Why Is Chlorine Used to Clean Swimming Pools?
Swimming pool sanitation is the process of ensuring healthy conditions in swimming pools, hot tubs, plunge pools, and similar recreational water venues. Proper sanitation is needed to maintain the visual clarity of water and to prevent the transmission of infectious waterborne diseases.
Sanitation methods include a water filter to remove pollutants, disinfection to kill infectious microorganisms, swimmer hygiene to minimize the introduction of contaminants into pool water, and regular testing of pool water, including chlorine and pH levels.
When chlorine solution is poured into water, various chemicals, including hypochlorous acid and hypochlorite ions, are formed. These kill the micro-organisms and bacteria present in the water by attacking the lipids — groups of naturally-occurring molecules which include fats, waxes, sterols, fat-soluble vitamins and other things — in the cell walls destroying, in the process, enzymes and structures inside the cells. This renders them oxidised and harmless.
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.
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 (t½) by four to sixfold.
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.
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.