Most people still assume that swimming pools are just holes in the ground, filled with water. In reality, there are actually many chemical processes, which occurs in a swimming pool, to keep the water clean and safe. Pathogens such as adenoviruses, Hepatitis A viruses, Shigella and Escherichia coli 0157, which can cause fever, diarrhoea and vomiting, must be eradicated. 3 There are many other chemicals involved in balancing the water as well. 4
Chlorination is the practice of adding chlorine to pool water, to destroy disease-producing microorganisms and other contaminants.
2,4 All forms of chlorine ionise in water to form hypochlorous acid (HOCL). 1,2,4 The HOCL formed, retains the oxidising property of chlorine, and can kill microorganisms by passing through their membranes and destroying their organelles. 1,4 However, HOCL further dissociates into hydrogen and hypochlorite ions, where the degree of dissociation depends on pH and temperature. 1,4 This makes pH control important, as hypochlorite ions are not as effective as HOCL in killing bacteria because of their negative charge. 1,4 At pH levels below six, HOCL dissociates poorly. 1,2,4 Then, between a pH of 6.
0 and 8.5, there is a very sharp increase in the dissociation of HOCL to form hypochlorite ions. 1,2,4 However, acidic water is corrosive. 1,2,4 Hence, a pH level of 7.5 is perfect, having 50% of dissolved chlorine as HOCL. 4
Chlorine compounds, in a solid or liquid state, are added to the pool manually or by erosion feeders.
2,4 Elemental chlorine, a poisonous gas, requires special equipments though. 2,4 It is cheap but extremely acidic, requiring a lot of base to counteract it. 1,4 Using an electrolysis cell, HOCL can be generated continuously if salt is added. 1
Dissolved chlorine exists as free available chlorine if it has not bonded with another compound. 2,4 When it has combined with nitrogen compounds, creating chloramines, it loses its ability as a sanitizer. 1,2,4 This compound, which causes eye burn and strong chlorine smell in pools, is eliminated by superchlorination.
2,4 These compounds are now called combined chlorine. 4 Superchlorination is a process to oxidize and remove all ammonia compounds in a pool by raising the chlorine level. 2,4 A threshold called breakpoint chlorination is reached when chlorine levels are ten times the level of chloramines. 4 All dissolved chlorine exists as free available chlorine at this point because the additional chlorine oxidises chloramines to nitrogen gas and nitrate ions. 1,4
HOCL dissipates under sunlight. 1 Cyanuric acid, used as a stabilizer, reacts with chlorine to produce dichloro(iso)cyanuric acid, which does not absorb the suns ultraviolet light.
1,4 When chlorine is used up in a pool, hypochlorite ions are released from the dichloro(iso)cyanuric acid to replace it, maintaining a constant amount of chlorine in the pool. 1
Water balance is the adjustment of the pH and Total Alkalinity of pool water. 4 The pH of pool water is a measurement of how acidic or basic the water is. 2,4 The pH is adjusted, using acids and bases, to reach an ideal pH range of 7.2-7.8.
2,4 This is because acidic water causes corrosion and eye irritation where else basic water causes the formation of scale. 1,2,4 Alkalinity level is the ability of the water to resist changes in pH and is known as the buffering capacity of the water. 1,2 Alkalinity is raised by bases. 4
Chlorinated pools do have drawbacks. They release trihalomethanes and nitrogen trichloride, resulting in accumulated concentrations around the pool. 3,5,6 Swimmers who inhale these toxins have increased risks of developing lung diseases, such as asthma.
5,6 Nitrogen trichloride triggers three proteins that destroy the cellular barrier protecting the lungs, unleashing an asthma attack. 6 Low pH levels in pool water can also cause dental enamel erosion. 5
There are other sanitizers available other than chlorine. 4 Ozone water treatment produces ozone molecules, which are very powerful oxidisers, by passing air over an ultraviolet bulb or creating a small lightning in an air-filled chamber. 4 Catalytic sanitation uses a non-electric ionizer, which doubles as an ozonator. 4 This self-eroding metallic cartridge releases silver cations and produces ozone molecules too.
4 Enzymes can also be used to break down organic matter in pools. .
