The ballasted flocculation system is a chemical process for enhanced reduction of suspended solids and biochemical oxidation demand. Fundamentally, the process is very similar to conventional coagulation, flocculation, and sedimentation systems used in water treatment plants.

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 micro sand (fine sand similar to silica powder) as seed for developing high-density floe, which is ballasted by the relatively high-density micro sand 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 optimised even with variations in flow and water quality.

The coagulation and flocculation processes may be used to condition or prepare the untreated water for the sedimentation process. Depending on the settling characteristics of the suspended material in the raw water, chemicals may be used to help the suspended materials stick together and form larger particles. In this process, a chemical called coagulant is rapidly mixed into the water. The coagulant helps the small suspended particles group together. Some of the types of coagulants typically used are alum, ferrous sulphate, ferric chloride, and ferric sulphate.