Welcome to our discussion on swimming pool shock – specifically, when should one shock the pool, and how much do I need to add?
This question is emailed to me quite often, so I thought it deserved an answer in print.
There are 3 main reasons to shock a swimming pool:
1). Remove chloramine bonds
2). Kill visible algae
3). Kill invisible pathogens
1). Remove Chloramine Bonds
Free Available Chlorine (FAC) is an attractive target for ammonia and nitrogen that gets into the pool water. When combined, FAC becomes ineffective as a sanitizer and causes eye irritation, or red eyes.
Although it seems counter intuitive, when your pool smells strongly of chlorine, chloramine bonds are likely at a high level, and a good pool shocking is indicated. There is another way to determine chloramine levels, however.
Using a DPD test kit will allow you to measure both FAC and Total Available Chlorine (TAC). Subtraction of Free from Total will yield the amount of Combined Available Chlorine (CAC), commonly known as chloramines. When a level of 0.3 ppm is reached, a good pool shocking is indicated.
Shocking the pool to a level that is 100x greater than your chloramine is effective at chloramine removal. For a level at 0.3 ppm CAC, add enough chlorine to reach a level of 30 ppm in the pool.
2). Kill Visible Algae
Chlorine is a great algaecide, I like to say. Pool shock, in the right amounts, will destroy all types of algae by slashing through their slimy outer shells, and disrupting their cellular processes. Pink, Green, Yellow – none is a match for chlorine, in the right amounts.
Shocking the pool to 30 ppm of free chlorine residual, in the presence of proper pH levels, will destroy most algae. For best results, the pool should be vacuumed before shocking, and brushed after shocking. Filter non-stop, backwashing as needed, and use clarifiers if needed.
3). Kill Invisible Pathogens
A pathogen is a disease-causing substance. Bacteria, Viruses and Parasites can live in pool and spa water. Your normal sanitation and filtration likely removes most pathogens, but if you want to be sure – a good pool shock would be in order.
Shocking the pool after heavy use, or an extended period of low or no chlorine, or if the water has not been filtered for some time – are all good reasons to shock the pool.
The amount of shock needed to remove pathogens is dependent upon the level of pathogens in the water. You can test for bacteria with a bacteria test kit, or you can just shock the pool to 30 ppm.
How Much Pool Shock Do I Need?
Breakpoint chlorination is a level of chlorine at which point molecular bonds are broken apart. Conveniently, this is also the point at which chloramines, algae and pathogens are removed from the water.
For most situations, a level of 30 ppm of chlorine will reach the threshold of breakpoint chlorination, and order will be restored.
Here’s a pool shock treatment chart to determine the amount of pool shock necessary to raise the chlorine level above the breakpoint threshold, which is usually around 30 ppm.
Not using enough chlorine, to not quite reach breakpoint, may not have the desired full effect.
Check for proper water balance, especially pH, in the range of 7.2-7.6, before adding the shock to the pool. High pH of 7.8-8.2 can render up to half of your pool shock completely inactive.
Proper filtration and circulation deserve some mention as well – pool shock doesn’t do it all alone. The pool water needs to be filtered to remove the microscopic remnants of organic matter that is destroyed.
Keep those questions coming!