Water softening

Under water softening means the elimination or masking of the water dissolved alkaline earth - cations Ca ²⁺ and Mg ²⁺ which the washing effect of detergents through the formation of calcium soaps reduce and disruptive scale deposits can lead in piping and apparatus. Of hard water is soft water generated. Softened water must not be confused with distilled or demineralized / fully demineralized water (DI water).

In addition to full softening, water is only partially softened. It is often sufficient if only the carbonate hardness is reduced or largely removed. This type of treatment is called decarbonisation .

Procedure

Cation exchange

Softening of the raw water by exchanging the alkaline earth ions for sodium ions.

A frequently used process is softening with cation exchange resin . The water flows through a container that contains a cation exchange resin. In this mainly the Ca ²⁺ and Mg ²⁺ ions are exchanged for an equivalent amount of Na ⁺ ions. The softened water now contains correspondingly more Na ⁺ and almost no Ca ²⁺ and Mg ²⁺ ions. All other ions remain in the water.

The chemical equation for softening the water for the absorption of calcium ions is:

{\ displaystyle \ mathrm {2 \ R-SO_ {3} Na \ + \ Ca _ {(aq)} ^ {2 +} \ \ longrightarrow \ (R-SO_ {3}) _ {2} Ca \ + \ 2 \ Na _ {(aq)} ^ {+}}}

With R − SO ₃ X: basic structure of a sulfonated resin.

The regeneration of an exhausted cation exchange resin is performed with a 8-12% aqueous sodium chloride solution ( regenerating ). During regeneration, the hardness (calcium and magnesium ions) absorbed by the ion exchange resin is exchanged for the sodium ions in the salt solution and, for example, the calcium is replaced again by sodium. The equation for this is:

{\ displaystyle \ mathrm {(R-SO_ {3}) _ {2} Ca \ + \ 2 \ Na _ {(aq)} ^ {+} \ \ longrightarrow \ 2 \ (R-SO_ {3}) Na \ + \ Ca _ {(aq)} ^ {2+}}}

The regeneration solution is rinsed with raw water from the exchanger tank and discarded. Dishwashers are equipped with such cation exchangers. The salt to be filled into the dishwasher is used to regenerate the built-in cation exchange resin. In the case of water softeners , regeneration often takes place automatically, at fixed time intervals, depending on the amount of water used or the quality of the treated water, which is described with the aid of the hardness liter . Since no softened water is available during regeneration in a single system (one tank), so-called double systems must be provided for the continuous supply of softened water. A double system consists of two separate tanks with cation exchange resin. These are operated in parallel. One of the containers supplies softened water, while the second can be regenerated.

Chemicals for softening water

Water softening substances are added to many household and industrial preparations. They enter into strong interactions with the alkaline earth cations, which are then no longer available for disruptive reactions. The alkaline earth cations are not removed from the water, but "masked". The water now shows the behavior of soft water within certain limits.

Compounds that exchange cations:

Zeolites , more precisely zeolite A , are found in most laundry detergents today .
Layered silicates are used as an alternative to zeolites.

Compounds that act through complex formation :

Triphosphates used to be used in detergents, but they lead to over-fertilization of water.
EDTA and NTA , both are also used in laundry detergents.
The anion of citric acid , citrate , can partly be used as a substitute for phosphates.
Short-chain polyacrylic acids are often used as a substitute for phosphates e.g. B. used in detergents.

Physical water softening

This term includes processes that use electric or magnetic fields to change the structure of the lime crystals so that deposits on pipes are reduced. Here z. B. magnets or current-carrying wires attached to the outside of water pipes or alternating voltage-controlled plates inside the device through which the water flows, placed directly in the path of the water. Some devices are still equipped with a “pre-filter”, which is actually a small ion exchanger .

In January 2000 the Stiftung Warentest published a study of such water treatment devices. In most cases no effect could be determined; Only the three devices with the filter cartridge that had to be changed regularly showed a significant reduction in limescale deposits in electric boilers. The result of an earlier test by Stiftung Warentest was thus confirmed.

A plausible hypothesis for the effectiveness of such procedures does not exist, and published “scientific” experiments on effectiveness cannot be reproduced. This includes a study commissioned by the Swiss Federal Office for the Environment, Forests and Landscape as part of a dissertation.

Further procedures

Removal of the temporary hardness (carbonate hardness) by boiling the water. This forms scale . The permanent water hardness remains in the water . This process is described in more detail under water hardness .

Precipitation by adding a suspension of calcium hydroxide (milk of lime); the calcium carbonate formed is precipitated as sludge during long-term decarbonisation and as so-called hard grain during rapid decarbonisation . The procedure only leads to the removal of the carbonate hardness.

One of the oldest processes for extensive softening is the lime - soda process. With this process, which was developed in the 19th century, the total hardness can be reduced to less than 2.0 ° dH (approx. 1.1 ° fH). Calcium and magnesium are preferentially precipitated as dolomite . The water is heated to accelerate the precipitation reaction. However, adding soda increases the sodium content of the water. This process was important for the production of low-hardness water for the operation of steam engines and steam locomotives. With the development and introduction of ion exchangers for softening, the process lost its importance and is no longer used today.

Internal feed water treatment e.g. B. in steam locomotives .

Web links

Christoph Caspari: Magnetic water softening. Web article with critical position on magnetic water softening

Individual evidence

↑ Stiftung Warentest: Physical water treatment - a punch in the water. January 2000 edition, pp. 59–63.

↑ Kalkkiller without effect , magazine test , 8/1985.

↑ Burkhard Wricke, Wolfgang Baumgardt ( DVGW -Technology Center Water): Study "Drinking Water Purifier " ( Memento from March 4, 2016 in the Internet Archive ). January 2003.

↑ Federal Office for the Environment FOEN: Electromagnetic water treatment. Retrieved October 3, 2019 .

↑ Regula Müller: Influence of electromagnetic fields on crystallization processes: practical applications in the sludge treatment of sewage treatment plants and in drinking water systems . Dissertation No. 12644 at the ETH Zurich 1998, doi: 10.3929 / ethz-a-001987748 .

↑ Watergadget: How does a water softener work? Guide to water softening

[1] Stiftung Warentest: Physical water treatment - a punch in the water. January 2000 edition, pp. 59–63.

[2] Kalkkiller without effect , magazine test , 8/1985.