Chlorine Demand is a pool water chemistry topic that is getting more attention due to changing climates, consumer's water chemistry and understanding of the problem. It is also becoming a greater issue in many private & commercial pools.
Chlorine Demand is defined as "the quantity of chlorine reduced or converted to inert or less active forms of chlorine by substances in the water." Faust and Aly's Chemistry of Water Treatment further state that, "since chlorine is a non-selective oxidant, almost any substance in the water...will react and consume chlorine." In other words, the more "stuff" dissolved in the pool water, whether organic or inorganic; chemical, vegetable or mineral; heavy, constant rainfall can cause a chlorine demand. Or more simply stated, not being able to successfully maintain a chlorine residual following a chock treatment. Certain household cleaners that are not specifically formulated for in-pool-water use will add components such as phosphates or nitrates will interfere with the pool's sanitizer causing a chlorine demand.
Another cause that may prove suspect, at least in our line of thinking, is the Chloramine issue. As you remember, chloramines are chlorine molecules that are chemically linked up (we call them contaminated) with organic waste such as nitrogen, ammonia, etc. More & more local water treatment authorities are using chloramines to sanitize the drinking water supply these days. Chloramines are pretty effective in killing pathogens in the drinking water supply. However, chloramines are not effective oxidizers. You've probably noticed a regular pink line in your home sink, shower or toilet bowl, especially over the past 4 to 5 years. Those spots need more regular cleaning. That's the same pink slime that is in your swimming pool! Chloramines are used because they are pretty effective and less "offensive" to those who want to rid our planet of chlorine. That premise is utter foolishness since you can't ban or get rid of an element! Anyway, as you top off your pool with the garden hose, you're putting more chloramines into your pool every time and aggravating an already serious problem. Please keep in mind that this issue is NOT pertinent to folks with well water (you have your own issues).
A sure sign that your are experiencing a chlorine demand is the inability to maintain a Free Available Chlorine level of about 1.0 to 3.0 ppm for more than a few hours; even after shocking or super chlorinating.
Since 1992, one of the pool industry's top Technical Services Department has gathered a huge amount of data concerning chlorine demand. It appears that the situation has become more widespread each year. Here are some actions (short version) that can help in dealing with Chlorine Demand.
Doing a proper Chlorine Demand Test. Your local pool Dealer should have a Chlorine Demand test station. When determining the ACTUAL Chlorine Demand, it is imperative that the solution & corrective action be accurate. Liken it to jumping across the Grand Canyon, if you miss, you miss! If it takes 20 lbs of Shock to break the chlorine demand, using 19 lbs will make the problem worse; 20 lbs or more of Shock will treat the issue. Maintenance of an adequate sanitizer being added to the swimming pool on a daily basis. Be sure to have the correct number of chlorine sticks or tablets dissolving into the pool. Check this as often as daily by visually examining the chlorine to see that it is eroding (whether in the skimmer or in a chlorinator) at a "normal" rate. Normal refers to what you have "normally" experienced in the past. Cool water, slower erosion/dissolution rate; warmer water, faster erosion/dissolution rate. The Pool MUST BE SHOCKED EVERY WEEK. PERIOD. Shocking oxidizes much of the "stuff" that was mentioned earlier. If you are using a solar blanket, REMOVE IT!After ANY chemical addition, the chemical reaction must have time to GAS-OFF in order to achieve the proper results. Keep the water balance proper. pH 7.2 - 7.6, Total Alkalinity 100 - 150 ppm, Calcium Hardness 200 - 300 ppm. Good water balance promotes good, efficient chlorine (sanitizer) performance.
Although a singular cause for Chlorine Demand has not been determined (there are many), we have found a common thread in many of these cases. One common thread is if a pool is kept closed longer in the spring (covered without a sanitizer). Heavy rainfall that has ammonia present will cause a Chlorine Demand. Accidental addition of household fertilizers or any compound that can be oxidized by chlorine will result in a Chlorine Demand.
In conclusion, Chlorine Demand needs our attention. Only careful monitoring and quick treatment may be successful for a clear pool. VISIT YOUR LOCAL POOL DEALER REGULARLY (4 times per season). If they don't have a chlorine demand test, find someone who does.
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