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Salt water chlorination is a process that uses dissolved salt (1000–4000 ppm or 1–4 g/L) for the chlorination of swimming pools 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 to produce chlorine gas or its dissolved forms, hypochlorous acid and sodium hypochlorite, which are already commonly used as sanitizing agents in pools. Hydrogen is produced as byproduct too.

The presence of chlorine in traditional swimming pools can be described as a combination of free available chlorine (FAC) and combined available chlorine (CAC).^[1] While FAC is composed of the free chlorine that is available for disinfecting the water, the CAC includes chloramines, 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).^[2] 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.^[1] 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.

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 haloketones and trihalomethanes (THMs) of those the predominant form being bromoform. Very high levels of bromoform—up to 1.3 mg per liter, or 13 times the World Health Organization's guideline values—have been found in some public saltwater swimming pools.^[3]

Manufacturers have been producing saltwater chlorine generators in the United States since the early 1980s, and they first appeared commercially in New Zealand in the early 1970s (the Aquatech IG4500).^[4]

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. 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 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^{[clarification needed]} 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 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.^[5] 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 hydroxide ions, and this requires the frequent addition of hydrochloric acid (HCl, also known as muriatic acid) to maintain pH.^[6]

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, and the cost of replacement cells. Salt is corrosive 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 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". 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 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.

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 or hydrochloric acid 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 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.

Swimming pool sanitation is the process of ensuring healthy conditions in swimming pools. Proper sanitation is needed to maintain the visual clarity of water and to prevent the transmission of infectious waterborne diseases.

Two distinct and separate methods are employed in the sanitation of a swimming pool. The filtration system removes organic waste on a daily basis by using the sieve baskets inside the skimmer and circulation pump and the sand unit with a backwash facility for easy removal of organic waste from the water circulation. Disinfection - normally in the form of hypochlorous acid (HClO) - kills infectious microorganisms. Alongside these two distinct measures within the pool owner's jurisdiction, swimmer hygiene and cleanliness helps reduce organic waste build-up.

The World Health Organization has published international guidelines for the safety of swimming pools and similar recreational-water environments, including standards for minimizing microbial and chemical hazards.^[1] The United States Centers for Disease Control and Prevention also provides information on pool sanitation and water related illnesses for health professionals and the public.^[2] The main organizations providing certifications for pool and spa operators and technicians are the National Swimming Pool Foundation and Association of Pool & Spa Professionals. The certifications are accepted by many state and local health departments.^[3]

Swimming pool contaminants are introduced from environmental sources and swimmers. Affecting primarily outdoor swimming pools, environmental contaminants include windblown dirt and debris, incoming water from unsanitary sources, rain containing microscopic algae spores and droppings from birds possibly harboring disease-causing pathogens.^[4] Indoor pools are less susceptible to environmental contaminants.

Contaminants introduced by swimmers can dramatically influence the operation of indoor and outdoor swimming pools. Contaminants include micro-organisms from infected swimmers and body oils including sweat, cosmetics, suntan lotion, urine, saliva and fecal matter; for example, it was estimated by researchers that swimming pools contain, on average, 30 to 80 mL of urine for each person that uses the pool.^[5] In addition, the interaction between disinfectants and pool water contaminants can produce a mixture of chloramines and other disinfection by-products. The journal Environmental Science & Technology reported that sweat and urine react with chlorine and produce trichloramine and cyanogen chloride, two chemicals dangerous to human health. An answer to the perennial question: Is it safe to pee in the pool? Nitrosamines are another type of the disinfection by-products that are of concern as a potential health hazard.^[6]

Acesulfame potassium is widely used in the human diet and excreted by the kidneys. It has been used by researchers as a marker to estimate the degree to which swimming pools are contaminated by urine.^[6] It was estimated that a commercial-size swimming pool of 220,000 gallons would contain about 20 gallons of urine, equivalent to about 2 gallons of urine in a typical residential pool.^[6]

Pathogenic contaminants are of greatest concern in swimming pools as they have been associated with numerous recreational water illnesses (RWIs).^[7] Public health pathogens can be present in swimming pools as viruses, bacteria, protozoa and fungi. Diarrhea is the most commonly reported illness associated with pathogenic contaminants, while other diseases associated with untreated pools are Cryptosporidiosis and Giardiasis.^[8][9] Other illnesses commonly occurring in poorly maintained swimming pools include otitis externa, commonly called swimmers ear, skin rashes and respiratory infections.

Contamination can be minimized by good swimmer hygiene practices such as showering before and after swimming, and not letting children with intestinal disorders swim. Effective treatments are needed to address contaminants in pool water because preventing the introduction of pool contaminants, pathogenic and non-pathogenic, into swimming pools is, in practice, impossible.

A well-maintained, properly operating pool filtration and re-circulation system is the first barrier, combating the contaminants large enough to be filtered. Rapid removal of these filterable contaminants reduces the impact on the disinfection system thereby limiting the formation of chloramines, restricting the formation of disinfection by-products and optimizing sanitation effectiveness. To kill pathogens and help prevent recreational water illnesses, pool operators must maintain proper levels of chlorine or another sanitizer.^[10][11]

Over time, calcium from municipal water tends to accumulate, developing salt deposits in the swimming pool walls and equipment (filters, pumps), reducing their effectiveness. Therefore, it is advised to either completely drain the pool, and refill it with fresh water, or recycle the existing pool water, using reverse osmosis. The advantage of the latter method is that 90% of the water can be reused.

Pool operators must also store and handle cleaning and sanitation chemicals safely.

Disease prevention should be the top priority for every water quality management program for pool and spa operators. Disinfection is critical to protect against pathogens, and is best managed through routine monitoring and maintenance of chemical feed equipment to ensure optimum chemical levels in accordance with state and local regulations.^[12]

Chemical parameters include disinfectant levels according to regulated pesticide label directions. pH should be kept between 7.2 and 7.8. Human tears have a pH of 7.4, making this an ideal point to set a pool.^[13] More often than not, it is improper pH and not the sanitiser that is responsible for irritating swimmers' skin and eyes.

Total alkalinity should be 80–120 ppm and calcium hardness between 200 and 400 ppm.^{[14] [failed verification]}

Good hygienic behavior at swimming pools is also important for reducing health risk factors at swimming pools and spas. Showering before swimming can reduce introduction of contaminants to the pool, and showering again after swimming will help to remove any that may have been picked up by the swimmer.

Those with diarrhea or other gastroenteritis illnesses should not swim within 2 weeks of an outbreak, especially children. Cryptosporidium is chlorine resistant.^[15]

In order to minimize exposure to pathogens, swimmers should avoid getting water into their mouths, and should never swallow pool or spa water.^[16]

Maintaining an effective concentration of disinfectant is critically important in assuring the safety and health of swimming pool and spa users. When any of these pool chemicals are used, it is very important to keep the pH of the pool in the range 7.2 to 7.8 – according to the Langelier Saturation Index, or 7.8 to 8.2 – according to the Hamilton Index; higher pH drastically reduces the sanitizing power of the chlorine due to reduced oxidation-reduction potential (ORP), while lower pH produces more rapid loss of chlorine and causes bather discomfort, especially to the eyes. However, according to the Hamilton Index, a higher pH can reduce unnecessary chlorine consumption while still remaining effective at preventing algae and bacteria growth.

To help ensure the health of bathers and protect pool equipment, it is essential to perform routine monitoring of water quality factors (or "parameters") on a regular basis. This process becomes the essence of an optimum water quality management program.

Conventional halogen-based oxidizers such as chlorine and bromine are convenient and economical primary sanitizers for swimming pools and provide a residual level of sanitizer that remains in the water. Chlorine-releasing compounds are the most popular and frequently used in swimming pools whereas bromine-releasing compounds have found heightened popularity in spas and hot tubs. Both are members of the halogen group with demonstrated ability to destroy and deactivate a wide range of potentially dangerous bacteria and viruses in swimming pools and spas. Both exhibit three essential elements as ideal first-line-of-defense sanitizers for swimming pools and spas: they are fast-acting and enduring, they are effective algaecides, and they oxidize undesired contaminants.

Swimming pools can be disinfected with a variety of chlorine-releasing compounds. The most basic of these compounds is molecular chlorine (Cl₂); however, its application is primarily in large commercial public swimming pools. Inorganic forms of chlorine-releasing compounds frequently used in residential and public swimming pools include sodium hypochlorite commonly known as liquid bleach or simply bleach, calcium hypochlorite and lithium hypochlorite. Chlorine residuals from Cl₂ and inorganic chlorine-releasing compounds break down rapidly in sunlight. To extend their disinfectant usefulness and persistence in outdoor settings, swimming pools treated with one or more of the inorganic forms of chlorine-releasing compounds can be supplemented with cyanuric acid – a granular stabilizing agent capable of extending the active chlorine residual half-life (t_½) by four to sixfold.^[17]

Chlorinated isocyanurates, a family of organic chlorine-releasing compounds, are stabilized to prevent UV degradation due to the presence of cyanurate as part of their chemical backbone. These are commonly sold for general use in small summer pools, where the water is expected to be used for only a few months and is expected to be regularly topped up with fresh, due to evaporation and splash loss. It is important to change the water frequently, otherwise, levels of cyanuric acid will build up to beyond the point at which the mechanism functions. Excess cyanurates will actually work in reverse and will inhibit the chlorine. A steadily lowering pH value of the water may at first be noticed. Algal growth may become visible, even though chlorine tests show sufficient levels.^[18]

Chlorine reacting with urea in urine and other nitrogen-containing wastes from bathers can produce chloramines. Chloramines typically occur when an insufficient amount of chlorine is used to disinfect a contaminated pool. Chloramines are generally responsible for the noxious, irritating smell prominently occurring in indoor pool settings. A common way to remove chloramines is to "superchlorinate" (commonly called "shocking") the pool with a high dose of inorganic chlorine sufficient to deliver 10 ppm chlorine. Regular superchlorination (every two weeks in summer) helps to eliminate these unpleasant odors in the pool. Levels of chloramines and other volatile compounds in water can be minimized by reducing contaminants that lead to their formation (e.g., urea, creatinine, amino acids and personal care products) as well as by use of non-chlorine "shock oxidizers" such as potassium peroxymonosulfate.

Medium pressure UV technology is used to control the level of chloramines in indoor pools. It is also used as a secondary form of disinfection to address chlorine-tolerant pathogens. A properly sized and maintained UV system should remove the need to shock for chloramines, although shocking would still be used to address a fecal accident in the pool. UV will not replace chlorine but is used to control the level of chloramines, which are responsible for the odor, irritation, and enhanced corrosion at an indoor pool.

Copper ion systems use an electric current across .500 gm bars (solid copper, or a mixture of copper and .100 gm or silver) to free copper ions into the flow of pool water to kill organisms such as algae in the water and provide a "residual" in the water. Alternative systems also use titanium plates to produce oxygen in the water to help degrade organic compounds.

An electrically operated water pump is the prime motivator in recirculating the water from the pool. Water is forced through a filter and then returned to the pool. Using a water pump by itself is often not sufficient to completely sanitize a pool. Commercial and public pool pumps usually run 24 hours a day for the entire operating season of the pool. Residential pool pumps are typically run for 4 hours per day in winter (when the pool is not in use) and up to 24 hours in summer. To save electricity costs, most pools run water pumps for between 6 hours and 12 hours in summer with the pump being controlled by an electronic timer.

Most pool pumps available today incorporate a small filter basket as the last effort to avoid leaf or hair contamination reaching the close-tolerance impeller section of the pump.

A pressure-fed sand filter is typically placed in line immediately after the water pump. The filter typically contains a medium such as graded sand (called '14/24 Filter Media' in the UK system of grading the size of sand by sifting through a fine brass-wire mesh of 14 to the inch (5.5 per centimeter) to 24 to the inch (9.5 per cm)). A pressure fed sand filter is termed a 'High Rate' sand filter, and will generally filter turbid water of particulates no less than 10 micrometers in size.^[19] The rapid sand filter type are periodically 'back washed' as contaminants reduce water flow and increase back pressure. Indicated by a pressure gauge on the pressure side of the filter reaching into the 'red line' area, the pool owner is alerted to the need to 'backwash' the unit. The sand in the filter will typically last five to seven years before all the "rough edges" are worn off, and the more tightly packed sand no longer works as intended ^{[citation needed]}. Recommended filtration for public/commercial pools is 1 ton sand per 100,000 liters water (10 ounces avdp. per cubic foot of water) [7.48 US or 6.23 UK gallons].

Introduced in the early 1900s was another type of sand filter – the 'Rapid Sand' filter, whereby water was pumped into the top of a large volume tank (3' 0" or more cube) (1 cubic yard/200US gal/170UK gal/770 liters) containing filter grade sand and returning to the pool through a pipe at the bottom of the tank. As there is no pressure inside this tank, they were also known as "gravity filters". These types of filters are not greatly effective, and are no longer common in home swimming pools, being replaced by the pressure-fed type filter.

Some filters use diatomaceous earth to help filter out contaminants. Commonly referred to as 'D.E.' filters, they exhibit superior filtration capabilities.^[20] Often a D.E. filter will trap waterborne contaminants as small as 1 micrometer in size. D.E. filters are banned in some states, as they must be emptied out periodically and the contaminated media flushed down the sewer, causing a problem in some districts' sewage systems.

As of 2020, several companies now produce regenerative media filters, sometimes called precoat media filters, which use perlite as the filtration media rather than diatomaceous earth. As of 2021, perlite can safely be flushed down the sewer and is approved and NSF listed for use in the United States.

Other filter media that have been introduced to the residential swimming pool market since 1970 include sand particles and paper type cartridge filters of 50 to 150 square feet (4.6 to 13.9 m²) filter area arranged in a tightly packed 12" diameter x 24" long (300 mm x 600 mm) accordion-like circular cartridge. These units can be 'daisy-chained' together to collectively filter almost any size home pool. The cartridges are typically cleaned by removal from the filter body and hosing-off down a sewer connection. They are popular where backwashed water from a sand filter is not allowed to be discharged or go into the aquifer.

Fabric Filters

Traditional pool filters vary in the micron particle sizes that they can capture. Fabric filters can capture particles smaller than that of standard swimming pool filtration systems. This type of filter connects where the water return to the pool after passing through a standard filter. They are usually in the form of a bag. With filtration levels as small as 1 micrometer, users can attain much cleaner water, when using a sand of cartridge filter. These levels are equal or better than that of a diatomaceous earth filter.

Automated pool cleaners more commonly known as "Automatic pool cleaners" and in particular electric, robotic pool cleaners provide an extra measure of filtration, and in fact like the handheld vacuums can microfilter a pool, which a sand filter without flocculation or coagulants is unable to accomplish.^[21]

These cleaners are independent from the pool's main filter and pump system and are powered by a separate electricity source, usually in the form of a set-down transformer that is kept at least 10 feet (3.0 m) from the water in the pool, often on the pool deck. They have two internal motors: one to suck in water through a self-contained filter bag and then return the filtered water at a high speed back into the pool water, and one that is a drive motor connected to tractor-like rubber or synthetic tracks and "brushes" connected by rubber or plastic bands via a metal shaft. The brushes, resembling paint rollers, are located on the front and back of the machine, and help to remove contaminating particles from the pool's floor, walls, and, in some designs, even the pool steps (depending on size and configuration). They also direct the particles into the internal filter bag.^[22][23]