@article{przepiora_hesterberg_parsons_gilliam_cassel_faircloth_1998, title={Field evaluation of calcium sulfate as a chemical flocculant for sedimentation basins}, volume={27}, ISSN={["0047-2425"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032076630&partnerID=MN8TOARS}, DOI={10.2134/jeq1998.00472425002700030026x}, abstractNote={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 (CaSO 4 · 0.5 H 2 O) 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 SO 4 concentrations of <250 mg L −1 .}, number={3}, journal={JOURNAL OF ENVIRONMENTAL QUALITY}, author={Przepiora, A and Hesterberg, D and Parsons, JE and Gilliam, JW and Cassel, DK and Faircloth, W}, year={1998}, pages={669–678} }
 @article{przepiora_hesterberg_parsons_gilliam_cassel_faircloth_1997, title={Calcium sulfate as a flocculant to reduce sedimentation basin water turbidity}, volume={26}, ISSN={["0047-2425"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031278959&partnerID=MN8TOARS}, DOI={10.2134/jeq1997.00472425002600060021x}, abstractNote={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.}, number={6}, journal={JOURNAL OF ENVIRONMENTAL QUALITY}, author={Przepiora, A and Hesterberg, D and Parsons, JE and Gilliam, JW and Cassel, DK and Faircloth, W}, year={1997}, pages={1605–1611} }