feature and stainless steel piping that optimizes performance and durability. The pre-engineered HDPE underdrain system employs a dual-media gravel-less design, yielding an increase in water depth atop the filters and adding 6 to 8 in. to the overall system freeboard.
The Leopold system consists of 30 dual cell filters (60 cells total) with a total filtration area of 15,660 sq ft. The two-part media consists of 17,128 cu ft of silica sand (856 tons), sized from 0.45 to 0.55 mm, and 31,320 cu ft (783 tons) of filter-grade anthracite, sized from 0.95 to 1.05 mm in a 22-in. depth. The pre-engineered underdrain is mechanically fixed and aligned end-to-end to create continuous parallel laterals in the approximate lengths of the filter cells.
The upgrade reduced the filter high-rate backwash down to only 5 minutes. Filter rewash time also drastically decreased, lasting 15 to 20 minutes on average. The new backwash cycle utilizes only about 90,000 gal for each dual cell filter.
“Before the rehabilitation, we were using 5% of our production water for filter backwash purposes,” said Stephens. “We are now using 1.8% of our production water.”
With the Leopold filter system, the plant is saving approximately 1,000,000 gal of water per day, which was previously used for backwash purposes.
Measurable reductions have been gained not only in the intervals, but also in the duration of the backwash cycles, along with the processed water and contingent electricity usage. The two-part media achieves greater efficiency and volume and a more thorough backwash from the air-scour technology that was first introduced by Leopold in 1977.
“In addition to longer intervals between backwashes, the new filters have brought the loss of head from 2.5 ft to just a 0.5-ft loss,” Stephens said. ”Replacing only the media in the old system would have restored the desired quality but not the net gain in quantity of production.”