Disinfection is the second subsection in the sanitation concept that sits atop our Golden Triangle. Here, we cannot talk about sterilization of pool water, as a sterile pool is not possible in reality. Dirt, dust, debris, organic and inorganic are always entering the your pool and there is a lag time, a latency period whereby for a certain period of time, microscopic life; a virus, bacterium, protozoa, or algae will still be alive.... even in the presence of swimming pool water that has been properly treated with chemicals, properly filtered, and one that has the proper circulation.
For instance: there is type of bacteria (Coliphage) that comes in human feces, one that is very resistant to normal concentrations of various disinfectants. This bacteria is one of the "bad" ones that can make you very sick. Because of this, lifeguards are required to shut down a commercial pool immediately upon sighting a piece of poop. After closing the pool the water must be super chlorinated for several hours. This bacteria is that tough! However, all forms of microscopic life have a certain amount of resistance to disinfectants, they don't die instantly! So... don't you drink pool water! Don't let your children drink pool water! Teach them to keep their mouths closed, and have them use nose clips and eye goggles. If I seem a little fussy here, remember how painful an ear infection, or an eye infection can be for a child or you! Certain skin conditions can arise also, but these aren't as critical. Check out this harmful bacteria chart: http://natsci.edgewood.edu/wingra/wingra_bacteria.htm
When we begin to talk about pool disinfection, it is obvious that disinfection is dependent upon and related to filtration and circulation. But at this point, I want to talk about which chemicals are going to be used to disinfect your swimming pool water, and not get side tracked into exploring these other relationships as of yet.
Chlorine is the chemical that is usually used to disinfect a swimming pool. The actual process that chlorine uses to disinfect water is called oxidation (the loss of electrons) and this is the same chemical process that you see when wood is burning or a piece of metal is rusting. It is this oxidation process that kills bacteria, viruses, protozoa, algae, and fungi.
Now would be a good time to describe the measuring protocols, or conventions, for water borne contaminates: dissolved and undissolved, organic and inorganic, mineral and chemical. All water except distilled, naturally contains these foreign substances. The concentration of these substances are measured in parts per million, parts per billion, or even parts per trillion. This is the generally accepted way of measuring "how much of anything is in water". Water test kits use this type of protocol. For instance: a chlorine test vial will have a color comparison chart that reads 1,2, or 3 parts per million.
Chlorine is the most commonly used chemical in swimming pools for disinfection. Chlorine is a member of the halogen family of chemicals, with bromine, iodine, fluorine and astatine finishing out the list. They are unique in that this particular periodic table group can exist as a gas, liquid, or solid on the earth's surface.
Iodine is used in swimming pools as an disinfectant when the swimmer, or swimmers have developed a sensitivity to chlorine. The main problem with iodine is that it turns the your pretty blue water, brown. Bromine is used sometimes in indoor pools when the smell of chlorine is intolerable due to ventilation issues. Bromine has a much heavier molecule than chlorine and so it tends to stay in the water and does not gas off as quickly as chlorine and so the odor from bromine is minimal.
Also, bromine is used therefore, in hot water applications such as spas and hot tubs. Chlorine, if used in spas and hot tubs, quickly becomes very annoying to you. When the spas blowers and jets are turned on, all your natural excitement, caused the bubbling and churning of the water, turns into a blue funk as soon as the chlorine begins to gas off, stinking up the area. They used chlorine gas in World War One after all! So don't use chlorine in your spa! Bromine in contrast to chlorine, will be barely detectable at all in the same exact situation.
Another negative quality that chlorine has is that once the chlorine has oxidized some organic material it becomes chemically changed into a compound that is not unlike tear gas! This changed chlorine is known as a "combined chlorine" or as a chloramine. Check out Wikipedia's article: (http://en.wikipedia.org/wiki/Chloramine). Chloramines are an unavoidable consequence of oxidation or the disinfection process. Chloramine is a very poor disinfectant. For example: ordinary table salt (sodium chloride) is a combined chlorine and we know that despite the fact that ocean water is salty, it would be some kind of joke to say that it has a minimal effect on the creatures that live there! Since it is actually absolutely necessary for their existence! Now, you and I know that if you increase the salt concentration above a certain threshold, as it is in the Great Salt Lake or the Dead Sea, then that will create an unlivable environment for all but the hardiest of creatures. Likewise, chloramines if allowed to increase past a certain level in pool water will cause eye, nose and skin irritation and will have an unpleasant odor. The solution to high chloramine levels is to super chlorinate or "shock" the pool in a very precise way, which will be covered in a later post.
This discussion about chloramine is leading up to the second positive attribute that bromine has over chlorine. That attribute is that once bromine has done it's job and a certain amount of bromine has been changed into a combined bromine, the combined bromine, unlike the combined chlorine, is still a effective disinfectant, and does not stink! So you get double duty from bromine!
So why is not bromine used in swimming pools instead of chlorine? Well, unfortunately, there is a real downside to using bromine as an disinfectant and that is bromine costs about twice as much as chlorine on a pound for pound basis. Although, in a spa or hot tub, the cost difference is negligible, in a swimming pool it becomes a real problem.
The next post will continue on with the positive and negative modifiers of disinfectant efficacy.
