You've been testing your pool religiously. Free chlorine is at 1.5 ppm, pH is locked in at 7.4, and everything should be fine. But the water looks off, a bit hazy, maybe slightly greenish. Your chlorine isn't where it should be, and no amount of regular dosing seems to bring it back. You've hit chlorine demand, and it's time to shock your pool.
Chlorine demand is one of the most misunderstood concepts in pool operations, and it's also one of the most important. Understanding how to identify it, measure it, and break it with shock treatment is the difference between a well-maintained pool and one that's headed toward a closure.
What Is Chlorine Demand?
Chlorine demand happens when something in your pool is consuming chlorine faster than normal. Chlorine molecules bond with organic contaminants (sweat, oils, urine, leaves, sunscreen), bacteria, and algae spores. When that happens, the chlorine gets used up. It's no longer available as free chlorine to protect your pool.
The problem is that a free chlorine reading on its own doesn't tell you how much chlorine has already combined with contaminants. That gap between what you're testing and what's actually working is chlorine demand.
A pool with unmet chlorine demand often smells strongly of chlorine, which is really the smell of chloramines. You might see cloudy water or the early signs of algae growth, even when the free chlorine reading looks adequate.
How to Identify Chlorine Demand
Start by running a complete water test. You need free chlorine, total chlorine, pH, alkalinity, and calcium hardness. The gap between total chlorine and free chlorine tells you how much chlorine is bound up. That's your combined chlorine, and it's the clearest sign of unmet chlorine demand.
Look for these signs. If your free chlorine is 2 ppm and your total chlorine is 4 ppm, you have 2 ppm of combined chlorine (demand). That's a problem. You want combined chlorine to stay below 0.5 ppm.
Once combined chlorine climbs above 0.5 ppm, it's time to act. Utah's code, for example, requires action when combined chlorine goes over 0.5 ppm. You'll know because the water quality starts to decline despite decent free chlorine numbers. Don't wait. High combined chlorine is what causes that harsh chemical smell and makes swimmers complain about red eyes.
Understanding Shock Treatment
Shock treatment means adding enough chlorine to overwhelm the demand and restore water quality. You add a large dose all at once to break through the combined chlorine and oxidize the contaminants.
At a high enough dose, free chlorine oxidizes the chloramines, mostly into nitrogen gas that leaves the water. Once you've broken demand, your regular chlorine dosing takes over to maintain normal residuals.
There are different types of chlorine shock: calcium hypochlorite (powder), sodium hypochlorite (liquid), and dichlor. Dichlor adds cyanuric acid with every dose, so calcium hypochlorite or sodium hypochlorite is the better fit for regular shocking.
Your shock dose depends on your combined chlorine level. The breakpoint rule is to raise free chlorine by about 10 times the combined chlorine reading. With 0.5 ppm combined chlorine, add about 5 ppm of free chlorine; with 1 ppm, about 10 ppm. A dose that falls short of breakpoint can leave you with more chloramines and a worse smell.
Running a Super-Chlorination Cycle
Super-chlorination is planned shock treatment. You plan it ahead of time, before there's a problem. Some facilities schedule it on a regular cycle during busy season. The goal is to break demand before it becomes visible in the water.
Follow these steps. Test your water in the morning. Calculate how much shock you need based on your pool volume and current combined chlorine. Add the shock when circulation is running at full speed and the sun is dropping (early evening is ideal, because you lose less chlorine to UV). Run your filters overnight and test again in the morning.
Combined chlorine should drop back below 0.5 ppm. If it doesn't, you've got a bigger issue. Either your dose was too low, you have an equipment problem, or there's heavy contamination. Run the filter more, add another half-dose if needed, and plan for a second shock the next day.
Post-Shock Protocol
After shock treatment, you'll want to run your circulation system hard. Keep filters running 24 hours if you can, especially overnight. The goal is to get the pool turned over as many times as possible while the shock is working and the oxidized contaminants are being filtered out.
Don't let swimmers back in until your combined chlorine is below 0.5 ppm and your free chlorine is back within your code's limits. The current Model Aquatic Health Code treats free chlorine above 10 ppm with bathers in the water as a reason to close, and California's code sets 10 ppm as the maximum. How long that takes depends on the dose and conditions.
Monitor your water closely for the next few days. Sometimes demand bounces back, especially in hot weather or after a heavy bather load. A second shock might be necessary sooner than usual.
Frequently Asked Questions
Can I shock my pool during the day?
You can, but you'll lose more chlorine to UV degradation. Evening or early morning is better. If you must shock during the day, do it later in the afternoon and ensure your stabilizer (cyanuric acid) is in the 30 to 50 ppm range to protect the chlorine.
What's the difference between regular chlorine and shock?
Chlorine shock is ordinary chlorine at a high dose. You could technically use your regular chlorine feeder to add a large amount, but shock products are made to dissolve quickly and work fast. Non-chlorine shock (potassium monopersulfate) also exists. It oxidizes contaminants but does not disinfect.
How often should I shock a commercial pool?
There's no single schedule. Some facilities shock weekly in peak season or after heavy bather loads. Check combined chlorine as part of your routine testing, and when it climbs toward 0.5 ppm, that's your signal to shock.
Will shock treatment damage my pool equipment?
No, when used correctly. Just make sure your circulation system is running and your filters are clean. High chlorine for 12 to 24 hours won't harm properly maintained equipment.
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