Swimming Pool Chemistry

Swimming pools are used recreationally and for competitive events. Since a participant is often fully immersed in the water, the water's hygiene is important. Water that is contaminated can cause illness.

In contrast to many natural water courses, the circulation and turnover of water in swimming pools can be slow. While the water in most below-ground pools is drawn, pumped through a filter and even a heater, and circulated back to the pool at a legislated rate, the time to replace the entire volume of water can still be less than in a free-flowing stream, pond, or lake. Additionally, the number of people using the limited water volume can be much greater than in a natural setting. If steps are not taken to keep microorganisms and other contaminants under control, the water can become a dangerous medium for the growth of the microbes, and for the continued presence of noxious compounds.

One approach to maintaining swimming pool water quality involves filters, which can remove microbes and larger debris from the water. The latter are removed by a filter basket that acts as a strainer. This process is purely physical; water enters the open end of the filter, and debris that is too large to pass through the mesh basket remains trapped.

A second filter utilizes physical and chemical means to trap smaller material. The filter contains a bed of special-grade sand or diatomaceous earth. Incoming pool water enters at the top of the sand. Typically, the water is then allowed to percolate down through the sand under the influence of gravity. During the slow downward journey, microorganisms and other particles stick to the sand or earth grains. The size, surface charge, and surface chemistry of water contaminants determines how avidly they associate with the sand or earth particles.

Over time, as debris collects, the ability of the filter bed to bind particles decreases. Then, by forcing water back through the bed in the reverse direction in a process called backwashing, the collected debris can be driven out. Once the sand or earth grains have settled, the filtering action of the bed is restored.

Today, the filter bed is sometimes replaced by a synthetic filtering material packaged in a cartridge that can be inserted and removed from the water flow.

Chemicals are also added to swimming pools to aid in keeping the microorganisms at permissible levels, and to maintain a balance of inorganic parameters.

Disease-causing (pathogenic) bacteria are controlled by the use of disinfectants. The most popular swimming pool disinfectant is chlorine, which is typically introduced in the form of a solid (calcium hypochlorite) or a liquid (sodium hypochlorite). (Instead of chlorine, a chemical called bromide can be used. Since it tends to be more expensive than chlorine, bromide is not the typical disinfection method of choice.) Less routinely, chlorine gas can be added to the source water. Predetermined amounts of the materials are added, based on the volume of water in the pool, to produce a final free chlorine concentration in the water when the solid dissolves and the liquid or gas disperses. The final chlorine concentration is important. If too little chlorine is added, for example, the chemical reaction that occurs can generate various forms of a compound called chloramine. Chloramines have an objectionable smell and can irritate the skin and mucous membranes such as the eyes.

Addition of the proper amount of chlorine will produce mainly hypochlorous acid in the ensuing chemical reaction. Hypochlorous acid kills bacteria by disrupting the structure of their cell walls and destroying the activity of enzymes inside the cells that are vital for life.

Hypochlorous acid is unstable and tends to lose its potency in sunlight. Thus, many outdoor pool operators will also add cyanuric acid to the water. Cyanuric acid associates with hyochlorous acid and stabilizes its structure, while not affecting its antimicrobial potency.

Another aspect of swimming pool chemistry concerns the pH level of the water—the acidity or alkalinity of the water. The level is measured on a logarithmic scale (each division on the scale is ten times different from the preceding or subsequent value) that indicates the balance between acid and alkaline molecules in the water. The pH scale ranges from 0 (extreme acidity) to 14 (extreme alkalinity). Swimming pool water should have a pH of 7.2 to 7.8; within this range, the antimicrobial action of chlorine is most efficient and the water is most comfortable to the user. Water that is too acidic can damage metal surfaces and is too harsh to skin, whereas water that is too alkaline causes build-up of deposits on surfaces and becomes objectionably cloudy.

The pH of swimming pool water can be easily measured using a number of portable devices. If necessary, adjustments to pH can be made by the addition of sodium carbonate or sodium bicarbonate, which raises the pH, or muriatic acid, which lowers the pH.

A swimming pool is an ever-changing environment. Thus, swimming pool chemistry needs to be frequently monitored and, if necessary, altered throughout the day.