Many people think that public swimming pools are emptied every evening and refilled with clean water for the next day. This may be suitable for a paddling pool in the garden but it is quite impractical for a large public pool, which takes hours to empty and days to refill, quite apart from the huge cost of heating fresh water to the required temperature. In fact, swimming pools are only completely drained for maintenance (perhaps once or twice a year), while for the rest of the time, the same water is endlessly circulated, with dirty water being filtered, disinfected, and heated before being returned to the pool.
The water in a swimming pool gets dirty because of the bathers who use it. The use of showers before swimming removes some dirt but bodily excretions such as sweat, mucus, and saliva are washed into the water while swimming. Urination is common, if often involuntary, among children.
Cosmetic residues from hair conditioners, deodorants and make-up also get washed off the skin while swimming. Foam rubber from pool toys, fragments of clothing, and even hair pins end up in the water.
The water treatment process reflects the multi-faceted nature of the pollutants. Larger objects which would damage the pumps are removed by a strainer basket. Small particles of dust and dirt are trapped by a large filter containing graded layers of sand, but to aid the filtration of microscopic particles a coagulant such as polyaluminium chloride is added to the water which encourages the smallest particles to clump together and become trapped in the sand.
Eventually the filter becomes clogged with a layer of residue and backwashing becomes necessary. This simply reverses the flow of the water through the filter and allows the accumulated debris to flow to waste. Before backwashing it is necessary to scour the filter with compressed air, which bubbles through the sand and loosens the dirt. Filters are backwashed in sequence so that at least one filter remains operational at all times.
In addition to filtration the water needs to be disinfected. This prevents the transmission of infection and curbs the population growth of micro-organisms that would thrive on the plentiful supply of microscopic food introduced by bathers. The most commonly used disinfectant is chlorine, however in its pure form chlorine is a dangerous and volatile gas so it is normally added to the pool using a chemical "donor". This is a stable compound (typically calcium hypochlorite) which reacts with water in a similar way to chlorine to produce hypochlorous acid. This acts as a disinfectant by reacting with pollutants and destroying them. Hypochlorous acid also breaks down spontaneously into two charged molecules (ions), one of which, the hypochlorite ion, also acts as a disinfectant.
The combination of hypochlorous acid and hypochlorite is known as free residual chlorine, normally maintained at a level between one and three parts per million. In destroying pollutants the disinfectant is itself destroyed, so it must be continuously replenished.
pH levels The effectiveness of disinfectants depends on the pH value of the water, a measure of its alkalinity or acidity. Chlorine becomes less effective as pH levels rise (increased alkalinity) but paradoxically the chlorine donor calcium hypochlorite itself increases alkalinity, so acid (typically sodium bisulphate) must be added on a regular basis to keep pH levels between 7.4 and 7.6. If the pH falls too low, the acidic water can corrode components, so sodium bicarbonate is occasionally added to increase alkalinity.
Dilution One effect of frequent backwashing is to remove water from the pool. Water is also lost when bathers splash or wring out their costumes. Pools are equipped with large "balance tanks" which top up the pool with fresh water. This dilution is not just incidental: it is an important part of controlling the level of pollutants, particularly those unaffected by filtration and disinfection. Public pools are expected to change as much as 30 litres of water per day per bather, which effectively replaces all the pool water over a period of time.