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General Water Quality Information

Swimming Pool/Spa Problems and Maintenance

What chemical ingredients are usually added to swimming pool water to increase alkalinity?

Although sodium bicarbonate (NaHCO3) is an acid forming salt, at near neutral pH values, it will increase total alkalinity with little impact on pH. If the water in your pool is soft (below 100 ppm) and acidic well water, it may require some adjustment for hardness as well as for pH and alkalinity. A pool professional can assist you in making these decisions. Shock treatment with calcium hypochlorite [Ca(OCl)2], a chemical disinfectant, can be used to increase calcium hardness.

What does cyanuric acid actually do in chlorinated swimming pool water to help stabilize chlorine and pH level?

Cyanuric acid performs two functions. It forms a weak bond with free chlorine in the pool water, effectively trapping it and slowing its escape to the atmosphere, and it also slows chlorine breakdown from the sun's UV rays. Hypochlorous acid, the primary chlorine-based disinfectant in water from addition of chlorine-based disinfectants, is readily decomposed by UV light. Cyanuric acid slows this process by absorbing UV wavelength light that strikes the pool surface. Properly managing cyanuric acid level in outdoor pools has been shown to reduce amount and frequency of chlorine additions needed to maintain residual chlorine levels for adequate disinfection. Even in small pools with a moderate bather load, cyanuric acid can significantly reduce the costs spent on chemical chlorine disinfectants.

What happens if a public swimming pool does not completely pass a health and safety inspection?

Depending on the seriousness of the violation, the pool may be required to suspend operations until the problem is corrected. Violations leading to pool closure include: elevated bacterial counts, low disinfectant levels, high water turbidity, or poor safety equipment, among others.

What happens if the chlorine stabilizer (cyanuric acid) concentration gets too high in my swimming pool water?

The chlorine will become locked in the chlorinated isocyanurates. This can render it ineffective as a disinfectant in the pool although a chlorine test kit indicates an adequate level of free available chlorine. A cyanuric acid level of 30 to 40 ppm is ideal for most pools. If cyanuric acid level gets above the desired concentration (above 60 ppm for most pools), you will have to replace part of the pool water to lower the stabilizer level.

What is a chlorine generator?

It is any device that uses electrolysis of salt water to generate chlorine. When direct current is passed through water containing NaCl (common salt) water molecules will decompose and hydrogen ions will migrate and collect electrons at the negative electrode (cathode). The hydrogen ions pair up to form hydrogen gas molecules. With some electrolytes oxygen is formed but in this case chlorine gas is generated at the anode (positive electrode) where chloride ions migrate, lose an electron and then pair up to form diatomic chlorine gas. Left behind in the water are sodium ions (Na+) and hydroxyl ions (OH-), which can significantly raise pH of the solution. If chlorine is added back to the solution, pH will be unchanged. Special salt water electrolysis systems can be designed to strip away both the hydrogen and chlorine gases. If chlorine is not removed, it will go into solution to form several chlorine species with the dominant species dependent on pH. At a pH significantly below 7, chlorine gas (Cl2) will be the dominant form, while hypochlorous acid (HClO) will be the dominant form at neutral pH values and hypochlorous acid ion (ClO-) will be the dominant species at higher pH values. At the higher pH values the solution may be considered to be an ionized solution of sodium hypochlorite NaOCl and sodium hydroxide (NaOH). This is why electrolysis of salt water is also called hypochlorite generation.

What is a conditioner or stabilizer that is added to swimming pools and what is its purpose?

This is a chemical, usually cyanuric acid, which is added to swimming pools to prolong the life of chlorine in water by slowing down chlorine decay due to sunlight. On a hot sunny day, 2 ppm of chlorine can be lost from a swimming pool in two hours through decomposition by sunlight. Some years ago, scientists found that if cyanuric acid, a weak organic acid, was added to water containing chlorine, it had a special stabilizing effect. The chlorinated isocyanurates that were formed significantly reduced chlorine loss but allowed the chlorine to be used up through disinfection while the cyanuric acid remains behind as a residual in the pool.

What is a salt-water swimming pool?

It is simply a swimming pool that has enough salt added to utilize on-site and in-line chlorine generation for effective pool disinfection. A salt-water pool is not chlorine-free nor does it contain water equivalent to the salinity of seawater, as some people assume. Salinity in a salt water pool is generally maintained at a level (3500 ppm) that is about one-tenth of that for seawater (35,000 ppm).

What is chloramination?

Chloramination is the process of mixing ammonia and chlorine to form chloramine, which is a weaker but longer lived disinfection agent than chlorine. The process is used for disinfecting drinking waters, pools and spas.

What is chlorinated lime [Ca(Cl)2:Ca(OH)2:H2O] used for?

Chlorinated lime is a very strong oxidizing agent that is active in alkaline and neutral solutions. It has many uses and plays a role in public health and environmental hygiene. It is used to disinfect drinking water, pool water and wastewater; to sanitize equipment and soil contaminated areas; as a whitening and bleaching agent in the textiles, leather, pulp and paper industries; and more recently as a strong oxidizing agent to neutralize certain hazardous products.

What is chlorinated lime?

Chlorinated lime is chemical that was first made by passing chlorine gas over slaked lime [Ca(OH2)] to form a chemical with the formula CaOCl2. It is used for bleaching and is commonly referred to as bleaching powder. Although the chemical is sometimes referred to as chloride of lime, this is not accurate because the chemical is actually a crystalline mixture of calcium hypochlorite [Ca(OCl)2] with alkaline calcium chloride [Ca(Cl)2:Ca(OH)2:H2O]. The mixture contains 30-35% active chlorine. Chlorinated lime is sometimes confused with calcium hypochlorite that is marketed under the trade name of HTH (high-test hypochlorite).

What is cyanuric acid and what is its association with swimming pools?

Cyanuric acid is a cyclic triazine chemical (C3H3N3O3) and a weak acid that is marketed as a chlorine stabilizer for swimming pools. This chemical binds somewhat with the hypochlorite ion to slow down its decomposition from UV rays in sunlight. As long as cyanuric acid level does not get too high, its primary beneficial affect is saving on chlorine dosing for a swimming pool. If the cuanuric acid level gets too high however, all chlorine is bound by the stabilizer and becomes ineffective as a disinfectant. There are other FAQs in this database on the use of cyanuric acid.

What is electrochlorination?

This is the name generally applied to electrolysis of seawater or a brine solution (fresh water + salt) where the intent is to produce chlorine gas or sodium hypochlorite. It is an electrolytic process where D.C. current is applied, usually through titanium elctrodes, to the saltwater or seawater in an electrochemical cell. Oxidation of chloride ions occurs at the anode to produce chlorine gas. Reduction of water occurs at the cathode to produce sodium hydroxide and hydrogen gas. In a single cell system, the chlorine gas reacts immediately with the sodium hydroxide to produce sodium hypochlorite while the hydrogen gas bubbles off. The overall reaction sustained by electrical energy is: 2NaCl(salt) + H2O ---> NaOCl(hypo) + NaCl(salt) + H2(hydrogen gas). Note: Hydrogen gas should be vented away due to risk of explosion. A number of chemicals can be produced and extracted from seawater using this type process. For chlorine production, the process is often referred to as on-site chlorine generation for disinfection of both swimming pools and drinking water sources.

What is high intensity UV for disinfecting pools and spas?

High intensity UV refers to greater operating output of a germicidal UV lamp expressed in milliamps (ma). A high intensity UV lamp operates at 700-900 ma, where a standard lamp used for typical household water disinfection operates at about 400 ma.

What is hydrogen peroxide?

Hydrogen Peroxide (H2O2) is a clear sharp-smelling and syrupy substance very similar in appearance to water (H2O). Like water it is made up of hydrogen and oxygen. The "per" prefix means extra or excess, so peroxide means extra or excess oxygen. In peroxides, the extra oxygen is covalently bonded to another oxygen atom, a rather unstable arrangement. It is this extra oxygen atom that gives H2O2 its useful properties. H2O2 has been used for many purposes including cleaning, bleaching, sterilizing, rocket fuel, animal feed treatment and in addition many miraculous claims about its health benefits have been made.

What is Legionella?

Legionella is a genus of bacteria. Legionellae (plural form) are aerobic, non-spore forming, rod-shaped, typically flagellated, gram-negative bacteria common to many moist or aquatic environments. At least 50 species and 70 serogroups have been identified. Legionella species are the causative agent of a water-borne pneumonia type human disease called Legionnaire's disease or legionellosis, and also the agent for a lesser form of the disease called Pontiac fever. Transmission is via aerosols due to inhalation of mist droplets containing the bacteria.

What is legionellosis?

Legionellosis is a collective term describing any illness caused by exposure to the bacterial pathogen Legionella. Legionnaires' disease and Pontiac fever are the two most common types of legionellosis, with Legionnaires’ disease being the more serious and primary ailment of focus. It is an environmental disease, with the causative agent transmitted from an environmental source (water or soil) to a host. It is not transmitted from person to person, thus, it is not a communicable disease.

What is meant by breakpoint chlorination of swimming pool water?

Breakpoint chlorination is the accepted method of removing chloramines and unwanted organic debris that is causing cloudy water and promoting algae growth in the swimming pool. Breakpoint chlorination is achieved by applying at least 10 times the amount of combined chlorine in the pool. The combined available chlorine (that combined with ammonia to form chloramines) is generally calculated by subtracting free available chlorine from total available chlorine, determined from special pool water test kits. Most pool professionals recommend that breakpoint chlorination, also referred to a shock chlorination, be performed when combined available chlorine (chloramine level) exceeds 0.2 ppm. This will improve bather comfort because chloramine levels above 0.2 ppm will irritate your eyes. Lowering chloramines will also make residual free chlorine a more effective disinfectant because chloramine has only about 1% of the sanitizing power of free available chlorine. More information on chlorination, breakpoint chlorination and shock treatment of pools is covered in pool maintenance handbooks and databases provided on the internet.

What is salt-water chlorination or a salt-water chlorinator?

Salt-water chlorination is the primary method for on-site generation of chlorine that can be used to sanitize a pool or spa or used to disinfect drinking water. The process works by passing an electric current through salty water to produce chlorine gas at the positive electrode (anode), the same method most commonly used for commercial chlorine production from seawater. This method is gaining in popularity because it reduces health risks associated with storage and handling of toxic chlorine-based chemical products. This is sometimes referred to as electrochlorination.

What is shock chlorination of drinking water or swimming pool and spa water?

Shock chlorination, also called super chlorination, is the process of disinfecting and/or completely sanitizing drinking water with a high concentration of chlorine. Shock chlorination is the most widely recommended means of treating bacterial contamination in privately maintained home water systems such as wells, springs and cisterns. In general, enough active chlorine is added to a drinking water system (including well, storage tank and all plumbing components) to maintain a chlorine concentration of at least 200 parts per million (ppm) for at least 6 hours (preferably 12 to 24 hours). Shock chlorination of water in swimming pools and spas requires about the same treatment time, but it is performed much more frequently, sometimes up to once or more times per week during heavy use, to rid the water of chloramines formed from reaction of chlorine with ammonia form of nitrogen that originates from human urea. In shock chlorination of swimming pool and spa water the desired free chlorine level is about 20 ppm (8 to 10 times the normal combined chlorine dose) instead of the 200 ppm level desired for sanitizing drinking water.

What is super chlorination or shock treatment used for in swimming pools?

This is the practice of adding 5 to 10 times the normal chlorine dose (up to at least 10 to 20 ppm or more) to pool water to destroy algae, or to prevent problems after heavy bather loads or severe rains. Shock treatment is also used to help clear up cloudy water and destroy chloramines in the water. If chlorine stabilizer (cyanuric acid) is not too high (above 60 ppm), the pool should be back to its normal residual chlorine level (3 to 5 ppm) within 24 to 48 hours. This should not be confused with shock chlorination of private drinking water where a temporary chlorine concentration up to 200 parts per million or higher is used to ensure complete sanitation of a well or other water source and all storage and distribution system components.

What is the available chlorine content of calcium hypochlorite granules or tablets that can be used for chlorination disinfection of water?

The available chlorine content in calcium hypochlorite, normally used in swimming pool disinfection and commonly referred to as HTH, is 65 percent by weight.

What is the best way to add acid to a swimming pool if the total alkalinity (TA) is too high (above 150ppm) and the pH keeps drifting above 7.6?

The two most common types of acid used to lower pH and alkalinity in swimming pools are muriatic acid (hydrochloric acid) in liquid form and sodium bisulfate as a powder. Some people prefer using the dry acid because of the hazards of handling the more corrosive muriatic acid, but most pool specialists say muriatic acid is the best because of how it interacts with other pool chemicals. In handling acids, it is always better to wear plastic gloves and glasses to protect against splattering concentrate into your eyes or getting it on your hands. The best and simplest way to add acid to a pool is to fill a two to five-gallon plastic bucket about half full with pool water. Then slowly pour about one quart of muriatic acid into the bucket of pool water. Now, slowly pour the bucket containing pool water and acid across the deep end of the pool. Care should be taken to make sure the recirculating pump is running and runs for about one hour after adding the acid. Care should also be taken to avoid spilling any acid on the concrete surface or other surfaces next to the pool because this could cause permanent damage.

What is the chemical reaction that takes place in a salt-water chlorinator?

It is the electrolysis of sodium chloride (NaCl) dissolved in water. Sodium ions are released at the negative electrode (cathode) and react with water to form sodium hydroxide (NaOH). Chlorine ions are released at the positive electrode (anode) to form chlorine gas that reacts with water to form hypochlorous acid. The chemical reaction can be written as: NaCl + HOH + electricity <===> Cl2 + NaOH + H2^ <===> NaOCl + HOH <===> HOCl + NaOH.

What is the difference between what is called stabilized chlorine and non-stabilized chlorine for swimming pools?

Cyanuric acid binds with and lengthens the life of chlorine in pool water. Chlorine products that contain cyanuric acid as an ingredient are referred to as stabilized chlorine products. These type of chlorine products are used to main consistent levels of chlorine in pool water. Chlorine products that do not contain cyanuric acid as an ingredient are referred to as non-stabilized chlorine products. NOTE: Once a certain level of cyanuric acid is reached in pool water, non-stabilized chlorine products can then be used because the cyanuric acid present will bind with and retard the loss of this chlorine. If cyanuric acid level gets too high, chlorine will be held too tightly to be available for chemical oxidation reactions and thus ineffective in water disinfection. The issue of cyanuric acid balance is explained in other FAQs within this database.

What is the HTH chemical used for disinfecting swimming pool water?

HTH is an acronym for high-test hypochlorite and is the calcium salt [Ca(OCl)2]of hypochlorous acid (HOCl). Both hypochlorous acid and hypochlorites release the hypochlorite (OCl-) ion in solution. This ion reacts vigorously as an oxidizing species with organic materials to serve as a strong bleaching agent and chemical disinfectant. HTH is the strongest solid-form chlorine-containing bleaching agent available.