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Softening Filtration is a water treatment process that employs a pressure filter vessel, or vessels, utilizing a single cationic ion exchange (IX) resin media. It is commonly used in commercial, industrial filtration and water treatment plants for removing hardness (calcium and magnesium).

Softening filters are typically designed to service flow at 4-10 gallons per minute (gpm) per square foot (ft2) of filter surface area.

The system of water softening requires incoming water to travel through a porous ion exchange (IX) media resin bed. This resin has the appearance and consistency of tiny plastic beads. These fine beads are constructed and treated so that each bead is exceptionally porous. The surface area is permanently chemically altered to be highly attractive to the offending minerals.

The surface sites of the resin have an affinity for minerals that have an electron charge (valence) of positive two and higher, such as calcium (Ca+2) and magnesium (Mg+2). Other minerals with a similar valence may also be removed. A complete analysis of the incoming water is essential to the proper sizing and operation of a water softening system.

Incoming water enters the water softener vessel that is filled with resin beads. The velocity of the water slows, spreading over the wider surface area of the resin bed, and travels through the millions of tiny beads. During this process, the minerals in the water are attracted to the resin bead surface areas. Water then exits the resin bed – freed of the laden minerals with little significant hydraulic head pressure drop.

However, as the surface area of each bed in the resin comes occupied by minerals, the effectiveness of the water softener gradually declines. A complete water softening system must include equipment to regenerate these resin beds. Usually, for industrial and municipal applications, there is a duplicate resin bed that can be engaged, so the initial resin bed has time to refresh. After water is diverted to the second bed, the regeneration of the first bed is commenced.

The alternative is to shut off the outflowing water during the regeneration process. This may be possible if the softened water demand is limited.

The resin has a much higher affinity for Ca+2 and Mg+2 ions but it will “give up” those when rinsed with water containing a very high concentration of sodium (Na+1) ions (i.e., very “salty” water) and the Na+1 ions replace Ca+2 and Mg+2 ions on the resin. Finally, the resin bed is flushed with water to remove excess salt before the bed is placed back into service (i.e., normal flow).

Softening filtration can have graded support media layers or the resin be directly installed over or on a distribution plate. The softening media is stored in the filter vessel with the bottom of the filter containing a porous collector or distributor.


Conventional gravity and pressurized softening filters operate in the downflow direction. The filter media is usually a 30-40 inches deep bed of ion exchange resin or sufficient resin to allow for 50% expansion during backwash.

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