Salt water chlorination is a process that uses dissolved salt (1,800–6,000 ppm) as a store for the chlorination system. The chlorinator uses electrolysis to break down the salt (NaCl). The resulting chemical reaction eventually produces hypochlorous acid (HCIO), and sodium hypochlorite (NaClO), which are the sanitizing agents already commonly used in swimming pools. As such, a saltwater pool is not actually chlorine-free; it simply utilizes a chlorine generator instead of direct addition of chlorine.
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The important distinction is that saltwater pools (usually) lack chloramines, referred to as combined chlorine. Chloramines are the irritants which give traditional pools the stigma of burning eyes and caustic smell. The electrolytic process burns off chloramines in the same manner as traditional shock (oxidizer). When chlorine levels are low in the pool, one possible cause is low salt (others can be higher-than-normal chlorine demand, low stabilizer or mechanical issues with the generator itself.) Salt count can be lowered due to splash-out, backwashing, and dilution via rainwater
Scientific research has shown that since saltwater pools still use chlorine sanitization, they generate unhealthy disinfection byproducts (DBPs) called trihalomethanes (THMs) the predominant form being bromoform. Very high levels of bromoform (up to 13-fold higher than maximum levels set by WHO) have been measured in public salt pools.[1]
Saltwater pools are relatively new to the U.S., but have been very popular for years in Australia. Many hotels and water parks have converted to saltwater systems. However, due to harmful salinification of natural wetlands certain places in the U.S. (e.g. Los Angeles) and parts of Australia have now passed legislation prohibiting the use of salt chlorinators.[2]
To super-chlorinate the pool, as is recommended monthly, the chlorinator and pump are left to run for 24 hours rather than the usual 8. Note that super chlorination is not a substitute for oxidation. It is recommended to shock–oxidize the pool weekly to break down bacteria and contaminants that may get in the pool as well as to help prevent algae. Super-chlorination is not strong enough to oxidize a salt water pool.
The chlorinator cell consists of parallel titanium plates coated with ruthenium and sometimes iridium. Older models make use of perforated (or mesh) plates, rather than solid plates. The electrolytic process naturally attracts calcium and other minerals to the plates. Thus, depending on water chemistry and magnitude of use, the cell will require cleaning in a mild acid solution (1 part HCl to 15 parts pool water) which will remove the buildup of calcium. Extremely excessive build up can reduce the effectiveness of the cell. Running the chlorinator for long periods with not enough salt in the pool can strip the coating off the cell which then requires an expensive replacement as can using too strong an acid wash.
Saltwater pools can also require stabilizer (cyanuric acid) to help stop the sun's UV rays from breaking down free chlorine in the pool. Usual levels are 50–75 pm. They also require the pH to be kept between 7.2 and 7.8 with the chlorine being more effective if the pH is kept closer to 7.2. The average salt levels are usually in the 3000-5000 ppm range, much less than the ocean, which has salt levels of around 35,000 parts per million (ppm).[3] The salt is poured across the bottom of the pool and swept with the pool brush until it dissolves. Concentrated salt solution should not be allowed into the skimmer as it causes the fuse to blow from overconductivity.
The benefits of salt systems in pools are the convenience and the constant delivery of pure chlorine-based sanitizer. The reduction of irritating chloramides versus traditional chlorinating methods and the "softening" effect of the electrolytic process reducing dissolved alkali minerals in the water are also perceived as benefits. For some people that have sensitivities to chlorine, these systems may be less offensive.
Downsides are initial cost of the system, maintenance, and the cost of replacement cells. Salt is a corrosive; it will damage some metals and some improperly-sealed stone, however as the ideal saline concentration of a salt-chlorinated pool is very low (<3,500ppm, the threshold for human perception of salt by taste; seawater is about ten times this concentration), damage usually occurs due to improperly-maintained pool chemistry or improper maintenance of the electrolytic cell. Pool equipment manufacturers will not warranty stainless steel products damaged by saline pools.
Calcium and other alkali precipitate buildup will occur naturally on the cathode plate, and sometimes in the pool itself as "scaling". Regular maintenance of the cell is necessary; failure to do so will reduce the effectiveness of the cell, which will in turn increase the salinity of the water to corrosive levels (as water flowing through the chlorinator will have salt added but not electrolyzed into chlorine). Certain designs of saline chlorinators use a "reverse-polarity" design that will regularly switch the roles of the two electrodes between anode and cathode, causing this calcium buildup to dissolve off the accumulating electrode. Such systems reduce, but do not eliminate, the need to clean the electrolytic cell and the occurrence of calcium scale in the water.
Sodium bromide can be used instead of salt, which produces a bromine pool. The benefits and downsides are the same as those of a salt system. It is not necessary to use a chloride based acid to balance the pH.