The modern compact water softener is a domestic appliance which contains a material, called ion exchange resin, to remove hardness minerals from the hard water supply. The removal of the hardness minerals eliminates scale in the hot water system, showers and on all sanitary fittings and prevents scum forming when using soap in baths, sinks and with laundry Although the basic principle has been used for many years, the design of the equipment, the materials and the sophistication of the control mechanisms have developed enormously.

A softener consists of three basic components: the resin tank (which contains the ion exchange resin), the brine tank (which contains salt which is dissolved to form brine) and the control head. As its name implies the control head contains the mechanism which determines when regeneration is to take place and which controls the operation of the valves during regeneration. In modern compact softeners the resin tank is usually located inside the brine tank. It is the brine tank which is the external 'cabinet' of the softener and so determines the size of the softener and although the dimensions may vary many can conveniently be installed under the kitchen sink. If this is not possible the design of the cabinet (brine tank) makes them acceptable as free standing equipment wherever it is most suitable for them to be installed.

There are basically two types of softener: single tank and twin tank. However, there are also increasingly sophisticated control systems which correspondingly provide increased economy of salt and water usage.

Single Tank

The single tank is the most common, and employs one single tank of resin. When the resin is regenerated the tank is taken out of service and a bypass valve opened. Thus only hard water is available while the regeneration takes place. The regeneration normally takes about 30 to 60 minutes and, in order to minimise the likelihood of hard water being used, the regeneration is programmed to occur during the night.

The simplest unit operates on a time control basis. Depending upon the hardness of the water supply, the capacity of the softener and the number of people in the residence, the number of days that the unit will operate is calculated. The softener is set when it is installed, to regenerate at, say, 2.00 a.m. at this calculated frequency. To ensure that soft water is always available, the interval between regenerations will be underestimated rather than overestimated.

The timer controlled softener will regenerate at the pre-set number of days regardless of the volume of water used. If water usage is low, because, for instance, one or more of the residents is away, there will be significant residual capacity at the time of regeneration, which is arguably wasteful of salt and regeneration water. Similarly, if there is an increase in usage of water, the capacity could be exceeded at the pre-set number of days and the water would go hard towards the end of the cycle. It is also advisable that the unit is switched off if no water is to be used for a period of time e.g. holidays. A timer is the simplest form of control and is generally the cheapest.

A volume-controlled softener will regenerate when a pre-set volume of water has been used (softened), regardless of the number of days this takes. This is to avoid under - or over - regenerating the softener when demand varies.

Some control systems use a microprocessor to monitor daily water usage and predict daily variations - such as washday - and ensure that regeneration takes place on the optimum night, this means that softened water is always available and also saves on salt and water used during regeneration.

With these control systems, there is often still some unused capacity when the regeneration is initiated. A further degree of sophistication can be incorporated using a technique called "proportional brining". This measures the actual quantity of water used before each regeneration and controls the amount of brine used for that regeneration so that it is just sufficient to restore the resin capacity, this results in the most efficient use of salt and water.

Twin Tank

A twin tank system uses two resin tanks rather than one. Usually, one is in service and the other on standby. The flow through the softener is monitored so that, when the "on-line" tank is exhausted, the control system swaps flow to the other tank and regenerates the one taken out of service. In this way, soft water is available even during regeneration, so the system continues to swap from one tank to the other, regenerating as and when necessary at any time of the day. Because the system only regenerates on volume, the total capacity of the resin is fully used for each service cycle giving optimum salt and water usage efficiencies.

Non-electric Softeners

Most units use electrically operated valves and timer or microprocessor. However, some manufacturers eliminate the need for an electric power supply by using a non-electric control head. They use water flow to monitor the volume of water used and use water pressure and flow to operate the valves in the control head.

Review

As with other kitchen white goods there is a range of water softeners from which to select the most suitable model. They all use the same defined and proven ion-exchange water softening process but with differing degrees of sophistication in operational and control technology, there is also a range of sizes and cabinet designs. Water softeners will operate satisfactorily for many years with minimal maintenance, a 10 to 20 year lifetime is not uncommon, and they can be moved from house to house. Many of our water softeners are now 24 years old ! and still working perfectly.