Fundamentals of Water Softening by Ion Exchange

Water is called "hard water" if it contains calcium and magnesium because these substances leave a hard scale on surfaces in contact with the water. The problem can be minimized by the use of a water softener, which operates on the principle of ion exchange.

A water softener tank typically is about 2/3 full with amber-coloured plastic resin beads. Fresh resin has sodium attached to the resin. As hard water enters the top of the tank it comes in contact with the fresh resin. The dissolved calcium and magnesium are attracted to the resin more strongly than sodium, so the resin lets go of the sodium and holds onto the hardness ions. The water softener also takes out iron and manganese, provided they are dissolved in the water and not suspended. Particulate matter is not exchanged and is not part of the process.

As water flows through the resin bed, the sodium is given up from the top of the bed first and the exchange process slowly works its way downward. Because ion exchange requires time to occur, there is an area within the bed where water is in the process of becoming softened. This is called the reaction zone. The depth of the reaction zone depends on factors such as flow rate, total dissolved solids, hardness, and resin particle size. When the resin bed is nearly exhausted, the reaction zone will be at the bottom of the bed. There will not be enough time for complete softening to occur, and hardness will enter the service line. When this begins to occur, the water softener must be regenerated.

The first step in a typical regeneration process is an upflow backwash to eliminate any particulate matter that may have collected in the resin bed. The backwash is typically for 10 minutes.

The actual regeneration occurs during the next step, which is downflow brining. Concentrated brine is drawn from the brine tank and blended with fresh water. This solution is strong enough to reverse the softening process. Calcium and magnesium are washed off of the resin beads as they are inundated by the brine solution. The hardness ions are rinsed out to waste. The brine eduction process requires about 10 to 30 minutes, depending upon the amount of salt used. After all of the brine has been drawn, fresh water continues to flow down through the resin bed at a low flow rate. This assures adequate contact time while rinsing out the brine solution. The rinse cycle lasts about 30 minutes after brine draw.

The last portion of the regeneration cycle is a downflow fast rinse or purge cycle. This 5 minute rinse at higher flow rate makes sure that no brine solution remains in the tank before returning the softener to service. The entire four cycles of regeneration require about 1-1/2 hours.

Each cubic foot of softening resin has a capacity of 30,000 grains of ion exchange capacity when dosed with 15 pounds of salt. A lower salt dosage of 6 pounds will yield a softening capacity of 20,000 grains. This lower salt dosage is more efficient, but requires more frequent regenerations. Most softeners are selected as a balance between equipment expense and operating expense. Other factors are maximum flow rate, pressure drop, maximum daily capacity and site requirements.