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Field Evaluation of Calcium Sulfate as a Chemical Flocculant for Sedimentation Basins

Dep. of Soil Science, North Carolina State Univ., Box 7619, Raleigh, NC 27695;
Dep. of Biological & Agricultural Engineering, North Carolina State Univ., Box 7625, Raleigh, NC 29695;
Orange County Planning Dep., 306 F Revere Rd., Hillsborough, NC 27278.

^* Corresponding author (dean]_hesterberg{at}ncsu.edu).

ABSTRACT

Sedimentation basins are built at construction sites to reduce the load of suspended solids in runoff water discharged into surface waters. However, these basins are not effective in reducing turbidity caused by fine suspended particles such as clay and silt. The objective of this field study was to evaluate the efficiency of moulding plaster (CaSO₄ · 0.5 H₂O) as a chemical flocculant for reducing the turbidity of water discharged from sedimentation basins equipped with a floating skimmer device. Following each of 14 rainfall events, sedimentation basins at two urban construction sites were either treated with moulding plaster or left untreated, and the turbidity and chemical properties of the impounded water and discharge water were monitored as the basins drained during a 50- to 70-h time period. Each sedimentation basin was equipped with a floating skimmer device that discharged water at a controlled rate from 5-cm below the surface of the impounded water. The turbidity of discharge water from untreated basins ranged from 100 to 1650 NTU (nephelometric turbidity units), while a surface-applied moulding plaster treatment of 450 to 520 mg L^–1 decreased the turbidity to <50 NTU. The time required for the discharge water from treated basins to reach either 100 NTU (2–20 h) or 50 NTU (5–52 h) was inversely proportional to the concentration of dissolved moulding plaster. Chemical flocculation using moulding plaster reduced the turbidity of discharge water to <50 NTU while producing dissolved SO₄ concentrations of <250 mg L^–1.