Swimming Pool Water Chemistry

WATER HARDNESS. Another factor in pool water problems is hardness, a measure of the calcium and magnesium content of the water. All water contains some natural hardness. The amount will vary regionally, and from source to source within a region. A certain amount is necessary in water to control its tendency to dissolve. If too little is present in a pool, the water will attack the materials of construction to satisfy its appetite. Hardness treatment will increase low water hardness and prevent etching, pitting and corrosion of surfaces and metallic components in such a situation. Scaling occurs if too much hardness is present. This is visible as crusty gray deposits and cloudy water, or visible as deposits in piping. A pool’s pipes, designed to accommodate a certain water flow at a certain pressure, will obviously not function properly if their diameters are decreased by scale formation on their inner surface (white rings inside the pipe). It’s a bit like a pool’s hardening of the arteries. Scale on pool surfaces is unsightly and unattractive. Scale in pool plumbing is disastrous and expensive, since it interferes with the circulation and filtration of the pool water.
YOU ARE WHAT YOU EAT: A LOOK AT CHLORINE CONSUMPTION
As you know, chlorine is very rapidly consumed by the action of sunlight in a swimming pool. Andy chlorinated sanitizer will produce HOCI, but up to 97% of that residual can be lost in 2 hours! What does this mean in actual pool operation? Well, when 4-5ppm of free available chlorine is added as calcium hypochlorite at 6 am, it’ll be completely gone by 12 noon. Whether or not anyone uses the pool. This same wasteful chlorine consumption occurs with chlorine gas and sodium hypochlorite. For the bulk of their swimming time, Mr. Or Mrs. Pool owner really have no assurance that the pool is sanitary, or that contamination will be rapidly eliminated as it enters the pool. What to do? The conscientious pool owner, aware of the importance of the free available chlorine residual, could operate his pool satisfactorily if he tested the water at noon, and added another 4-5 ppm residual for safe afternoon swimming. He’d then have to repeat the process at 6 pm so the entire family could enjoy the pool in the evening. Three times a day. Whew! Or, he could install a bulky and complex chemical feeding system to constantly add sanitizer. He would also probably need a separate feeder to add a pH adjusting chemical, while large quantities of the unstabilized sanitizer are being added. Sound unwieldy? It is. And there’s a better way. You guessed it. Our pool owner could use a sanitizer, add the label-directed 1-1.5 ppm of free available chlorine, and be assured that it would keep the pool water clear and sparkling.
CHLORAMINES, THE GREAT IMPOSTER
Since all chlorinated sanitizers react with water to produce HOCI, chlorine consumption depends on the amount of contamination that is present - not the brand that’s used. Enough sanitizer must be added to meet the chlorine demand of the water before a measurable residual can be maintained. This amount depends on the amount of contamination present in make up water, plus whatever is added by bather loading, rain, dust and other external sources. One particularly troublesome type of contamination is nitrogenous wastes from bathers bodies. Whether they are as simple as ammonia in urine, or as complex as the components in perspiration or saliva, they present special problems when they accumulate in pool water. These contaminants react with HOCI to form compounds called chloramines, or combined chlorine.
CHLORAMINES
The combined chlorine reaction begins with one unit of ammonia, combining with one unit of HOCI to form monochloramine (NH2CI). This reacts with another unit of HOCI to form dichloramine and finally with a third unit of HOCI to produce trichloramine (NCI3). We’re not through yet. It takes a fourth unit of HOCI to finally convert the original molecule of ammonia into harmless nitrogen gas (N2), water and chloride ion (CI-) and a fifth unit of HOCI before a free available chlorine residual can be measured. These chloramines cause plenty of trouble in pool water. Why? Because they are stable and persistent. The monodi and trichloramine from this first unit of ammonia will survive and accumulate with the chloramines formed from subsequent units of ammonia. This is actually chlorine consumption, because HOCI combined with ammonia forms chloramines. Chloramines have very poor sanitizing power, so algae and bacteria can grow. In fact, they have such poor pool sanitizing power that they would be rated at only 0-10 on a relative activity scale with HOCI rated at 10,000. Quite a difference. It’s been estimated that chloramines could provide germ fee water if they were present at a concentration of at least 25 - 50 ppm. But this would create additional problems in a swimming pool, because they’re very pungent and irritating, causing eye irritation and chlorine odors at very low concentrations.
FUNNY IT SMELLS LIKE CHLORINE
Unfortunately, the chlorine odors generated by chloramines lead many people to think that too much chlorine has been added. So, they stop adding chemicals - and problems grow worse. These symptoms are a signal to test and adjust pH, and add enough chlorine to oxidize all the chloramines, establishing a free available chlorine residual. It’s often very difficult to convince a pool owner that insufficient chlorination is the cause of chlorine odors, eye burn and algae. That’s because she probably tested her pool when a problem was noticed, and got a very positive chlorine test - according to her test kit. This is the most confusing problem caused by combined chlorine: certain test methods measure it as part of a total chlorine residual. If you find a yellow color in the comparator and a clear liquid reagent, then it’s clear that this pool is being tested by the orthotolidine, or OTO method.
OTO
This OTO method has some advantages that have made it popular and widely used. It also has some major deficiencies. The fact that it cannot easily distinguish free from combined chlorine makes it a very doubtful aid to pool operation. Even worse, it creates a false sense of security, leading to erroneous diagnosis of pool problems, which delays remedial action. The pool owner could test once, twice, or three times daily and still have no idea whether enough free available chlorine was present to protect the quality of the water. A 1 ppm residual measured by OTO will provide far less protection to pool than a 1 ppm residual measured by the DPD method.
DPD
OTO, DPD - Welcome to "Acronym City". Actually, there’s a very solid reason for using DPD: it will distinguish free from combined chlorine. A DPD residual will be fee available chlorine, not some combination of HOCI, mono- di- and trichloramine. It will effectively protect the pool from contamination. The method is simple and rapid.
A DPD NO. 1 tablet is dissolved in a measured amount of pool water to produce a colored solution.
This color is compared with the color standards in the comparator to determine the amount of FAC that is present.
The amount of chloramine that is present can easily be determining by adding a DPD NO. 3 tablet to the same test sample and determining whether any additional color develops.
The difference between this total chlorine measurement and the FAC test result equals the chloramine content of the water.
EXAMPLE:
DPD No. 3 minus DPD No. 1 = Chloramines
1.5 ppm - 1.3 ppm = 0.2 ppm chloramines
Total - Free = Chloramine residual
This test method indicates precisely when remedial action is necessary to prevent the accumulation of chloramines in pool water. We do not suggest that you try to pronounce or memorize its chemical name. (Di ethyl-paraphenylene di amine). Just remember what to do next.
SUPER CHLORINATION
The remedial action that should be taken when the DPD test detects the presence of chloramines is to super chlorinate. (Some people call it shocking a pool). Super chlorination simply involves the addition of 5 - 10 times the normal daily dose of chlorine. Super chlorination should be routinely performed on very hot days under heavy loads… after heavy rains… or if DPD test shows combined chlorine. As a general rule of thumb, it ought to be performed at least every week when the temperature is over 80 degrees, and every other week when it’s 80 degrees or under. A more practical rule is to super chlorinate whenever a DPD test measures 0.2 ppm chloramine in the pool. A tenfold excess of chlorine should be added. Super chlorination is a remedial action if it must be performed to remove gross amounts of chloramines, correct eye and nasal discomfort, or destroy a visible algae growth. But it can be far more economical than using 15 ppm to remove 1.5 ppm. It also eliminates waiting, vacuuming, scrubbing and backwashing to remove algae that will surely have appeared while chloramines accumulated. Most of us would agree that super chlorination is terrific. It works beautifully. But eventually, if you just keep loading chemicals and contaminants in a pool, you’ll have problems. Which brings us to the next topic of conversation…
FRESH WATER AND TDS
TDS is not an abbreviation for "Tough day on Saturday", or "Take a dip Steve". It stands for Total Dissolved Solids. TDS are a measure of all the dissolved chemicals in the water. Whether they’re natural components of source water, residues of treatment chemicals, bathers’ wastes, or wind and rain borne atmospheric pollutants, they stay in the water and concentrate. Eventually, TDS will cause staining, scaling, reduced chlorine efficiency, and erratic pool behavior. All pool water contains total dissolved solids. If a drop of TDS water could be magnified, it might show Ca (calcium) and Na (sodium), representing dissolved chemicals. Although a dissolved chemical is not visible to the naked eye, it does occupy space in the water. Take table salt, for instance. It’s visible in the shaker and invisible in a water solution - but it reappears if the water is boiled away. It hasn’t disappeared, it’s just been dispersed as submicroscopic particles called ions. In ideally stabilized water, HOCI has a direct route to reach algae, bacteria and germs that it must destroy to provide sanitary water. The ions of the dissolved solids are widely dispersed, and don’t hinder this action. Unfortunately, water doesn’t remain in this ideal condition for very long. Even the residues of sanitizers consumed in the various categories of the California field test produced substantial amounts of dissolved solids during one season of treatment. And these TDS accumulations are the chemical residues from sanitizer treatment only. Other materials add even more dissolved materials.
ALPHABET SOUP
It’s interesting to note that stabilizing and using chloroisocyanurate produces the least amount of TDS. When more chemicals are required, more residues result. Eventually, you get microscopic alphabet soup. Several seasons of adding chemicals and bathers to water that’s already exposed to a variety of natural contaminants can create a very crowded body of water. Obviously, as this alphabet soup gets more crowded it’s difficult for HOCI to perform efficiently. And algae, bacteria and germs that are not eliminated will cause problems. It’s difficult to predict exactly how fast TDS ill accumulate, and at what concentration they’ll cause trouble. But it’s been estimated by the NSPI chemical treatment and process committee that TDS should be maintained at less than 1500 ppm. Concentrations in excess of that may cause problems.
NOTE: When problems with TDS occur, untrained home owners and overworked service people often find a convenient culprit in over stabilization. (There’s really no such thing.) Cyanuric acid is easy to measure, and gives a test result that can be interpreted individually, to explain almost every water quality problem. A cloudy, green pool will almost certainly have one or two ppm in excess of some arbitrary maximum. The problem pool water will be dumped to lower the cyanuric acid content, and the problem will magically disappear.
WHAT IS THE PROBLEM, ANYWAY?
What isn’t usually recognized is that any dissolved solids, chloramines, or pH buffers will be correspondingly reduced when a pool is dumped. It’s impossible to remove only one kind of dissolved material in water that’s all gone. Total dissolved solids have not received the attention they deserve for causing pool problems, because they aren’t easy to measure - and there’s a tendency to forget they’re in the water. Additionally, they’ve never received the kind of study and publicity that have surrounded cyanuric products. It has been proven that cyanuric acid causes no ill effects in pool water… but that it may, by it’s accumulation, signal the onset of problems due to TDS in the pool. Apparently this proof has been convincing, since the leading proponent of the over stabilization theory has built a cyanurate plant !
LESS IS MORE
The fact is, there’s only one practical way to remove dissolved solids from a pool. That’s to remove a portion of the water in which they’re dissolved. Removal of 100 gallons of water removes 1.7 pounds of dissolved solids from a 10,000 gallon pool containing 2,000 ppm TDS. If the cost for control of TDS accumulation is calculated, the partial water removal procedure becomes the ultimate bargain in pool operation.
The recommended rate of water removal per week is 1-3%.
In a 10,000 gallon pool, this represents 100 gallons per week, or 4,000 gallons in a ten month season.
At .53 per 1000 gallons, this would cost $2.12. Since many municipalities levy a sewage charge of 120% of the cost of the water, an additional $2.54 is added.
The total cost for replacing 40% of the used water in the pool is $4.66.
If this cost seems excessive, compare it to the initial investment in the pool, the cost of chemicals that don’t perform efficiently, and the expense of dumping, acid washing, refilling and balancing. You’ll find that water is the cheapest chemical that can be added to a pool.
Information on this page is intended as a basic guideline only. Consult the instructions on your specific brand of chemicals for proper use and safety instructions.

 

 

whew....