The river water is "coagulated."Chemicals are added to the water to cause smaller particles in water to join together . This makes them heavier so that they will settle to the bottom of the basin.
The water mixed to make sure the added chemicals are well blended and react with all of the smaller particles. The particles combine to form "floc" which settle to the bottom of the basin.
The newly joined particles or "floc" settle by gravity and are removed from the bottom of the mixing tanks.