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 can 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.

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 addressing the problem immediately.

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. See Lab Troubleshooting for more information.

Chlorine 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 deplete chlorine steadily.

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, which is usually contaminated with algae, metals, and other debris, should not be used to fill pools. Well water may have some of the same problems. Even a local municipal water supply can create high chlorine demand, especially if it contains chloramines. In these cases, 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.

High bather loads
A large crowd using the pool over a few days can boost bacteria and oxidizable compounds in the water to unusually high levels. Requiring all swimmers to shower before swimming can help prevent this. Since this isn't going to be likely most of the time, shock immediately following parties or other occasions where many people have enjoyed the pool.

The best way by far to break a chlorine demand is to perform a chlorine demand test. This test is contained in a separate lab available to BioGuard Dealers. (For lab test instructions, see Chlorine Demand Test Instructions.) 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. (Check environmental factors such as water tables etc. before proceeding with a drain. Consult the pool manufacturer or builder before draining significant amounts of water from the pool.) Performing this test can prevent a great deal of frustration by allowing you to make an educated decision on how to proceed. If a chlorine demand test is performed, you can enter the test results in the encyclopedia tab titled "Chlorine Demand" and it will automatically calculate the proper amount of Burn Out or Burn Out 35 for you.

If you do not have the Chlorine Demand Test station, call BioGuard and place your order. In the meantime, you can use the following procedure to try and eliminate the demand until it comes in!

1. Shock the pool with 3 lb. of Burn Out per 10,000 gallons or 3 lb. of Burn Out 35 per 6,000 gallons all at one time.
   
2. Circulate the water continuously. Three hours after adding the Burn Out or Burn Out 35, test the water for chlorine. If it is not greater than 3 ppm, repeat step one.
   
3. Continue shocking and retesting every three to four hours until you can maintain a 3 ppm free chlorine reading for 24 hours.

NOTE: There is no way without the Demand test station to determine how many of these shock applications the demand will take.

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Prevention of Chlorine Demand

The best way to prevent chlorine demand is to follow one of the Three Step Programs. Consistent sanitation, regular shocking, and the application of an algae preventative are the best weapons against any kind of problem, chlorine demand included.

To minimize the risk of chlorine demand from outside contaminants, shock the pool after periods of heavy use and avoid getting chemicals in the pool, such as lawn sprays, fertilizers, pesticides and other pollutants.

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