Traditional metal-based coagulants, such as alum or iron salts, react with alkalinity in the water to form metal hydroxide floc particles which subsequently adsorb onto and agglomerate suspended and colloidal particles. If the coagulant’s alkalinity demand exceeds the available alkalinity, the metal hydroxide cannot completely precipitate, leading to incomplete clarification. In turn, this will reduce filter run lengths by causing increased headloss and early turbidity breakthrough, as well as post-precipitation of the coagulant.

This issue can be resolved by either adding alkali (usually lime) prior to coagulant addition, or by switching to a coagulant having a lower alkalinity demand. Due to their chemical structure, aluminum chlorohydrate based coagulants have significantly lower alkalinity demands than either alum or ferric sulfate, and thus are preferred for treating low alkalinity raw waters.