CALCIUM-SULFATE AS A FLOCCULANT TO REDUCE SEDIMENTATION BASIN WATER TURBIDITY

A high-suspended solids load in surface waters is one of the biggest w ater quality problems in the Piedmont region of the southeastern USA. Sedimentation basins at construction sites are designed to reduce susp ended solids in discharged water, but they are not effective in reduci ng turbidity, A survey of sedimentation basin water at two urban const ruction sites showed that turbidity levels during a 9- to 12-mo period were always greater than the 50-NTU (nephelometric turbidity units) s tandard adopted in North Carolina for surface waters. Furthermore, wat er chemistry varied over time, with pH ranging from 5.8 to 8.9 and ele ctrical conductivity (EC) ranging from 3.0 to 23.0 mS m(-1). Laborator y experiments demonstrated that temporal variations in the water chemi stry were likely caused by contact with concrete and crushed stone. La boratory flocculation experiments were completed to evaluate the effic iency of calcium sulfate compounds (hemihydrate, agricultural gypsum, and phosphogypsum) as chemical flocculants for reducing the turbidity of sedimentation basin water from two field sites and for different pH conditions, Moulding plaster (hemihydrate) was a more efficient flocc ulant than agricultural gypsum and has fewer environmental restriction s on its use than phosphogypsum, Moulding plaster application rates of 350 to 700 mg L-1 were needed to reduce the turbidity of unstirred se dimentation basin water to 50 NTU within 3 h after addition of the flo cculant. To achieve a given turbidity level, less flocculant was requi red for longer flocculation and settling times, or when sedimentation basin water had a lower pH.