Texas Commission on Environmental Quality (TCEQ). Class C Water License Exam

Which chemicals are commonly used as coagulants in surface water treatment?

1. Alum and ferric chloride

2. Chlorine and ozone

3. PAC and sodium hypochlorite

4. Activated carbon and lime

The correct answer is: Alum and ferric chloride

Alum and ferric chloride are widely recognized as effective coagulants used in surface water treatment. Coagulation is a crucial process in water treatment, as it helps to remove suspended solids and colloidal particles from water. This is achieved by neutralizing the charges on these particles, allowing them to clump together, or coagulate, into larger aggregates that can be more easily removed through sedimentation or filtration. Alum, which is a form of aluminum sulfate, works by creating a gelatinous mass when it dissolves in water, trapping suspended particles within it. Ferric chloride, on the other hand, provides iron ions that also promote coagulation by similar mechanisms. These coagulants are favored in water treatment facilities because they are effective at a range of pH levels and help improve water clarity and quality. The other listed chemicals, such as chlorine and ozone, are primarily disinfectants used for killing pathogens rather than for coagulation. PAC, which stands for polyaluminum chloride, is sometimes used in conjunction with chemicals like ferric chloride but does not replace the basic coagulant function of alum or ferric chloride. Sodium hypochlorite, while an important disinfectant, does not serve as a coagulant. Similarly,