The effectiveness of polymers and additives on reducing suspended sediment

ABSTRACT:

Nonpoint source pollution, particularly suspended sediment, is a major threat to water quality. One effective new water clarification technology for the removal of suspended sediment from stormwater is the application of sediment-flocculating polymers in waterways or sedimentation ponds. Due to temporal variations in stormwater flows and sediment loads, effective polymer application rates are difficult to determine. A series of experiments were conducted to determine the effectiveness of different types, forms and concentrations of polymers, and mixing speeds on removal of suspended sediment. In addition, calcium, aluminum and citric acid were evaluated to determine their ability to increase the efficacy of polymers to remove suspended sediment. Four polymers (Magnafloc LT27, SoilNet 926, SoilNet 934, and SoilNet 945) were studied with a silt loam soil. Solution forms of polymers were highly effective immediately after application with more than 95 percent suspended sediment reduction regardless of mixing speed. However, granular forms of polymers were effective in reducing suspended sediment by 95 percent or more only at mixing speeds ≥130 rpm. The effectiveness of the polymers to reduce suspended sediment was related to sediment load and polymer concentration. At sediment concentrations ≥10000 ppm, polymer concentration of 1 ppm reduced suspended sediment more than 90 percent after 2 minutes of settling and more than 95 percent after 5 minutes of settling. At low sediment concentrations (2000 ppm), greater polymer concentrations (5 and 10 ppm) decreased the efficiency of the polymers significantly. At the same low sediment concentration, polymer:citric acid:AlCl(OH)₅ and polymer:Ca:AlCl(OH)₅ at ratios of 1:1:1 increased the polymer efficiency from 62 percent to 85 percent while maintaining the solution pH above 8 and close to the initial pH.