How does a Water Softener work?

Water Softeners work by a process called ion exchange (or base exchange), scale forming minerals – Calcium and Magnesium – are removed on a resin bed and are replaced by Sodium ions. Periodically the ion exchange resin bed is regenerated (cleaned & refreshed) by rinsing with brine (obtained from common salt). Water softeners are usually used to treat cold water, but can be used to treat hot water up to 65oC providing special heat resistant components are used.

Ion Exchange - In detail

The process of ion exchange firstly relies on an ion exchange resin. This is usually in the form of crosslinked polystyrene beads (styrene blended with divinyl benzene during polymerization). Benzene (C6H6) is what is known as an Aromatic Hydrocarbon or Arene, which means one of it's free hydrogen atoms can be replaced by Sulphur during Sulfonation. This involves passing either Suphur Trioxide gas or Sulphuric Acid over the resin beads, after which the resin bed forms a polystyrene sulfonic acid - essentially a poly bead if you like, with fixed sulphur sites attached on the benzene crosslinks. These sulphur sites are the key to the whole ion-exchange process.

When a brine solution is passed over the resin, univalent sodium is drawn to the sulfonate, which remains in place via a weak ionic bond (Na2SO4). When hard water is passed over this resin, now rich in Sodium Sulfate, the calcium and magnesium (which are both higher in the reactivity series than sodium) forces the displacement of the sodium ions, and forms Calcium/Magnesium Sulfate (CaSO4 or MgSO4) instead. Sodium ions are released into the water as a result in a chemically equivalent amount to that of the harness ions removed, and whilst well below the taste threshold in terms of quantity, the addition of a small amount of sodium may be an issue for newborn babies, the elderly and people with certain medical conditions (i.e. particular dialysis patients).

This process continues until the resin has no more sodium ions to displace - At this stage the resin is exhausted, and the ability to remove calcium and magnesium stops. Ion Exchange resins though are unique because they can be easily regenerated back to their original state. This process involves drawing a highly concentrated brine solution back over the resin bed of at least 10% sodium chloride. The concentration of the sodium displaces the calcium and magnesium from the Sulphate, resulting in the resin bed taking up it's original form (Na2SO4), and the free minerals forming Chloride salts in the water (Caclium Chloride - CaCl2 or Magnesium Chloride MgCl2), which are flushed away during this cycle to drain.

This process of Ion Exchange can be repeated hundreds and hundreds of times with the same media - the media itself doesn't have a lifespan per se, and it's common for units in the UK to be in service from 5 all the way up to 20 years. However, chlorine is particularly aggressive at breaking down the skeletal structure of the polystyrene beads, causing oxidation, which manifests itself as the bead blowing (i.e. turning to powder). If, for example, your water company conduct some super-chlorination (i.e. pipe flushing) for a limited amount of time, it is entirely possible this can degrade the entire resin bed in a matter of hours.

For that reason it is heavily recommended that for industrial machines chlorine pre-treatment be installed, and for domestic machines that softeners are switched onto bypass when 'pipe flushing' notices are received from your water board. Also avoid harsh chemicals if and when cleaning brine tanks - any bleach based product high in chlorine could seriously decrease the life of the softener even if only trace amounts make it to the resin bed.

From a practical perspective
A water softener consists of three main parts - a resin chamber (filled with ion exchange resin), a brine storage tank filled with salt (usually in tablet form), and a valve head, which essentially is the brains of the entire operation (it controls the entire process). Under normal operating conditions the valve head diverts incoming water down the riser tube to the bottom of the resin chamber. Here, water rises up through the media - removing calcium and magnesium ions in the process, back up to the top of the tank, where the valve head diverts the water back out to your buildings pipework.

The valve head usually cannot monitor the water hardness - the technology required to do this costs a great deal of money and isn't easily incorporated into such a machine. Instead the softener uses either a timer to regenerate the resin at set intervals (usually early morning when no water is required), or a meter which triggers regeneration based on a set 'volume' of water passing through the valve head.

Once the valve head reaches the regeneration point and it is assumed the media is exhausted the unit starts to regenerate. On most machines this can be set to happen immediately or at a specific time - although with the latter you obviously will vend hard water until that time is reached.

When regenerating four keys steps take place:

Resin bed backwash
Brine Draw
Resin bed Rinse
Bring Tank Refill

When installing a softener it is important to realise that because the initial salt is required to charge the media, you may not get fully softened water straight away. This is because on the initial regeneration the water doesn't have a long enough contact time with the salt to form brine of a high enough concentration. This matter is usually resolved at the next regeneration once the sodium has had sufficient contact time to dissolve.