Ferric Sulphate is used as a coagulant over a wide range of pH, typically any pH value between 4.5 and 12. Most raw waters, whether they originate from a reservoir or from a river, will contain pollutants in both colloidal and suspended forms. Colloidal systems tend to be extremely stable and will generally have a particle size in the range 10-9m to 10-7m, whereas suspended solids will generally have a particle size above 10-7m.

The addition of Ferric Sulphate to a colloidal system ensures the introduction of highly charged metal ions, this destabilises the colloidal system and allow the formation of flocs as the hydroxide radicals of iron bond together. Use of Ferric Sulphate offers a significant cost saving.