Chlorine Demand: Disappearing Chlorine Reading

In recent years the most frequently reported pool maintenance problem is the rapid loss of chlorine in pools, even when the water is balanced and crystal clear.
difficulty in holding a chlorine reading, particularly in the spring season, is the often the result of an unusually high chlorine demand that can require 5 – 10x normal shock treatment or more! the term “chlorine demand”, commonly used in the water treatment industry, describes the quantity of free chlorine needed to establish and maintain a chlorine residual (testable chlorine reading) in water for a day or longer. before focusing on chlorine demand as the reason for disappearing chlorine readings check a few basic factors: First, make sure the pH is at least 6.8. low pH will cause chlorine to become unstable in water, emerge as chlorine gas (ie. “gas off”) and be lost in the air. second, check the conditioner/stabilizer (cynauric acid) level. it is not usual for an unstabilized pool (less than 25 ppm of conditioner) to lose 2 – 4 ppm of chlorine per day. third, recognize that the warmer the water and the brighter the sun, the greater the rate of chlorine loss. it is estimated that biological growth nearly doubles for every twenty degree (20F) increase in water tempreature. all of these factors will make a huge impact on chlorine usuage. Effective filtration followed by filter cleaning or backwashing is also required before total chlorine demand can be considered. dead algaeand dead mircoorganisms are oxidized by chlorine as long as they are still in the water or trapped in the filter. the same is true for organic debris of all kinds. through vacuuming, filtering and removal of all particles from the system by backwashing the filter is a must. if both pH and conditioner readings are in range AND if the filter is clean AND the chlorine still disappears, then examine the recent history of the pool, including winterizing techniques and algae episodes.

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