How to prevent and treat depleted sanitizer levels in pool water
By Karen Rigsby and Zach Hansen
Chlorine demand is defined as ‘the inability to maintain a chlorine residual in a pool or spa’ even after repeated application of a chlorinating product. It is one of the most frustrating problems a pool owner can experience, as continued application of product seems to make no difference in water quality or in the ability to maintain a sanitizer residual.
Think of it like an overdrawn bank account. For example, if the account carries a balance of -$50, at least $50 must be deposited to break the deficit before a positive balance is available to spend against. Chlorine demand works similarly, (i.e. an excess of oxidizable material in the water results in a negative chlorine deficit, thus chlorine must be added to break the deficit before a positive chlorine residual can be carried).
Treating chlorine demand can seem like an insurmountable task. In many cases, the exact cause of the demand is impossible to determine and may not be truly relevant to the treatment needed. Knowing what causes chlorine demand can, however, be relevant to preventing future problems.
What causes chlorine demand?
This is a difficult question to answer because a countless number of substances can contribute to chlorine demand. These include (but are not limited to) bacteria, algae, ammonia (NH3), urine, sweat, health and beauty products, as well as bather and animal waste. These contaminants can enter the water in numerous ways, including source/rain water, bathers, animals, fertilizers, plants/leaves and industrial pollution. The following looks at a few of the most common sources.
Bather waste
Obviously, having swimmers enjoy the pool is the primary reason for having one. At the same time, however, bather waste can be some of the most difficult compounds to oxidize or break down.
Urea is the major nitrogen (N) containing contaminant in bather waste and it is extremely slow to oxidize. Chlorinated intermediates are formed, which require additional oxidation. These reactions are very slow and will use up any available hypochlorous acid (HClO). Another component of bather waste is creatinine (C4H7N3O), a metabolic waste product normally excreted in urine. Just as with urea, oxidation of this particular compound is extremely slow and intermediate compounds will need to be oxidized.
Personal care products
Compounds such as diethanolamine (DEA, C4H11NO2), which are found in health and beauty products (e.g. cosmetics and suntan oils), will react with hypochlorous acid in pool water. The resulting product is a chlorinated-DEA compound, which would be considered an organic chloramine that could contribute to poor water quality.
Environmental contamination
Leaves and other plant material that enter the pool are a source of contamination. This material can introduce algae spores, bacteria, dirt and other types of pollution, which will affect how much chlorine is needed for sanitation.
Products not intended for pool use
Often pool water becomes contaminated with fertilizers or other products that may have an adverse effect on the pool water. For example, fertilizer contains a high level of ammonia, which reacts directly with hypochlorous acid and causes chlorine levels to deplete rapidly.
This list is not meant to be exhaustive. Chlorine demand is likely caused by a combination of different types of contaminants, so the treatment time and difficulty can vary.
How to prevent and treat depleted sanitizer levels in pool water
What is chlorine lock?
Chlorine lock is defined as the inability of chlorine, present in pool water, to provide sanitation or oxidation. In the past, it has often been mistakenly attributed to the chlorine attached to a cyanuric acid (CYA, [CNOH]3) molecule. Free available chlorine (FAC) is susceptible to ultraviolet (UV) degradation from exposure to the sun, which causes the chlorine to break down, making it unable to perform its job sanitizing the water.
Cyanuric acid is a molecule that is added to stabilize chlorine and protect it from UV degradation, which is why it is often referred to as a stabilizer. When added to the pool, cyanuric acid forms a weak bond with chlorine that shields it from UV rays and allows it to remain in its active form for much longer.
In theory, if chlorine is attached to the cyanuric acid molecule then it is not available for sanitation or oxidation. However, cyanuric acid does not truly ‘lock up’ chlorine, as the bond is very weak and therefore, allows the free chlorine to easily leave the molecule when it is needed to sanitize or oxidize contaminants. True chlorine lock occurs when chlorine becomes attached to a nitrogen containing contaminant, such as those described above that can contribute to chlorine demand.
The chlorine-nitrogen bond in something like chloro-urea, chloro-creatinine or chloro-ammonia is much stronger than the chlorine bond with cyanuric acid, so it never gives it up. The subsequent combined chlorine, also known as a chloramine (NH2Cl), has a fraction of the oxidative power of free available chlorine, so oxidative activity is lost.
This is easily demonstrated by performing a simple water test. When using N, N-diethyl-p-phenylenediamine-1 (DPD-1) reagent, any chlorine that may have been bound to the cyanuric acid molecule easily reacts with the testing reagent and shows up on the test. Chlorine bound to something like ammonia, urea or creatinine does not, which is why a retest should be performed with a different DPD reagent to obtain a value for combined chlorine.
How is chlorine demand treated?
The easiest and most common method to treat chlorine demand is to simply add a chlorinating product. However, as described above, it is important to understand that the amount of chlorine added has to be enough to overcome the ‘deficit’ that exists in the water. Otherwise, the chlorine added will only go toward reducing this deficit and not establishing a positive FAC residual.
A second option for treating chlorine demand is to replace some of the contaminated water with fresh water. When draining and refilling, however, first verify the water being added is not the source of chlorine demand contamination in the first place. If the water has an excess of ammonia, it will react with the chlorine and mitigate the benefits of reducing chlorine demand.
In most instances, adding fresh water will lower the total dissolved solids (TDS) level, which will not only lower the contaminant level, but may also help clear the water. Remember, adding fresh water does not completely satisfy the chlorine demand—it only lowers it. But doing so may make treatment more manageable.
In situations where the chlorine demand is accompanied by cloudy water, a flocculation treatment may reduce the demand by physically removing some of the contaminants from the water. Although similar to water replacement, this process does not completely cure the demand but may make it easier to treat.
When treating a chlorine demand problem, it is important to keep timing in mind. For instance, checking the water’s chlorine residual a few hours after treatment could show the presence of free chlorine. One might assume the demand is broken and no further product application is needed; however, if slow-reacting contaminants are present in the 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 important to add the entire amount of sanitizer as instructed and continue to test frequently to make sure the free chlorine residual holds at one to four parts per million (ppm) for 24 to 36 hours.
How is chlorine demand prevented?
Having the pool on a maintenance system that includes continuous sanitation, along with routine oxidation and algae prevention, is the best way to prevent problems. Continuous sanitation provides consistent protection against disease transmission, while routine oxidation helps clear the pool water of contaminants that can accumulate to react with chlorine and contribute to chlorine demand.
Most systems recommend a once per week application of a shock oxidizer, but there are instances where more frequent application is needed. These include heavy bather loads, rain, warmer than average temperatures and any time there is suspected contamination of the pool water (e.g. fertilizer or pollutants).
Designing a maintenance program specific to the characteristics of each pool will help to prevent problems before they begin. By also adding an algae preventative to the system, it helps make the sanitizer’s job easier and contributes to better water quality.