The W.R. Wise Water Treatment Plant in Greenwood County (S.C.) provides drinking water for 45,000 people from Lake Greenwood, the plant’s 3.5 billion ft3 (99 million m3) impoundment. The lake is also used to generate hydroelectric power and for recreation. In the spring, the plant became just the fourth water utility ever to receive Phase IV “Excellence in Water Treatment” recognition from the Partnership for Safe Water, a national volunteer initiative developed by the U. S. Environmental Protection Agency and other water organizations. Water suppliers in the Partnership strive to provide their communities with drinking water quality that surpasses the required federal standards. Phase IV is the program’s highest possible level of performance and signifies optimized plant performance.
Six vertical turbine pumps pull up to 39 mgd (150,000 m3/d) of raw water out of the lake and push it through three mains 0.25 mi (.040 km) to a central main where the volume is metered as it enters the plant. After the flow is metered, chlorine is added to oxidize iron and manganese and begin disinfection. Disinfection byproduct concentrations are closely monitored and if those concentrations or total organic compound concentrations are deemed too high, chlorine dioxide is substituted for chlorine.
Next, alum enters the water via a rapid-mix chamber to adjust the pH as needed. After mixing, a splitter box directs the flow into 18 flocculation and sedimentation basins. Most of the flocculation basins are three-stage horizontal paddle wheel flocculators. The sedimentation basins contain sludge collectors capable of removing sludge to a center drain with an automatically opening and closing plug valve.
Sedimentation effluent spills over V-notch weirs into a common filter influent flume and diffusers add more chlorine for disinfection and algae control.
The plant has 11 conventional dual media filters rated at 4.0 gal/min/ft2 (163 L/min/m2) that are used continuously. Operators initiate an automated backwash system when the filter effluent turbidity exceeds 0.10 NTU, 6 ft (1.8 m) of head are lost, or the filter has operated for 96 hours — operating time is the most common reason for a backwash.
After filtration, the plant has the ability to add chemicals to regulate the pH and hardness and to add fluoride. Lime is currently fed as part of the plant’s corrosion resistance program. Finished water pH is elevated to 8.0 and hardness to about 30 ppm.
Finally, eight vertical turbine pump stations push the water into the distribution system under pressure. One 24-in. (600-mm) and two 30-in. (750-m) water mains connect the plant to the distribution system.
Solids flow from the sedimentation basins into sludge holding basins. After settling, the water is decanted from the top of the basins and discharged back into Lake Greenwood. The plant maintains a National Pollutant Discharge Elimination System permit for pH, chlorine, total suspended solids, and aluminum for the decant water.
The solids next enter two sludge thickeners where they are further dewatered to between 4% and 5% solids. Polymer is added to the thickened sludge and it is discharged to one of 34 sand drying beds. A front-end loader removes the dried sludge, which is landfilled at the Greenwood County Landfill.
In August 1995, Tropical Storm Jerry dumped 10 to 15 in. (254 to 381 mm) of rain on upstate South Carolina, causing Lake Greenwood to swell and raw water quality to drop. The plant temporarily lost coagulation and almost violated allowed turbidity levels in its finished water. Operators quickly got the problem under control, and the close call led to substantial changes in the plant’s philosophy and operation of the treatment process.
More stringent operating procedures have significantly refined the coagulation process. Operators now have the authority to monitor and alter coagulant chemical doses as needed. To ensure the operators remain sharp on when to alter the doses, the plant holds refresher training each summer.
As a result of the changes, settled water turbidity, on average, has stayed well below 1 NTU and filtered water turbidity has stayed at or below 0.10 NTU.