Factors affecting ballasted flocculation reactions

Ballasted flocculation represents a relatively new technology that shows promise of bringing improvements to the field of water and wastewater treatment. Ballasted flocculation involves the addition of a ballasting agent (high-density microsand, sp gr = 2.65) to a chemically stabilized and coagulated suspension of particulate solids. Tests were conducted to determine the effect of eight independent parameters on the settling velocity of the resulting floc. Measurements showed settling velocities ranging from approximately 100 m/h for 0.5-mm diameter particles to as high as 380 m/h for particles having effective diameters up to 7 mm. Settling velocities for discrete microsand particles and discrete ballasted flocs were found to fit conventional settling theory with reasonable accuracy. The most important factors contributing to the benefits of ballasted flocculation are the large floc sizes that can be maintained, the greater roundness of the floc particles, and a lower shape factor for the ballasted floc, which all contribute to higher settling rates. Higher settling rates allow for substantially smaller sedimentation units and decreased capital costs.