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 your test kit only measures free chlorine. It doesn't tell you how much chlorine is already bound up fighting contaminants. That gap between what you're testing and what's actually working is chlorine demand.
A pool with high chlorine demand will feel sticky when you walk across the deck. The water might smell strongly of chlorine (that's actually the smell of chloramines, not chlorine). You might see cloudy water or the early signs of algae growth. All of this is happening despite what looks like adequate free chlorine in your test.
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 chlorine demand.
Here's what you're looking for. 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 1 ppm, it's time to act. 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're not just bumping up chlorine a bit. You're adding a large dose all at once to break through the combined chlorine and oxidize the contaminants.
The chlorine breaks the bonds holding the combined chlorine together, oxidizing it into gas that escapes from the pool. Once you've broken demand, your regular chlorine dosing takes over to maintain normal residuals.
There are different types of shock: calcium hypochlorite (powder), sodium hypochlorite (liquid bleach), and dichlor (which includes stabilizer). For commercial pools, calcium hypochlorite is the most common choice. It's stable, affordable, and effective.
Your shock dose depends on your combined chlorine level. If combined chlorine is 0.5 to 1 ppm, target a breakpoint of 2 to 3 ppm free chlorine above your normal target. If it's higher than 1 ppm, you're likely looking at 5 to 10 ppm above normal.
Running a Super-Chlorination Cycle
Super-chlorination is planned shock treatment. You do it deliberately, not in crisis mode. Many facilities schedule it once a week or once every two weeks, especially during busy season. The goal is to break demand before it becomes visible in the water.
Here's the process. 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 to normal operating range. Most facilities wait at least 12 hours after shock before opening.
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?
Shock is just chlorine in a high dose. You could technically use your regular chlorine feeder to add a large amount, but shock products (calcium hypochlorite, dichlor) are formulated to dissolve quickly and work fast.
How often should I shock a commercial pool?
Most facilities shock once a week, more frequently during peak season or after high bather loads. Check your combined chlorine levels weekly. When it starts climbing, 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.
Master Pool Chemistry and Get Certified
Chlorine demand isn't something to fear. It's something to manage. CPO PRO teaches the real-world chemistry behind shock treatment and super-chlorination, with a PHTA certification at the end. Virtual class every other Saturday, or on-site for your team.
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