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Ion Exchange

Ion Exchange units are usually installed as Point of Entry (POE) systems. They are generally placed at the entrance of the water system into the house to service all or most incoming water.

The most common example of an Ion exchange unit is a water softener, which are primarily used to remove calcium, magnesium and low quantities of barium, cadmium, copper, iron, manganese, radium, zinc, and other metallic, positively-charged ions.  Other specialized Ion exchange units also remove iron, nitrates, sulfates, and various toxic metals from water. Understanding how ion exchange works and what contaminants different units will remove can help you make informed decisions about water treatment by ion exchange.

Ions are atoms, small particles that are the building blocks for molecules. Ions have a weak electrical charge.  These positive and negative charges act similarly to the north or south pole of a magnet.  The electrical charge on an ion can be either positive (+) or negative (-). Valence is the term that describes the category of the electrical charge on a dissolved ion such as positive 2 or positive 3. If the contaminant has a positive charge, it would be called a cation, and would be removed by use of an Ion Exchange media called a cation exchange resin. If the contaminant has a negative charge, it would be called an anion, and the appropriate treatment media would be called an anion exchange resin. Ions with a valence of =>2 are removed with a IE water softener.

During the recharge cycle a concentrated brine solution loads positively charged sodium ions onto the cation exchange resins which are usually made of porous beads manufactured from polystyrene.  Negatively charged chloride or hydroxide ions coat the anion exchange resins. 

As water passes over the resins they exchange ions (sodium and chloride) which have weaker bonds to the resin with ions from within the water which have stronger bonds.  When water to be treated passes through the ion exchange unit, ions in the water are attracted by either a positive or a negative charge to the ions in the resin bed.   Since the ions from within the water are usually attracted more strongly to the resins than they were held in the water, they are in effect removed from the water in the exchange process. 

In this way the calcium and magnesium ions responsible for hardness are removed from the water and attached to the resin, and the sodium ions from the resin are added to the water. 

Eventually a point is reached when few sodium and chloride ions remain on the resin and no further calcium or magnesium is removed from the water. At his point the unit needs to be recharged or backwashed with the brine solution.  Because the brine solution is so concentrated, it is able to strip the accumulated ions picked up from the water.  

Water Softeners based on Ion Exchange Units are effective at reducing: 

  • calcium, magnesium
  •  (some) inorganic iron, nitrate

Categorizing Hardness:

Suffice to say that there are several categorizations of hardness.  The US Department of the Interior has produced a simple, easy to read chart:

Considerations:

Maintenance of cation exchange units is largely confined to restocking the salt supply for the brine solution. With manual and semiautomatic models, the consumer will also have to start the regeneration cycle. Typically 6 - 7 pounds of salt (sodium chloride) per cubic foot of IE resin are used during the regeneration cycle. Salt is cheap,  heavy, and needs to be filled into a container usually about 30-40 " high. Monthly operating expenses range from $3 to $13 if sodium is used and from $5 to $24 if potassium is used.  Consider maintenance costs.

If not regenerated on a regular basis the resin bed may become contaminated with slime or impurities  and become unusable.  If this happens, the resin must be replaced with new material. Consider maintenance

If the water is cloudy or otherwise discolored, it is likely that some, or many, of the contaminants are in a solid form. Solid particles are not removed by Ion Exchange and can clog the treatment media. Consider testing and mechanical filtration to remove suspended contaminants.

Most units are not recommended for concentration of iron above 3 ppm (ratings apply to ferrous iron ONLY) as resins can become clogged with iron if the raw water contains a high concentration of dissolved iron. Backwashing or reversing the normal flow of water through the treatment unit may be required to remove the iron. If Iron is a problem special additives can be included in the brine regenerating solution to help minimize this condition. Consider your water test results and maintenance costs.

High concentrations of dissolved hydrogen sulphide also can foul the resin bed of an ion exchange water softener. When a hydrogen sulphide odor occurs in treated water (softened or filtered) and no hydrogen sulphide is detected in the non-treated water, it usually indicates the presence of some form of sulfate-reducing bacteria in the system. Water softeners provide a convenient environment for these bacteria to grow. A "salt-loving" bacteria, that uses sulfates as an energy source, may produce a black slime inside water softeners.

While there is controversy concerning disposal of the waste brines into septic systems, however,  no credible proof of problems with softener disposal into septic systems has been published that we are aware of.  Studies by the Water Quality Association (CWQA) indicate that waste brine and purged contaminants do not injure leach fields or septic tanks when hardness levels are 10 or below.. 

Softening hard water can reduce the quantity of cleaning products needed by as much as 50 percent. The life of the water heater, plumbing system and water-using appliances may be extended.  Of additional benefit are the better results in laundry, dishwashing, and personal grooming. Consider other indirect savings

As mentioned above softeners need to be recharged below are some of the differences between some of the automatic and semi-automatic controls available:

  • Water softeners with CLOCK-TYPE (TIMER) controls regenerate according to a fixed time schedule (every 3 days, or 6 days, etc.) regardless of the amount of water actually used in the home. This is based on a "guess" of how much water you might use daily. Often these units regenerate too soon, wasting salt. Or too late...allowing  hard water into the home.
  • Water softeners with SIMPLE METER-TYPE CONTROLS regenerate after a fixed amount of water has passed through the meter. This is a better method than the timer softeners, since it is based on actual water usage. However, these units have a preset calculated reserve capacity to carry through the day until the unit regenerates at night. If your usage increases, the reserve capacity may not be enough and you can run out of soft water.
  • Water softeners with SENSORS have an electronic sensing device imbedded in a fixed point low in the softener resin. Since the resin exhausts from top to bottom, when the sensor detects hard water, it signals the valve to recharge that night (again, usually after midnight). This design, like the meter, is using actual water usage to determine recharge intervals, so it is an improvement over the timer type. The "reserve capacity"
    is the amount of resin left below the sensor. Since the sensor is in a fixed position, the reserve capacity is a fixed amount.

    A final note about controls; while sensor driven electronics are the most efficient, when they fail they may be costly to replace or require the unit to be removed and fixed off-site.

Water Conditioner Valve Styles

  • Hydraulic; powered by the water pressure within the system.
  • Cam; an electric motor driven cam rotates and actuates valves
  • Solenoid; electric solenoids actuate valves 

Sizing of Softeners can be confusing. First, the capacity of the resin media will deteriorate over time due to contaminants. The best method of comparing softener capacity is to consider the number of cubic feet of resin and the resin used. Measurement of softeners by grains of removal capacity may be misleading.  This is due to the relationship to the amount of salt used to regenerate the media to the grains of hardness that may be removed.  A softener can be regenerated with 25 pounds of salt or it may be regenerated with as little as 5 pounds of salt with effectual grains removal capability 150% of the same media. Simple math shows that you use 5 times the amount of salt to get only 1.5 times improvement in capacity.  A costly waste of salt! Your water conditioner will last for years, take your time to consider the merits of one over the other.

Resins are made of polystyrene in the form of porous beads.  Resin bead size is referred to mesh size.  Good resins range from 16 to 40 mesh. and shape, Resin bead shape is also important, beads should be perfect spheres and not include broken material because broken and deformed beads can get past the distributor into the water system with a resultant loss of capacity over time.  Ask questions, ask look at the media within!

Health Consideration:

The major disadvantage of water softeners may be that they remove beneficial calcium and magnesium and substitute sodium. The sodium added to softened water is normally a relatively small fraction of the sodium intake from other dietary sources and is probably not a problem for healthy people. However, people on restricted salt diets should consult their physicians before using softened water from ion exchange units for drinking and cooking. To remove the sodium problem Potassium chloride salt substitute is now widely available. 

How much will I use:

This simple chart may help you estimate the amount of material necessary to regenerate a IE.  

Hardness Removed

Sodium (Na+ or Potassium (K+) added


10 mg/L as CaC03
10 mg/L as CaC03
 
 Sodium (Na+) added = 4.6 mg/L
 Potassium (K+) added = 7.6 mg/L 

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Last modified: November 13, 2018