Problem: Chlorine demand
Cause: Chlorine demand is defined as the inability for pool water to maintain a chlorine residual—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, including source water, rain water, animals, fertilizers, plants/leaves, industrial pollution, and bathers (during the season). Any contaminant that can be oxidized will react with chlorine and deplete the level. During the off-season, run-off or rain water can be a concern if it contaminated with materials such as ammonia, fertilizer, or industrial pollution.
Mesh covers, in particular, allow these contaminants to enter the pool water. In addition, source water is likely to contain ammonia, and this will be an issue when topping off the pool upon opening.
Chlorine demand is most likely caused by a combination of different types of contaminants, so the treatment time and difficulty could vary.
Treatment: As mentioned previously, if the water is swampy, a floc can be used to help drop material to the bottom of the pool, allowing some of the contaminants to be physically removed.
When starting treatment, the water should be balance and the filtration system should be working properly. Chlorine should then be applied until a residual can be maintained. It is important to keep timing in mind when treating a chlorine demand issue. For instance, checking the chlorine residual a few hours after treatment may 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 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 add—as per the instructions—the entire amount of recommended product and continuing to test frequently to ensure the free chlorine residual holds between 1 and 4 ppm for 24 to 36 hours. If the chlorine demand includes algae growth, an algaecide should be applied.
Prevention: To prevent problems with chlorine demand it is important the pool is closed properly. The water must be balanced and have a chlorine residual, preferably on the high end (approximately 4 ppm). A winter algaecide should also be added. In milder climates, or those with warmer winters, a mid-season treatment will also be necessary.
When the pool is opened, a treatment program that includes routine shocking/oxidation should be used to eliminate contaminants and keep the water clear. Typically, the shock/oxidizer products should be applied weekly. However, once the season is in full swing, more frequent application may be necessary based on weather or bather load.
