Colloids are agglomerates of atoms or molecules whose sizes are so small that gravity has no effect on settling them but, instead, they stay in suspension. Because they stay in suspension, they are said to be stable. The reason for this stability is the mutual repulsion between colloid particles. They may, however, be destabilized by application of chemicals. Coagulation is the unit process of applying these chemicals for the purpose of destabilizing the mutual repulsion of the particles, thus causing the particles to bind together. This process is normally applied in conjunction with the unit operation of flocculation. The colloid particles are the cause of the turbidity and color that make waters objectionable, thus, should, at least, be partially removed.

This chapter applies the techniques of the unit process of coagulation to the treatment of water and wastewater for the removal of colloids that cause turbidity and color. It also discusses prerequisite topics necessary for the understanding of coagulation such as the behavior of colloids, zeta potential, and colloid stability. It then treats the coagulation process, in general, and the unit process of the use of alum and the iron salts, in particular. It also discusses chemical requirements and sludge production.

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