Coagulation and Flocculation Calculations
Following screening and the other pretreatment processes, the next unit process in a conventional water treatment system is mixing, when chemicals are added during what is known as coagulation. The exception to this situation occurs in small systems using groundwater, where chlorine or other taste and odor control measures are often introduced at the intake and are the extent of treatment. The term
refers to the series of chemical and mechanical operations by which coagulants are applied and made effective. These operations are comprised of two distinct phases: (1) rapid mixing to disperse coagulant chemicals by violent agitation into the water being treated, and (2) flocculation to agglomerate small particles into well-defined floc by gentle agitation for a much longer time. The coagulant must be added to the raw water and perfectly distributed into the liquid; such uniformity of chemical treatment is reached through rapid agitation or mixing. Coagulation results from adding salts of iron or aluminum to the water and is a reaction between one of the following (coagulants) salts and water:
• Alum — aluminum sulfate • Sodium aluminate • Ferric sulfate • Ferrous sulfate • Ferric chloride • Polymers
Flocculation follows coagulation in the conventional water treatment process. Flocculation is the physical process of slowly mixing the coagulated water to increase the probability of particle collision. Through experience, we see that effective mixing reduces the required amount of chemicals and greatly improves the sedimentation process, which results in longer filter runs and higher quality finished water. The goal of flocculation is to form a uniform, feather-like material similar to snowflakes — a dense, tenacious
that entraps the fine, suspended, and colloidal particles and carries them down rapidly in the settling basin. To increase the speed of floc formation and the strength and weight of the floc, polymers are often added.