Back to the Basics: The Benefits of Liquid Sodium Hypochlorite for Swimming Pools
By Terry Arko
Overwhelming product choices for pool water treatment
In today’s world, the consumer has become overburdened with product choices. Everything from food, beverages and clothing to media, technology and personal care products have a dizzying variety of personalized options available: when is the last time you ordered a plain, black coffee at a coffee shop? The same has occurred for the treatment of swimming-pool water. The primary task for a pool service professional, or even any DIY homeowner, is to ensure that the water in the pool is clean and safe for the swimmers. The very apparent failure of an off-balance pool when it goes green is a clear Don’t Swim sign, but we in the industry know that just as dangerous, invisible microbes and bacteria can leave a pool unswimmable, even though it may look okay.
Chlorine has been a long-standing workhorse to sanitize swimming pools and keep them free from bacteria or algae growth. Going back in time, the treatment of pool water was mostly simple. Two primary forms of chlorine compounds were widely used: liquid chlorinating compound (sodium hypochlorite, or bleach) and calcium hypochlorite dry, in both commercial and residential pools. The water was balanced primarily with muriatic acid. As swimming popularity increased, more pools were built and more swimmers entered the water. Longer hours at the aquatic facility and more swimmers in the backyard pool put a huge strain on the water quality. Products to address the surge began to fill the market.
More chlorinating options became available, led in the DIY market by trichlor tablets. Trichlor tablets differ from sodium hypochlorite and calcium hypochlorite because they are blended compounds that contain cyanuric acid (CYA). Cyanuric acid is a stabilizer that prevents chlorine in pool water from being broken down by sunlight. In the commercial realm, feeder systems became more prevalent and sophisticated. Ozone, UV and advanced oxidation processes (AOPs) soon became treatment options (joined by specialty chemicals) to handle everything from algae to evaporation prevention. The maintenance of the backyard pool evolved from a simple to a potentially complicated and confusing process.
How tablet convenience replaced liquid practicality
While calcium hypochlorite and liquid sodium hypochlorite were enjoying mainstream popularity in backyard pools, the fact remained that both forms of hypochlorite do not contain a stabilizer like CYA. That meant that in the summer sun, the free chlorine created by these compounds didn’t last very long. In fact, nearly all the chlorine from sodium hypochlorite or calcium hypochlorite could be destroyed in about four hours by direct summer sunlight. Since this problem began to be recognized, the practice of adding extra amounts of chlorine in the late afternoon or evening was incorporated. The issue was partially solved when Monsanto began to produce and distribute a chlorine stabilizer known as cyanuric acid (CYA). CYA stabilizer could be added to the pool to allow for longer residuals of chlorine during the sunny days of summer. The addition of the CYA stabilizer allowed chlorine from sodium hypochlorite or calcium hypochlorite to last up to eight times longer.
At levels of just 30 ppm, the CYA helped to significantly slow the degradation of chlorine from UV sunlight, which was an incredible benefit. Soon after, the trichlor tablets of stabilized chlorine (known as isocyanurates) began to make their way into residential swimming pools. The selling point of these tablets was convenience for the owner. The tablets could be added to an inline feeder or a floating container (floater). The main advantage of trichlor tablets was having the stabilizer that protects chlorine from the sun built right into the tablet, making it an instant convenience for the modern pool owner. As this system of treating backyard pools grew in popularity, liquid sodium hypochlorite was pushed out as the main source of pool sanitization. Liquid was now mainly used as a backup or shock.
Perceived dry chlorine convenience causes complications
While the two-in-one convenience of trichlor tabs continued to grow, there was something going on beneath the surface that pool pros and homeowners were beginning to observe. Early on there was a suspicion that higher levels of CYA could block the effectiveness of the chlorine, causing what was known as chlorine lock. Anecdotally, it was becoming certain that pools treated with trichlor tabs, with increasing amounts of CYA, were becoming a struggle to maintain. It seemed more of a challenge to keep free available chlorine levels in control and many pools had algae problems toward the end of the season. Users of trichlor tablets soon became aware of some side effects that led to complications in water treatment. The amount of CYA being released from the continual dependence on trichlor tablets as the primary means of chlorination bore a closer look. (More than half of an eight-ounce trichlor tablet consists of CYA. Over 54 percent by weight is CYA with the other 46 percent being chlorine and binders.) It was realized that too much CYA came from trichlor tablets with every application, which was far more than needed to stabilize a pool.
Despite the clear chemical argument for using (but limiting) CYA, there was a large contingent of users under the impression that more was better, while others were beginning to see the need to drain and dilute to keep CYA levels down. Regardless of the stance on the CYA debate, one point was clear: trichlor tablets made the maintenance of pool water more complicated and costly. Another important point was that they were very acidic, which led to using more soda ash to balance pool water. It was clear to see that the perceived convenience of trichlor tablets led to a lot of complications for both service pros and pool owners.
Liquid sodium hypochlorite and saltwater generators
Moving into the 90s, devices began to become more prevalent. A technology that became popular was the saltwater chlorine generator. These were sold to many new pool owners under the auspice of being a non-chemical pool product that relied only on salt to purify the water. Many of those new pool owners were not aware that the salt unit installed on their new pool was basically a small home version of a large-scale liquid sodium hypochlorite factory.
Liquid sodium hypochlorite is made at chemical plants by first using a process of electrolysis to split the sodium chloride molecule. Simple salt is divided into chlorine and sodium hydroxide, then blended with water to form liquid sodium hypochlorite. The saltwater generators on swimming pools do the same thing on a smaller (and less efficient) scale. So, they really are more correctly referred to as chlorine generators.
Liquid sodium hypochlorite is still recognized as one of the best back-up sanitizers for chlorine generator systems. The main reason for this is due to the byproduct that comes from the use of sodium hypochlorite. Let’s look at the different types of chlorine and their byproducts:
- Calcium hypochlorite: byproduct left in water is calcium chloride
- Trichlor: byproduct left in water is cyanuric acid (CYA)
- Liquid sodium hypochlorite: byproduct left in water is sodium chloride
The only byproduct left from using liquid sodium hypochlorite to backup a chlorine generator pool is sodium chloride, better known as salt. Of all the types of assistive chlorine that can be used with a saltwater chlorine generator, liquid sodium hypochlorite is the simplest and most consistent with the system. The use of liquid sodium hypochlorite as a backup in a chlorine generator pool is best, due to the fact that it will not add any calcium scale or additional CYA to the system.
Like any type of chlorine-sanitized swimming pool, one using a chlorine generator will still need to maintain a certain level of CYA to prevent rapid burnout of chlorine from the sun’s UV rays. CYA levels in a salt pool are good at a level of 30-50 ppm. At times, a chlorine generator may need a backup of manually added chlorine to keep the water quality good. This could be due to heavy swimmer load, equipment failure or other inert items such as leaves and dirt being introduced into the pool water.
Liquid sodium hypochlorite and secondary systems
There are several sophisticated secondary sanitizer options that have gained in popularity recently: ozone, UV and AOPs. None are yet approved by US EPA as primary sanitizers, however. Even though they are beneficial to oxidize and inactivate many pool-water contaminants, they are unable to leave a measurable residual in the water. That means chlorine still needs to be the primary sanitizer, with a residual between one and four ppm. CYA levels need to be controlled in pools with these treatment types in order to get the desired residuals to ensure protection from bacteria in the pool.
As with nearly all other pool types, 30-50 ppm of CYA is the recommended level in these systems. At a CYA level of 60, it would take 4.5 ppm of chlorine to inactivate bacteria, which is outside what is required by US EPA rules. Since the main purpose of secondary devices is to allow for complete disinfection with lower amounts of chlorine, the use of trichlor in these treatment options is a less than ideal solution. Again, liquid sodium hypochlorite is a preferred choice for these systems because it provides manageable levels of free chlorine without byproducts that will reduce the effectiveness of UV, ozone or AOP.
The benefits of a simple liquid sodium hypochlorite system
Liquid sodium hypochlorite has been proven throughout the history of pool chlorination to be one of the most cost-effective, easiest and safest ways to disinfect pool water. Perhaps this is a good time to return to the simplicity of liquid for effective pool treatment. Benefits of liquid sodium hypochlorite include:
- Affordable and easy
- Safe for storage (as it is non-flammable and non-combustible)
- Immediately available sanitizer
- Leaves a measurable residual of free chlorine
- Does not contain calcium or cyanuric acid
- Very beneficial as a backup to chlorine generator systems
- Ideal for use with secondary sanitizing systems like ozone, UV and AOP
Liquid sodium hypochlorite highly recommended for the COVID-19 summer
Most public aquatic facilities are reopening slowly and with restrictions. That means many families are choosing to stay home, making the backyard pool a center of activity. Liquid sodium hypochlorite is noted by the CDC as one of the most effective sanitizers for water disinfection and surfaces as well. The agency’s Safe Water page states: “Although a number of other disinfectants (calcium hypochlorite, ozone, UV, solar disinfection) and treatment processes (filters, slow sand filtration) have been investigated, sodium hypochlorite appears to offer the best mix of low cost, ease of use, safety, and effectiveness in areas where there is enough water to drink and water is not excessively turbid. These characteristics are the reasons why most water treatment systems in the US and Europe have been using chlorine for disinfecting drinking water for nearly 100 years.”
The liquid sodium hypochlorite used for swimming pools is a concentrated form of household bleach. The bleach used for laundry is typically a six-percent strength. Swimming-pool liquid sodium hypochlorite is available in strengths ranging from eight to as high as 12.5 percent. This is great for the swimming pool because liquid is ready to disinfect immediately without any need to dissolve or break down. That means that bacteria and algae are quickly disrupted. In relation to COVID-19, the CDC has stated that there is no evidence that the virus can be transmitted in treated swimming pools.
While there are several options for pool treatment, it’s important to make sure the right option is used. Liquid sodium hypochlorite (chlorine) for pools remains one of the most practical, effective and safest options for keeping pool water clean and clear this summer.
APSP Service Tech Manual, 5th Edition. Association of Pool and Spa Professionals. 2019
Lowry, Robert W. Basic Training Manual. IPSSA. 2016
The Condensed Chemical Dictionary, 10th Edition. Van Nostrand Reinhold Company 1981
Griffiths, Tom. The Complete Swimming Pool Reference. 1994
White, George Glifford. White’s Handbook of Chlorination and Alternative Disinfectants. 2010
About the author
Terry Arko has more than 40 years of experience in the recreational water industry, working in service, repair, retail sales, chemical manufacturing, technical service, commercial sales and product development. He has written over 100 published articles on water chemistry and has been an instructor of technical courses for over 25 years. Arko is a voting member on the board of the Recreational Water Quality Committee (RWQC) and serves as a board member for the California Pool and Spa Association (CPSA). He is also a Certified Pool Operator instructor with the Pool Hot Tub Alliance (PHTA). Arko is currently working as Technical Content and Product Training Manager for HASA Pool, makers of HASA Sani-Clor. He can be reached at firstname.lastname@example.org