Coagulation and Flocculation 3 3.1 Introduction
Visible cloudiness was the driving force behind the earliest water treatments, as many source waters contained particles that had an objectionable taste and appearance. To clarify water, the Egyptians reportedly used the chemical alum to cause suspended particles to settle out of water. This is called coagulation and thought to have been used as early as 1500 BC, and is still today one of the most key water treatment stages. The performance of physical separation processes such as sedimentation, flotation and filtration is reliant on the characteristics of the particles they are trying to remove. This is typically in terms of the particle size, shape, and density as well as the particle charge, all of which are controlled to some degree by the coagulation and flocculation processes. The process is a simple but very important stage in most water treatment works and its goal of the agglomeration of fine particles and colloids into larger particles is a well established means of removing turbidity, natural organic matter and other soluble organic and inorganic pollutants.
This agglomeration process is referred to by a number of terminologies that depend on either the application or the mechanism of operation. Terms such as coagulation and flocculation are often used with specific meanings, to refer to different aspects of the overall process. For instance, in water treatment coagulation is often taken to mean the process whereby particles in water are destabilised by dosing certain chemical additives (coagulants) and perhaps the rapid formation of small agglomerates. Flocculation is then the process in which destabilised particles and small agglomerates are encouraged to collide with each other (in a stirred or flowthrough reactor) to form agglomerates (flocs). This is shown schematically in Figure 3.1.