What is the predominating coagulation mechanism for raw water having high turbidity and high alkalinity?

ANS:C - Sweep coagulation

Option (C) is correct.

The coagulation of metallic salts releases hydrogen ions as well as coagulant species. These hydrogen ions neutralize alkalinity. In water with high turbidity and high alkalinity the pH will be relatively unaffected by coagulant addition. Because of the high alkalinity, absorption and charge neutralization will be less effective than in waters of low alkalinity. Higher coagulant dosage should be used sweep coagulation.