FUNDAMENTALS OF BALLASTED FLOCCULATION REACTIONS

Ballasted flocculation represents a relatively new technology that shows promise of bringing major improvements to the field of water and wastewater treatment. Ballasted flocculation involves the addition of a ballasting agent (high-density microsand @ ρ = 2.65 g/cc) to a chemically stabilized and coagulated suspension of particulate solids. The ballasting agent coagulates simultaneously with the chemical precipitate and particulate solids to form “ballasted” flocs. The ballasted flocs are heavier and larger than chemical flocs and can be removed faster by sedimentation.An intensive series of tests was conducted to determine the effect of varying concentrations and density of ballasting agent on the settling velocity of the resulting floc. Measurements showed settling velocities ranging from about 100 m/hr for 0.5 mm diameter particles to as high as 380 m/hr for particles having effective diameters up to 7 mm. Measured settling velocities for pure microsand particles and ballasted flocs were found to fit conventional settling theory with reasonable accuracy.The test program indicated that the ballasting agent did not seem to be involved in the chemical reaction but was simply incorporated into the chemical floc by contact flocculation or differential settling (or differential momentum) mechanisms. The most important factors contributing to the benefits of ballasted flocculation are greater settling rates caused by increased floc density, the large floc sizes that can be maintained at a given mixing rate, and the greater “roundness” of the floc particles and therefore a lower shape factor. Increasing amounts of ballasting agent improved settling velocity up to the point that the floc could not hold additional amounts. Therefore, there seems to be an optimum amount of ballasting agent associated with each combination of raw water turbidity, coagulant dose, and polymer dose. This optimum point can be expected to vary with the type and concentration of chemical floc formed.