Assessing Alternative Media for Ballasted Flocculation

Most current commercial applications of ballasted flocculation use silica sand to increase floc size and density. Other ballast media with different specific gravity may offer advantages such as increased applicable superficial velocity or increased particulate matter removal. This study assessed the relative effect of five ballast media on ballasted flocculation/settling performance: anthracite, recycled crushed glass, conventional silica sand, garnet sand, and magnetite sand, with a common $d_{50}$ of 150 μm but variable specific densities of 1.45, 2.58, 2.62, 3.93, and 5.08, respectively. Based on microscopic observations and assuming discrete particle removal in an ideal settler, mean superficial media settling velocities were respectively calculated as 35, 73, 74, 122, and $137\mathrm{m}/\mathrm{h}$. These values do not account for the effect of lamellae or other specific geometries of different patented clarifiers (e.g., CoMag, Densadeg, Sirofloc, and Actiflo). Although the use of magnetite sand allows the total suspended solids load to increase by more than twofold compared to silica sand, the residual turbidity increased after settling as the mixing intensity needed to maintain denser media in suspension was augmented. Consequently, the lowest residual turbidity (0.78 NTU for surface water and 1.38 NTU for wastewater) was observed when anthracite was used as the ballast medium. The ballast media geometry did not significantly affect turbidity removal and settling velocity. Hence, recycled crushed glass was identified as a potential alternative to conventional silica sand despite its higher angularity.