Which compound is not typically used as a coagulant in water treatment?

Which compound is not typically used as a coagulant in water treatment?

• A. Sodium chloride
• B. Sodium aluminate
• C. Ferrous sulfate
• D. Aluminum sulfate

Answer: (A)

Coagulation in water treatment relies on positively charged species that neutralize the negative surface charges of tiny particles so they can come together into larger flocs and settle out. The common coagulants are metal salts that hydrolyze to form positively charged aluminum or iron hydroxide species, which destabilize colloids and help them aggregate. For example, aluminum sulfate (alum) and sodium aluminate generate aluminum-based hydrolysis products that act as coagulants, while ferrous sulfate or ferric salts supply iron-based flocs that aid aggregation. Sodium chloride is a neutral salt of sodium and chloride ions. It does not carry the positive charge needed to neutralize particle surfaces, and it does not form the kinds of hydroxide precipitates that bind and grow flocs. As a result, it does not function as a coagulant and can even increase salinity without improving clarification. That’s why sodium chloride is not typically used as a coagulant, whereas the other compounds listed are used to promote coagulation and help remove suspended material.

Coagulation in water treatment relies on positively charged species that neutralize the negative surface charges of tiny particles so they can come together into larger flocs and settle out. The common coagulants are metal salts that hydrolyze to form positively charged aluminum or iron hydroxide species, which destabilize colloids and help them aggregate. For example, aluminum sulfate (alum) and sodium aluminate generate aluminum-based hydrolysis products that act as coagulants, while ferrous sulfate or ferric salts supply iron-based flocs that aid aggregation.

Sodium chloride is a neutral salt of sodium and chloride ions. It does not carry the positive charge needed to neutralize particle surfaces, and it does not form the kinds of hydroxide precipitates that bind and grow flocs. As a result, it does not function as a coagulant and can even increase salinity without improving clarification. That’s why sodium chloride is not typically used as a coagulant, whereas the other compounds listed are used to promote coagulation and help remove suspended material.