What Is Water Softening? Hard Water Explained

"Hard water" is water that contains dissolved mineral constituents, especially calcium and magnesium. Hard water does not pose a direct health hazard, but it causes many everyday inconveniences, such as scale deposits on water-using appliances and reduced washing efficiency of soaps and detergents.

The salt content of hard water is usually expressed as hardness, measured in degrees. One degree corresponds to 10 mg of calcium oxide (CaO) per liter of water. Water with a hardness above 8 degrees is generally called hard water; groundwater such as well water and spring water, which contains more dissolved salts, belongs to this category. In hard water, calcium and magnesium salts exist in the form of bicarbonates, carbonates, sulfates, chlorides and nitrates.

Hardness and Its Origin

As water evaporates and falls as rain, it absorbs and dissolves pollutants in the atmosphere; when it lands on the ground, it dissolves surface contaminants; and as it seeps underground or flows into rivers, it continuously dissolves the minerals and chemicals it comes into contact with. In this water cycle, water picks up calcium (Ca) and magnesium (Mg) ions, which create water hardness.

Hardness generally refers to the combined concentration of calcium and magnesium ions in water, expressed in mmol/L. If 1 L of water contains 10 mg of CaO, or substances equivalent to 10 mg of CaO — for example 7.1 mg of MgO — its hardness is said to be 1 degree. Typically, water of 0–4 degrees is called very soft, 4–8 degrees soft, 8–16 degrees moderately hard, 16–30 degrees hard, and above 30 degrees very hard.

Industrial Hazards of Hard Water

Hard water can cause serious damage and even danger in industry. For example, scale accumulation in industrial boilers leads to poor heat transfer and wasted energy, and uneven heat transfer may even cause an explosion.

Temporary Hardness

If water has temporary hardness, boiling decomposes the calcium or magnesium bicarbonates it contains into insoluble calcium carbonate and sparingly soluble magnesium hydroxide precipitates. Once these precipitates are removed, the hardness is reduced and the water is softened.

Methods for Permanent Hardness

1) Ion exchange: specific cation exchange resins replace the calcium and magnesium ions in water with sodium ions. Because sodium salts are highly soluble, scale formation caused by rising temperature is avoided. This is the most common standard method, with stable and accurate results and a mature process; it can reduce hardness to zero. Softening equipment of this type is usually called an ion exchanger (mostly sodium ion exchangers, since sodium-form resins are typically used), a water softener or a softening unit.

2) Membrane separation: nanofiltration (NF) and reverse osmosis (RO) membranes can intercept calcium and magnesium ions, fundamentally reducing hardness. The method is effective and stable, and the treated water has a wide range of uses; however, it requires relatively high feed pressure, and both capital investment and operating costs are high. It is generally not used solely for softening purposes.

3) Lime method: lime is added to the water. It is mainly used to treat large flows of high-hardness water and can only reduce hardness to a certain range.

4) Electromagnetic method: an electric or magnetic field is applied to the water to change the characteristics of the ions, thereby altering the deposition rate and physical properties of calcium (magnesium) carbonate and preventing the formation of hard scale. Its advantages are low investment, easy installation and low operating cost. However, the effect is not sufficiently stable, there is no unified evaluation standard, and because its main function is merely to influence the physical behavior of scale within a certain range, the usable time and distance of the treated water are limited. It is mostly used for commercial circulating cooling water (such as central air conditioning) and cannot be applied to industrial production or boiler feed water (its mechanism has also not been genuinely verified in theory).

5) Chemical dosing: special antiscalant chemicals are added to the water to change how calcium and magnesium ions combine with carbonate ions, so that scale cannot precipitate and deposit. Many antiscalants are available for industrial use today. This method requires a relatively small one-time investment and is widely adaptable, but operating costs are high for large water volumes. Because chemicals are added, the use of the water is greatly restricted — in most cases it cannot be used for drinking, food processing or industrial production — and it is rarely applied in civil settings.

Chemical Softening Processes

The process of removing or reducing calcium and magnesium salts in natural water is called water softening. The main methods are reagent softening and ion exchange.

① Lime softening: quicklime mixed with water to form milk of lime is added to the water to eliminate temporary hardness. The reactions are:

Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃↓ + 2H₂O

Mg(HCO₃)₂ + 2Ca(OH)₂ → Mg(OH)₂↓ + 2CaCO₃↓ + 2H₂O

MgSO₄ + Na₂CO₃ → MgCO₃↓ + Na₂SO₄

CaCl₂ + Na₂CO₃ → CaCO₃↓ + 2NaCl

The milk of lime also precipitates magnesium, iron and other ions from the water and promotes the coagulation of colloidal particles. However, this method cannot completely soften the water; it is suitable for water with high carbonate hardness where thorough softening is not required, and it can also serve as a pretreatment stage for other methods.

② Lime–soda softening: a mixture of milk of lime and soda ash is used as the softening reagent. Soda ash removes the permanent hardness of water, as shown in the reactions above.

③ Combined softening: milk of lime and soda ash serve as the primary softening agents, with a small amount of trisodium phosphate as an auxiliary. Trisodium phosphate reacts with the salts responsible for both temporary and permanent hardness to form insoluble salts that precipitate out.

Ion Exchange in Depth

Ion exchange can be regarded as a special adsorption process. A sodium-form cation exchanger adsorbs various cations from the solution while releasing its own sodium ions into it, thereby softening the water. There are many kinds of exchangers: inorganic ion exchangers have a small exchange capacity, and sulfonated coal, once widely used in industry, is being phased out. Organic high polymers, known as ion exchange resins, are now in common use. An ion exchange resin consists of an organic polymer matrix and exchangeable cations or anions, and is classified as either a cation exchange resin or an anion exchange resin. Cation exchange resins are further divided into sodium-form (R–Na), hydrogen-form (R–H) and ammonium-form (R–NH₄) types according to their exchange groups. Ion exchange is a reversible process: when hard water flows through sodium-form exchange resin, Ca²⁺ and Mg²⁺ ions are exchanged as follows:

2R-Na + Ca²⁺ = CaR₂ + 2Na⁺