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.
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.
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.