Although these two processes are often presented as the same treatment, important differences can be found. In order to explain these differences in a simple and effective way, we will first present the coagulation and flocculation processes.
To begin with, coagulation/flocculation are treatment techniques used to reduce the total suspended solids (TSS) and total dissolved solids (TDS) present in the water. These separation techniques, called physicochemical separation, work by adding a chemical product; a coagulant and a flocculant.
Coagulation
When the coagulant is added, the dissolved elements and colloidal particles in the water agglomerate to form more physically massive particles. In this way, the total dissolved solids decrease considerably as they become suspended solids. Once the coagulation stage is complete, the vast majority of particles invisible to the naked eye (colloid and dissolved matter) become apparent.
To be part of the suspended solids "family", the dissolved solids and colloids must agglomerate to be more massive and be perceived as a floating mass in the water. Therefore, the first step in coagulation/flocculation is the addition of a coagulant to allow the smaller particles to gain mass.
Flocculation
While coagulant attacks dissolved matter, the addition of flocculant will attack suspended matter. In the same agglomeration principle as coagulation, the flocculant will allow the suspended solids to gather together to increase their mass. Flocculation differs from coagulation in that the particles become massive enough to be treated with flotation or sedimentation treatments. In other words, the particles can store oxygen to float and be removed during the flotation process or they can sink and be removed during sedimentation.
Miscellaneous fact
Interesting aside regarding the coagulation process and how it works. Coagulants provide an electrostatic charge to decrease or increase the electrostatic charges of colloidal particles in water. The addition of too much coagulant, whether anionic or cationic, can lead to complete electrostatic reversal of the water mass and return the colloidal particles to their original states. In other words, too much coagulant will have no effect on the total dissolved solids in the water. On the other hand, adding too much flocculant can affect the quality of the water by making it very viscous.