The Ballasted Flocculation System
The ballasted flocculation system is a chemical process for enhanced reduction of suspended solids and biochemical oxidation demand (BOD). Fundamentally, the process is very similar to conventional coagulation, flocculation, and sedimentation systems used in water treatment plants (see figure, p. 33, and Table 1, p. 32).

As in conventional water treatment technology, a coagulant is used to destabilize suspended materials entering the process and a flocculation-aiding polymer is added to aggregate solids into larger masses. The resulting floc is removed by settling.
    The ballasted flocculation system enhances this process by adding microsand (fine sand similar to silica powder) as seed for developing high-density floe, which is ballasted by the relatively high-density microsand and so more easily removed by settling. The benefit of the ballasted floc process is the ability to achieve good solids removal performance at a very high surface overflow rate. The process can be rapidly started and optimized even with variations in flow and water quality.
   In addition to the ballasted flocculation basins, a flow splitter—screening facility, a disinfection basin, and a chemical storage and feed facility were constructed. The ballasted flocculation process is protected by fine screens, which remove trash, debris, and rags from the process stream. After screening, the flow passes through the ballasted flocculation basins.
   Each 76,000-m3/d 20-mgd) basin includes a coagulation chamber, an injection chamber, a flocculation (maturation) chamber, and a sedimentation chamber. The coagulant (ferric chloride) is added to wastewater flow at the excess-flow splitter-screening facility. A diffuser disperses ferric chloride at the entrance of the 105-cm (42-in.) excess-flow treatment basin influent pipes, ahead of the coagulation chamber. In the coagulation chamber, solids entering the ballasted flocculation process are destabilized. The coagulated water then enters the injection chamber, where It is mixed with polymer and microsand.
   With most of the floe forming in the flocculation chamber, the ballasted floc settles in the sedimentation chamber, which is equipped with lamella plate settlers.