Which type of polymer is most suitable as a coagulant and filter aid?

• A. Anionic
• B. Cationic
• C. Nonionic
• D. Amphoteric

Answer: (A)

When a polymer is used in coagulation and filtration, its charge dictates how it interacts with particles and with the coagulant-created flocs. Anionic polymers bring negative charges that can adsorb onto the positively charged hydroxo species formed by the coagulant, then extend to bridge multiple particles together. This bridging creates larger, stronger flocs that settle more quickly and are easier to filter. As a filter aid, these long, negatively charged chains form a network that traps fine particles while maintaining a permeable cake, improving filtration rate and cake stability.

Cationic polymers are strong flocculants but can over-neutralize surfaces or reverse charge in some systems, which can complicate filtration. Nonionic and amphoteric polymers rely less on charge interactions and more on other mechanisms, making their performance less predictable in a coagulation–filtration setup. So, the anionic type is the best fit for serving both coagulation and filtration roles.