chemical precipitations

In water treatment, chemical precipitations (with the exception of those involved in coagulation-floculation) are mainly used to eliminate, by insolubilisation, one or more mineral compounds that adversely affect water utilisation, including: hardness (Ca²⁺ and Mg²⁺), metals, some anions such as SO₄^2–, PO₄^3–, F^–…

Therefore, in every case, the target will consist of injecting into the water, in the form of a soluble reagent, the ion that, combined with the unwanted compound, will form a precipitate that is as insoluble as possible.

For given temperature and pH conditions, residual solubility will be governed by the solubility product (Ks see ionisation) of the precipitating compound. Thus we can adjust the residual value of the unwanted ion by shifting the balance through a reagent overdose (exceeding the stoichiometric amount).

However, this is an expensive practice (high reagent consumption) that leaves an excess of reaction ion in the treated water and this can become a problem in some cases.

In the case of Ca and Mg, it is not only these ions which can become problematic (overconsumption of detergents or taste of water) but mainly calcium carbonate precipitation (scaling). Therefore it is this precipitation potential which is a determining factor (produced from the concentration of Ca and CO₃). Furthermore Ca and Mg tend to be favourable for human health and are only a “problem” when content is high or when used for cleaning purposes. Often the terms commonly used are not suited to what actually occurs when “softening” and “carbonate removal” are the two aspects of calcium carbonate precipitation. However softening using resins or carbonate removal by degassing CO₂ may be performed and are two phenomena remotely-related to chemical precipitation.