Over the last few years, I am amazed at how many people do not understand how to operate a dealkalizer system properly or efficiently. This includes some water treatment companies that are selling the ion exchange equipment and replacement resin. In this discussion, I am only going to review Strong Base Anion resin in the Chloride Form.
In many parts of the country, there are water supplies that have high alkalinity water. By that I mean waters that have total alkalinity between 200-250 ppm as CaCO3. There are many areas of the country that have over 250 ppm total alkalinity.
There are different types of strong base dealkalizer resin. Most of them are either Type 1 or Type 2 resins. There is also a Type 3 resin. This discussion will include the more common Type 1 and 2 resins. Type 1 resin is prone to organic fouling which is normally not an issue with municipal water supplies however this can be a problem if other water sources are used. Type 1 resin has slightly lower capacity than Type 2 as well as a lower regeneration efficiency. Type 2 resin has poorer chemical and physical stability than Type 1 resin.
Resin life is greatly affected by the operation of the exchange equipment, the water source used for regeneration, water temperature, the salt impurities in the regenerant brine, TEA (total exchangeable anions), and regeneration frequency. Typical resin life span for Type 1 anion resin is 3 to 5 years and and 2 to 3 years for Type 2 resin.
The most common thing that can shorten the life of Type 1 and Type 2 resins is fouling caused by hardness ions. There are many boiler plants that have bulk brine systems which were installed to eliminate the handling of bag salt by operating personnel as well as reduce the cost per pound of salt. Both of these are good reasons for installing bulk brine systems. What is NOT good is that the bulk brine produced typically uses rock salt which is high in impurities including calcium and magnesium hardness salts. These salts can and will foul exchange sites on the resin beads and will eventually leach out into the water used for boiler makeup causing increased usage of phosphate and chelant chemicals.
To try and compensate for this loss of capacity, the water treatment rep may recommend the use of caustic soda to enhance the regeneration efficiency. In the presence of hardness, the addition of caustic soda during regeneration can precipitate calcium carbonate or magnesium hydroxide in the resin bed causing further reduced exchange capacity. For this reason, high purity salt should be used instead of rock salt for dealkalizer brine makedown to avoid premature resin replacement. This recommendation may initially create push back from the customer however it should be at least discussed PRIOR to the sale of a dealkalizer system.
Another common mistake that is made when a dealkalizer system is first set up is the service run is sometimes set too low. This results in wasted regenerant water and salt due to more frequent regenerations than necessary and further deterioration of exchange capacity due to resin bead breakage and resulting loss of resin fines during backwash.
New dealkalizer SBA resin in the chloride form typically has an exchange capacity of 10,000 grains/cubic foot. In actual practice, most often you will see the regeneration set point at 90% of the full exchange capacity or 9,000 grains/cubic foot at a salt dosage of 5 lb/cubic foot of resin.
Another consideration in setting up a dealkalizer system is to consider the affect TEA (total exchangeable anions) has on the exchange capacity. The TEA includes PO4, SO4, SiO2, NO3, and chloride ions. Why would chloride ion affect exchange capacity? The answer is that high concentrations (usually 100 ppm) of chloride ions can interfere with ion exchange efficiency just as high sodium levels in softener influent can interfere with exchange efficiency.
I actually had an equipment sales manager tell me a few years ago that he always set the regeneration set point at 6,500 grains/cubic foot for Indianapolis city water as “this was was the point at which alkalinity increased at the end of the service run”. Did he bother to check the amount of hardness in the brine, the TEA, or the water source (there are 4 different water sources used by the municipality having different TEA’s), or if caustic soda was added during regeneration?
One final thing that needs to be mentioned, ONLY soft water should be used for dealkalizer regeneration.