Saltwater pool
Encyclopedia
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.
s (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.
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.
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). 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.
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.
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
(1,800–6,000 ppm) as a store for the chlorination
Chlorination
Chlorination is the process of adding the element chlorine to water as a method of water purification to make it fit for human consumption as drinking water...
system. The chlorinator uses electrolysis
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
to break down the salt (NaCl
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
). The resulting chemical reaction eventually produces hypochlorous acid (HCIO
Hypochlorous acid
Hypochlorous acid is a weak acid with the chemical formula HClO. It forms when chlorine dissolves in water. It cannot be isolated in pure form due to rapid equilibration with its precursor...
), 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.
Distinctions
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 rainwaterConcerns
Scientific research has shown that since saltwater pools still use chlorine sanitization, they generate unhealthy disinfection byproducts (DBPs) called trihalomethaneTrihalomethane
Trihalomethanes are chemical compounds in which three of the four hydrogen atoms of methane are replaced by halogen atoms. Many trihalomethanes find uses in industry as solvents or refrigerants. THMs are also environmental pollutants, and many are considered carcinogenic...
s (THMs) the predominant form being bromoform
Bromoform
Bromoform is a pale yellowish liquid with a sweet odor similar to chloroform, a halomethane or haloform. Its refractive index is 1.595 . Bromoform is produced naturally by phytoplankton and seaweeds in the ocean and this is thought to be the predominant source to the environment...
. Very high levels of bromoform (up to 13-fold higher than maximum levels set by WHO) have been measured in public salt pools.
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.
Operation
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
Ruthenium
Ruthenium is a chemical element with symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most chemicals. The Russian scientist Karl Ernst Claus discovered the element...
and sometimes iridium
Iridium
Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...
. 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
HCL
HCL or HCl can stand for:* Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia* Hardware compatibility list...
to 15 parts pool water) which will remove the buildup of calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
. 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
Cyanuric acid
Cyanuric acid or 1,3,5-triazine-2,4,6-triol is a chemical compound with the formula 3. Like many industrially useful chemicals, this triazine has many synonyms. This white, odorless solid finds use as a precursor or a component of bleaches, disinfectants, and herbicides...
) 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
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
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). 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.
Benefits
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
Sodium bromide
Sodium bromide is an inorganic compound with the formula NaBr. It is a high-melting white, crystalline solid that resembles sodium chloride. It is a widely used source of the bromide ion and has many applications.-Synthesis, structure, reactions:...
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.
External links
- About: A Salt-Water Pool is Often the Pool of Choice
- Swimming pool control with Linux (discusses techniques and economics of chlorination via salt generation)
- Saltwater Pools