The main application for ferric chloride today is water treatment. It is mainly used to remove phosphates from wastewater, but it is also used as a coagulant (to flocculate particles) and to remove sulphides (such as hydrogen sulphide) from wastewater. As a coagulant, it is not only used in wastewater treatment, but also in drinking water treatment, as well as industrial and process water treatment. Apart from wastewater treatment, ferric chloride is also used for etching (e.g. of printed circuit boards) and in organic synthesis as a catalyst for the Friedel-Crafts reaction. In the past, it was also used as an astringent (hemostatic agent) and was therefore used in medicine.

The active ingredient iron undergoes various chemical reactions. For example, iron can combine with phosphate ions or sulphide ions. These substances are poorly soluble in water and are therefore precipitated as solids. In addition, a number of iron hydroxides are formed with water. Both the iron ion and the iron hydroxides act as coagulants with their positive charges. This reduces the repulsion of particles in the water and the particles join together to form larger ones, which are then heavier and can settle more easily.

Iron ions and chloride ions disperse rapidly in the water. Iron reacts with phosphate or sulphide or forms hydroxides with the water. Ultimately, a precipitated sludge is formed which can be separated from the water. Chloride ions remain dissolved in the water.

PAC is the acronym for polyaluminum chloride. It is a metal salt solution, similar to ferric chloride. The function and mode of action is therefore also similar to ferric chloride, with a few differences:

PAC is partially neutralized. This means that some chlorides are replaced by hydroxide ions. As a result, aluminium hydroxide chloride ions are formed, which can combine into larger units ("polymerize"). These polymers have a higher charge and therefore a better flocculation effect. The product also reacts less acidically. PAC is therefore used both for phosphate precipitation in waste water and for coagulation/flocculation in waste water, drinking water, industrial water and swimming pool water.
Another application is the paper industry: aluminum has been an important auxiliary material in paper production since the 19th century. In the past, aluminum sulfate ("alum") was mainly used for this purpose, but in the last 20-30 years many processes have been converted to PAC, as this has certain advantages for the modern paper process.