How's this for a bit of misinformation: "Every once in a while, you've got to shock the pool water by adding an extra-large amount of chlorine to get rid of algae and other contaminants that might otherwise be left to grow."
This is an actual bit of advice that was overheard being given to a pool owner by the counterperson at a home-supply store that happened to sell pool chemicals — and as advice goes, it's dead wrong!
True, you do have to periodically shock pool or spa water. And true, it is often done with chlorine — although "an extra-large amount" is not really an accurate guideline to be followed.
But the reason given for shocking is the most inaccurate portiion of a generally inaccurate statement. Shocking the water may, indeed, help prevent the spread of algae, but the true purpose of this periodic treatment is to rid the water of organic compounds that irritate swimmers and bathers and tie up chlorine to prevent it from performing its task as a sanitizer.
Yet the notion persists throughout many corners of the pool and spa industry that occasional, "super" doses of chlorine are necessary, but the exact reason for this is as much a matter of custom and tradition as it is a matter of scientifically based water chemistry.
As a pool service professional, it's your job to understand the true reasons for doing what it is you do to treat water. And you also should be prepared to explain your practices to your customers.
And for this, you need a thorough knowledge of shock treatment — the hows, whys and wherefores of this often misunderstood practice.
For years, operators of public swimming pools have heard swimmers complain that "there’s too much chlorine in the water," when, in fact, the problem was too little chlorine.
The most noticeable symptom of this condition is the odor — that obnoxious "chlorine" smell that assails you the moment you get near the pool — especially in a heavily used indoor public pool.
Then, once in the water, you are attacked by the other symptoms of the condition: red eyes and sore nose. And if, like most of the public, you are unaware of the way chlorine works to sanitize water, you are drawn to the inescapable conclusion that too much chlorine is to blame.
Yet if you open up a gallon jug of chlorine bleach or pool chlorine and take a whiff, it smells rather fresh and clean — nothing like the dreadful aroma that attacks you in that public pool. From this alone, it should be obvious that something other than chlorine is the culprit behind that sore nose, those red eyes and that awful smell.
And, indeed, something else is to blame.
You can call it chloramine; or combined chlorine; or organic contamination; or bather waste — the fact is that an improperly sanitized pool contains irritating levels of chemical compounds that need to be removed before they become a problem.
When a chlorine compound is added to swimming pool or spa water, it reacts with water to form the compounds known as hypochlorous acid and hypochlorite ion. Together, these compounds are known as "free available chlorine" or "free chlorine."
The primary reason for adding chlorine to swimming pool or spa water is to disinfect or kill possibly harmful microorganisms. But once the free chlorine has joined with ammonia and nitrogen compounds to form combined chlorine, its ability to disinfect is hindered. It actually takes 25 parts of combined chlorine to do the work of 1 part of free chlorine.
Ridding the water of chloramines is a 3-step process:
Using the DPD test, you first determine the free chlorine level in the water using the DPD #1 tablet. After recording the reading, the DPD #3 tablet is added to the same sample, and the result is recorded. The second reading is the total chlorine level in the water. If the reading is higher than the free chlorine reading, then the difference between the two represents the level of combined chlorine in the water. If the readings are the same, then no combined chlorine is present. The total chlorine level cannot be less than the free chlorine level.
If you use bromine for disinfection, you still must be concerned with organics. Even though combined bromine is a good disinfectant and does not smell or irritate eyes, it is not as efficient as chlorine in removing organic waste.
And if they are allowed to build up, ammonia-like waste products can still be irritating — creating such conditions as jock itch or skin rash.
Once you know how much combined chlorine is in the water, you have to add about 10 times that amount of free chlorine to get rid of it.
In other words, it your test reveals that you have 1 ppm of combined chlorine in the water, you will have to add 10 ppm of available chlorine in the water to get rid of it.
The process of breakpoint chlorination or the destruction of organic waste in the water is known as "oxidation."
When chlorine is introduced into swimming pool or spa water containing ammonia, the hypochlorous acid (free chlorine) quickly begins to oxidize the ammonia to form a combined chlorine compound called monochloramine.
As more chlorine is added, the monochloramine is oxidized by the additional chlorine to form dichloramine. As even more free chlorine is added, it attacks or oxidizes the dichloramine to form trichloramine. Finally, as even more free chlorine is added, the trichloramine is broken down or oxidized to simple nitrogen and chloride salt, completing breakpoint chlorination. Any chlorine added after this point will be free chlorine, as long as no new contamination of the pool or spa occurs.
The accompanying chart breakpoint chlorination in a graphic way. Let's discuss each of the points on the curve: First, the addition of chlorine to a pool containing ammonia (1) results in a growing production of combined chlorine (2). Once enough chlorine has been added to oxidize all of the ammonia, the combined chlorine rapidly breaks down (3). At this time, little, if any, chlorine remains in the water, because it has all broken down into nitrogen and chloride (4). Any chlorine added after this point will form free chlorine (5), because there is no ammonia to react with the chlorine now being added.
Superchlorination to truly achieve breakpoint — the destruction of all organic waste — can be a very tricky thing to accomplish. If not enough chlorine is added, the combined chlorine problem is only made worse. When this happens, eye burn and skin irritation are not reduced, but rather raised to very high and very irritating levels. If too much chlorine is added, it may take days to drop to safe levels (less than 5 ppm) before bathing can be resumed.
By chemical calculation, chemists know that it takes 7.6 parts of chlorine to oxidize 1 part of ammonia. But a number of other factors will affect that ratio: Other organics or products in the water will consume some of the added chlorine so that 7.6 parts are not enough. To be practical, it is best to add 10 parts of chlorine for each part of ammonia.
The problem is figuring out exactly how much of a given chlorine product you will need to provide that 10-ppm level. Using a pocket calculator, you can do this easily, but you've got to remember that 1 gallon of water weighs 8.34 pounds and that ppm (parts per million) is a weight-to-weight ratio.
We're going to assume that you know how many gallons are in your customers' pools or know how to calculate this figure. Once you know this, you must:
For example, if you need 11/2 pounds of available chlorine, and you're using cal-hypo (65-percent available chlorine), you divide 1.5 by .65, which gives you 2.3 pounds.
If you use liquid chlorine (12-percent available chlorine), you need 1 1/2 gallons, because a gallon of liquid chlorine (sodium hypochlorite) provides almost exactly 1 pound of available chlorine.
By now, you are probably beginning to realize that superchlorination is not a simple matter. To do it right, you cannot simply add "some" chlorine — you might actually be making the problem worse. You have to calculate the required amount and add the correct dose.
Non-chlorine shock treatments go a long way toward simplifying the process. Using chemical compounds called monopersulfates, these products oxidize contaminants without going through the mono- di- and trichloramine chemical process. So they can be added on a 1-to-1 ratio: Each pound of product added will oxidize 1 pound of contaminant.
Non-chlorine shocks are a bit more costly than chlorine compounds, but they eliminate much pool downtime because they work rapidly and do not require time to dissipate.