Pool owners frequently seek out "Chemical Free" alternatives to pool water sanitization. Many claims in favor of these systems and methods have been made (usually by the manufacturers of such systems) often without adequate scientific proof of their efficacy. Let's take a look at them and see how well they hold up.
The notion of a "Chemical Free" pool is a bit of a misnomer. If you want sanitized water, as defined by the EPA, you should know that there are only three EPA approved primary sanitizers -- chlorine, bromine, and biguanide (aka Baqua, SoftSwim, Revacil, etc.). These are your only choices - period. Everything else is a supplemental sanitizer that MUST be used in conjunction with one of these three if you want sanitized water! Why are these the only three approved sanitizers? These are the only sanitizers with fast kill times that also leave a residual in the water for ongoing protection.
Let's look at some of these alternative sanitizers and see if they "hold water".
First there are the 'metal ion' and 'mineral' systems that might also include oxidation by oxygen (not ozone). Some of them are active and use electricity and metal electrodes to generate the ion and others use an erosion feeder with metallic salts like silver nitrate and copper sulfate that slowly dissolve to put the ions in the water. These added metals (usually copper, silver, and/or zinc) can hardly be considered 'chemical free.' Metals do have algaestatic and bacteriostatic properties but they have very slow kill times so they look better on paper then in actual use. They can be very effective for drinking water purification since the water can sit in the tank for enough time for pathogens to be killed before being used (and this water purification usage is often cited as a tribute to their effectiveness) BUT water purification is a closed system, while a swimming pool is an open system, meaning anything can and will drop into the pool at any time, (such as bird droppings, sweat, urine and feces from bathers, dogs jumping into the water, etc.) For this reason a residual EPA approved sanitizer with fast kill times such as one of the three above is needed.
Many of these metals systems used to be advertised as chemical free or chlorine free, but this is no longer permitted in most countries for obvious reasons. They might also use the term 'minerals' to make it sound healthy and spa like! - minerals are good for us, aren't they? How about if the 'minerals' are copper sulfate and silver nitrate? Still sounding healthy and chemical free? Ionizers do basically the same thing by electrolysis of copper and silver electrodes. A new twist is combining copper ionization with titanium electrode oxygen generation! - pure pseudoscience!
What about copper systems - chemical based? (See ionizers and mineral systems above.) Again, they are not primary sanitizers and need to be used with a chlorine residual. Most of this type of product (chelated copper sulfate) is actually sold as algaecide and not sanitizer. Of course there is that 'chlorine is bad and I want something safer' market to tap into, and there are plenty of well-intentioned people who buy into the hype. It makes for slick marketing but knowing what you know now, would you rather have chlorine or copper sulfate in your water? Since chlorine is an effective sanitizer and oxidizer and copper is not (and copper can and will stain pools and turn hair green) then give me the chlorine!
The biggest drawback to metal ion systems, besides the fact that they are NOT primary sanitizers, is that they can stain pools and people--green hair is caused by copper.
An interesting thing about these metal systems, is that they are not permitted in Australia even with reduced chlorine levels but must be used with normal chlorination levels. Why? Research has shown them to be ineffective at quickly killing some of the 'nasties' that can get in to a pool. Also, most state Health Departments do not recognize them as suitable sanitizers for commercial installations. Why? Once again, they are not effective without a residual sanitizer. Remember, copper is a very effective algaecide and zinc is almost as effective so if you have one of these systems your water will probably look clear most of the time because algae won't grow but other things can! Clear water does not automatically mean safe water! These systems leave you feeling like you are doing something good for your family, but what you are potentially doing is exposing them to dangerous bacteria and pathogens.
What about ozone and/or UV? Can't that reduce or eliminate the need for chlorine? In a word, No, and for a very simple reason. Ozone and UV have NO residual effect so again, a residual sanitizer is still needed. The only place these will kill pathogens is in the contact chamber (where the water is exposed to the ozone or UV), not in the pool. Ozone will destroy chlorine but will oxidize organics so it's a two edge sword. You will generally have higher chlorine consumption with ozone than without and it does not allow you to run lower chlorine levels because there is no residual effect from the ozone. The higher bather to water ratio in a spa makes ozone much more useful there than in a pool because it does oxidize organics. Ozone is also toxic in large enough quantities to actually sanitize pool water. For many, ozone is more useful in bromine systems since it will activate the bromide into bromine sanitizer so it works with bromine instead of against it. However, it will cause bromates to form and they are a suspected carcinogen in drinking water!
UV light can kill pathogens and some of the units also produce some ozone but this only occurs in the contact chamber and there is no residual effect in the pool. Once again, more useful in a spa than a pool. UV might be of some value in an indoor pool in conjunction with chlorine to help destroy persistent chloramines.
Not convinced yet?
What about Enzymes? They can help reduce or prevent a scum line but are NOT sanitizers?
Hydrogen peroxide? Hydrogen peroxide is NOT effective as a stand-alone sanitizer. To have a high enough level of hydrogen peroxide in the water to act as a sanitizer would cause extreme bather discomfort, to say the least! In the US it is used as an oxidizer in conjunction with biguanide.
MPS (potassium monopersulfate)? An oxidizer, NOT a sanitizer! Some metal-based systems used to say that you could use strictly MPS and no chlorine was needed. This has changed and they no longer say that in the US (and most other countries as well).
"Proprietary mixtures" that make all kinds of claims and have a lot of pseudo sciences are nothing more than snake oil. If they won't tell you what is in it you don't want to use it! If they say that you need to shock or use a bit of chlorine it is NOT a primary sanitizer!
What about magnets, electronics devices that generate 'standing waves' in the water, nascent oxygen, etc? If you believe any of this stuff works then you may be beyond help.
Chlorine gets a lot of bad press but anyone who follows the methods we outline here and at TFP know that a properly chlorinated pool does not fade your bathing suits, make your eyes red, or smell like chlorine! It actually is a lot safer than the alternatives when it comes right down to it.