FUNDAMENTALS OF WATER SOFTENING BY ION EXCHANGE
Water that contains calcium and magnesium ions is called "hard water" because calcium and magnesium can combine with other ions and compounds to leave a hard scale on the surfaces they touch. An ion exchange water softener can reduce or eliminate hardness problems.
A typical water softener has a pressure tank partially filled with ion exchange resin. Our brand of resin, called Cullex® resin, consists of highly porous, amber colored, plastic beads loaded with "exchange sites" that preferentially remove hardness ions and replace them with sodium, a "soft" ion. A softener system also includes a brine tank to provide a source of sodium for regenerating the resin and hydraulic controls to direct the flow of water through the softener during service and regeneration.
At the beginning of the softening cycle, sodium ions occupy the resin's exchange sites. As water passes through it, the resin's stronger attraction for the hardness ions causes it to take on the hardness ions and give up its sodium ions. Iron and manganese are considered hardness and they are removed also, provided they are in solution. Ion exchange cannot remove suspended matter.
As water flows downward through the resin bed, the resin at the top of the bed gives up its sodium first. The exchange process is not instantaneous, so exchange occurs in a band called a "reaction zone". The reaction zone's depth depends on incoming water hardness and TDS, flow rate, water temperature and resin particle size. When the reaction zone's leading edge reaches the bottom of the resin bed and hardness passed into the service line, the resin has become "exhausted" and it must be regenerated before it can remove hardness again.
The regeneration cycle starts with backwash, an upward flow that loosens the resin bed and flushes out suspended particles. Backwash usually lasts about 10 minutes.
Regeneration occurs when a solution of sodium chloride (salt) brine is passed through the resin in a downward direction. An eductor draws concentrated brine from a storage tank and dilutes it to the right concentration. Brine draws lasts from 10 to 30 minutes depending on salt dosage (weight of salt per volume of resin). A large excess of sodium ions causes the resin to release its hold on hardness ions picked up during the preceding service cycle and returns the resin to its sodium state.
The brining step is followed by a slow downflow rinse to displace spent brine from the resin. It also carries the hardness removed from the resin to drain. The rinse rate is regulated to ensure correct contact time between the salt and the resin. Slow rinse usually lasts about 30 minutes.
A final fast downflow rinse, or purge, flushes all remaining brine from the tank. It lasts about 5 minutes.
Each cubic foot of Cullex resin can remove about 30,000 grains of hardness when dosed with 15 pounds of salt (one liter of resin can remove 69 grams of hardness when regenerated with 240 grams of salt). A lower dosage of 6 pounds per cubic foot yields about 20,000 grains of capacity (46 grams capacity at 96 grams of salt per liter). This lower dosage is more salt efficient, but it requires more frequent regeneration. Most softeners are selected on the basis of a balance between capital cost and operating cost.
