Swimming Pool Blog
by Mark Garcia January 10, 2014
The Pool Shock Handbook
Continuing a short series on 'Shocking Pool Topics', today we look at when to shock, and how much shock to use.
These best methods of pool shocking are applicable to any pool, whether you are shocking a small pool, or a 400,000 gallon Olympic sized pool.
In this article, we'll approach shocking from the standpoint of why you are shocking the pool. There are 3 main reasons for shocking a pool, covered below.
But first, let's briefly discuss the various types of swimming pool oxidizers, or disinfectants - or as I prefer to call them - pool shocks.
Types of Pool Shock
Granulated chlorine powder is made by introducing enough chlorine gas to a solution of hydroxides to reach a certain concentration. The salts are dried and crystallized into a form of bleaching powder - invented in 1799!
Chlorine pool shocks include Calcium Hypochlorite, Lithium Hypochlorite, Dichlor and Sodium Hypochlorite, also known as liquid chlorine. They are sold in several different strengths, and will list strength as the percentage of available chlorine. Cal Hypo has the most bang for the buck, along with liquid chlorine, or bleach. Dichlor is a stabilized form of pool shock, containing cyanuric acid, or conditioner. Lithium burns very clean, without adding calcium salts to the water.
Non-chlorine pool shock, also known as Potassium Monopersulfate, is a strong oxidant made with active oxygen and several salts, dried to a fine powder. It's useful in a chlorine-free pool, or for shocking without adding calcium or stabilizer to the water. Chlorine-free shock is great at removing impurities, contaminants and chloramines, but not so good at killing bacteria or removing algae.
3 Reasons to Shock a Pool
1. Shocking for Chloramine Removal
Chloramines, also known as Combined Chlorine, occur when a chlorine molecule combines with ammonia or nitrogen in the pool water. The attachment makes the molecule sluggish, and it becomes ineffective as a sanitizer. Furthermore, it becomes irritating to skin and eyes, and makes the water smell.
Raise the chlorine level in your pool high enough, and you will break apart these chloramine bonds, removing chloramines from the water. To effectively remove chloramines, many sources say that you need 10x the amount of combined chlorine, but for complete removal I may double that amount, to compensate for stabilizer (cyanuric acid) levels, or after active pool use, or when treating algae.
To determine the presence of chloramines, a DPD test kit is used, along with a little formula. First one measures for Free Chlorine (FC), and then adds another reagent to test for Total Chlorine (TC). If the sample gets darker in the test vial, this shows a difference between Total and Free chlorine. Subtracting Free from Total yields Combined Chlorine (CC).
For example; If we test Free Chlorine at 1.0ppm, and find our Total Chlorine is 1.4ppm, the difference is 0.4ppm - of combined chlorine. FC - TC = CC. Now, how much chlorine is needed to shock the pool for chloramine removal? Some would say that the calculation is just 10x the amount of chlorine. But this is primarily based on monochloramine removal, and does not account for pH level, and the level of stabilizer, or cyanuric acid in the water.
Here's a real world chart to shock the pool for chloramine removal. Use it to figure out how much pool shock to use, based on your level of combined chlorine in the water. Remember this chart assumes a good pH, and a relatively low CYA level. More shock will be needed for higher levels of stabilizer in the pool, and for pH levels above 7.5.
You'll also need to know your pool size in gallons, and do some math to multiply these amounts (based on 10000 gallons), for the size of your pool. Oh, and in case you have forgotten, there are 16oz to a 1lb bag of shock, and 128oz in a gallon of bleach.
2. Shocking for Bacteria Removal
Bacteria, fungus, protozoa and viruses and even parasites can enter the pool water from swimmer waste or from organic contaminants. Chlorine pool shocks in the correct amount kill these organisms by attacking the cell wall, which exposes vital cell constituents and terminates cell function.
How much chlorine is needed to kill bacteria in swimming pools? It depends on the type of bacteria, and the extent of the contamination. Most types of bacteria found in pools, such as E-coli or Pseudomonas are fairly easy to kill with just 2-3 ppm of chlorine. However, the CDC recommends that for pool fecal incidents involving diarrhea, a level of 20 ppm be maintained for 13 hours, to eliminate protozoans, such as Crypto.
Like our charts mention, 20ppm requires a good pH level, a relatively low level of cyanuric acid. A tepid temperature, of 70-80 degrees, is also assumed. High pH, High Stabilizer or High temperatures will require a higher concentration of chlorine, and/or a longer reaction time - to effectively remove bacteria in swimming pools.
3. Shocking for Algae Removal
Algae can require large amounts of chlorine for a complete removal. It depends on the severity of the algae bloom, and other factors, such as high pH and Stabilizer levels, or a very warm water temperature, the amount of organic debris in the pool, and the effectiveness of the pool filtration and circulation.
After considering the variables described above, refer to the chart above for treating algae blooms in pools. Light colored or isolated algae may only require 10 ppm to eradicate, medium blooms should double the dose, and for dark green or yellow algae blooms, adding enough pool shock to reach 30 ppm may be necessary.
A less calculated approach is to balance the chemistry, clean the pool and shock until the water turns a blue-grey color. When shocking the pool to remove algae, it's best to not hold back, but rather 'hit it hard'. Using mechanical methods is usually necessary - brushing the walls and floor, vacuuming and backwashing, to remove algae cells from the pool, and flush them out of the system.
Thanks for Reading!