The term shocking covers a vast array of scenarios in swimming pools.
aquamagazine.com
This article says there are three different methods of chlorine "shock", despite the universal use of the term, and each type of "shock" refers to specific processes and are used in different situations:
1) Superchlorination: increasing the level of chlorine to sufficiently sanitize a pool, clear hazy water and deal with limited algae growth. When superchlorinating, chlorine is added to raise the measured free chlorine level to 10 to 20 ppm.
2) Hyperchlorination: used in situations when a pool needs to be completely disinfected, in which the chlorine level is raised to 20 to 40 ppm. One such situation would be a case of suspected contamination from a chlorine resistant germ such as cryptosporidium (crypto). Crypto is commonly spread in pools from an accidental fecal release.
3) Breakpoint chlorination: used specifically for the purpose of breaking apart and removing combined chlorine or chloramines. Breakpoint chlorination is NOT used to clear green pools or deal with crypto. In the pool industry [historically and incorrectly] the ratio of chlorine to chloramine is 10 to 1. [Breakpoint only applies to inorganic chloramines starting with ammonia, and does not apply to urea, creatinine, and other organic compounds.]
You will often see the pool industry use a breakpoint chlorination 10x rule of thumb, stating 10 chlorine (FC) to 1 Combined Chlorine (CC), and this is completely wrong and results in over-chlorination.
Technically, if the compound to break apart is ammonia, you need 0.5x. If the compound is urea, then 3x at most. Practically, "just keep adding chlorine to the pool until the FC starts to hold, though it may be in two phases -- the first where the chlorine is consumed very quickly (in less than a minute) as it converts ammonia to monochloramine -- and then a slower second phase that takes hours where it converts monochloramine to nitrogen gas (about 4 hours if CYA is in the water) or oxidizes partially degraded CYA."
Richard Falk's (chem geek) writings about breakpoint chlorination:
There is a molar relationship of 3:2 for chlorine to ammonia. Chlorine is measured in ppm Cl2 units, where molecular chlorine has a molecular weight of 70.906 g/mole. Ammonia is measured in ppm N units, where atomic nitrogen has a molecular weight of 14.0067. Therefore, in terms of a chlorine to ammonia ppm (weight) ratio, it is (3*70.906)/(2*14.0067) = 7.593. In practice, due to side reactions producing nitrate, the actual weight ratio needed for chlorine oxidation of ammonia is 8 to 10. This is where the 10x pool industry rule-of-thumb comes from.
Now let's look at how this very valid rule was misapplied in the pool industry. The pool industry took this rule and applied it against Combined Chlorine (CC). The first major flaw is that CC is measured in molecular chlorine units (i.e. ppm Cl2), NOT ammonia nitrogen units (i.e. ppm Nitrogen). So there is no factor of 70.906/14.0067 = 5.062 weight difference. The second major flaw is that CC already has chlorine combined with ammonia - presuming it is mostly monochloramine, which should be the case if one starts with ammonia. So 2 of the 3 initial chlorine would have already been used up combining with the 2 ammonia. The molar ratio of what is left is only 1:2, not the original 3:2. In practice, it would take a little more than this 0.5x amount, but the point is that it is nowhere near the presumed 10x rule.
Even if one goes through this same analysis using chlorination of urea (instead of ammonia), one doesn't get to more than 3x at the most. The 10x rule is completely wrong in its application to CC because (1) the unit of measurement of CC is 5 times larger than that of ammonia so takes 1/5th as much chlorine compared to ammonia and (2) chlorine is already part of CC so it takes less chlorine to further oxidize it.
The pool industry got it wrong decades ago and everyone has been following the 8x to 10x rule like lemmings ever since (again, the rule IS correct against ammonia in ppm Nitrogen units). The breakpoint chlorination "10x" rule only applies to the ammonia measured in its own units of measurement (ppm Nitrogen).
The largest nitrogenous component of bather waste is urea, not ammonia, so the biggest problem with CC in higher bather-load pools is mostly due to a buildup of urea in the water. Chlorine combines with urea rather slowly so urea concentrations can build up at which point the intermediate CC (monochlorourea) can show up. If you have a lot of built up urea and try to raise the FC level to get rid of the CC, the CC level may rise instead of fall. Increasing FC can have the CC decrease IF the chemical to be oxidized is ammonia, but the CC can INCREASE if the chemical to be oxidized is urea, because urea is much slower to combine with chlorine.
In spas, most of the chlorine is used to oxidize bather waste. In between soaks, however, the chlorine level is kept lower, but right after a soak a lot of chlorine is added to oxidize the bather waste. This is where an ozonator can help cut that chlorine usage roughly in half if the spa is used every day or two. Unfortunately, ozone also reacts with chlorine, so in between soaks, if the spa isn't used frequently, the chlorine demand is at least doubled due to the ozone. The ideal situation would have the ozonator turn on right after one ends one's soak and stays on for 12-24 hours depending on how long it takes to oxidize the bather waste. The ozonator would then turn off until after the next soak. That way chlorine can be kept at a low 1-2 ppm FC level with 30-40 ppm CYA the entire time so provides for disinfection in the background but is not the primary chemical used for oxidation of bather waste.
Source:
Breakpoint Chlorination
Also note that CYA has a strong blunting effect on chlorine shocks:
"Pools with high levels of CYA would require much more chlorine to achieve breakpoint based on the binding of CYA to HOCl. There are current studies stating that for chlorine to be effective in the presence of CYA, there must be a ratio of 20 to 1. Meaning 1 ppm chlorine for every 20 ppm of CYA... in the presence of high CYA, chlorine will not be as effective and that includes when using breakpoint. It is because of this that tri-chlor and di-chlor forms of shock should not be used when attempting breakpoint chlorination."
The article says chlorine shocks are used to get rid of 2 types of chloramines: inorganically bound, and organically bound. "Breakpoint is an efficient way to deal with inorganically bound chloramines. However, it is not very effective at removing the organic bound types. Proactive methods of oxidation, such as regular superchlorination using unstabilized chlorine or the addition of ozone and UV, can help prevent formation of the organic bound chloramines."
So we can see that organic chloramines are the bigger problem, requiring proactive measures to prevent them from building up. Ed Lightcap added a suggestion in the comments below the article: "I would add potassium monopersulfate to your "proactive methods of oxidation" to help prevent the formation of organic chloramines."
However, it should be noted that potassium monopersulfate aka MPS and/or trace amounts of compounds in MPS products are well-documented in dermatology literature to cause severe contact dermatitis in some people. Anyone experiencing chemical burn type itching or unexplained hot tub itch should discontinue MPS usage (be aware that products like SoftSoak Trio's weekly packets contain MPS not indicated on packaging but indicated in MSDS).
After doing any of these 3 chlorine shocks, it's probably a good idea to open the spa cover, run the jets for 10-20 minutes, and allow the area to air out in order to dissipate any disinfection byproducts (DBPs). DBPs may have harmful/carcinogenic effects to humans when inhaled.
