Greenville puts dissolved air flotation to the test
Last summer, the Table Rock and North Saluda Water Treatment Plant went online in Greenville, S.C., becoming the largest U.S. treatment plant to apply dissolved air flotation (DAF) to the treatment of potable water. With a capacity of 75 million gallons per day, the plant treats an average of 50 mgd and serves 350,000 customers.
The Table Rock and North Saluda Reservoirs had served as Greenville’s unfiltered source of potable water for 75 years. However, when the Surface Water Treatment Rule mandated the filtration of all previously unfiltered supplies, Greenville Water System (GWS) began investigating ways to meet the new requirements.
GWS needed a cost-effective method of treating water that was characterized by low turbidity and periodic alga blooms. It hired Kansas City, Mo.-based Black & Veatch to evaluate various treatment processes (and, subsequently, to provide preliminary engineering, design and construction-related services). Based on the company’s recommendations, the utility decided to test the DAF technology.
When compressed air is injected into a DAF basin, the small air bubbles create a “float blanket,” forcing the floc particles to the surface. Reciprocating scrapers remove the residuals from the surface, and the residuals undergo further treatment and are ultimately discarded. The technology minimizes water waste because the discarded solids have a lower water content than the solids produced by other processes.
DAF had been applied to water and wastewater treatment in Europe, but experience with the technology was limited in the United States. Therefore, GWS chose to construct a pilot plant to evaluate DAF’s effectiveness — both on its own and in combination with other treatment processes.
Representatives of GWS traveled overseas to tour DAF facilities. “They met with experienced operating staff at a number of DAF water treatment plants in the UK to establish how best to apply DAF technology in Greenville,” explains Lynn Stovall, general manager for GWS. The utility also added Reid Crowther, a Calgary, Canada-based consulting firm, to the design team.
Pilot testing, which lasted six months and used portable equipment, demonstrated that DAF, followed by deep-bed, mono-media filtration, could meet GWS’s treatment needs. In September 1996, the utility broke ground on its new DAF plant, as well as a full-scale pilot facility where testing could continue.
The plant incorporates treatment facilities for coagulant addition, DAF, filtration and disinfection, as well as facilities for water storage/pumping and washwater/residuals management. The natural slope of the 94-acre plant site was incorporated into the design, allowing GWS to harness the power of gravity and thereby minimize pumping and energy requirements. Additionally, GWS had to convert existing mains, which were designed for treated water, to allow for raw water transmission.
Construction of the Table Rock and North Saluda Water Treatment Plant, including the main improvements, cost Greenville $70 million. Since the plant began operating, it has exceeded treatment goals and has served as an educational example to other utilities considering the use of DAF technology.
For more information about the plant, contact Lynn Stovall, general manager for the Greenville Water System, at (864) 241-6155.