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Pool Residual Sanitizer Options Compared: 2021

Here is an important truth for a pool or spa owner: if your water isn’t clean, you don’t have a pool or spa. No one wants to swim or soak in dirty water. And it’s not just unpleasant to look at: water can make you sick if it’s not disinfected and sanitized. Luckily, there are plenty of options for cleaning the water. All of these involve some type of chemical “residual.”

What is a residual sanitizer?

The big trick of a residual sanitizer is that it is right there in the water, all around the swimmers. Unlike a filter or UV system that the water must pass through to be cleaned. 

A residual not only maintains the pool, it actively kills bacteria as it is introduced to the water. This is really important when bathers are in the water. A residual can prevent the spread of germs that can make multiple people sick. Some residual options also prevent algae and biofilm from building up as well.

A residual is usually fed into the water or dispensed in some way. Once you maintain the proper residual level in the water (usually measured in parts per million—ppm) the water is kept clean at all times. The residual is available to sanitize, even if the pump is off and the chemical is not being fed into the water 24/7.

There are many options for a residual sanitizer. Let’s break them into a few categories to explain the pros and cons of each option.

TLDR: Here’s a quick summary!

Use mineral systems as a complement to chlorine. You can try biguanide to truly go “chlorine free,” just make sure you understand the quirks. For the most reliable and effective residual, chlorine is still the way to go. Keep reading for details!


1.  Supplemental Residuals

What we are calling supplemental residuals do actively sanitize, but they do not work on their own. They need a backup, generally chlorine, to keep the water clean. That’s why we consider them an addition because they need to work alongside something else.

The typical supplemental residual would be some type of metal—copper and/or silver. These are usually called mineral sanitizers or ionizers. A “mineral” sanitizer dispenses the metals into the water with a floater, feeder, or skimmer. An ionizer can be inline or a floater. It uses an electrical charger to ionize the metals into a solution. Ionizers can also be certified to the NSF 50 standard.

With each of these, the metals do not oxidize contaminants. (Oxidize basically means to burn up and destroy.)  But supplemental mineral residuals can give bacteria a taste of their own medicine by infecting them, disrupting them, and eventually causing membranes to fall apart. [link] These are proven methods, used since ancient times, to keep the water clean. But they do have some drawbacks.


Minerals and ionizers work very slowly compared to strong oxidizers. This is one of the reasons they can really only function as backup, not a primary sanitizer. They may also cause staining if the pool gets out of “balance” with pH and hardness measurements. And unlike stronger residuals, they will not break down chloramines in the water.

But these systems are very popular and can have benefits. They work with chlorine to make it more effective and are typically very good at controlling algae. Minerals or ionizers can also reduce the amount of chlorine required. But use caution—reducing chlorine may not mean what you think!

Let’s be super-clear here. In the US there are specific guidelines for the residual of free chlorine required in the water. For residential pools, this is 1-3 ppm. This does not change with the addition of a mineral sanitizer. Even NSF 50 certified ionizers still require 1-3 ppm per the standard. There is no agreed-upon regulated standard for below 1ppm of chlorine in a pool. 

But by assisting chlorine, minerals can reduce the amount of chlorine needed to maintain 1-3ppm. So they may actually reduce how much chlorine you need to store, add and buy for the pool to be maintained. Plus, they can help chlorine be more effective by acting as a backup residual.

2.  Alternative Residuals

A less common alternative to chlorine that has been around for a while is polyhexamethylene biguanide (PHMB), or simply Biguanide. One of the most exciting things about this chemical is that it is the rare, true chlorine-free alternative.

While biguanide does not oxidize, it does actually effectively sanitize. More or less, it pulls bacteria apart into pieces and helps the dead bacteria clump and filter out of the water. While it does work more slowly than chlorine, it is considered effective and recognized as a stand-alone sanitizer.

Since it does not oxidize, PHMB must be used with a second chemical. Hydrogen peroxide is used periodically to oxidize contaminants and help keep the water clear. Peroxide is not great at maintaining a residual, but it is a good partner with Biguanide. It is a strong chemical but can feel less “harsh” to work with than chlorine.


Both PHMB and Hydrogen Peroxide are liquids, so they are very easy to work with. They are easy to measure and don’t require any kind of pre-dissolving. There are no strong chemical smells or effects in the water. In fact, the water will probably feel very smooth and silky. It really is a totally different experience than chlorine.

The downside is while they are gentle on you, the chemicals are also gentle on bacteria. They kill, but much more slowly. Biguanide is not a recommended treatment for a Cryptosporidium outbreak. If there was a suspected exposure to crypto, the recommended solution would still be chlorine. Adding chlorine will require a whole process to reset the pool and use Biguanide again since they are not compatible.

PHMB has a tendency to cause foaming which can be an annoyance. It will also require much more regular and thorough filter cleaning since it is dependent on the filter to pull out the “dead” bacteria bodies.

It is also very common over time to develop a problem with water molds and “slime.” These build up in the plumbing first so by the time you see it, it’s probably already a big problem. And the best way to get rid of these outbreaks is—you guessed it—chlorine.


3.  Sanitizing Residuals


This is one of the largest categories and the most common sanitizing method for pools or spas. In fact, chlorine is hands down the most popular method, and with good reason. It’s not hard to use, inexpensive, and ridiculously effective. 

This chemical has been used and improved for over 100 years and it is still a staple of pool water, spa water, and municipal water systems. It’s popular because it works and the upsides really outweigh any downsides.

There are many ways to get chlorine in the water, but they will all have similar features. Residential pool standards require 1-3 parts per million which isn’t very much when you think about it! That small amount of chlorine will oxidize contaminants in the water. 

It’s important that the 1-3ppm amount is free chlorine. That is the level recommended by every standard in the US. At this level you have a buffer, the pool can handle multiple swimmers without overloading the chlorine and you are always protected. But it must be free chlorine, not combined chlorine that occurs with chloramines.

We are going to focus on chlorine, but it is also possible to sanitize with Bromine. As a “halogen” chemical, it sanitizes in a very similar way to chlorine. Bromine is more commonly used for spas or indoors since it cannot be stabilized to protect it from the sun. It can be very effective…but that’s for a separate post.

You may have heard of or experienced uncomfortable chlorine pools, smelly chlorine pools, etc. This is usually caused by chloramine build-up (not free chlorine). The great news is you can control it. And you probably are not allergic to chlorine. (link)

There are a few ways to add chlorine to the pool:

– Trichlor tabs: These tabs are used in a feeder or floater and slowly erode, adding chlorine to the water. They may contain other additives to help the pool, and will always contain CYA to protect the chlorine from the sun.

– Calcium Hypochlorite tabs: Relatively new, these chlorine tab formulations require special feeders. But they do not contain CYA, which can make it easier to control the CYA level in the pool.

– Chlorine granules: This is a chlorine powder, available in trichlor, dichlor, or cal hypo. Scooping out granules is much less convenient, but is technically a way to add chlorine to the pool. More commonly this could be a way to “shock” or super-chlorinate the pool.

– Liquid chlorine: This old standby is still a favorite of many pool owners and pros. It is usually inexpensive, easy to add with no mixing, and is pretty effective. But storage can be a hassle with lots of bottles hanging around.

Don’t forget: if you are using chlorine tabs, never mix different tab types together.


Salt pools, or chlorine generators, take the buckets and bottles of chemicals mentioned above out of the equation. With these systems, a small amount of salt in the water is used to generate chlorine right on-site. You can have an endless supply without handling any harsh products, and there are no additives to deal with. This puts you in total control.

These systems have lovers and haters. In fact, we did a whole video and a detailed post about common “complaints” with salt pools. You should check it out—it’s a great chance to see what’s really going on with these systems.

In addition to convenience, Powerclean Salt systems from CMP are simple to operate, durable, and easy to maintain. Every salt system has to be cleaned, we just made it easier. If you went to the trouble of setting up a salt system to get rid of chemicals around the house, why would you want to keep a jug of acid around for cleaning? Instead of dangerous acid, you can clean the cells with the simple tool we provide. Or just use a paint stir stick in a pinch.


While most chlorine “problems” can largely be avoided, [link] some people just do not enjoy the chlorine experience. And of course, even if you choose chlorine, you may have to deal with some level of strong chemical around the pool. Using a chlorine generator can greatly reduce this. 

With any chlorine system, you will need to keep on top of chloramines. This combined chlorine can be unpleasant and can build up indoors. There are also growing concerns about disinfection by-products (DPBs). The good news is that for regular pool uses, DBPs don’t usually reach a dangerous or concerning level. And the use of an advanced sanitizer (more info below) can greatly reduce the formation of DBPs by reducing the organic load in the pool.

The upside of chlorine is very high, though. It is hands-down the most effective residual sanitizer you can find for a pool. It is the top way to keep a pool safe for swimmers. And the truth is almost all of the negative effects are mitigated by proper use and/or a supplemental sanitizer (like we said, more info below!)

What to use besides a residual?

Other than biguanide mentioned above there really isn’t a true chlorine-free option for residuals. However, you can get the most out of a chlorine system by adding a powerful advanced sanitizer. 

The chlorine residual works in the body of water. A second system like UV, Ozone or AOP treats the water before it gets to the pool. 

Powerful disinfection or oxidation takes the water to a whole new level. Chloramines are reduced and removed. With ozone and AOP, contaminants are destroyed much faster than with chlorine alone. And chlorine-resistant bacteria are broken down, making swimmers safer.

Find The Right System For You With The Best Residual And The Best Advanced Sanitizer. Check out all of the options at c-m-p.com/pool-products/pool-sanitizers.

More Posts & Related Content

The Essential Guide to NSF 50 for Pool & Spa Sanitizers

Is Salt Water Bad for a Swimming Pool?

Water Is Awesome: A Guide to Understanding the Relationship Between Humans and High-Quality H2O

How Do I Make My Pool Water Crystal Clear?


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