Parsons Main tackles
riverbank erosion, ash pond pH problems
South Carolina Electric and Gas (SCE&G) has encountered problems with riverbank erosion and ash pond pH control at its Wateree Station, near Eastover, S.C.
After many years, riverbank erosion had reached a point where vital plant equipment and structures were threatened, and it became imperative that the bank be stabilized.
The station?s discharge permit required a stricter control of the pH range of the facility?s ash pond effluent.
SCE&G requested a study from Parsons Main to assess the riverbank erosion rates and to evaluate slope stabilization systems for technical merit and cost-effectiveness.
The assessment included topographical surveys, soil borings, soil testing and a slope stability analysis. Five slope stabilization schemes were evaluated before choosing to stabilize the slope and cover it with a surface projection of either Rip Rap or Armoflex, depending on the application area along the bank.
Also at the station, two 80-acre ash storage ponds are maintained. Effluent from the ponds discharges into the Wateree River at a rate of 5 million to 10 million gallons per day.
Biological activity in the ponds was causing seasonal high pH levels and the South Carolina Department of Health and Environmental Control is reducing the permitted discharge to a pH range of 6.0 to 8.5.
The manual pH control system SCE&G was using treats only the low end of the pH range by pumping caustic soda into an outlet, where it is mixed by turbulent flow before discharge into the river.
Parsons Main evaluated upgrading the treatment system for both the upper and lower pH range, looking at systems using sulfuric acid and carbon dioxide. Sulfuric acid inject proved to be the most cost-effective.
The new treatment system includes an acid storage tank, acid and caustic metering pumps, a pH controller and probe, and a 24-foot diameter open-top mixing tank with a 15-hp agitator. The mixing tank is connected to the outlet structure by a transition duct.
All effluent now passes through the mixing tank where acid or caustic is added before it is discharged into the river.