Particle Destabilization in Highway Runoff to Optimize Pollutant Removal

Sedimentation column studies and simulations using particle size distribution suggest that low removal efficiencies of smaller particles in highway runoff would be obtained using sedimentation if coagulation-flocculation is not performed. Coagulation-flocculation studies, using metal salts (alum and ferric chloride) and one organic polymer in three molecular weights, were evaluated over the 2004–2005 storm seasons. Only the first flush or approximately the first hour of runoff was coagulated. Efficiencies were quantified with particle size distribution measurements and turbidity. Results with low dosages of metal salts were ineffective and did not improve water quality. High dosages of metal salts using a sweep floc mechanism were effective in dramatically lowering runoff turbidity, but resulted in large quantities of sludge production and required pH control. A cationic organic polymer at low dosages (<10mg∕L) was effective in coagulating highway runoff and reducing particle charge. Extended mixing time was required to achieve low turbidities (∼5 NTU). A combination of organic polymer, followed by small doses of alum (<10mg∕L), reduced mixing time and produced high quality effluent.