2. Coagulation

2.1 Basic principles

The difficulty in separating colloids present in water by simple decantation or filtration stems from the fact that these particles, which are of mineral (clays), plant (humic or fulvic acids, algae), animal (zooplankton), human (fatty suspensions, ERU discharges) or industrial (various wastewaters) origin, are present in water as electrically charged particles (always negatively charged in natural waters). The mutual repulsion exerted by these particles prevents them from agglutinating under the effect of natural forces of attraction (known on this scale as Van der Waals forces) and thus acquiring a sufficient size to become settleable or filterable (figure