Chemistry » Equilibria of Other Reaction Classes » Precipitation and Dissolution

# The Role of Precipitation in Wastewater Treatment

## The Role of Precipitation in Wastewater Treatment

Solubility equilibria are useful tools in the treatment of wastewater carried out in facilities that may treat the municipal water in your city or town (see the figure below). Specifically, selective precipitation is used to remove contaminants from wastewater before it is released back into natural bodies of water. For example, phosphate ions $${\text{(PO}}_{4}{}^{\text{3−}})$$ are often present in the water discharged from manufacturing facilities. An abundance of phosphate causes excess algae to grow, which impacts the amount of oxygen available for marine life as well as making water unsuitable for human consumption.

Wastewater treatment facilities, such as this one, remove contaminants from wastewater before the water is released back into the natural environment. (credit: “eutrophication&hypoxia”/Wikimedia Commons)

One common way to remove phosphates from water is by the addition of calcium hydroxide, known as lime, Ca(OH)2. As the water is made more basic, the calcium ions react with phosphate ions to produce hydroxylapatite, Ca5(PO4)3OH, which then precipitates out of the solution:

$$5{\text{Ca}}^{\text{2+}}+{\text{3PO}}_{4}{}^{\text{3−}}+{\text{OH}}^{\text{−}}⇌{\text{Ca}}_{5}{{\text{(PO}}_{4})}_{\text{3}}\text{·}\text{OH}(s)$$

The precipitate is then removed by filtration and the water is brought back to a neutral pH by the addition of CO2 in a recarbonation process. Other chemicals can also be used for the removal of phosphates by precipitation, including iron(III) chloride and aluminum sulfate.

View this site for more information on how phosphorus is removed from wastewater.

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