Coagulation

The purpose of coagulation is to promote the removal of particulate impurities, particularly nonsettleable solids and color, from the water being treated. Coagulation is a physical and chemical reaction occurring between the alkalinity of the water and the coagulant added to the water, which results in the formation of insoluble flocculated particles. It is caused by the addition of a chemical (coagulant) to neutralize the charges on the small suspended (colloidal) particles and cause them to clump together into larger particles that will settle out or can be filtered out.

Coagulation is affected by pH, depending on which coagulant is used. The best pH for coagulation usually falls in the range of five to seven. The proper pH range must be maintained because coagulants generally react with the alkalinity in the water. Residual alkalinity in the water serves to buffer the process (prevents pH from changing) and aids in the complete precipitation of the coagulant chemicals. For instance, the complete precipitation of alum will not occur if the alkalinity is too low. Lime or soda ash can be added to increase alkalinity beyond its saturation point. However, too much alkalinity will slow down the removal of organics, which are more easily removed at low pH. 

Coagulants are classified as primary or secondary coagulants (coagulant aids). Primary coagulantscause the particles to become destabilized and clump together. Secondarycoagulants add density and toughness to slow-moving floc particles so they don’t tear apart and appear later in the distribution system.

Primary coagulants are characterized as either metallic salts, such as aluminum sulfate, ferric sulfate and ferric chloride, or as synthetic organic polymers. They can also appear as a blend of metallic salts and polymer blends, such as aluminum chlorohydrate or polyaluminum chloride. These coagulants react with ions in the water and enough coagulant must be added to exceed the solubility of the metals. When solubility is exceeded, the result is floc formation that will then absorb onto the turbidity in the water. Cationic polymers are also used as primary coagulants. These polymers are positively charged and have the ability to adsorb on negatively charged turbidity particles and neutralize the charge. They can also form interparticle bridges that collect the particles in the water.

Secondary coagulants, such as anionicand nonionicpolymers, are used to toughen up the floc particles so they don’t degrade in the treatment process. Anionicpolymers are negatively charged poly-electrolytes that are primarily used as coagulant aids. Nonionic polymers possess a neutral charge and are also used as coagulant aids.

Primary and secondary coagulation is the start of most surface water treatment schemes and needs to be optimized to use the least amount of chemical possible. Jar tests are normally used to determine the proper pH and correct dose of coagulant. If the jar tests or some other sort of process control is not being used and things remain the same, the treatment plant is not being utilized to its full potential. Optimize the coagulation processes in the treatment plant by reducing chemicals, therefore, reducing costs.

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