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Calcium Sulfate as a Flocculant to Reduce Sedimentation Basin Water Turbidity

Dep. of Soil Science, North Carolina State Univ., Box 7619, Raleigh, NC 27695;
Dep. of Biological and Agricultural Engineering, North Carolina State Univ., Box 7625, Raleigh, NC 27695;
Orange County Planning Dep., 306 F Revere Rd., Hillsborough, NC 27278.

^* Corresponding author (dean_hesterberg{at}ncsu.edu).

ABSTRACT

A high-suspended solids load in surface waters is one of the biggest water quality problems in the Piedmont region of the southeastern USA. Sedimentation basins at construction sites are designed to reduce suspended solids in discharged water, but they are not effective in reducing turbidity. A survey of sedimentation basin water at two urban construction sites showed that turbidity levels during a 9- to 12-mo period were always greater than the 50-NTU (nephelometric turbidity units) standard adopted in North Carolina for surface waters. Furthermore, water chemistry varied over time, with pH ranging from 5.8 to 8.9 and electrical conductivity (EC) ranging from 3.0 to 23.0 mS m^–1. Laboratory experiments demonstrated that temporal variations in the water chemistry were likely caused by contact with concrete and crushed stone. Laboratory flocculation experiments were completed to evaluate the efficiency 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 flocculant than agricultural gypsum and has fewer environmental restrictions on its use than phosphogypsum. Moulding plaster application rates of 350 to 700 mg L^–1 were needed to reduce the turbidity of unstirred sedimentation basin water to 50 NTU within 3 h after addition of the flocculant. To achieve a given turbidity level, less flocculant was required for longer flocculation and settling times, or when sedimentation basin water had a lower pH.