Ion-exchange water softening, sometimes referred to as Base-exchange water softening, is a chemical process whereby scale-forming mineral salts naturally present in the water, are replaced with alternative mineral salts which cannot lead to scale formation. Since these pre-existing mineral salts are those responsible for causing hard water, the process of replacing them is known as ‘softening’. Ion exchange water softening is extremely effective at preventing scale for a wide range of applications, and is a straightforward process which can be carried out quietly and efficiently in most buildings.

The mineral salts that cause hard water, i.e. water that can lead to scale formation in water systems, are calcium and magnesium. These are both naturally present in water, and their quantities depend on the location and source of the water. The hardness of water is dependent on how much contact time water has had with calcium and magnesium after it has fallen to the earth as rain. For example, areas close to the east coast with very chalky terrain will produce hard water, because water falls on this land and percolates through the earth, dissolving these hardness forming minerals. The softest water is found in areas such as the Scottish highlands, where rain falls on hard rock strata, and runs off quickly, with little time to dissolve any minerals.

Once dissolved, these minerals form ‘ions’, which are particles possessing either a negative or positive charge. The fundamental component of a Ion exchange water softener is an ‘ion exchange resin’, whose job it is to ‘exchange’ the ions in the water. The resin is pre-charged with sodium ions, which will be released into the water upon capture of the undesirable calcium and magnesium ions. Each ion exchange resin has a finite capacity of water it can soften, before needing to be recharged with sodium ions.

The process of recharging the ion exchange resin with sodium is known as ‘regeneration’. This is effected by passing a high strength brine through the resin bed, forcing the ion exchange resin to release the hardness ions it has captured, and re-adopt fresh sodium ions. Waste water from this process is passed to drain, thus removing the calcium and magnesium ions from the system.

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Ion exchange water softeners come in many shapes and sizes. They generally consist of a pressurised vessel filled with softener ‘resin’, an automated regeneration control valve system  incorporating brine injection and drain flow control device, and a brine storage container. Some are regenerated at regular timed intervals, where others feature a water meter and regenerate when a fixed volume has flowed through them

WATER SOFTENER RESIN

Softener resin is a porous plastic bead made from a lattice type structure of styrene, and where the styrene lattices cross each other are the active exchange sites. The beads come in a variety of sizes, generally around 0.3-1.2 mm in diameter, although other size ranges are available for more specialised uses.

New resin comes pre-regenerated with Sodium (Na) and after a good backwash and rinse to remove manufacturing debris can be used immediately.

The active sites on the resin attract Calcium (Ca) and Magnesium (Mg) ions and release Sodium (Na) ions. The resin has an exchange capacity related to the amount of Brine used during regeneration, after this point the Sodium ions are all used up and no further exchange takes place, so hardness begins to break through to service.

BRINE INJECTOR

Most water softener resins use some version of a venturi device to draw brine into the softener during regeneration. A venturi consists of a specially engineered cone which speeds up the velocity of the water flowing through it, the cone then opens up into a much larger chamber which has a connection to the brine draw line. The sudden change of velocity as the water enters the chamber causes a low pressure (vacuum) and the brine is drawn in to fill the vacuum.

Some specialist water softeners use a pump to inject the brine solution under pressure into the softener vessel.

BRINE LINE FLOW CONTROL

On water softeners fitted with a ‘salt bin’ (as opposed to a bulk saturator), once the regeneration is completed and the brine has been drawn, the salt bin must be refilled with water ready for the next regeneration. The Brine line flow control (BLFC) controls the refill rate.

DRAIN LINE FLOW CONTROL

During regeneration water is directed down the drain. The Drain line flow control (DLFC) controls the rate of flow of water to drain during the Backwash and Fast rinse phases.

SALT BIN

The salt bin (brine bin) is used to hold the salt and brine solution used for regeneration

WATER METER

Some water softener resins are equipped with a water meter used to measure the flow through the softener and initiate regeneration once a pre-set capacity of water has flowed.

Other ion exchange softeners may be fitted with a  simple timer, and regeneration is triggered after a preset time interval.

CONTROL VALVE SYSTEM.

Some water softeners use a multiport valve assembly with moving pistons to control regeneration flows. Others use a bespoke manifold system using large automated valves