by Rob Cox, January 14, 2011
Shocking vs. Super-Chlorinating your swimming pool
Shocking the pool - to many people this means to pour in their chlorine powder once per week - whether they need it or not. Although a healthy practice from a sanitation standpoint, regular and unnecessary shocking may lead to a buildup of chloramines and an unnecessary lightening of the wallet.
Superchlorinating the pool - is this different from shocking the pool? Many think not, and use the terms interchangeably. I therefore offer this distinct distinction:
Shocking: Shocking the pool occurs when you have reached what's known as breakpoint chlorination. This is a threshold that is achieved when the ppm (parts per million) of chlorine reaches 10X the level of chloramines present in the pool water. If you go only 90% of the way there, and not actually reach this threshold of breakpoint chlorination, you will only contribute to a worsening problem - that of creating more chloramines (a chloramine is a combination of a chlorine molecule joined with an ammonia or a nitrogen).
When breakpoint chlorination is reached, imagine a lighting bolt ripping through the water, "shocking" everything in the pool (at a microscopic level). With such a high level of chlorine in the water, every molecule floating around is broken apart, and every pathogen or bacteria strain floating around suffers from the shock, with the invasive chlorine molecule overpowering it, and ripping it apart from the inside out. If we could watch the process on the big screen, it would surely look rather violent.
Super-Chlorination: This is a bump to your chlorine level, just a boost, to handle growing algae or a large crowd having used the pool, or a larger crowd expected. You can use less pool shock (granular chlorine) during a super chlorination - the goal is not to reach the threshold of breakpoint chlorination, but to bring up the chlorine level to triple or quadruple the normal amount for a short period of time.
Super-chlorinating your pool - bringing the chlorine residual up from 1.0ppm to 5.0ppm, for instance, won't help remove chloramines. Chloramines are those molecules mentioned above that have very little sanitizing power, smell bad (like chlorine), and make the eyes red. If you have ever been to a pool that smelled strongly of chlorine and was making your eyes hurt - you may have thought that they have too much chlorine in the pool. Actually they have too much combined chlorine in the pool. Free Available Chlorine (FAC), in and of itself, does not smell, nor burn the eyes. It sounds counter-intuitive to the layman, but a pool that smells of chlorine and burns the eyes - needs a lot more chlorine added to the water - to reach the point of breakpoint chlorination, where these combined chlorine molecules can be separated. The only way to remove chloramines (and the smell/burn) is to bring the chlorine level up to 10X the amount of the chloramine level, at which point, all molecular bonds are violently broken.
How do I know if I need to shock? Well, aside from the burn and itch - pool owners can use a DPD pool water test kit. As opposed to OTO type test kits with a yellow color, the pink color DPD test kits allow you to test for FAC (Free Available Chlorine) and TAC (Total Available Chlorine). The difference between these two measurements is your level of combined chlorine, or chloramines. It has been established, by Taylor, that anything over .3ppm difference between TAC and FAC is a high enough level of chloramines to warrant shocking the pool. Basically, if you see your FAC reading get any darker when you add DPD reagent #3, you should shock the pool. If there is no color change between these two tests - no shocking is needed.
As mentioned at the start of the article, many people shock the pool weekly, or twice monthly - usually because they were told to do so by their pool store, or they heard it somewhere. Usualmente, this is an unnecessary exercise and wasted money. If you need to shock the pool - to reach breakpoint chlorination, because of a high chloramine level or an extreme algae condition, by all means go for it. Follow package directions and add a little extra - just to make sure you have reached the threshold of breakpoint chlorination.
However, if you are just gearing up for a party, or just enjoyed a high bather load, or spot a patch of algae blooming in the corner, you may not need a full shocking (generally 1 pound or 1 gallon per 10000 gals), but can use only one bag to superchlorinate the pool.
Here's some tips on using granular pool shock safely: