Salt water chlorination is a process that uses dissolved
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...
(1000–36,000
ppm or 1–36 g/L) for the
chlorination Chlorination may refer to:
* Chlorination reaction
In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transform ...
of
swimming pools
A swimming pool, swimming bath, wading pool, paddling pool, or simply pool, is a structure designed to hold water to enable swimming or other leisure activities. Pools can be built into the ground (in-ground pools) or built above ground (a ...
and
hot tubs. The chlorine generator (also known as salt cell, salt generator, salt chlorinator, or SWG) uses
electrolysis in the presence of dissolved
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...
to produce
chlorine gas
Chlorine is a chemical element with the Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate betwee ...
or its dissolved forms,
hypochlorous acid
Hypochlorous acid (HClO, HOCl, or ClHO) is a weak acid that forms when chlorine dissolves in water, and itself partially dissociates, forming hypochlorite, ClO−. HClO and ClO− are oxidizers, and the primary disinfection agents of chlorine sol ...
and
sodium hypochlorite
Sodium hypochlorite (commonly known in a dilute solution as bleach) is an inorganic chemical compound with the formula NaOCl (or NaClO), comprising a sodium cation () and a hypochlorite anion (or ). It may also be viewed as the sodium s ...
, which are already commonly used as
sanitizing agents in pools.
Hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
is produced as byproduct too.
Distinction from traditional pool chlorination
The presence of chlorine in traditional
swimming pools
A swimming pool, swimming bath, wading pool, paddling pool, or simply pool, is a structure designed to hold water to enable swimming or other leisure activities. Pools can be built into the ground (in-ground pools) or built above ground (a ...
can be described as a combination of free available chlorine (FAC) and combined available chlorine (CAC).
While FAC is composed of the free chlorine that is available for disinfecting the water, the CAC includes
chloramine
Chloramines refer to derivatives of ammonia and organic amines wherein one or more N-H bonds have been replaced by N-Cl bonds. Two classes of compounds are considered: inorganic chloramines and organic chloramines.
Inorganic chloramines
Inorgan ...
s, which are formed by the reaction of FAC with amines (introduced into the pool by human perspiration, saliva, mucus, urine, and other biologics, and by insects and other pests). Chloramines are responsible for the "chlorine smell" of pools, as well as skin and eye irritation. These problems are the result of insufficient levels of free available chlorine, and indicate a pool that must be "shocked" by the addition of 5–10 times the normal amount of chlorine.
In saltwater pools, the generator uses electrolysis to continuously produce free chlorine. As such, a saltwater pool or hot tub is not actually chlorine-free; it simply utilizes added salt and a chlorine generator instead of direct addition of chlorine. It also burns off chloramines in the same manner as traditional shock (oxidizer). As with traditionally chlorinated pools, saltwater pools must be monitored in order to maintain proper water chemistry. Low chlorine levels can be caused by insufficient salt, incorrect (low) chlorine-generation setting on the SWG unit, higher-than-normal chlorine demand, low stabilizer, sun exposure, insufficient pump speed, or mechanical issues with the chlorine generator. Salt count can be lowered due to splash-out, backwashing, and dilution via rainwater.
Health concerns
Research has shown that because saltwater pools still use chlorine sanitization, they generate the same
disinfection byproducts (DBPs) that are present in traditional pools. Of highest concern are
haloketone
In organic chemistry, an α-haloketone is a functional group consisting of a ketone group or more generally a carbonyl group with an α-halogen substituent. α-haloketones are alkylating agents. Prominent α-haloketones include phenacyl bromide ...
s and
trihalomethane
In chemistry, trihalomethanes (THMs) 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 environmenta ...
s (THMs) of those the predominant form being
bromoform
Bromoform (CHBr3) is a brominated organic solvent, colorless liquid at room temperature, with a high refractive index, very high density, and sweet odor is similar to that of chloroform. It is one of the four haloforms, the others being fluorofor ...
. Very high levels of bromoform—up to 1.3 mg per liter, or 13 times the
World Health Organization
The World Health Organization (WHO) is a specialized agency of the United Nations responsible for international public health. The WHO Constitution states its main objective as "the attainment by all peoples of the highest possible level of ...
's guideline values—have been found in some public saltwater swimming pools.
Manufacturers have been producing saltwater chlorine generators in the United States since the early 1980s, and they first appeared commercially in
New Zealand
New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...
in the early 1970s (the Aquatech IG4500).
Operation
The chlorinator cell consists of parallel
titanium
Titanium is a chemical element with the Symbol (chemistry), symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resista ...
plates coated with
ruthenium
Ruthenium is a chemical element with the 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 other chemical ...
and sometimes
iridium
Iridium is a chemical element with the symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal (after osmium) with a density of ...
. Older models make use of perforated (or mesh) plates rather than solid plates. Electrolysis naturally attracts calcium and other minerals to the plates. Thus, depending on water chemistry and magnitude of use, the cell will require periodic cleaning in a mild acid solution (1 part
HCl to 15 parts water) which will remove the buildup of
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...
compound crystals, such as calcium carbonate or calcium nitrate. Excessive buildup can reduce the effectiveness of the cell. Running the chlorinator for long periods with insufficient 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 or 1,3,5-triazine-2,4,6-triol is a chemical compound with the formula (CNOH)3. Like many industrially useful chemicals, this triazine has many synonyms. This white, odorless solid finds use as a precursor or a component of bleach ...
) to help stop the sun's
UV rays from breaking down free chlorine in the pool. Usual levels are 20–50 ppm. 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 ppm.
Salinity – Dissolved Salts, Measuring Salinity
windows2universe.org
In swimming pools, salt is typically poured across the bottom and swept with the pool brush until it dissolves; if concentrated brine is allowed into the return-water system it can cause the chlorinator cell to malfunction due to overconductivity.
Salt water chlorination produces an excess of hydroxyl ions, and this requires the frequent addition of hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
(HCl, also known as muriatic acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the digestiv ...
) to neutralise the alkalinity. The initial chlorine chemistry is as follows.
4NaCl → 4Na+ + 4Cl− Salt dissolves in water.
4Na+ + 4Cl− → 4Na + 2Cl2 By electrolysis.
4Na + 4H20 → 4Na+ +4OH− + 2H2 Reaction of metallic sodium with water.
2Cl2 + 2H2O → 2HClO + 2H+ + 2Cl− Hydrolysis of aqueous Chlorine gas.
2HClO → HClO + ClO− + H+ Dissociation of hypochlorous acid at pH 7.5 and 25C.
4NaCl + 3H2O → 4Na+ + HClO + ClO− + OH− + 2Cl− + 2H2 Net of all the above.
Addition of Hydrochloric Acid to restore the pH to 7.5
HCl + 4Na+ + HClO + ClO− + OH− + 2Cl− +2H2 → HClO + OCl− +H2O + 4Na+ + 3Cl− +2H2.
4NaCl + HCl + 2H2O → HClO + OCl− + 4Na+ +3Cl− + 2H2 Net of the last two.
Benefits and disadvantages
The benefits of salt systems in pools are the convenience and the constant delivery of pure chlorine-based sanitizer. The reduction of irritating chloramines versus traditional chlorinating methods and the "softening" effect of electrolysis 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.
Disadvantages are the initial cost of the system, maintenance
Maintenance may refer to:
Biological science
* Maintenance of an organism
* Maintenance respiration
Non-technical maintenance
* Alimony, also called ''maintenance'' in British English
* Champerty and maintenance, two related legal doct ...
, and the cost of replacement cells. Salt is corrosive
A corrosive substance is one that will damage or destroy other substances with which it comes into contact by means of a chemical reaction.
Etymology
The word ''corrosive'' is derived from the Latin verb ''corrodere'', which means ''to gnaw'', ...
and 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
Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appr ...
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 typically will not warrant 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
Scaling may refer to:
Science and technology
Mathematics and physics
* Scaling (geometry), a linear transformation that enlarges or diminishes objects
* Scale invariance, a feature of objects or laws that do not change if scales of length, energ ...
". Regular maintenance of the cell is necessary; failure to do so will reduce the effectiveness of the cell. Certain designs of saline chlorinators use a "reverse-polarity" method that will regularly switch the roles of the two electrodes between anode
An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic ...
and cathode
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in wh ...
, 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.
As chlorine is generated, pH will rise causing the chlorine to be less effective. Many systems with chemistry automation can sense the rising pH and automatically introduce either CO2
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
or hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
in order to bring the pH back to the target level.
Automation systems will also manage levels of sanitizer by monitoring the ORP or redox levels of the water. This allows only the needed amount of chlorine to be generated based on the demand.
Sodium bromide
Sodium bromide is an inorganic compound with the formula Na Br. 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.Michael J. Dagani, Henry J. Ba ...
can be used instead of sodium chloride, 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. Also, bromine is only effective as a sanitizer, not as an oxidizer, leaving a need for adding a "shock" such as hydrogen peroxide or any chlorine-based shock to burn off inorganic waste and free up combined bromines. This extra step is not needed in a sodium chloride system, as chlorine is effective as both a sanitizer and an oxidizer. A user would only need to "super chlorinate" or increase chlorine production of the cell occasionally. That would normally be less than once a week or after heavy bather loads.
References
{{DEFAULTSORT:Saltwater Pool
Swimming pools
Water treatment
Chlorine