WATER CHEMISTRY FOR SWIMMING POOLS

WATER CHEMISTRY FOR SWIMMING POOLS

Proper water chemistry is essential to maintaining safe and consistent swimming pool operation. Chemicals used in swimming pools include: Disinfectants to destroy harmful or otherwise objectionable organisms; Alkalinity and pH Adjusters to maintain a consistent acid-base relationship and acid buffering capacity; Chlorine Stabilizer to prevent unnecessary loss of chlorine; Algicide to kill and prevent algae, and Filter Aids to help remove foreign material. Following is a discussion of various factors which affect water chemistry, how they affect swimming pools and how to use pool chemicals to restore a properly balanced water chemistry.

Contents:

• pH
• Total Alkalinity
• Calcium Hardness
• Total Dissolved Solids
• Disinfection of Pool Water
  • Chlorine
  • Breakpoint Chlorination
  • How pH Affects free chlorine
  • Stabilizer-Cyanuric Acid
  • Bromine
  • Biguanide Disinfectants
  • Supplimental Disinfection Equipment
• Suggested NSPI Standards-Swimming Pools
• Suggested NSPI Standards-Spas
• Algae Control

pH

pH is the single most important element in swimming pool water chemistry. It affects every other chemical balance in pool water.

pH is a measure of hydrogen ion (H+) concentration in water. It indicates the relative acidity or basicity of pool water. pH is measured on a scale of 0 (strong acid) to 14 (strong base) with 7 being the neutral pH.

In pools a slightly alkaline pH of 7.4 to 7.6 is most desirable because this range is most comfortable to the human eye and provides for optimum use of free chlorine while maintaining water that is not corrosive or scale forming.

If pH is too low (below 7)

• Water becomes acidic
• Chlorine residuals dissipate rapidly
• Eye irritation occurs
• Plaster walls are etched
• Metal fittings, pump impeller, heater core may corrode
• Dissolved metals may leave stains on walls
• Rapid Loss of alkalinity

If pH is too high (above 8)

• Chlorine activity is slowed and inefficient
• Scale formation and discoloration of pool walls
• Water becomes cloudy
• Filter is overworked
• Eye irritation may occur

pH Adjustment
To avoid the problems listed above, pH must be maintained between 7.2 and 7.8. The most desirable level for pH is between 7.4 and 7.6. If pH is too low - run alkali demand test if available. Raise pH by adding soda ash (sodium carbonate). Never add more than 2 lbs per 10, 000 gallons in a single treatment. Be sure the pump is running when chemicals are added. Allow to recirculate then retest to determine if further treatment is necessary. Caustic soda (sodium hydroxide) is sometimes used with chemical feed pumps to raise pH. If problems with low pH persist, it may be necessary to raise total alkalinity to stabilize the pH.

If pH is too high - run acid demand test if available. pH is lowered by adding muriatic acid (hydrochloric acid) or sodium bisulfate. Carefully add acid at the deep end of the pool. Try not to pour acid near pool walls or fittings. Remember: When using or diluting acids, " do as you oughta, add the acid to the water" (never add water to acid)
NOTE: 10 lbs. sodium bisulfate is roughly the same as 1 gal. muriatic acid.

Total Alkalinity (TA)

Total alkalinity is closely associated with pH but rather than a measure of hydrogen ion concentration it is a measure of the ability of a solution to neutralize hydrogen ions. Expressed in parts per million (ppm), total alkalinity is the result of alkaline materials including carbonates, bicarbonates and hydroxides - mostly bicarbonates. This acid neutralizing (buffering) capacity of water is desirable because it helps prevent wide variations in pH whenever small amounts of acid or alkali are added to the pool. Total alkalinity is a measure of water's resistance to change in pH.

Total alkalinity should be maintained in the range of 80 to 150 ppm.

If total alkalinity is too low:
pH changes rapidly when chemicals or impurities enter the water. pH may drop rapidly, causing etching and corrosion.

If total alkalinity is too high:
pH becomes difficult to adjust. High pH often occurs causing other problems such as; cloudy water, decreased disinfectant effectiveness, scale formation and filter problems.

Raising total Alkalinity - Total alkalinity can be raised by the addition of bicarbonate of soda (sodium bicarbonate, baking soda). 1.4 lbs. bicarbonate of soda per 10, 000 gallons will raise total alkalinity 10 ppm.

In some cases, soda ash can be used to raise total alkalinity. Pound for pound, soda ash raises alkalinity 60 percent more than sodium bicarbonate and is cheaper than sodium bicarbiante. The problem with using soda ash to increase alkalinity is it drastically increases pH. This can cause cloudy water and scale formation. Soda ash should only be used to increase total alkalinity if you also need to increase the pH or if only small increases in alkalinity are needed.

Chemical manufactures are now marketing a total alkalinity increaser which combines the effects of sodium bicarbonate and soda ash. The product sodium sesquicarbonate or sodium hydrogen carbonate affects total alkalinity more than sodium bicarbonate, but does not cause quite as much increase in pH as soda ash does.

Lowering total Alkalinity - Total alkalinity can be lowered by adding muriatic acid or sodium bisulfate. Acid may be added in doses of up to 1 quart per 10, 000 gallons. Total alkalinity tests and further required additions of acid can be made every 2 hours.

Calcium Hardness:

Calcium hardness is a measure of the dissolved calcium salts in water. Under normal conditions this should not be a problem in properly operated swimming pools. Estimates of the proper range of calcium hardness vary widely but the ideal level for plaster pool is generally considered to be about 250 ppm. If calcium hardness is very low then water may leach calcium from pool walls causing pitting of the plaster surface. Very high calcium hardness may contribute to scale formation and clouding of the water. To raise calcium hardness - add calcium chloride.

To lower calcium hardness anhydrous trisodium phosphate may be used. One pound trisodium phosphate per 10, 000 gallons will lower calcium hardness 11 ppm. Use in small increments or clouding may occur. Another method of lowering calcium hardness is to simply drain off part of the pool water and dilute the remaining water with fresh make up water.

Total Dissolved Solids (TDS)

After a pool has been in use for a time, dissolved solids may begin to accumulate. These unfilterable solids include body wastes, suntan lotion, stabilizer, chlorines, algicide, dirt, pollen, etc. Normally this is less of a problem with outdoor pools because of rain water and no use during winter months. Indoor pools sometimes have a buildup of dissolved solids requiring draining the pool and refilling with fresh water. Most pools should be drained after 3 to 5 years. Ideally pool water contains under 450 ppm total dissolved solids.

DISINFECTION OF POOL WATER

A proper balance of the previously described water chemistry factors will provide water that will not damage pool components and is non-irritating to swimmers. It is then necessary to provide for disinfection of the water to prevent the spread of disease organisms from person to person and prevent unwanted growth of bacteria and algae in the pool.

Chlorine
The most commonly used disinfectant for swimming pools is chlorine. In its elemental form chlorine is a heavy greenish yellow gas which is so toxic that is has been used as a weapon in chemical warfare. Because of the extremely high potential for injury or death from improper use of chlorine gas, a number of chlorine compounds have been formulated to provide chlorine in forms that can be handled and used safely by swimming pool operators.

The following forms of chlorine are commonly used in swimming pools:

Because of the special hazards associated with the use of gaseous chlorine, its use has been prohibited at public swimming pools in North Carolina.

Calcium Hypochlorite: granular or pelletized 65% available chlorine

WARNING - do not use in closed tablet feeders designed to use other forms of chlorine

Sodium Hypochlorite: Liquid bleach 12.5% available chlorine

Trichloroisocyanuric Acid: sticks or tablets 90% available chlorine

Other forms of chlorine less commonly used are lithium hypochlorite, potassium dichloroisocyanuric acid, and sodium dichloroisocyanuric acid.

Free Chlorine Residual is the amount of chlorine in the pool which has not reacted with substances other than water. It is the chlorine which is available to disinfect pool water and oxidize organic substances. Free chlorine residual should be maintained between 1 and 3 ppm.

Combined Chlorine is chlorine in the pool which has reacted with substance other than water and is no longer available in its free state. Some combined chlorines are bactericides but they contribute little to the disinfection process. Chlorine combined with ammonia produces chloramines which cause eye irritation and an objectionable chlorine odor. For this reason combined chlorine residual should be kept to a minimum preferably below 0.2 ppm.

Total Chlorine residual is the concentration of free chlorine plus combined chlorine. To determine combined chlorine residual test for free chlorine and total chlorine.

Breakpoint Chlorination is the process by which combined chlorine and some organics are "burned out" of the pool by addition of large amounts of chlorine. The reaction of chlorine with ammonia to form chloramines occurs in several stages with free chlorine consumed at each stage. If enough chlorine is added to the water the total chlorine residual will rise to a point that forces the reaction of chlorine with ammonia to go rapidly to completion. Compounds of nitrogen and chlorine are released from the water and the apparent residual chlorine decreases. The point at which the chlorine residual suddenly drops is called the breakpoint. When enough chlorine is added to pass the breakpoint, combined chlorine compounds disappear, eye irritation potential and chlorine odors disappear, and the chlorine remaining in the water is all in the free state.

Superchlorination: In order to prevent buildup of chloramines in the pool it is necessary to periodically add large amounts of new chlorine in an effort to pass the breakpoint. Public swimming pools should be supechlorinated about once a week. The amount of chlorine needed to reach the breakpoint will vary depending on the amount of organic material introduced by bathers and on the level of free chlorine maintained in the pool. If the amount of combined chlorine is known then the amount of new chlorine needed is ten times the amount of combined chlorine. When combined chlorine residual is not known, superchlorination is accomplished by adding 10 ppm of new chlorine to the pool. Ordinarily calcium hypochlorite at a dose of at least 1 lb. per 10, 000 gallons is used for superchlorination. The chart below shows the amounts of various chlorine compounds which can be used to introduce 10 ppm of chlorine to the pool.

Non-chlorine Shock Treatments Several products have been developed which oxidize organics without the use of chlorine. Pools which use those products can accomplish the reduction of organics without closing the pool for any longer than it takes to dissolve and distribute the chemicals. Those products are more expensive than chlorine but may be preferred where it is necessary to keep a pool open.

How pH affects free chlorine residual Chlorine reacts with water to form Hypochlorous acid (HOC1). The reaction is different for each form of chlorine but hypochlorous acid is produced by each of those reactions and is the form in which chlorine serves best as a disinfectant. Hypochlorous acid is a weak acid and easily dissociates to an ionized hypochlorite state as shown below.

Suggested NSPI Standards - Spas

ALGAE CONTROL

Algae are tiny plants that bloom and grow in swimming pools if nutrients are present and a sufficient level of free chlorine is not maintained. Below are descriptions of the three most common algae problems in swimming pools.

Green Algae The most common algae in swimming pool floats in water and coats pool surfaces. Left unchecked green algae will very quickly turn the pool water pea green.

Mustard Algae settles on pool walls and causes a slimy yellow film.

Black Algae appears in "buds" or clumps attached to tile grout, corners, steps and pool surfaces.

Solution:
Green Algae - is very susceptible to chemical treatment. Superchlorinate with 10 to 20 ppm chlorine in the evening. Keep the filter running and brush the pool walls and bottom. Periodically check chlorine and maintain above 3 ppm until water clears. Using an algicide containing quaternary ammonia the next morning will help prevent the return of green algae.

Mustard Algae - is much more resistant to chemical treatment and clings more tightly to pool walls than green algae. Adjust pH and superchlorinate as for green algae then brush diligently. Later vacuum the pool, check chlorine and superchorinate again if necessary. Mustard algae will generally return unless treated with a special mustard algicide or a copper based algicide. Algicide should be added in the morning to treat algae in daylight - its most active period.

Black Algae - is very difficult to get rid of. It can be controlled to some extent by frequent superchlorination and diligent brushing with a stiff brush. Spot treatments can be made by turning off the recirculation pumps and pouring granular chlorine directly on recently brushed spots. Trichlor tablets can also be rubbed on recently brushed areas to spot treat. Black algae can usually be controlled with the use of strong alicides and maintenance of relatively high free chlorine residual, but complete removal of black algae may require draining and cleaning the pool.

Note: Algae blooms are a problem best avoided. Maintaining proper water quality and frequent brushing of pool walls will deprive algae of the opportunity to get started.

Source: http://www.deh.enr.state.nc.us/ehs/chem.htm