Water softening
Limescale removal
The number one enemy of any water circuit is limescale: in addition to causing extensive damage to pipes, ceramics and appliances, it also entails an unnecessary waste of energy. Although there are other techniques to counter its formation, ion exchange softening is among the most effective.
Ion-exchange resin softening
Ion exchange
Ion exchange is a water treatment technique that involves an exchange of substances between the water and a specific material that is commonly known as resin.
The chemical entities that are exchanged are ions (chemical species that have an electrical charge). The ion exchange can be oriented towards positive ions (cations) or negative ions (anions). Since calcium and magnesium are positive ions, it will be an exchange of cations and the resins used are known as cationic resins.
Ion exchange resins
Ion-exchange resins are synthetic polymeric materials whose molecular structure has a cross-linked organic skeleton into which functional groups capable of exchanging ions are inserted.
On gross inspection, they appear as a porous granular material.
Ion-exchange resins
Microscopic structure of ionic resins: the black lines represent the carbon skeleton onto which the active ionic sites are attached.
On the left, a cationic resin that exchanges sodium (Na+) and on the right an anionic resin that exchanges chloride (Cl-)
Phases of ion exchange
Ion exchange is a reversible process. This means that the resins can be regenerated.
In the case of calcium and magnesium softening, they are replaced by sodium. Indeed, the latter does not precipitate like carbonate and does not cause issues of limescale build-up.
It is possible to restore the sodium in the resin with a regeneration process in which the resin is brought into contact with a regenerant: in this case sodium chloride.
In this way, resins are guaranteed a long life.
1 - Operating phase
The water passes through a column filled with resin. The flow rate adjustment allows an optimal water-resin contact time for ion exchange to be achieved
2 - Regenerative phase
a) backwash: moves the resin and eliminates possible preferential channels created by the water during exchange
b) regeneration: the regenerant is brought into contact with the resin, and the active exchange sites are restored
c) final wash: the regenerant residues are washed from the resin
Advantages of softening
The hardness of the water is lowered and it is brought to an optimal value that allows for water sufficiently rich in calcium and magnesium (useful for the human body), but that does not cause limescale build-up issues.
- Energy savings due to improved boiler efficiency.
- Extension of the life of household appliances.
- Elimination of white limescale coating on crockery and sanitary wares.
Indeed, limescale deposits leads to damage to pipes, appliances and boilers.
It is estimated that the use of very hard water in boilers causes a decrease in efficiency that leads to an increase in heating costs of 15-20%.
Hard water also discourages the dissolution of soaps and simultaneously attacks the fibres of fabrics: these two effects combined are responsible for damage to clothes.
The hardness is generally expressed in French degrees (°F).
A French degree corresponds to 10 mg/l of calcium carbonate (CaCO3).
The entire salt content is then expressed as if it were all derived from calcium carbonate.
Water hardness classification:
- up to 7 °F: very soft
- 7 °F to 14 °F: soft
- 14 °F to 22 °F: moderately hard
- 22 °F to 32 °F: fairly hard
- 32 °F to 54 °F: hard
- over 54 °F: very hard.
