Superchlorination (Shock Treatment)

To oxide out the ammonia and nitrogen compounds and other contaminants requires adding much more chlorine than daily dosages, or adding the non-chlorine shock treatment potassium monopersulfate when combined chlorine levels are above 0.2 ppm. How much of these oxidizers are needed depends on several factors- amount of water in pool, spa or hot tub, condition of water (cloudy, green, dirty), water temperature and other variables that must be considered. There is not exact or across-the-board rule for amounts to add; although there are commonly accepted amounts that could be used as a starting point when you need to shock water. pH and total alkalinity must be in the accepted ranges before adding oxidizers. Oxidizers added to water with lower pH or TA could cause problems, as the water is already in or near the corrosive ranges and oxidizers can also be corrosive, be sure pH is up to 7.4 to 7.6 and TA is up to 100 ppm before adding additional chlorine or non-chlorine shock treatment. If pH or TA are too low, the minerals in the water can come out of solution when superchlorinated or shocked with oxidizers. Higher pH and TA will slow down the effectiveness of the additional chlorine, but non-chlorine type oxidizers.

Green algae
The recommended amount of liquid chlorine needed to superchlorinate an average, 18,000- gallon pool is from 1½ to 2 gallons of 12½% chlorine. For this same amount of water, use 1½ to 2 pounds of non-chlorine shock, or 2 to 2½ of calcium hypochlorite. In spas and hot tubs, liquid chlorine or calcium hypochlorite chlorides are not recommended, due to the high pH of both and higher calcium content of calcium hypochlorite. I recommend non-chlorine shock and/or lithium hypochlorite, not stabilized sodium dichloro. Stabilizers are not needed in spas or hot tubs because they are covered most of the time, retain heat and have very little exposure to direct sunlight. To superchlorinate with lithium, add from 1½ to 2 ounces for each 500 gallons of water. This does not need to be pre-dissolved because of the warmer water. If using non-chlorine shock, add from 1½ to 2 ounces for every 500 gallons of water.
There are no free lunches- each sanitizer or oxidizer used will have some positive and negative factors.

Using liquid chlorine where the pH is high can raise pH; you may have to compensate for this by adding acid. The same is true when you use calcium hypochlorine and lithium chlorines. The non-chlorine shock, potassium monopersulfate, will not affect pH. Positive factors: Liquid chlorine will remove chloramines and add additional amounts of chlorine to water. This chlorine residual is a fast-acting and excellent algaecide, germicide bactericide and disinfectant; it is also less expensive than some other chlorines. Negative factors: High pH, 13, bulky, short shelf life; may require 24 hours or more for chlorines to drop back down to safe swimming levels (4 ppm before swimming is recommended).

Calcium Hypochlorite
Is fast acting, granular chlorine with the same properties as listed above for liquid chlorine. Positive: Ideal in areas with lower water hardness, lower pH, and TA as a basic sanitizer for pools. Negative: high pH (11.5) and high calcium content can leave visible residue in pool. Highly combustible if water or moisture are allowed to saturate or contact dry concentrations.

Lithium
Positive: fast dissolving, safer to store and transport, ideally used to activate bromine. Negative: Lower available chlorine per pound. They are more expensive than other chlorines, and they have a higher pH- 10.5

An important consideration when using any chlorine to oxidize out chloramines, is that if you don’t add enough chlorine to completely knock out these contaminants, then you can be adding more chloramines and more possible problems. Enough chlorine must be added to bring the chlorine as high as 10 ppm to completely oxidize out the chloramines.

Non-chlorine shock will oxidize shock out chloramines even if not enough is added and it will not add to the problem. Another advantage of non-chlorine shock is that swimming can be allowed shortly after adding- about one hour. One disadvantage of non-chlorine shock treatment is that it can remove the combined chlorines and also activate out the free available chlorine, leaving no sanitizer in the water. If chlorine levels are above 3 ppm, I use non-chlorine shock treatment; fill up the floater with chlorine tablets and superchlorinate as soon as possible. To be sure you added enough oxidizers tablets to do the job thoroughly, test for combined chlorines the next day. If less than 0.2 ppm combined chlorines, then you have removed some or all of the chloramines. The rule of thumb for remembering when to oxidize using chlorines or non-chlorine shock treatment is when the combined chlorines are over 0.2 ppm. Test combined chlorine the following day to determine if enough oxidizers were added.

Breakpoint Chlorination
Is referred to as adding enough chlorine to bring up to 10 ppm or higher; refers to adding up to 5 ppm chlorine for each ppm of ammonia. This is basically the same as superchlorination.