Waterguide - Softening

How does water softening work?

In the softening unit, calcium ions (Ca2+) and magnesium ions (Mg2+), which are hardeners present in natural water, are exchanged for sodium ions (Na+). Ion exchangers are used for this process. The ion exchangers consist of spherical synthetic pellets based on polystyrene. They are also commonly referred to as exchange resins. An ion exchanger has a limited capacity to absorb hardeners from raw water. Once a certain volume of water has flowed through, the ion exchanger is exhausted and has to be regenerated. The higher the water hardness, the more frequently an ion exchanger has to be regenerated. The regeneration process is carried out using dissolved sodium chloride (NaCl).

What are the steps involved in water softening?

The following stages make up a complete cycle of a softening unit:

  • Operation
  • Backwashing
  • Salting / slow washing
  • Fast washing

What happens during the operation stage of a softening unit?

During the operation stage, the water is fed in at the upper nozzle and continuously conveyed over the resin bed from top to bottom until the resin is no longer able to exchange the hardeners for sodium ions. In this state, the resin is referred to as “exhausted”. The resin becomes loaded with hardness components, with the resin depletion starting at the top and expanding downwards through the resin bed. A water meter immediately detects when the resin is exhausted, as it records how much soft water is produced..

What does “backwashing” mean in a softening unit?

During the backwashing stage, the resin bed is loosened by water flowing upwards from the bottom to the top, and resin debris and suspended solids are carried away through the upper nozzle. This process lasts around five to ten minutes.

How is the softening unit “salted”?

In the “salting” stage, brine is poured over the resin bed from top to bottom. The grit in the area of the lower nozzle helps to evenly distribute the flow of brine throughout the entire tank cross-section. The hardeners on the resin are, for the most part, once again exchanged for sodium ions. A solution consisting of unused NaCl and a combination of CaCl2 and MgCl2 produced by the ion exchange process leaves the filter through to the channel connection.

What does the “slow washing” stage entail?

During this stage, the motive water from the water jet injector continues to flow over the resin bed once the brine has been extracted. The solution consisting of the above-mentioned salts that is still present in the resin bed is forced out of the exchange tank.

As the salting and slow washing processes proceed without the central control valve changing its position, the two steps are referred to as the “salting/slow washing” stage.

This phase lasts around 45 to 55 minutes.

 

What defines “fast washing” during the softening process?

In the final regeneration phase, the “fast washing” stage, the water is conveyed over the resin bed from top to bottom like in the operation stage. The resin bed once again has its original packing density and the solution residue in the resin grain is rinsed away. This means the softening unit is ready for operation again. The process lasts around 15 to 25 minutes, after which the filter is ready for operation.

The figure shows the operation and regeneration of the softening process.

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