Aggregation of Suspended Rainfall-Runoff Particles: Evaluation of pH and Velocity Gradients

The efficacy of particle agglomeration can significantly influence water treatment unit operation selection and efficiency. While the study of natural particle agglomeration's qualitative existence and quantitative kinetics and time to equilibrium have been long studied in such areas as drinking water and wastewater solid/liquid phase separation, the study of agglomeration in surface water runoff is just commencing in response to increased federal interest in the pollutant impact of rainfall-runoff. Through an assumption of spherical particles and a measured site rainfall-runoff particle specific gravity of 2.51, several granulometric indices were calculated for collected storm events and their trends observed over time in order to evaluate the degree to which agglomeration and disaggregation occur in fluid shear conditions. Changes in particle size distributions (PSD) from rainfall-runoff samples collected from under the 1-10 overpass directly over City Park Lake in Baton Rouge, LA were measured using laser-diffraction particle analysis. Various influencers of aggregation kinetics, namely pH, ionic strength, and Camp and Stein's velocity gradient (G), were evaluated for 17 separate rainfall-runoff samples over the naturally-occurring rainfall-runoff pH range of 5.5-9.0. Through observation of various rainfall-runoff sample pH values with fixed G-values and various G-values at a natural pH, it was observed that agglomeration rates tend to accelerate at lower pH values. Further, agglomeration rates are optimized when mixing rates avoid particle sedimentation and fluid shears that can cause disaggregation.