Ademars is the best ...

Ademars is the best explanation. Most contaminants in natural waters are negatively charged - bacteria viruses and sediment. Coagulation occurs when an appropriate amount of a positively charged chemical or material such as aluminium ions (often in the form of alum) or cationic polyelectrolyte or ferric salts are added to neutralise the negatively charged contaminants, this reaction occurs rapidly. Flocculation follows and as said below is the agglomeration of the now neutralised particles. Flocculation may be enhanced by gentle mixing and sometimes the addition of a polyelectrolyte. The objective is to increase the particle size so that it can be filtered directly or be able to settle quickly.