Chlorine Demand
Definition of Chlorine Demand
High chlorine demand is the inability to keep adequate chlorine in pool water, even though the water is balanced and properly maintained. Various contaminants increase oxidation levels, consuming chlorine faster than it can be replaced by automatic feeders or normal shocking. Symptoms include slimy or slick pool walls and cloudy water (although water can be clear and still have a high chlorine demand). A sudden drop in cyanuric acid can be an indicator that there is a chlorine demand problem, as the CYA reading is often masked by the demand. If this occurs, do not add stabilizer.
Another form of chlorine demand is a high level of combined chlorine that cannot be broken by successive shock applications. Often this is caused by the presence of ammonia in the water. This can come from fertilizers, pesticides, and even fill water.
High chlorine demand can be frustrating to solve. It may be caused by a number of factors and is especially common during spring start-up. For outdoor pools subject to many environmental factors, it may not be possible or feasible to isolate the exact cause. However, this is less important than beginning immediate elimination.
NOTE: A zero chlorine reading does not necessarily indicate that there is no chlorine in the pool. The chlorine level may be so high that it is bleaching the color out of the reagent.
Common causes of Chlorine DemandChlorine resistant algae, fungus or bacteria: An infestation can exhaust normal chlorine levels and require a specially formulated algicide to solve the problem. Often, the growth is not visible on the pool surface itself. Pink slime and water mold are notorious for growing first inside lines, skimmers, and behind light niches before becoming visible. Growth in these areas can continue to deplete chlorine levels steadily until removed.
Nitrogen contamination: Lawn fertilizers and other nitrogen products in pool water produce a high level of chloramines, which require larger amounts of chlorine for oxidation.
Source water: Lake water is usually contaminated with algae, metals, and other debris, and should not be used to fill pools. Well water may have some of the same problems. Even local municipal water supplies can create high chlorine demand, especially if the contain chloramines. Every time fill water is added to the pool, chloramine and nitrogen levels rise, requiring more chlorine. Shocking after fill water addition helps prevent excessive chloramine levels in this situation, as does avoiding the addition of large amounts of fill water at a time.
Rain and Pollution: Contaminants from factories, highways, airports, and other sources may be deposited in pool water, especially during rainfalls. Clouds sometimes transport pollution over long distances. Rain and wind also carry algae spores, leaves and other debris which raise chlorine demand. During the winter, stagnant water in uncovered pools exposed to air, rain, and snow often develop chlorine demand problems. Older pool covers will develop pin holes as the fabric weave stretchs from the weight of heavy water and/or the force of strong wind. These small holes allow fine particles and the contaminated water on top of the cover to pass through to the
winterized pool water under the cover.
Keep in mind…
High bather loads: A large crowd using the pool over a few days can introduce a lot of undesirable compounds, (such as perspiration, cosmetics and suntan oils), into the water. Requiring all swimmers to shower before swimming can help prevent this. Shock after any heavy bather load.
Eliminating Chlorine Demand
The best way by far to handle chlorine demand is to perform a chlorine demand test. This test is contained in a separate lab available to BioGuard Dealers. This test can give the exact amount of chlorine needed to break the demand. In some cases, this may be quite high. It may be a more feasible option to perform a partial drain and refill with fresh water to reduce the demand. Performing this test can prevent a great deal of frustration by allowing you to make an educated product application.
Prevention of Chlorine Demand
To prevent chlorine demand, maintain proper chlorine residuals consistently, shock regularly, and avoid contaminating the pool with chemicals or foreign objects.
Keep in mind…
S Some contaminants are oxidized quickly and some are oxidized very slowly
• It depends on the nature of the contaminant
S It may be impossible to determine the exact cause of chlorine demand, however it is not usually relevant to treatment
