Shock chlorination: when you need it, how to do it, when to swim again

What a shock achieves

In everyday running, chlorine gradually neutralizes whatever swimmers and the surroundings bring in. When the load rises suddenly, chlorine cannot keep up and chloramines, that is combined chlorine, build up. A shock raises free chlorine for a few hours to a level that breaks down these compounds and kills algae and bacteria before they settle in. A rule of thumb says that breaking down chloramines needs free chlorine at roughly ten times the combined chlorine.

When you need it

• After a storm, hail or Saharan dust; what else to do is in our article on the pool after a storm.
• After a day with many swimmers or a party.
• When the water looks dull or the walls feel slippery.
• When combined chlorine rises or the water smells strongly and stings the eyes; a chlorine smell is caused by chloramines.
• When opening the pool for the season, after being away or after a heatwave.
• When algae appear.

In a normal summer, many owners do a mild shock every one or two weeks. It is not a rule: if the readings are good and the water is clear, it can be done less often.

Step by step

1. Test the pH and bring it to 7.2–7.4. With high pH, the shock loses much of its strength.
2. Work out the water volume and the dose from the packaging. For a routine shock, the target is usually free chlorine of around 10 mg/l; algae or green water need more and a repeat.
3. Wait until the late afternoon, so the sun does not break down the chlorine.
4. With the pump running, pour liquid chlorine slowly in front of the return inlets, or first dissolve granules in a bucket of water and pour the solution around the edge. Never put granules directly onto the liner.
5. Brush the walls and steps so the chlorine reaches everywhere.
6. Leave the circulation running all night. If you have a solar cover, keep it off until the chlorine drops, because very high concentrations damage it.
7. Test in the morning. If chlorine has fallen almost to zero, the load was heavy and the shock should be repeated.

Which product, and when you can swim again

For shocking, products without stabilizer are preferred, such as liquid chlorine or calcium hypochlorite, so the water is not loaded with cyanuric acid. Dichlor granules are convenient, but frequent use raises the stabilizer level. We compare the different forms in our article on pool chlorine. There is also chlorine-free shock with active oxygen, which oxidizes organic matter and lets you get back in the water sooner, but it does not deal effectively with algae.

Swim again when a test shows free chlorine at 1–3 mg/l and pH at 7.2–7.6. After a mild evening shock this is usually the case the next day; after a strong shock it may take one to two days. Do not go by the clock; test.

Never put calcium hypochlorite in the skimmer or in a feeder containing trichlor tablets: contact between them can cause a violent reaction and fire. More in safe storage of pool chemicals.

Questions

See also