Reducing Turbidity of Construction Site Runoff: Coagulation with Polyacrylamide

Abstract

The U.S. Environmental Protection Agency is developing a nationwide standard for turbidity in construction site runoff. It is widely expected that conventional measures for erosion and sediment control cannot meet this standard; consequently, innovative practices for managing sediment on construction sites must be developed. One emerging practice is the use of polyacrylamide (PAM) to improve sediment control by promoting flocculation of particles in runoff. Effective use of this practice requires an understanding of how the properties of soil and PAM affect flocculation. The objective of this study was to develop such an understanding. A protocol for creating modified synthetic stormwater runoff from soil samples was developed, and synthetic runoff suspensions were created with soil from six Texas construction sites. Particle size distribution was used to compare the synthetic runoff suspensions with grab samples of stormwater from one site. Flocculation tests were performed on the synthetic runoff suspensions with PAM doses ranging from 0.03 to 10 mg/L. The polymers used included anionic PAMs of 0% to 50% charge density and 0.2 to 14 Mg/mol molecular weight and one copolymer. A neutral PAM and the copolymer were the most effective in reducing the turbidity of all the synthetic runoff suspensions below 200 nephelometric turbidity units at doses of 10 mg/L. Hardness tests indicated interparticle bridging to be the bonding mechanism. The high-molecular-weight (HMW) anionic PAMs were effective on only two of the six synthetic runoff suspensions. As the charge density of these HMW PAMs increased, their effectiveness decreased.