Salt water chlorination
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Salt water chlorination is a process that uses dissolved salt (2,500–6,000 ppm) as a store for the chlorination system. In order to achieve this concentration, the chlorine generator (also known as salt cell, salt generator, salt chlorinator) uses electrolysis to break down approximately 200-500 pounds of 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.
Distinction from traditional pool chlorination
The presence of chlorine in traditional swimming pools can be described as a combination of free available chlorine (FAC) and combined available chlorine (CAC). While FAC is composed of the free chlorine ions that are available for sanitizing the water, the CAC includes chloramines, which are formed by the reaction of FAC with amines (introduced into the pool by human perspiration and urine). 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, however, the generator continuously produces free chlorine ions, eliminating the formation of CAC. Electrolysis 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, higher-than-normal chlorine demand, low stabilizer, sun exposure, or mechanical issues with the generator. 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 the same unhealthy disinfection byproducts (DBPs) that are present in traditional pools. Of highest concern are haloketones and trihalomethanes (THMs). Among these, bromoform has been found in swimming pools at up to 13-fold higher than maximum levels set by the World Health Organization.
Many people consider saltwater chlorine generators to be a new item. However, manufacturers have been producing salt chlorine generators in the United States since the early 1980s. The use of saltwater chlorine generators has however grown greatly since the early 2000s. Many hotels and water parks have converted to saltwater systems.
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 cleaning in a mild acid solution (1 part HCl to 15 parts pool water) which will remove the buildup of calcium. 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 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 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 and Downsides
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.
Downsides are initial cost of the system, maintenance, and the cost of replacement cells. Salt is a 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 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.
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 Muriatic 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 redux 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.
- Mixed metal oxide (MMO) electrodes
- Woodhurst, Kevin. "Chlorine Generators: A Salt-Water Pool is Often the Pool of Choice". Retrieved 19 February 2013.
- David Short, Fran J. Donegan (2012). Pools and Spas: Planning, Designing, Maintaining, Landscaping. Upper Saddle River, NJ: Creative Homeowner. p. 239. ISBN 1580113915.
- "Controlling Chloramines in Indoor Swimming Pools". NSW Government Health. 3. Retrieved 20 February 2013.
- Beech et al 1980 Nitrates, Chlorates and Trihalomethanes In Swimming Pool Water, Am J Public Health 70(1): p79-82
- Salinity - Dissolved Salts, Measuring Salinity, windows2universe.org.