Normally, your pool is kept clean by chlorine from chlorine tabs, a salt chlorinator, a cartridge system, or some other slow-release, low-dose system. But when the chlorine in your pool is not doing the trick, you may need to shock your pool. But when it comes to shocking your pool, there are choices. Many pool owners are left wondering what the best way is to shock their pools. In this article, we will compare chlorinated shock vs. non-chlorine shock for swimming pools.

What is Shock?

Have you ever gone to an indoor pool, taken a deep breath, and been hit with the strong smell of chlorine? At first, you may think that the chlorine smell means the pool is clean. After all, there must be plenty of chlorine in that water to make all that smell. In fact, the chlorine you are smelling is not the type of chlorine that keeps the pool clean.

There are two main types of chlorine in your pool. Free chlorine is chlorine that is available in your pool to do its job, oxidizing organic matter and sanitizing the water. Combined chlorine is chlorine that has already done its job and is no longer useful. Combined chlorine is also called chloramine. It creates that notorious chlorine smell, and in high enough doses it can be toxic to swimmers.

When the combined chlorine level in your pool water rises to more than about .3 ppm (parts per million), you will need to shock your pool. Shock, both the chlorinated and chlorine-free variety, is a powerful oxidizer. Adding shock to your pool will oxidize and destroy organic material, giving a boost to your free chlorine. It will also oxidize the combined chlorine and allow it to gas off, reducing the total chlorine in your pool and making it more pleasant to swim in.

Types of Chlorinated Shock

There are three main types of chlorinated shock. Each one will do the trick, but they do have some significant differences. The main types are liquid chlorine (sodium hypochlorite), cal-hypo (calcium hypochlorite), and lithium (lithium hypochlorite). These all contain chlorine as the main ingredient, but the other ingredients affect how they work. Depending on your situation, one type of chlorinated shock may be better suited to your pool than another.

So to understand when to use each type of shock, let’s look in more detail at the three types of chlorinated shock.

Liquid Chlorine

Liquid chlorine is relatively inexpensive. It has the highest pH of any of the chlorinated shock formulas. In high doses that can be a bit of a problem, bleaching out paint, vinyl liners, and other soft or sensitive surfaces. However, it requires a relatively low dose to be effective, about 1 gallon per 10k gallons of pool water. After using liquid chlorine, it is recommended not to enter your pool for 24 hours. Once liquid chlorine oxidizes, it turns to simple salt water.

Cal-Hypo

Calcium Hypochlorite comes as a powder and is available in different strengths. Like liquid chlorine, it is a relatively inexpensive option for shocking your pool. It has a high pH, though not quite as high as liquid chlorine. For this reason, it should be avoided or used with care on any pool surface that is prone to bleaching, especially vinyl liners. It requires a relatively low dose, about 1 pound per 10k-15k gallons of water, depending on the concentration you are using.

Cal-Hypo will increase your free chlorine without increasing you cyanuric acid (CYA). This can be a benefit but requires a little extra care. CYA acts as a sort of sunscreen for chlorine, protecting it from the destructive force of UV rays from the sun. If you are using cal-hypo, it is best to add it to the pool in the evening, so it has a chance to work while the sun is low and overnight. Cal-hypo also adds calcium to the water. If your pool is already high in calcium, it may be worthwhile to look for another type of shock.

After adding cal-hypo it is recommended to wait 18-24 hours to use your pool, or until the pool has reached 2-4 ppm of total chlorine.

Lithium

Lithium is the most expensive option for shocking a pool. However, the benefit is that it dissolves instantly. This makes it ideal for vinyl-lined pools or other surfaces prone to bleaching. It also leaves no residue and won’t add calcium to your pool. If you already have hard water, lithium may be a good option. However, lithium shock requires higher doses than other shocks, as much as 1 pound for every 8k gallons of pool water. Due to its price and higher required dosage, it is used almost exclusively for smaller residential pools. Commercial pools rarely use it.

As with other chlorinated shocks, it is best to wait 24 hours to use your pool after adding lithium shock.

Dichlor and Trichlor

Another type of chlorinated shock is dichlor and trichlor. Unlike the other types of chlorinated shocks, dichlor and trichlor are called stabilized chlorine. That is because they have been formulated to withstand the destructive force of UV rays. As stabilized forms of chlorine, they are often used in slow-release pucks, tablets, or sticks to sanitize outdoor pools. But they are also available in granulated form. Dichlor and Trichlor have an exceptionally high level of available chlorine, which makes it ideal for dealing with heavy-duty algae blooms.

Non-Chlorine Shock

Non-chlorine shock contains a combination of oxygen and potassium. It may be known as Oxone, Potassium Monopersulfate, or Potassium Peroxymonosulfate, and is usually abbreviated MPS.

There are a lot of potential benefits to non-chlorine shock. It dissolves immediately upon hitting the water and leaves no residue. You need about 1 pound per 10k gallons of pool water. It works to oxidize organic matter from dead skin cells, sweat, sunscreen, dirt, leaves, and other sources. It also oxidizes chloramine.

Unlike chlorinated shock, chlorine-free has a pH level of 9 (neutral is 7). That’s somewhere between ocean water and hand soap. Chlorinated shocks have a pH closer to household cleaners. Because chlorine-free shocks have a lower pH than their chlorinated counterparts, swimming can resume just 15 minutes after the shock it added. It also will not bleach out pool liners, paint, or your swim trunks.

The main disadvantage of non-chlorine shock is that it does not kill algae of pathogenic bacteria (the stuff that makes you sick). For this reason, it is best used when the water is murky or the combined chlorine is too high. But if you are trying to deal with an algae bloom or unusually heavy pool use (lots of swimmers), non-chlorine bleach will not help much.

Conclusion

Non-chlorine shock is a good option if you want to get rid of organic debris and murky water and go right back to swimming. It is also effective to reduce combined chlorine. If you have an algae bloom or are worried about extra bacteria from a heavy load of swimmers, only chlorinated shock will do the trick. When using most forms of chlorinated shock it is best to add the shock in the evening, so the UV rays from the sun don’t break down the chlorine before it can have its full effect.

If you have questions about shocking your pool or you have ongoing problems with murky water or an algae bloom, contact The Pool Butler . We’ll be happy to answer your questions and give you a free quote for pool cleaning and maintenance. Here’s to happy and safe swimming!

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