Beginners Guide to Pool Chemicals and Water Chemistry
Water Sanitising Components
As you probably know, the occasional addition of new water -- or complete water replacement in the case of spas -- isn't enough to keep the water clean and clear of contaminants.
Your goals are water sanitation and water balance. In other words, you want your levels of sanitisers (such as chlorine or bromine) and your levels of pH, total alkalinity, water hardness and total dissolved solids to all fall within acceptable limits.
Chlorination
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
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
Oxidation is the process of chemically removing organic debris, such as body waste, particulate matter and perspiration, from the water. 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
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.
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 For now, we will consider only the effect of PH on sanitation.
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.
Bromine
Although bromine in its elemental form is a liquid, it is not available for swimming pool disinfection in that form. 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.
Ozone
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.
Water PH
PH is a measure of how acidic or basic the water is. PH is a logarithmic scale from 0-14, with 7 being neutral. Below 7 is defined as being acidic, while levels above 7 are said to be basic or alkaline. Everything that enters your pool has a pH value and so regular monitoring is essential. To balance the pH in pool water you need to adjust the water with the addition of pH increasers (Soda Ash is the most common) or pH decreases (Dry Acid) to achieve the ideal range of 7.2 - 7.8.