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Swimming Pool Water Chemistry

Water Sanitizing

Back to the Table of Contents

Water Sanitizing Components

As you probably know, the occasional addition of new water -- or wholesale water replacement in the case of spas -- isn't enough to keep the water clean and clear of unwanted and often microscopic contaminants.

But worry not! Pool and spa chemists have spent years developing a variety of tests and chemical-treatment methods to keep your pool and spa safe and sparkling clean.

Your goals here are water sanitation and water balance. In other words, you want your levels of sanitizers (such as chlorine or bromine) and your levels of pH, total alkalinity, water hardness and total dissolved solids to all fall within acceptable limits.

And learning to keep these areas in check isn't as complicated as it may seem. To simplify things, we'll explain the relevant topics one at a time.

Sanitizers Sanitizers, or disinfectants, are hardworking chemicals. They also have a killing streak in them -- which is exactly what you want.

The important thing to keep in mind is, that for them to do their duty, they have to be present in the pool and spa water at all times. There always needs to be a certain minimum amount -- called a residual -- of sanitizer in your water.

What we're saying is, to make sure your pool and spa are protected from bacteria and their friends, you need your sanitizers on 'round-the-clock duty.

When you add your sanitizer, therefore, you should be adding enough to kill any existing contaminants in the water -- and create a residual that can neutralize any dirt, debris or germs that may get into the water before you next treat it.

That said, let's take a look at some of the most common sanitizing chemicals and equipment available on the market today.

Bromine Also available on the market today is a chemical sanitizer called bromine. Do note, however, that bromine cannot be stabilized with cyanuric acid. As much as 65 percent of a bromine residual can be depleted by the sun in a two-hour time period. Because there is no known way to retain a reliable level of bromine when exposed to sunlight, many experts recommend it for indoor pools or spas only.

FYI: The acceptable range of bromine for your pool or spa is from 2.0 to 4.0 ppm.

Bromine also has no odor, and dispensing it your spa via a feeder allows it to dissolve at a slow, constant, desirable rate. Just be sure to remove the feeder when your spa is in use.

A final note: When using bromine as your sanitizer, you will need to occasionally shock the water with large doses of another chemical to oxidize waste material still in the water. (We'll explain this in more detail below.)

Chlorine Chlorine has been somewhat of a wonder drug for pool and spa environments. It has the power to kill bacteria and algae -- and works extremely well in aqueous environments.

Chlorine is not invincible, however. Like most chemicals, it has a threshold -- a point at which it has used up all its sanitizing power and can no longer protect your pool and spa water.

Chlorine levels are also heavily influenced by evaporation, splash out and destructive UV rays, not to mention a low pH. (You'll read about this last factor in more detail below.)

Once added to the water, the "free available" chlorine -- that portion of the chlorine with the killing power -- will sanitize and oxidize the water by attacking undesirables such as bacteria, algae, sweat and oils from your skin, residual soaps, shampoos, perfume and, yes, urine.

As it uses up its killing potential, the chlorine becomes ineffective or it combines with the contaminants and remains in the pool and spa water in the form of chloramines.

FYI: It is the chloramines in your pool and spa water -- not too much "good" chlorine -- that causes a chlorine-like odor and can irritate your eyes and skin.

Indeed, when a pool or spa exudes a chlorine odor and you begin to hear complaints of skin and eye irritation, that is a loud warning that there is not enough chlorine in the water. If this is the case, you should test the water and add the appropriate amounts of sanitizer as soon as possible. (Check out the Testing Your Water section to learn more about this process.)

The recommended level of free available chlorine to keep in your pool is between 1.0 and 3.0 ppm or parts per million. For spas, the recommended level of free available chlorine is between 1.50 and 3.0 ppm.

(For the definition of parts per million and other pool- and spa-related terms, see the Glossary of Terms section).

Another note: We'll address how you measure chlorine and other water-balance levels in another section.

Chlorine is extremely susceptible to sunlight and needs to be regularly monitored. But just as we use sunscreen to protect our skin from the sun, chlorine uses a sunscreen of cyanuric acid. Used this way, cyanuric acid is also commonly called a stabilizer or conditioner.

With its help, chlorine retains its effectiveness. Without going into the chemical whys and wherefores, know that acid works to help keep a fairly consistent chlorine level (a residual) in the water.

Some chlorine-based sanitizers are sold with a dose of cyanuric acid already mixed into the product. One such product is trichlor tablets, which are usually placed in a floater, chemical feeder or in the skimmer basket. Another commonly used product is sodium dichlor, which is a granular substance usually dispersed directly into the pool or spa water, or added via the skimmer.

Oxidizers OK, you've been reading about oxidizers and oxidizing -- and still have no idea what they are or what they do. Let's find out.

Oxidizers work with sanitizers to rid your pool and spa water of pollutants. By definition, sanitizers kill things like algae and bacteria but they work very slowly and inefficiently when trying to remove waste products such as sweat, skin oil, shampoos, soap and urine. This is where you need separate oxidizers.

Oxidizers destroy these undesirables. They do their part by breaking down the contaminant's structure, rendering them vulnerable. The sanitizers can then attack and kill the organisms.

Some products, such as chlorine, act as both a sanitizer and an oxidizer. Bromine, on the other hand, does not oxidize very well and therefore needs the aid of a separate oxidizing chemical to properly clean your pool and spa water.

The marriage of sanitizers and oxidizers is an almost foolproof method of keeping invaders out of your pool and spa water.

Sanitizers - Alternative Although chlorine and bromine are the most commonly used sanitizers, there are a few alternatives available to also consider. One such alternative is a chlorine generator.

Wait, we just discussed chlorine! How is this product different?

The liquid or granular chlorine most people are familiar with is added directly to pool or spa water or dispensed through a feeder or similar automated system. Chlorine generators, conversely, actually create chlorine in the pool or spa without you having to measure out any chemicals.

These generators are electrical devices that manufacture chlorine from salt added to the water. The resulting chlorine gas is then put directly into the water through the circulation system.

Chlorine generators create a residual, which can be tested with a DPD or OTO test kit. (These test kits are described in more detail in a later section.)

Another alternative sanitizing source is ozone. No, it's not just the atmospheric layer that protects us from the sun's harmful rays. Rather, the ozone used in pools and spas is a modified version of that gaseous oxygen.

To its advantage, ozone works quite well as a sanitizer and an oxidizer. . Because it is a form of oxygen, ozone doesn't last long in an aqueous environment. Once it does its job of ridding the pool and spa water of bacteria and other unwanted matter, the ozone then reverts back to oxygen and either dissolves into the water or escapes into the air.

Because it cannot create a residual, an ozonator must be used in conjunction with small amounts of chlorine or bromine -- how much chlorine or bromine depends on how long the ozonator is run each day.

One other common kind of sanitizing equipment is an ionizer. This system introduces silver and copper into your pool or spa water through the circulation process. It works well as a sanitizer but does require the addition of an oxidizer.


This is the most commonly used method of sanitation today. Bromine and iodine are other members of the halogen family of chemicals also used to sanitize water. Other chemicals include ozone, silver and copper compounds. Ultraviolet light is a nonchemical disinfectant. Each of these methods will be covered to some extent, but chlorination is presently the most widely accepted means of treating pool water.

All chlorine - regardless of whether it is introduced as a gas or as a dry or liquid compound when added to water, does exactly the same thing: It forms hypochlorous acid (HOCI) and hypochlorite ions (OCI-).

HOCl is the killing form of chlorine; OCI- is relatively inactive. However, together, they are free available chlorine (FAC).

Because each of the many chlorinating agents produces the same active form of chlorine, we can ignore the source for now and deal with the process of chlorination in general.

HOCI is an extremely active, powerful chemical. It not only destroys such harmful organisms as bacteria, algae, fungi, viruses, etc., it also destroys impurities that are not removed by filtration. These two processes are called sanitation and oxidation.


Sanitation is the process of destroying organisms that are harmful to People. These organisms, referred to as pathogens, include bacteria, fungi, viruses, etc. Chlorination also controls algae (which are not usually harmful themselves, but may harbor pathogenic organisms). In addition to being unsightly, algae can cause the surfaces around the pool to become slippery and unsafe.

While each of these organisms may require different amounts of HOCI for control, the required amount for public swimming pools is often established by local health officials. Very often, local codes will specify an FAC residual of 1.0-3.0 parts per million (ppm), but some might vary from this.


Oxidation is the process of chemically removing organic debris, such as body waste, particulate matter and perspiration, from the water. The process is similar to burning trash in air. It is not important to understand the chemistry involved; it is sufficient to know that enough chlorine in water will chemically "burn" impurities.

The use of chlorine to clean up water is a supplement to filtration, discussed in another chapter. Filters remove the dirt and debris suspended in water, but even the best filter cannot remove dissolved impurities because they are not physically separate from the water. If the water looks dull or hazy, even though the filter system is operating properly, the operator should consider a shock treatment to oxidize the organic impurities and restore the clarity of the water. Although there are some non-chlorine shock treatments available, the most common method used to shock water is superchlorination.


Superchlorination is a term that describes an extra large dose (usually 8 to 10 ppm) of chlorine to oxidize organic compounds and kill and remove algae and other contaminants from the water. This is the same as using three to six times the normal dosage of a chlorinating agent. For example, a 50,000-gallon pool requires about four gallons of liquid pool chlorine (12% Available Chlorine) or six pounds of a granular chlorinating compound such as calcium bypochlorite (65% Available Chlorine).

As mentioned, HOCI is the form of chlorine that provides sanitation. Because it is an extremely active chemical, however, it also reacts with organic impurities. When there is enough HOCI present, the impurities are completely oxidized. Combined chlorine is formed when there is an insufficient supply of HOCI or when there is a very high level of organic impurities. Combined chlorine compounds can be oxidized by increasing the HOCI level in the water. The point at which all the organic impurities are oxidized is called the breakpoint. The addition of sufficient chlorine to reach this point is known as breakpoint chlorination.

Combined Chlorine

Combined chlorine is formed by chlorine combining with ammonia and other nitrogen-containing organic compounds. Some sources of these compounds include perspiration, urine, saliva and body oils. These combined forms of chlorine, also called chloramines, are still disinfectants, but they are 40 to 60 times less effective than free available chlorine.

Chloramines kill slowly, so when they are formed in swimming-pool water, the FAC is no longer present for "instant kill" sanitation. Free available chlorine and combined chlorine exist together in many pools. There are simple tests to measure the levels of each. These will be described in detail in another chapter.

In addition to reduced effectiveness against bacteria, chloramines cause eye irritation and the so-called 'chlorine odor" that swimmers complain about. Chloramines have a foul, irritating odor; free chlorine in water in normal concentration has no discernable odor.

Breakpoint Chlorination

When chloramines are known to be present, either by test (over 0.2 ppm combined chlorine) or because of a foul chlorine odor, the continued addition of chlorine causes a corresponding rise in measurable chlorine residual, but eventually, a point is reached at which addition of chlorine causes a sudden drop in residual. This phenomenon is accompanied by a reduction in eye irritation and chlorine odor.

Investigation reveals that when the total concentration of chlorine in the water reaches seven times the amount of combined chlorine, the oxidation of chloramines and other organic compounds is complete. The point of residual concentration at which this sudden reaction occurs is called the breakpoint. Chlorine remaining or added after the breakpoint is reached exists as free residual chlorine, and all the combined residual is oxidized. The breakpoint varies in its speed and amplitude, depending upon the organic matter present. In some waters, the breakpoint is hardly discernible.

The practice of periodic superchlorination is actually an attempt to pass the breakpoint to rid the water of an accumulation of combined chlorine and potential chlorine-consuming compounds.

chem-fig1.gif (4140 bytes)

Figure 1 - Graph of breakpoint phenomenon

Inorganic and Organic Chlorine

Inorganic chlorine compounds such as calcium hypochlorite, sodium hypochlorite and lithium hypochlorite, as well as chlorine gas, are greatly affected by direct sunlight. They require the addition of cyanuric acid to be stabilized when used in outdoor pools. These products are recommended for indoor swimming pools.

Organic chlorine compounds are products combined with cyanuric acid. Sodium dichloro-striazinetrione ("dichlor') and trichloro-s-triazinetri-one ('trichlor) are both popular stabilized chlorines for water treatment of outdoor pools.

Calcium hypochlorite will support combustion and must never be mixed with carbonated drinks, oils of any type or an organic chlorine product such as "dichlor' or 'trichlor' - explosions can result.

chem-fig2.gif (2422 bytes)

Figure 2 - Disinfection time for free and combined chlorine residuals at various pH values.

Effect of pH on Chlorine

The pH of water has a definite effect on the efficiency of chlorine as well as on the corrosive properties of water (covered later in this chapter.) For now, we will consider only the effect of pH on sanitation.

It can be seen in Table 2.a that free chlorine is most efficient in pH ranges below the ideal range of 7.2-7.6. Some pool operators do, however, maintain pH levels higher than the ideal range. They should also maintain appropriately higher FAC levels to provide the same concentration of the active HOCL form.

For example, at a pH of 8.0, 21% (about 1/5 of the FAC is in the active form. At that pH level, it would take 2.5 ppm of FAC to provide about 0.5 ppm of HOCI. At a pH of 7.5, about 1/2 (50%) of the FAC is in the active HOCI form. At that pH level, it would take only 1.0 ppm of FAC to provide the same 0.5 ppm of HOCI. For this reason, many authorities recommend that the pH of pools be maintained in the range between 7.2 and 7.6 and as close to 7.5 as practical. These conditions are also considered to be most comfortable for the swimmers' eyes and skin.

Hypochlorous Acid

Killing Agent

Active, but unstable form

Hydrogen Ion Hypochlorite Ion

Inactive, but stable form

% Chlorine as HOCI pH % Chlorine as OCI-
90 6.5 10
73 7.0 27
66 7.2 34
45 7.6 55
21 8.0 79
10 8.5 90

Chlorine Gas (Cl2)

Chlorine gas has 100% available strength, is very toxic and can be lethal if an operator is overcome by it. This form of chlorine is the most economical, pound for pound, but the laws regarding safety practices and the intricate feeding equipment it requires make it a feared method of water sanitation. Chlorine gas for pool use is contained under pressure in steel tanks as large as one ton. The gas is green in color and heavier than air.

Strict adherence to the following practices is required for minimum safety:

  1. Chlorine tanks should always be stored indoors in a fire-resistant building.

  2. Tanks and chlorinator must be kept in a separate room with a vent fan capable of complete air exchange in one to four minutes.

  3. Tanks must be chained or strapped to a rigid support to prevent accidental tipping.

  4. A self-contained air supply gas mask must be immediately available in case of emergencies.

  5. The chlorinator and all tanks should be checked daily for leaks. A small amount of ammonium hydroxide (household ammonia) on a piece of cloth produces white vapor in the presence of chlorine.

Sodium Hypochlorite (NAOCl)

Sodium hypochlorite is a clear, slightly yellow liquid solution used in dilute form as common household bleach. In its commercial form, it provides 12% to 15% available chlorine. The chemical is usually introduced to pool water through a chemical feeder, but it can be poured directly into the pool for a quick increase in chlorine residual. It has no sediment or precipitate and raises Total Dissolved Solids (TDS). This chemical has a pH of 13 and causes a significant increase in the pool's pH. The occasional addition of muriatic acid or sodium bisulfate can correct the increased pH, however.

Sodium hypochlorite is not stable in storage and gradually loses strength, especially in sunlight. If stored in a dark, cool room, it has a one-month shelf life.

Dilute solutions of sodium hypochlorite can be used for poolside sanitation and for disinfecting and cleaning decks. The chemical should not be spilled directly on clothes and should be immediately washed off if it gets on the skin. However, its safety and low cost has made this a very popular chlorinating agent.

Calcium Hypochlorite (Ca(OCl)2)

Calcium hypochlorite is available in granular or tablet form. It provides 65% available chlorine by weight and remains stable if stored in a dry, cool area. The chemical can be dissolved and introduced to the pool as a liquid, or it can be added in dry form. When applied directly to the pool, it may cause a temporary cloudiness. Direct applications should be broadcast evenly over the water surface to avoid bleaching the pool bottom as a result of having a concentrated amount in one spot. This must be done when no bathers are in the pool.

This chemical, when contaminated by or mixed with an organic compound, can produce a fire. A good rule is never to mix calcium hypochlorite with another chemical or store it in anything but the original container. Mix the chemical into water not water into the chemical. Calcium hypochlorite should not be handled with bare hands and must be kept off the operator's clothes.

As a chlorinating agent, calcium hypochlorite will slightly increase pool pH. It has a pH of 11.8. Operators of gas-chlorinated pools often keep a supply on hand for emergency use or for a quick charge when superchlorinating. Dissolved in water, calcium hypochlorite can be used as a sanitizing agent for decks and locker rooms.

%AVAILABLE CHLORINE 100% 12.15% 65-75% 35% 56% OR 62% 90%
(pH < 1.0)
(pH 13.0)
(pH 11.8)
(pH 10.7)
(pH 6.9)
(pH 2.9

Lithium Hypochlorite (LIOCl)

Lithium hypochlorite is a newer entry in the field of chlorinating agents. Its cost is greater than other hypochlorites, and it provides only about 35% available chlorine. However, the chemical is fast dissolving and totally soluble in water, and pH tends to rise more slowly with its use than with other chemicals. It has a pH of 10.7. Lithium hypochlorite should not be mixed with organic compounds, but it is safer to store and use than calcium hypochlorite.

Chlorinated lsocyanurates
(Stabilized Chlorine)

This family of chemicals is in wide use for swimming pool chlorination. The family is composed of sodium dichloro-s-triazinetrione and trichloro-s-tria zinetrione - chlorine compounds that contain cyanuric acid (stabilizer). The dichlor is more soluble and provides 56% or 62% available chlorine, depending on formulation. dichlor provides 90% available chlorine and is used when a slow release of chlorine over a period of time is desired. The dichlor compound has little effect on pH, while trichlor is extremely acid (pH 2.8-3.0). Dichlor can be added directly to the pool; trichlor is generally fed through an erosion-type feeder, but never through the skimmer basket.

The major effect of cyanuric acid on bypochlorous acid (HOCI) is to keep it from being decomposed by ultraviolet (LTV) light such as contained in sunlight.

Because it is readily decomposed by LTV light, the dosage of a chlorinating agent that is sufficient for an indoor pool is dissipated rapidly in an outdoor pool. Cyanuric acid bonds with the available chlorine in a manner that does not use up the chlorine. At high stabilizer levels (over 100 ppm), chlorine's efficiency may be reduced. The operator should consult the local codes and manufacturers' recommendations on the proper use of stabilizers.

Chlorination Summary

  • Chlorination both sanitizes and cleans the water by oxidizing organic impurities.

  • A free chlorine residual of 1.0-3.0 ppm is preferred. Combined chlorine should not exceed 0.5 ppm and may be destroyed by breakpoint chlorination.

  • Proper pH control (7.2-7.6) provides better chlorine efficiency.

  • When cyanuric acid is used in an outdoor pool, chlorine consumption is reduced, because the chlorine degradation caused by LTV light is reduced.

  • There are many chlorinating agents available but each provides HOCI, the active chemical specifically for disinfecting.


Other members of the halogen family - bromine and iodine - are used for treating pool water. New developments in the use of ozone, ultraviolet light, ionization of salts and other chemical compounds continue to provide new challenges and techniques in water treatment.


Although bromine in its elemental form is a liquid, it is not available for swimming pool disinfection in that form. Elemental bromine is a heavy, dark brown, volatile liquid with fumes that are toxic and irritating to eyes and respiratory tract. For pool sanitation, bromine compounds are sold in two solid forms - a two-part system that uses a bromide salt dissolved in water and activated by addition of a separate oxidizer; and a one-part stick or tablet that contains both bromine and an oxidizer and is dispensed by an erosion-type feeder.

The chemistry of bromine is similar in many respects to the chemistry of chlorine; however, bromine cannot be used for shock treating.

Bromine has a pH of 4.0-4.5. When bromine is added to water and an oxidizer is present, the bromine forms hypobromous acid (HOBR) and bypobromite ions (OBr). Like chlorine, the percentage of each is affected by pH. However, the effect is not as dramatic as it is with chlorine. Table 2.c displays the effect of pH on bromine. Like chlorine, bromine combines with organic impurities to form combined bromine or bromamines. However, combined bromine is still an effective sanitizer, and it does not smell. Because of this, bromine is popular for spas.

Hypobromous Acid
% Bromine as HOBr
pH OBr-
Hypobromite Ion
% Bromine as OBr-
100.0 6.0 0.0
99.4 6.5 0.6
98.0 7.0 2.0
94.0 7.5 6.0
83.0 8.0 17.0
57.0 8.5 43.0


Ozone was first used as a water sanitizer in France in the early 1900s. It is the most popular method of treating drinking and pool water throughout Europe. Ozone is one of the strongest oxidizers available for treating swimming and spa water and is growing in popularity in the U.S.A. It kills bacteria and oxidizes organic compounds including chloramines, soaps, oils and bather wastes and does not alter the water's pH. Due to limitations of the amount of ozone that can be economically introduced to pool and spa water, algae growth is not eliminated and may possibly increase.

Two systems have been developed for the production of ozone for pool and spa water. The most common European method - and the most expensive - is the Corona Discharge. This method generates ozone by exposing pressurized, dried air to high-voltage electricity. The ozone gas is then directed into the bottom of the pool and seen as very small bubbles rising to the surface. The ozone can be introduced into a separate chamber or directly to the pool.

The Ultraviolet (LTV) or photo-chemical method of ozone production passes the pressurized, dried air next to a UV bulb within a chamber, where the UV rays bombard the oxygen molecules and produce ozone. A single LTV lamp is capable of treating approximately 10,000 gallons of water. The UV method is more common for spas or private small pools and is less expensive to operate. The LTV method cannot match the output of the Corona Discharge method.

Once it has done its job as an oxidizer, ozone reverts to oxygen and improves both smell and taste of the water. This factor is worth noting, especially for indoor pools. Ozone has to be continually generated, because its effectiveness is about 22 minutes while the gas remains in the water. Ozone offers no continuous residual, and because it is not effective against algae growth, most manufacturers recommend that ozone be introduced with a halogen such as chlorine or bromine.

Testing of ozone residual is in parts per billion (ppb). Test kits are available that use DPD as the reagent and color calibrations that indicate readings from 0 to 100 ppb. Testing with the use of an Oxidation Reduction Potential (ORP) meter is effective, and a minimum level of 650 millivolts is recommended. Because ozone of the smallest residual is effective and adequate for treating water, when combined with chlorine or bromine, a test reading of either is an indication of the effectiveness of the ozone.


Electrical devices (chlorine generators) have been developed that produce chlorine from salt dissolved in the pool water or within a separate salt-solution (brine) chamber. This method is becoming popular in Australia and some European countries. Because of their use on spacecraft, development of metals that are used for electrodes has been rapid. There is a real future for this concept in sanitizing water.

Brine Method

The brine type of chlorine generator passes 4 to 16 D.C. voltage of about 20 amps through two chambers. One chamber houses the positive (anode) electrode within a brine solution of salt and distilled water, while the second chamber houses the negative (cathode) electrode, also in water. Chlorine is formed as a gas at the anode and sodium hydrox ' ide (caustic soda) at the cathode. Chlorine gas is drawn off through a tube and introduced into the pool or spa water. Some systems use the caustic soda as a neutralizer for pH balance. This design is similar to those used for producing chlorine gas commercially and can be designed for any size pool.

In-Line Method

The in-line type of chlorine generator is located within the pool or spa circulation system, past the filters. A low concentration of salt is added directly to the pool. The electrolytic cell has been designed for smaller pools or spas and requires a current of 5 to 15 amps with 5 to 35 volts D.C. The production of chlorine gas, caustic soda and hydrogen gas passes directly into the pool or spa water.

The water passing the electrodes is superchlorinated and thereby reduces the build-up of chloramines. The caustic soda aids in balancing pH. The control unit is capable of reversing the charge on the electrodes to eliminate scale build-up. The chlorine generated from salt will, after use, revert back to salt, and the process starts all over again. The addition of salt is reduced greatly by this recycling.


Chlorine generators activated by ORP sensors have produced a system that reacts to fluctuations in swimmer load and oxidizing demands for algae prevention. In-line units in large pools have been coupled with electronic control systems to offer a means of continuously treating the water.


The bactericidal properties of silver nitrate and Argyrol are well known in medical practice. Silver ions are introduced to water by electrolysis or by passing a current through a silver electrode. The primary limiting factors in its use in swimming pools are the high cost of silver and the fact that its bactericidal action is quite slow. Silver is also photo-sensitive and may cause a black deposit to form on the walls.


Products are available to oxidize body wastes and organic materials and to reduce combined chlorine or chloramines. These products are not sanitizers and are used as an alternative to superchlorination for oxidizing only. Non-chlorine oxidizers are useful for clearing cloudy water in heavily used pools located in areas where health codes place upper limits on free available chlorine readings.


The greatest use of chemical products is in the bactericidal treatment of pool water. Many special conditions, however, create requirements for additional chemical treatment.


Aluminum sulfate (Al2(SO4)3), commonly called alum, is used as a filter aid and as a coagulant and settling agent for water turbidity. Alum "floc' is a white, gelatinous substance that attaches to freefloating matter in water to form larger, heavier-than water particles, which settle to the bottom of the pool. Alum floc is especially effective on sand filter beds. The floc partially fills the voids in the sand bed and holds organic debris in its suspended gelatinous coating.

Alum is introduced as a filter aid at the most convenient entry point ahead of the filter. The chemical feed, hair and lint strainer or skimmer are effective points of introduction. However, to coagulate particles in pool water, a powdered alum is broadcast over the pool surface at the rate of two ounces per square foot of surface area and is permitted to stand overnight or for a minimum of two hours. After standing, the pool should be vacuumed with minimal agitation to prevent the floc from breaking apart. It is recommended when using alum as a water clarifier or a filter floc that the pool water be adjusted to pH of 7.6-7.8.

Flocculant aids, with a combination of ingredients, sold under various trade names, have been used to produce a heavier or more stable floc. Colloidal silica, a clay called bentonite and a new family of organic polyelectroloytes are available.


The chemistry of algaecides is complex, because 46 species of clean-water algae exist. Some algaecides work better on one kind of algae than on another.

Planktonic clean-water algae float on the surface. Other types attach themselves to rough spots on the pool walls and floor and are very difficult to remove. Clean-water algae may be blue-green, red, brown or black and can cause tastes, odors, turbidity and slippery spots, as well as increased chlorine demand.

Sunlight, temperature, pH, bacteria, chlorine residual and the mineral content of the water affect the presence and growth rate of algae. Algae can be introduced to a pool by wind-borne debris, rain and falling leaves, or they may be present in the source from which the pool is filled.

Preventing algae growth by chlorination is usually not a problem, but removing existing algae from a pool can be difficult. If algae get a firm start on the side or bottom of a pool, draining the pool is sometimes more practical if the local water table is not too high to allow it. The pool should be thoroughly washed down with a chlorine solution.

Sunlight is necessary to the growth of algae, so it is a much greater problem in outdoor pools. If not controlled, algae can spread rapidly, turning an entire pool dark green in as little as a day or two.

Pools that consistently maintain a high freechlorine residual concentration are seldom troubled by algae. Maintaining free available chlorine and superchlorinating are the best preventative measures. Combined chlorine is not as effective as free chlorine in preventing algae growth, and bromine and iodine are even less effective.

An algae inhibitor is a commercial product that acts as a penetrating or wetting agent to allow the chlorine to be more effective. Algae inhibitor is said to control all types of algae growth and provide a stable backup system to chlorine. It is not pH sensitive, does not evaporate, concentrates on surfaces of the algae and is a powerful wetting agent.

Chelating or Sequestering Agents

Many stains around main drains and inlets have to be cleaned by hand, but the stains can be kept from returning. Sequestering agents increase the ability of water to hold metals in solution instead of precipitating out to form stains. Chelating agents remove iron and other metals from the water and the pool walls. Pools with high iron content use a chelating agent as part of routine water treatment.


There are commercial acids and biodegradable detergents that effectively clean D.E. filter bags and filter sand. Each product has its own ability to degrease and rejuvenate filters. Each filter system has its own individual solution for doing the job.


Foam or sudsing is a chronic problem for most spas. Occasionally, a box of detergent is thrown into the water as a prank, and a defoamer made specifically for pools and spas is necessary to remove the foam. A bottle of defoamer is handy to have on hand as a safeguard.

Cyanuric Acid

Chlorine in outdoor pools must be shielded from the degrading effects of the sun's ultraviolet (UV) rays. Cyanuric acid is used in outdoor pools with the inorganic chlorines such as calcium hypochlorite, sodium hypochlorite, lithium hypochlorite and chlorine gas.

It is recommended that the cyanuric-acid level be 30-50 ppm with a maximum of 100 ppm and a lower limit of 10 ppm. The level of cyanuric acid is reduced by dilution due to filter backwashing, bather dragout or dumping. To meet health codes, pools above 100 ppm need to be drained of about 20% of volume, and fresh water added. Four pounds of cyanuric acid will offer a reading of 50 ppm in 10,000 gallons of water. Cyanuric acid has a pH of 4.0, and if added to a gas-chlorinated pool, the addition of one-third of a pound of soda ash per pound of cyanuric acid is recommended.

'Chlorine Lock,' a term given to a condition once thought to be produced from high cyanuric-acid levels tying up free available chlorine, has been proven false by the industry. Generally, high cyanuric-acid levels of 400 ppm or higher are associated with excessive Total Dissolve Solids (TDS) or combined chlorine or cbloramines and not "Chlorine Lock".