Pool Water Chemistry, Part 1: Pool Sanitization

With a basic understanding of pool chemistry, the air quality problems of indoor pools become easier to comprehend. The two sides of the equation–water chemistry and air handling–are both critical to achieving healthy indoor air quality (IAQ) in a swimming pool.

Covered in this article:

• What does chlorine do in a pool?
  • Sanitization
  • Disinfection
  • Oxidation and Reduction
  • When chlorine falls short, it needs help
• Summary

This is a multi-part article, so let's first talk about the chemistry of keeping water clean and safe. This article is a brief overview of what is going on in the water. If you want a deeper dive into the chemistry aspects, we invite you to read our sources, which we have hyperlinked to throughout the article. 

What does chlorine do in a pool?

Chlorine is the most commonly-used sanitizer in the world, by far. It is an excellent sanitizer and disinfectant, and a decent oxidizer too. But compared to its strength as a sanitizer, it is a relatively weak oxidizer.

We can compare chlorine to stronger oxidizers such as ozone, hydroxyl radicals, hydrogen peroxide and others using the chart below from this source.

This chart shows relative strength of these oxidizers compared to Cl₂, chlorine.

As you can see, the chart compares these other oxidizers to chlorine. Ozone is 1.5x stronger. Hydroxyl radicals are 2.06x stronger. Bromine is a 20% weaker oxidizer than chlorine. We share this only to illustrate a point: chlorine is not used in pools because of its oxidation power; rather it is used because of its undeniable strength as a sanitizer and disinfectant.  So what's the difference between sanitation, disinfection and oxidation?

Related: Ozone for Swimming Pools

Pool Sanitization

"To sanitize means to reduce microorganisms of public health importance to levels considered safe, based on established parameters, without adversely affecting either the quality of the product or its safety." - Allan Pfuntner, M.A. REHS

The efficacy of a sanitizer is gauged on its ability to reduce the contamination level within a specified amount of time, not necessarily eliminate it entirely. In terms of public health, that standard is generally accepted as 99.9%, a 3-log reduction in contaminants within 30 seconds. For food contact surfaces, that standard is 99.999%, a 5-log reduction. For pool sanitization, we need water to be safe enough to be in direct contact with.

Disinfection

"Disinfection, in contrast, must destroy or irreversibly inactivate all specified organisms within a certain time, usually 10 minutes. Some chemicals may function as both sanitizers and disinfectants." - Allan Pfuntner, M.A. REHS

Disinfection, unlike sanitation, means destroying (or inactivating) every infectious microorganism. In the swimming pool business, just think of it this way. Sanitation means to kill any microorganisms to at least a 3-log reduction, and disinfection means to kill disease causing microorganisms completely, within a span of 10 minutes. Fortunately, hypochlorous acid (HOCl), the strong form of chlorine in water, is very good at doing both disinfection and sanitation in swimming pools.

Oxidation and Reduction

Oxidation is the process of taking an electron from a molecule, and sometimes replacing it with oxygen. Reduction is what occurs when a substance gains an electron, which is confusing, because it sounds opposite. The acronym to remember for this is OILRIG:

Oxidation Is Loss, Reduction Is Gain.

We call these reactions redox reactions, for reduction/oxidation. 

In swimming pools, oxidation is a term broadly used for removing contaminants that are non-living, like bather waste and metals. If contaminants are living, the process would be sanitation or disinfection. Oxidation is how chlorine 'burns up' non-living organics like sweat, mucous, body oils, urine, saliva, and bather products like cosmetics, sunscreen, lotions, deodorants, etc.

Sanitization/disinfection kills living microorganisms and contaminants, while oxidation handles non-living contaminants.

We can measure oxidation and overall chlorine efficacy with electronic probes that measure millivolts (mV) of conductivityl in the water. The term is Oxidation Reduction Potential (ORP). ORP is measured on almost every pool chemical controller, which provides a real-time picture of how chlorine is performing.

When chlorine falls short, it needs help

Chlorine is such a phenomenal sanitizer and disinfectant, it takes hardly any to keep bacteria or algae under control. According to Robert Lowry, just 0.05 ppm of HOCl is enough chlorine to disinfect a pool. The issue

There are several things that can weaken chlorine, however. Things like higher pH, too much cyanuric acid, or chlorine simply has too much to manage and depleted. When chlorine falls short, it needs help conquering the oxidant demand. This process, in short, is known as breakpoint chlorination.

The breakpoint chlorination curve

The first thing chlorine goes after is metals, which it oxidizes. That's up to point (A). Chlorine is reduced by these metals, and then begins oxidizing nitrogen and carbon-based contaminants. Carbon-based contaminants are primarily bather waste and other non-living organics. Quality enzymes can break down and digest these substances with ease, which is why we strongly recommend using enzymes in commercial swimming pools. Enzymes take an enormous amount of the non-living organic burden off of chlorine, so it is freed up to do what it does best: pool sanitization and disinfection.