How often should I shock my pool?

Cindy in the Pool

The simple answer is “Whenever it needs it”. The real question is “How do I know when my pool needs a shock treatment?”

Why Shock?

First, it helps to understand what a shock treatment is and why we need to do it.

When clients (both canine and human) enter the pool, they introduce organics into the water. These include sweat, saliva, skin cells, body lotions, deodorants, soil, urine, loam, and a host of other things. Mother nature also contributes by blowing leaves and other debris into outdoor pools. These organics provide a food source for bacteria to live on. Although there are both good and bad bacteria, the health department generally insists that we get rid of ALL of them.

So we add sanitizers to the pool. When you use chlorine, ozone, or bromine, these compounds actually do two things: 1) they kill the bacteria, and 2) they break down the contaminants, helping to remove them from the water so they no longer provide a smorgasbord for new bacteria.

This is accomplished by oxidation – an oxygen molecule “attacks” the contaminants. You may have heard of oxidation in relation to a rusty old car, and it’s exactly the same thing. Just as oxidation can turn metal into dust, it will also break down the organics in the pool water.

Killing bacteria is fairly quick and easy, but dissolving the contaminants takes a bit more time and muscle. The problem comes when the sanitizer “falls behind”. If there is a choice between a live bacterium and the organic food source, the oxygen molecule will tend to attach to the food rather than the bacterium. So your sanitizer ends up working on the less important task. Unfortunately, while the sanitizer is working on the contaminant, the bacteria can multiply and you can end up with a bacteria “bloom”. If your water is cloudy before a shock treatment, but clears up afterwards, this is probably what is happening.

You could avoid shock treatments almost entirely by increasing the level of an oxidizing sanitizer so that it never falls behind. This makes sense if contaminants are coming into the pool at a fairly steady rate, but it is wasteful if the problem is just the occasional exceptionally dirty dog. In addition, some sanitizers (particularly minerals and UV) don’t use oxidation at all, so they are of no use in removing organics.

The purpose of a shock treatment, then, is to remove the organics and other contaminants from the pool. It is NOT intended as a replacement for sanitation, but it will help sanitizers be more effective.

When To Shock?

If you are chlorinating a pool, it’s quite easy to measure when a shock treatment is needed. Your test kit should include tests for both free and total chlorine. The free chlorine is what is available for killing the bacteria. The total chlorine is the combination of free chlorine plus what has already been “spent” for dissolving organics. When the level of total chlorine exceeds the amount of free (usually by 2 ppm or more), then you know that you are falling behind and need to shock.

We’ve all walked into a public pool and been knocked over by the chlorine smell. It might surprise you to learn that this was probably the result of NOT ENOUGH chlorine, rather than too much! The level of organics has gotten out of hand, and what you smell is the byproduct – a combination of chlorine and ammonia, or chloramines. A shock treatment is needed to deal with the organics, followed by an increase in chlorine on a daily basis to prevent the buildup in the future.

No matter what sanitizer you are using you can certainly schedule regular shock treatments, but there is no single formula to determine how often. In general, if your water is getting cloudy before the treatment and clearing up afterward, then you need to shorten the cycle and/or increase your daily dose of an oxidizing sanitizer. The smaller your pool, the more often you will need to shock. An 800 gallon spa may require daily shock treatments.

You may also want to shock:

  • After exceptionally heavy use.
  • After a heavy rain or a windstorm that blows debris into the pool.
  • After a water change.

This last one may surprise you – you’d think that the water coming from your tap is clean and pure. It probably IS safe from live bacteria, but there may still be organics. I always shock after a water change.

What Kind of Shock?

There are two main categories of shock treatment: chlorine and non-chlorine. The former is simply raising the chlorine to a very high level – typically around 10 ppm. This provides enough oxidation to dissolve the organics AND rid the pool of bacteria that may have been busy multiplying during the imbalance.

The trouble with chlorinated shocks is that you cannot use the pool until the chlorine level drops back to normal levels – around 3 ppm. This can take some time, and you must test the pool water to ensure the level has dropped. In addition, if you are using an alternative sanitizer, it may not be compatible with the chlorine shock treatment.

Non-chlorine treatments generally use potassium monopersulfate. This will NOT kill bacteria, but it is very effective at oxidizing the contaminants. The labels claim you can swim 15 minutes after treating, although I still prefer to wait overnight.

Did You Know?

  • You should leave the cover off the pool during a shock treatment. This is because nitrogen gas is expelled as a by-product, and you want to let it escape.
  • If you use a flocculant (a blue clarifier), add it after the shock treatment has had time to work.
  • You’ll probably need to vacuum the pool afterwards, since the oxidized materials will tend to sink to the bottom of the pool.
  • Certain sanitizers may reduce or eliminate the need for shock treatments, but shock treatments do NOT replace sanitizers.

Answer courtesy of:

The ACWT Pool Guru:
Karen Hunter
Woodinville , WA 98077