Challenge

The city of Rock Hill, South Carolina's 24-MGD surface-water treatment plant struggled with a variety of filter performance and capacity issues. With three generations of filter designs dating back as far as 1899, upgrades from 1912 to 1994 incorporated additional cells as capacity needs increased. In the late 1990s, daily filter backwashes used excessive amounts of water, especially during a drought season. With warm seasonal temperatures, particles and contaminants become sticky and difficult to remove from the dual media filter bed. To remove them effectively, there was a tendency to increase backwash duration, resulting also in loss of some anthracite media in the filter, affecting filtering performance.

Solution

The city’s engineering consultant evaluated a number of treatment technologies and, in 2004, selected both the MULTIWASH® washtrough process and MULTIBLOCK with Laser Shield™ underdrains. While cleaning the filter media using air and water, the consultant found it could increase times between filter backwashes from one a day to once every 72 hours, improve on line performance and minimize media loss, and include this design as the basis for a final 60-mgd design build-out potential.

The 12 refurbished filters were finished in spring 2006. The combined total area of the filters is 4,295 square feet with five filter cells of 18ft wide x 20ft long, one cell 17ft wide x 21ft long, and 6 cells 17ft wide x 20ft long. They're operated at higher filter rates (6 gpm/sq ft), increasing the plant’s capacity to the city’s new 36 mgd goal.

Results

The Rock Hill water treatment plant uses less water for backwashing, while achieving cleaner filter beds following backwash. It also has enjoyed decreased backwashing frequency and longer filter runs.

Previously, when backwashing one filter cell once a day, an estimated 54,000 gallons of filtered water was consumed per backwash, and over a three-day period, 162,000 gallons would have been consumed for only one of the 12 filter cells. With the filter runs increased to three days between backwashes, a savings of 108,000 gallons of produced filtered water is achieved for each filter backwash with less wastewater generated.

This approach provided the city with a cost-effective solution for expansion and enhancement of existing facilities without the need for additional structures.