Water Treatment Plant

Coagulation is defined as the water treatment process of increasing the tendency of small particles to attach to one another and to attach to surfaces such as the grains of a filter bed. Many surface water supplies contain particles that are too small to settle out of solution on their own. These small particles often carry a small negative charge which make the particles repel each other like similar ends of a magnet. Water system operators must use chemicals in order to neutralize these small charges, help the particles attach to one another, and become heavy enough to settle out of solution.

A coagulant chemical is injected to neutralize these small negative charges and then the water is rapidly mixed. The rapid mixing disperses the coagulant and also increases the interaction of these small particles.

History of Coagulation in Drinking Water Treatment

Coagulation has been an important process in high-rate filtration plants in the United States since the 1880s. Aluminum and iron salts have been used in the coagulation process since the beginning. These salts are still the most commonly used coagulants today. Common coagulants used today include aluminum sulphate (alum), ferric sulphate, ferric chloride, and sodium aluminate. Synthetic organic polymers were introduced in the 1960s. Depending on your system’s water quality, it may be necessary to employ a combination of two or more coagulants.

Aluminum and ferric coagulants put a large number of positive ions in the water. These ions begin to form flocs, which then attract the negatively charged particles in the water. As the particles collide in the mixing area they begin to stick together a form larger and larger flocs. Temperature, pH, alkalinity, and the amount of turbidity in the water control the reactions of aluminum and ferric salts in the water.

Factors Effecting Coagulation Process

The water’s pH has a large impact on the coagulants effectiveness during the destabilization process. The optimal pH range for coagulation is 6 to 7 when using alum, and 5.5 to 6.5 when using iron. A large amount of coagulant may be required to lower the pH into the optimal range for high alkalinity waters. This results in high chemical costs and large amounts of sludge produced in the treatment process. It therefore may be cost effective to add acid in addition to the coagulant to bring the pH down and reduce the amount of coagulant required in this situation.
Water temperature also impacts the coagulation process because it effects the viscosity of water. Both alum and ferric salts form flocs at a slower rate as the water temperature decreases.