The second phase, flocculation, must continue for 30 to 45 minutes for best results. In this second phase, the floc particles grow and form into firm, dense settleable particles. The amount of alum required to produce a floc varies with the concentration, size and the type of particles in the water. Generally, the more turbid the water, the greater the amount of alum necessary. The smaller suspended particles are more difficult to coagulate than the coarser particles and require a larger quantity of alum for a given turbidity.
Clear water is usually much harder to treat than muddy water.