The correct answer is: B. 30 to 45 minutes.
Coagulation is a process that is used to remove suspended particles from water. It is a chemical process that involves adding a coagulant to the water, which causes the particles to clump together and form larger particles called flocs. These flocs can then be removed from the water by sedimentation or filtration.
The detention period in coagulation tanks is the amount of time that the water is allowed to stay in the tank before it is discharged. The detention period is important because it allows the coagulant to react with the particles in the water and form flocs. The longer the detention period, the more time the coagulant has to react with the particles, and the larger the flocs will be.
However, the detention period cannot be too long, or the flocs will start to break down and the particles will be released back into the water. The optimum detention period for coagulation tanks is typically 30 to 45 minutes.
Option A is incorrect because the detention period is typically too short. Option C is incorrect because the detention period is typically too long. Option D is incorrect because the detention period is typically not measured in days.