FIELD-EVALUATION OF CALCIUM-SULFATE AS A CHEMICAL FLOCCULANT FOR SEDIMENTATION BASINS

Sedimentation basins are built at construction sites to reduce the loa d of suspended solids in runoff water discharged into surface waters, However, these basins are not effective in reducing turbidity caused b y fine suspended particles such as clay and silt. The objective of thi s field study was to evaluate the efficiency of moulding plaster (CaSO 4 . 0.5 H2O) as a chemical flocculant for reducing the turbidity of wa ter discharged from sedimentation basins equipped with a floating skim mer device, Following each of 14 rainfall events, sedimentation basins at two urban construction sites were either treated with moulding pla ster or Left untreated, and the turbidity and chemical properties of t he impounded water and discharge water were monitored as the basins dr ained during a 50- to 70-h time period. Each sedimentation basin was e quipped with a floating skimmer device that discharged water at a cont rolled rate from 5-cm below the surface of the impounded water, The tu rbidity of discharge water from untreated basins ranged from 100 to 16 50 NTU (nephelometric turbidity units), while a surface-applied mouldi ng 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 prop ortional 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.