Coagulation and Flocculation in Water and Wastewater Treatment

John Bratby

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Coagulation and flocculation are essential processes in a number of diverse disciplines, including biochemistry, cheese manufacturing, rubber manufacturing, and in water and wastewater treatment. Many people are probably familiar with the general concept of coagulation from the well known chemically caused clotting of blood when exposed to air. Another well known example of coagulation is the hard-boiling of an egg, in which the protein coagulates and hardens by heat. In water and wastewater treatment, coagulation and flocculation phenomena are extremely important. In the area of potable water treatment, clarification of water with coagulating agents has been practiced from ancient times, using a variety of substances – most notably crushed seeds. The Egyptians as early as 2000 BC used almonds smeared around a vessel to clarify river water. Flocculation was achieved by plunging an arm into the vessel. The early Romans were also familiar with alum, although perhaps not for water treatment. However, its use as a coagulant by the Romans was mentioned in ca. 77 AD. By 1757, alum was used for coagulation in water treatment in England, and more formally for the treatment of public water supplies in 1881 (Faust & Aly, 1998). In modern water treatment, coagulation and flocculation are still essential components of the overall suite of treatment processes. From an engineering survey of the quality of water treatment at over twenty operating water treatment plants, Cleasby et al. (1989) concluded: Chemical pretreatment prior to filtration is more critical to success than the physical facilities at the plant.