What is the necessity of following coagulation theory in the water purification of turbid water?

Dear friends, we are lifting water from open surface for our drinking water purpose. During the rainy season, the turbidity levels of water often exceeds 55 NTU's. Experts says the reason could be the soil erosion. We are suggested to opt for several purification treatment methods by them. I want to know which is the best method to treat the turbid water? Also I want to know what is the necessity of following coagulation theory in the water purification of turbid water and how does it helps in removing the turbidity? Your views are very much appreciated. Best.