March 9, 2020
By Alicia Stephens
In an era of technological advancements and equipment upgrades, pool and spa chemistry is often viewed as being an unchanging element of water care.
However, over the past several years, chemistry has actually undergone many changes, including an increase in saltwater pool care, a focus on ‘low sanitizer residual’ systems, and an overall increase in problem pools/spas due to an ever-changing environment. Throughout all of these changes, one problem that remains prevalent and continues to frustrate pool/spa owners and professionals alike is chlorine (Cl) demand.
This is defined as the inability to maintain a chlorine residual in a pool or spa even after repeated application of a chlorinating product. There are an infinite number of substances that can contribute to chlorine demand. These include (but are not limited to) bacteria, algae, ammonia, urine, sweat, health and beauty products, and bather and animal waste. These contaminants can enter the water in a number of different ways.
Determining the cause of chlorine demand in a particular pool may seem like an insurmountable task. In many cases, the root cause of the demand will be impossible to uncover; however, it is usually not truly relevant to the treatment needed. There are many misconceptions about what actually causes chlorine demand, as well as when a demand is present and how it should
Is the chlorine working?
One of the most common misconceptions about chlorine demand is the thought that the chlorine is not working when it is applied to the pool/spa. Consumers often complain the chlorine they are using is weak or ineffective because they keep adding it to the water, but nothing seems to happen. By nothing, they are referring to constant addition of product but not establishing a free chlorine residual.
In reality, the lack of residual is caused by an overload of contamination in the pool/spa that depletes the amount of chlorine available to sanitize the water. It often appears as if chlorine is not working, while in reality it is working overtime to try and overcome the impurities in the water.
Water contamination is reduced as more chlorine is added; however, the inability to maintain a residual will continue until all chlorine reactive pollutants are removed from the pool water. If the contamination is substantial, it often takes a large amount of chlorine not only to eliminate the problem, but also to re-establish a chlorine residual in the water.
Do phosphates and nitrates consume chlorine residuals?
The second common misconception is phosphates and nitrates in the pool eat up chlorine residuals and, as a result, contributes to chlorine demand. Hypochlorous acid (HOCl), or free available chlorine (FAC), reacts easily with many different types of materials. By looking at the chemical structure of some contaminants, one can predict whether or not there will be an interaction with chlorine.
All atoms have what is referred to as a preferred ‘oxidation state’ or ‘oxidation number.’ This is simply a number assigned to a particular atom based on its chemical properties. For example, the preferred oxidation number for chlorine is -1. Atoms in an oxidation state that are not preferred are very reactive, while atoms in their preferred oxidation state are stable and are much less reactive.
It is not important for one to know how the oxidation numbers are determined, but knowing what they are is very helpful. It may sound complicated at first, but it is an extremely useful way for scientists to predict which chemical reactions are likely to occur.
In hypochlorous acid (or FAC), chlorine actually has an oxidation number of +1, which is not preferred. Because chlorine is constantly trying to reach its preferred state of -1, it is very reactive. This is why hypochlorous acid is such a great oxidizer. When it oxidizes other material, the chlorine atom ends up where it wants to be at -1.
Because of oxidation numbers, there are compounds that do not tend to react with hypochlorous acid. For example, the nitrogen in nitrate (NO3-) is already where it wants to be at +5. The same is true for phosphate (PO₄³⁻). In the orthophosphate molecule, the phosphorous atom is also where it wants to be at +5. This makes these compounds quite stable and unlikely to react with hypochlorous acid. If the material does not react with hypochlorous acid, then it does not contribute to chlorine demand. If orthophosphate or nitrate reacted with chlorine and caused a chlorine demand, then these compounds would be removed when shocking the pool—this does not occur.
If a free chlorine residual is lacking, is more salt required?
The third misconception about chlorine demand concerns saltwater pools. A common issue in these pools is a lack of chlorine residual, which is the first sign of chlorine demand. In many cases, the first reaction is to add more salt. There is a misunderstanding that the only thing necessary to maintain a saltwater pool is, in fact, salt. While it is certainly necessary, it is a stable element of saltwater pool chemistry, and salt levels do not fluctuate rapidly enough to cause a sudden inability to maintain a chlorine residual without a significant amount of fresh water being added.
Most chlorine generation cells have an acceptable range of salt that allows free chlorine to be created. Often, a fluctuation of up to 500 parts per million (ppm) is still within range for the effective generation of free chlorine. Adding more salt is usually not the answer to re-establishing a chlorine residual.
As a result of the ‘just add salt’ mentality, chlorine demand is often overlooked when dealing with a lack of residual in a saltwater pool. These are still chlorine pools that can suffer from a chlorine demand the same way a traditional chlorine pool can. However, while a chlorine demand is certainly possible in a saltwater pool, a properly functioning chlorine generator cell is provides a steady source of chlorine and oxidation of contaminants.
This makes a chlorine demand less likely to occur, although certainly still a possibility. If a lack of chlorine residual is an issue with a saltwater pool, other sources of trouble should be considered. For instance, the pump/cell run time, size and age of the chlorine generator cell, and temperature of the water. All of these can lead to reduced chlorine output and low chlorine residual. Further, scale buildup on cell plates is common because of the water balance environment within the cell itself. Scale formation on the electrolytic plates can severely limit the ability to produce chlorine, leading to lower residuals and the increasing possibility of chlorine demand.
As illustrated, chlorine demand can be caused by a combination of different types of contaminants, so the treatment time and difficulty could vary. Therefore, it is important service professionals keep timing in mind when they are treating a demand. Checking the chlorine residual a few hours after treatment could show the presence of free chlorine, and one might assume the demand is broken and no further product application is needed. However, if slow-reacting contaminants are present in the pool water, the chlorine can be used up as they continue to react. As a result, the chlorine residual will end up at zero as more time passes, which means the demand is not truly broken. This is why it is very important to continue to test the water frequently, and be sure the free chlorine residual holds at 1–4 ppm for a full 24 to 36 hours.
For saltwater pools, relying on the boost button to provide the additional chlorine needed to treat a demand can cause increased stress on the chlorine generator cell and fail to provide the amount of chlorine necessary to satisfy the demand. Adding chlorine from an alternate source, such as a shock product, is more effective.
Limited options are available
Unfortunately, there is no easy cure for many chlorine demand situations. In most cases, there are still only two options.
The first is to apply the appropriate amount of chlorinating product (usually determined through testing), and the second option is to replace some of the water in the pool/spa with fresh water that has no chlorine demand.
In some cases, a floc treatment may reduce the demand by physically removing some of the contaminants from the water. That said, a floc treatment or water replacement does not actually cure the demand—it only lessens it. Therefore, it is necessary to re-test and apply the newly recommended amount of chlorinating product.
Of course, the best course of action is always prevention. Keeping pool owners on a system that includes routine oxidation as well as application of a maintenance algaecide will help keep pool water clear and free from contaminants that can contribute to chlorine demand. It is also important for service professionals to know when additional oxidation is needed. Most systems recommend a once-per-week application of an oxidizer, but there are instances when more frequent application is needed. These include heavy bather loads, rain, warmer-than-normal temperatures, and any time there is suspected contamination of the pool/spa water (such as fertilizer or pollutants). Designing a maintenance program specific to the characteristics of each pool/spa will help to prevent problems before they begin.
Alicia Stephens is the education and training manager for Biolab Inc. In her 19 years with the company, she has focused primarily on education, training, and development, as well as technical support and new product research and integration. Currently, Stephens supports all education and training initiatives for the Biolab Pro Dealer division. She can be reached via email at firstname.lastname@example.org.
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