Improving Water Quality in Construction Site Runoff: Optimal Mixing Time and Dose for Flocculating Suspended Sediment with Polyacrylamide

Abstract:

Sediment, a major water pollutant, can harm ecosystems and water resources. Sediment in construction site runoff can be controlled through flocculation using anionic, linear polyacrylamide (PAM), but there is little information on optimizing these applications. We conducted laboratory experiments to determine the optimal mixing times for varying concentrations of two types of polyacrylamide, APS 705 and FA 920, which are blends of polymers with a range of molecular weights that are anionic and neutral, respectively. These were selected from a variety of PAMs previously screened for flocculation potential. Soil from three active construction sites in the Piedmont region of North Carolina were used in the testing. A mixing speed of 300 revolutions per minute (RPM), the maximum available for the paddle mixer we used, was the most effective at reducing turbidity with PAM. Turbidity was reduced with increased mixing times up to a point, after which little additional benefit was evident. For all three soils tested, turbidity decreased as mixing time reached 1-2 minutes at polymer doses of 1, 5 and 10 mg L^-1, with no substantial reduction with further mixing. At a polymer dose of 0.5 mg L^-1, however, turbidity tended to increase beyond 5 minutes of mixing time, possibly because excessive shear forces destroyed sparsely linked floc. For both PAMs, 1 mg L^-1 and a mixing time of 2-3 min appeared to be sufficient to achieve the most effective turbidity reduction.