Water softening

Notes about principles and plants

The scale

The water available from well and other sources usually contains many dissolved substances of several origin and type.
This bulletin specially concerns the presence in water of calcium and magnesium bicarbonates.

Their presence in water can cause serious problems, according to their concentration and the utilization of water.
The quantity of calcium and magnesium bicarbonates (soluble substances) are chemically balanced in water with calcium and magnesium carbonates and with carbon dioxide.
An increasing of the temperature of the water causes the developing of part of the carbon dioxide and the subsequent breaking of the previous balance. The carbon dioxide is a gas, hence the higher is the temperature of water, the lower is its solubility.
The subsequent chemical reaction produces other carbon dioxide in order to rebuilt the balance again, this way the calcium and magnesium bicarbonates are transformed in calcium and magnesium carbonates, which are less soluble substances.
The deposits of calcium and magnesium form the so called “scale”.

Carbonic balance in water

Ca(HCO3)2 CaCO3 + CO2 + H2O
calcium bicarbonate
calcium carbonate
(less soluble)
carbon dioxide
(gas not very soluble)

It is interesting to note that the breaking of the balance features already at a not very high temperature; for example, whether the temperature of water is 10-15°C (50-59° F), an increasing of temperature up to 30-35° (86-95°F) can already start the reaction.

The salts of calcium and magnesium dissolved in water is the “hardness” of water.
The hardness of water is usually measured as ppm (part per million) of calcium carbonate; it is also used the French degree (°Fr), where 1 °Fr = 10 ppm of calcium carbonate.
There are many processes to remove calcium and magnesium off the water; this bulletin concerns the most diffused of them, the softening by ion exchange resins.

Ion exchange resins for softening

The resins are organic substances, industrially prepared. Their special characteristic is that they have a “mobile” functional group, it means a part chemically balanced between the resin itself and the ions dissolved in the water.

Ion balance of the resins

R-Na + Ca R-Ca + Na
resin in sodium form
calcium resin in calcium form

When water containing calcium and magnesium flow across the resins in sodium form, they are exchanged with sodium which is released in water. This is the purpose of the softening process: the calcium and magnesium (their carbonates cause scale) are replaced with sodium, where the sodium carbonate is perfectly soluble.
Naturally, sooner or later, the resins will not be able to replace calcium and magnesium, since there will not be any more (or a very little quantity) ions of sodium available for exchange.

These resins, without any sodium ions, is exhausted, and it is required to re-built the original sodium form to start the exchange process of softening again. It is required a regeneration of the resins.
The regeneration simply means that some water, with a very high concentration of sodium ions, flows through the resins and exchanges back sodium against calcium and magnesium, released in water.
The regeneration water is a solution of sodium chloride (common kitchen salt); it flows through the resins and is carried to drain with addiction of calcium and magnesium released from resins.

The softeners

The softeners are the equipment that allow to make the softening of the water by ion exchange process.
Simply the softener can also be a housing filled with resins; the water simply flows through the resins and is softened.
But the softener must also allow to make the regeneration of the resins; hence there are several types of softeners, different not only for the dimensions but for the way the regeneration is featured.

The most simple softener has an opening to introduce the salt directly above the resins, after that water will flow through the resins to slowly dissolve the salt. This is the most simple softener and basically the features are still always the same; now a concentrated solution of salt (brine) is drawn inside the softener by an injector, therefore using the hydraulic energy of the water itself to make the suction of brine. This way allows not to open the housing of the resins.

Now, most of the softeners are fully automatic, with several and different modes to run the regeneration:

  • by time schedule, allowing to set how often and the time of day of the regeneration
  • by volume, where the regeneration starts after that a pre-set quantity of water has been supplied
  • a combination of both, (volume schedule but only at a pre-set time of day)

Besides, dual systems softeners are also commonly used; they allow the continuous feeding of treated water, 24 hours a day, no stop. There is always a column on service, while the other one is on regeneration or stand-by. The regeneration (exchange of working between the columns) is usually set by volume schedule.
Obviously, the correct selection of the automatic mode of a softener depends on the application of the softener itself.

Utilization of soft water

“Hard” water, the water containing high quantity of calcium and magnesium, causes many problems.
The most common of them surely is the scale deposits along the surface of boilers, heater exchangers, laundry machines, etc. The scale inhibits the correct thermal exchange, since it is a very good thermal insulator (the thermal conductivity of the scale is approx 1/100 of the one of the iron).
The scale makes also difficult the flowing of water along the pipes, since it reduces the size of them, can cause the block of valves, mixers, can make the corrosion easier. Besides, hardness of water can have a reaction with chemicals in many chemical industrial processes.

It must also be noted that utilization of hard water for washing makes rough and coarse the clothing and the hair, makes dry the skin because the scale can clog the pores, makes different the taste of meals and drinks. Besides, it is required to use larger quantity of soaps and detergents for washing and laundry machines.

For this last point, it can be underlined that the utilization of soft water allows a sensible reduction in the quantity of detergent used and therefore a shorter quantity of waste, as a sure advantage for environmental protection.

Wherever the hard water can cause problems, the utilization of soft water can be recommended.
As residential utilization, drinking water can also be softened, with “hygienical” advantages (taste of meals and drinks, smoothness of the cloths), but also to protect the household plants (heating system, laundries, etc).

Local laws in many countries have special regulations or restrictions concerning the softening of drinking water, but is generally allowed everywhere.

It is obvious that the softening of water must not be confused as “purification” of water, as sometimes happens. The softening of water does not change any characteristics of water, included the total salinity, except the contents of calcium and magnesium, which is replaced by sodium.