Waterguide - Chloride

What is chloride?

Chloride is the short name for the chlorine anion (Cl-). Chlorine (symbol Cl, atomic number 17, atomic mass 35.5 g/mol) is in the 7th main group and belongs to the halogen elements. As chlorine reacts easily, it combines easily with other elements, especially metals such as sodium, potassium and magnesium.

In what form is chloride to be found in the Earth's crust?

The amount of chloride in the solid crust of the Earth is 130 g/t. That is a relatively small amount compared to the large amounts of sodium and potassium in the Earth’s crust. The most common compounds that contain chloride are with the aforementioned metals: NaCl, KCl and MgCl2. As chloride dissolves very easily in water, some waters may exhibit large concentrations of it, depending on the source of the water.

Seawater is where you will most commonly find chlorides, as well as in the largest quantities. Chloride is also what gives seawater its strong salty taste.

Chloride in water

In communal waste water, the concentration of chloride is usually higher than in unpolluted waters, as chloride enters waste water through the metabolism of humans. The chloride that is produced in the regeneration of softening units in industry and trade and from domestic dishwashers also contributes to the higher concentration of chloride in waste water.

Salt and potassium mining is another major source from which chloride can enter waters. Chloride in salt mine waste stored outside can get washed away by rain and seep into surface water.

Precipitation from ocean spindrift also contains a chloride concentration of around 2 mg/l. The concentration of chloride in seawater is 19 g/l, which makes chloride the predominant anion in it.

Groundwater generally contains only around 100 mg/l of chloride, but groundwater from chloride-containing geological strata (Zechstein and salt domes) and sedimentary rock (rich with sodium chloride) sometimes have a very high concentration of chloride.

To a lesser extent, chloride sometimes makes its way into groundwater through the use of fertilisers, liquid manure and slurry on agricultural land. Near roads, towns and cities, chloride can enter groundwater from road salt.

Chloride in water treatment

Chloride is highly soluble, which is why it does not cause membrane scaling in reverse osmosis. Chloride is the anion that is normally used to even out the ionic balance of the feedwater.

The World Health Organisation (WHO) specifies that the limit for chloride in drinking water should be 250 mg/l. If the concentration of chloride is any higher than this, it becomes noticeable, as the water acquires a salty taste. If necessary, reverse osmosis can be used to reduce the concentration of chloride in the drinking water treatment process (possibly by treating just part of the current).