How Are Swimming Pools Kept Clean?

Swimming pools are a miracle of science and hygiene; thanks to the wonders of chlorine—a poisonous gas that’s been coopted to keep pool water clean and sparkling—your local pool can operate safely without risk of spreading potentially dangerous water-borne pathogens.

But keeping that pool water clean and swimmable requires a lot of maintenance. While open bodies of water, such as lakes, rivers, and oceans, have constant influx and outflow of water to help dilute and push out impurities, swimming pools don’t have such robust natural means of filtration and purification. Instead, they rely on mechanical filtration and a carefully calibrated stew of chemicals to prevent illness.

In a 2007 paper, “Clear Waters and a Green Gas: A History of Chlorine as a Swimming Pool Sanitizer in the United States,” chemist Kevin Olson of Montclair State University in New Jersey, writes that “beginning in the 1890s, a hygienic reform movement grew in larger cities both in the United States and Europe, the goal being to promote health through cleanliness by providing the urban poor with public baths.”

This movement coincided with the rise of the textile and paper industries, both of which used chlorine as a bleaching agent. It didn’t take long for this inexpensive byproduct to find its way into municipal water systems and swimming pools as a means of keeping them free of infectious pathogens. Olsen writes that “credit for the first use of chlorine to disinfect potable water goes to British scientist Sims Woodhead, who used ‘bleach solutions’ as a sterilizing agent during an 1897 typhoid outbreak in Maidstone, Kent.”

Though that measure was intended to be temporary, chlorine’s efficiency made it an ideal chemical agent for keeping water clean, and it didn’t take long for it to find its way into swimming pools. “As near as the author can determine,” Olsen writes, “the first attempt to sterilize a pool in the United States with chlorine was made at Brown University in Providence, Rhode Island,” when the Colgate Hoyt Pool opened on March 2, 1903. Add the installation of a filtration system that turned the brownish water clear, and the modern image of a swimming pool was born.

To keep the pool water clean and safe, pool operators must maintain sanitizing chemicals within strict parameters. This is why you’ll see a lifeguard or pool manager testing the pool water every hour at your local pool. As organic matter—think sweat, urine, and microscopic bits of fecal matter—leach off the human body into the water, it uses up the disinfectant qualities of the chlorine. That chlorine needs to be replaced continually to keep the water from getting really gross.

Provided the appropriate pH and chlorination levels are maintained, your local pool may recirculate the same water for years. Many pool companies recommend a complete drain, clean, and refill of pools every five to seven years. This is dependent on how heavily the pool is used and other factors, including state regulations.

However, the Centers for Disease Control and Prevention recommends draining and replacing “portions of the water on a weekly to monthly basis, depending on usage and water quality.” The water should also be shocked daily or weekly depending on water quality; a municipal pool in heavy use at the height of summer will likely need more upkeep than a seldom-used backyard pool.