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 (i.e. “gas off”) and be lost in the air. Second, check the conditioner/stabilizer (cyanotic 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 temperature. All of these factors will make a huge impact on chlorine usage. Effective filtration followed by filter cleaning or backwashing is also required before total chlorine demand can be considered. Dead algae and dead microorganisms 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. A wide variety of pollutants, both organic and inorganic can be oxidized by free available chlorine and lower a chlorine reading. Some of the visible ones include decomposed leaves and debris, algae, fertilizers, bird droppings and contaminants in rain.
The longer those visible pollutants remain in the water, during a long winterization period, for example, the greater the likelihood that a high chlorine demand will occur in the spring.
The use of a solid winter cover to prevent visible pollution, brushing and vacuuming frequently and filtering constantly will remove much of the chlorine demand and save time and money spent on chemicals.
Even if all visible pollutants are quickly vacuumed or filtered and the water is crystal clear, soluble pollutants such as mineral ions, algae spores, bacteria, other microorganisms and inorganic and organic nitrogen compounds can use up large quantities of chlorine. Long such as nitrate phosphate, cyan rate or borate is not part of the list of the list of soluble pollutants and do not contribute to chlorine demand. The procedure to determine chlorine demand requires specific testing equipment. The idea is simple: Raise the chlorine level of the sample of water to be tested to a very high level, 100 ppm for example, wait up to 24 hours under controlled conditions, and retest the water sample for free chlorine, 20 ppm for example. The difference, 80 ppm, is the total chlorine demand.
Once the total chlorine demand has been established the choice of chlorine shock treatment can be made. Based on cost and control over the water balance the most wisely used shock treatment choice is hit hard, a full strength calcium Hypochlorite product. One pound of Nuclo hit hard (68% calcium hypochlorite) in 10,000 gallons of water imparts 8 ppm of chlorine to water. In the example above, 10 lbs. of hit hard would be required in 10,000 gallons of water to satisfy the chlorine demand and establish a chlorine reading. It is important to apply the entire dose over a few days time because the lab test in our example indicated the water used up 80 ppm in only 24 hours and the water needs the complete dose. If concerns over bleachable surfaces such as vinyl liners and fiberglass exist, the treatment can be spread over few days. If the shock treatment expense is judged to be too high, compare it to the cost of draining a large portion of the water and refilling with fresh water. If the structural integrity of the pool is doubtful or if fresh water is expensive or of poor quality containing minerals that will require sequestering treatments, then proceed with the required shock treatment. Now let’s consider role that nitrogen bearing pollutants have no chlorine demand, chloramines and breakpoint chlorination. Within the range of pollutants that make up chlorine demand, nitrogen, in all its forms including inorganic nitrogen (ammonia type) and organic nitrogen (amino acids and proteins) is the largest polluting group. Ammonia nitrogen reacts almost instantly with chlorine to form chloramines, a unique part of chlorine demand.
Disappearing chlorine readings can arise from the development of chloramines, requiring breakpoint chlorination. Chloramines develop slowly during the season and are routinely oxidized in 2-3 ppm of chlorine. If they build to higher levels then breakpoint chlorination is needed. There is a real difference between “chlorine demand” and “breakpoint chlorination.” Breakpoint chlorination only refers to the amount of chlorine needed to oxidize chloramines as determined by DPD testing. Chlorine demand can be dosed over a few days time but breakpoint chlorination must be dosed immediately. Use this simple calculation to determine breakpoint chlorination:
Chloramines level X 10= ppm of chlorine needed to achieve breakpoint chlorination.
For example, 2 ppm of chloramines requires 20 ppm of chlorine, about 2.5 lbs. of hit hard per 10,000 gallons of water. Retest the next day for free chlorine.
RECAP: avoiding the introduction of soluble and insoluble pollutants is the best way to minimize a high chlorine demand. The best advice is to keep water balanced and well sanitized to prevent algae, winterize carefully, and open early in spring. Proper use of algaecides, clarifies and mineral and stain control products will also help minimize the development of high chlorine demand. Remind: when using conquest reduce the chlorine level to 1 ppm or less to maximize the effectiveness of the treatment.

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About amerimerc

Wichita, Kansas
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