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Ion Exchange Water Softening


Introduction

Principles of Ion Exchange to Soften Water
Ion Exchange Unit Components
Plumbing Requirements
Unit Selection
Operation and Maintenance
Costs
Other Considerations

Ion Exchange Process

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Introduction
Ion exchange units that replace calcium and magnesium ions from water are known as water softeners. They may also remove varying amounts of other inorganic pollutants such as metals, but they will not remove organic chemicals, pathogens, particles, or radon gas. Water softener units work most efficiently with particulate-free water.

 

Principles of Ion Exchange to Soften Water
Calcium and magnesium ions are atoms having a positive electrical charge, as do sodium and potassium ions. Ions of the same charge can be exchanged. In the ion exchange process, a granular substance (usually a resin) that is coated with sodium or potassium ions comes into contact with water containing calcium and magnesium ions. Two positively charged sodium or potassium ions are exchanged (released into the water) for every calcium or magnesium ion that is held by the resin. This “exchange or trade” happens because sodium or potassium are loosely held by the resin. In this way, calcium and magnesium ions responsible for hardness are removed from the water, held by the resin, and replaced by sodium or potassium ions in the water. This process makes water “soft.” Eventually, a point is reached when very few sodium or potassium ions remain on the resin, thus no more calcium or magnesium ions can be removed from the incoming water. The resin at this point is said to be “exhausted” or “spent,” and must be “recharged” or “regenerated.”

Ion Exchange Unit Components
A water softener can be as simple as a tank to hold the exchange resin, together with appropriate piping for raw (inlet) and treated (outlet) water. Modern water softeners include a separate tank for the brine solutions used to regenerate the resin, additional valves to back-wash the resin, and switches for automatic operation.

Plumbing Requirements
New homes are plumbed for water softeners. Plumbing old homes for soft water can be very expensive. Not all the water coming into a home needs treatment. If the water is classified as very hard (10.5 or more grains per gallon or 180 or more mg/L of calcium carbonate), a house point-of-entry (POE) treatment may be needed. Otherwise, only the hot water supply should be treated to reduce formation of calcium deposits (scale) in the water heater and pipes.

Personal preference may also influence the decision to treat hot and cold supplies going to the laundry room, showers, and sinks. Toilets and outdoor faucets should not receive softened water. Softened and untreated water may also be “blended” to produce water with a lower hardness and to decrease the amount of water that must be treated.

Unit Selection
The selection of a water softening unit depends on the hardness of the raw water and the amount of water to be softened. There are manual, automatic, semiautomatic, and fully automatic units that differ in the degree of resin regeneration automation. First, the number of fixtures in the home that will require softened water must be determined. Then, all the fixtures' flow rates need to be added up. Note that conventional faucets use 3-5 gallons per minute (gpm) and conventional showers use 5-10 gpm. (Newer, water-saving fixtures may use only half these amounts.)

Operation and Maintenance
Maintenance of water softeners is largely confined to restocking the salt supply for the brine solution. Semiautomatic models require either a manual start of the regeneration cycle or regular service for a fee.

The resin should never wear out. If resins are not regenerated on a regular basis, at the proper intervals, they may become contaminated with slime or impurities and require replacement. Resins can also become clogged with tiny particles of iron if the raw water contains that mineral. Back-washing, that is, reversing the normal flow of water through the treatment unit, may be required to remove the iron. Alternately, special additives may be added to the brine to help minimize this condition.

Costs
The initial cost of water softeners depends on the total hardness of the water, the degree of desired automation, the volume of water to be treated, and other design factors. Retail prices range from approximately $300 for a one-tank system capable of removing 12,000 grains between recharging to more than $1,000 for a two-tank system capable of removing 48,000 grains between recharging.

Operating costs depend on the frequency of resin regeneration. Only salt made specifically for ion exchange units should be used. This salt costs about $3.50 for a 40-pound bag. Electrical costs should be considered as part of operating expense for ion exchange units. Seek units that are energy efficient as expressed by their Energy Efficiency Rating (EER).

An often overlooked environmental cost of water softening systems is that they degrade the quality of reclaimed and gray water by increasing water salinity (TDS). Remember, most of the additional sodium or potassium that is used in water softeners, and the brackish water produced during resin regeneration are discharged into the sewage or gray water systems and, eventually, into the environment.

Other Considerations
-- House and yard plants should not be irrigated with soft water. This is due to its disproportionate ratio of sodium or potassium to calcium and magnesium ions. In general, water with a high sodium/calcium ratio has an adverse affect on soils, and plants are more stressed because soft water has a higher salinity and may lack calcium and magnesium (necessary plant nutrients).
-- Soft water may not be as healthy to drink as hard water for persons that are on a low sodium diet.
-- The taste of soft water may not be as pleasant as hard water.

Go to other treatment methods:
Particle and Microfiltration Filters
Activated Carbon Filters
Reverse Osmosis
Distillation
Disinfection of Drinking Water
Other Treatment Methods


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