A page with substantiated pool facts, explanations and instructions without commercial interests. I could not find it on the internet and that is why I made this page. This page is intended to get a quick and good view of facets of swimming pool management, to reduce costs and to keep health aspects optimal.
This is an article about a complex topic, written to the best of my knowledge and common sense. Do you see errors or do you have useful additions? Then leave a comment at the bottom of this page please.
How does saltwater chlorination work?
Salt has the formula NaCl. That is, every salt molecule is made up of a sodium atom and chlorine atom. Salt, dissolved in swimming pool water, can be split into pure sodium and chlorine using an electric current. The chlorine generator does this for you. The chlorine gas that results from this electrolysis dissolves in the water and keeps your pool clean. That is the principle but …
What happens to the sodium? That reacts with the water to caustic soda. You could think that the salt is used up and that you have to add salt from time to time. Well, that is an extremely bad idea, as you will only get more sodium in your swimming pool over time. The smart solution is very simple: Add hydrochloric acid because hydrochloric acid is the name for hydrogen chloride and reacts with the sodium hydroxide solution to salt while the used chlorine is added again. The balance has been restored, the amount of salt in the water is again equal to the value before the electrolysis.
This is the most important principle, the golden rule, of a swimming pool with a chlorine generator and every owner and manager should understand this. In anticipation of the explanation: the acidity must be close to the optimum value 7.2.
The complexity of management
You can measure various parameters and then influence them with chemicals. However, these chemicals also influence each other so that caution is required. Just an example: The acidity needs to be lowered and hydrochloric acid goes into the pool. The alkalinity must be increased for which sodium bicarbonate is used. The result is limited because the hydrochloric acid reacts with sodium bicarbonate to form salt, which ultimately increases salinity.
Conclusion: Do everything with policy, in small steps and measure again. You don’t have to be a swimming pool guru to get perfect readings. These values are ultimately more or less a guarantee for an excellent swimming environment with only limited costs for chemicals, pumping hours and man-hours.
Also do not deviate from the mentioned chemicals, you might solve one problem but you will get another problem. Does the package not state what is in it? Then don’t buy it. For example, chlorine blocks contain calcium with which the hardness increases undesirably, some pH-lowers contain sulphates that you also do not want to have in the bath. Using certain cyanuric acid compounds excessively can turn your pool into a poison pit.
This article is not intended to be complete and there are parameters that are not mentioned, such as the temperature. There are also chemicals that are aimed at combating algae and may be a great addition to the arsenal below.
The shopping list
Here are all the substances listed. Other substances are in principle harmful unless you have enough chemical knowledge to oversee the consequences. So read the package really well. Is it the right substance? Have other substances been added?
You may not need all substances listed below.
- Salt: recycling medium for chlorine gas
- Sodium carbonate: raise pH
- Sodium bicarbonate: increase pH and alkalinity
- Hydrochloric acid: lower pH and alkalinity, supplement consumed chlorine
- Aluminum sulfate: flocculant for reducing hardness
- Calcium chloride: increase hardness
- Poly(Dimethylamine-Co-Epichlorohydrin): kill algae
- Cyanuric acid: UV protection of free chlorine
Where can you buy everything? Finding a swimming pool store with a good knowledge of chemistry is a blessing. There are also chemical companies that want to compete. For example, for Germany, the Netherlands and France it is worthwhile to look at https://hoefer-shop.de/.
PPM stands for “parts per milion”. Volume parts? No, mass parts. This corresponds to milligrams per liter, mg/l. A thousand milligrams is 1 gram and a liter of swimming pool water weighs (~) 1 kg. And, mg/l is the unit that is also on test strips.
Volume or: “What is the total content of your pool?” It is a given and make sure you know how much it is. Suppose you have a bath of 5 m wide and 10 m long. The depth is 1 m, a transition to 2 m and the deep part is just as large as the shallow part. The average depth is then 1.5 m and the volume is 5 * 10 * 1.5 = 75 m3, cubic meter. The mass is 75 tons, 75 000 kg. The next question is how much one millionth is? 75 000 000 grams / 1 000 000 = 75 grams. Remember that number, it is a fixed characteristic of your pool. So if you want to add 100 ppm of a substance, this means that in this case you need 75 grams * 100 ppm = 7.5 kg of substance.
The parameters are the variable values of your swimming pool. You can change some values directly. Other values also affect other parameters.
Salt is the recycling medium for chlorine gas. Salinity is a nice word for salt concentration. The recommended values vary considerably. Very salty also means a corrosive environment – think of pipes. A value of 2500 to 6000 ppm is reasonable with an optimum value of 4000 ppm. Be sure to check your chlorinator manual here, Zodiac LM3 minimum is 4000 ppm for example. Suppose you refill a pool with the volume data mentioned before, then 1 ppm is 75 grams and 4000 ppm is equal to 4000 * 75 = 300 kg, so 12 bags of 25 kg.
Turn off the chlorine generator before adding salt. The chlorine generator cannot withstand high salt concentrations, it can cause a short circuit between the electrodes. Never add salt directly in skimmers. Empty bags in the pool onto the bottom and dissolve it with a broom for example. Residues dissolve automatically. Wait 24 hours before turning on the chlorine generator so that the pump has ensured that the salt has dissolved properly.
FC, Free chlorine. This is the actual disinfectant. It is chlorine gas dissolved in water. That is not entirely the truth, the dissolved gas becomes hypochlorous acid that destroys bacteria, virusses as well as algae. The effect is only optimal if the pH value and all other measured values are in the correct range. The pH also influences the rate of degradation and the production of harmful chlorine compounds.
- < 1 ppm: Let the chlorine generator work a little harder.
- > 3 ppm: Turn the chlorine generator off or let it work less hard. Don’t do anything further, the value drops automatically, the free chlorine breaks down over time.
Cyanuric acid – or CYA – protects the free chlorine FC from UV rays. It is therefore not necessary at all in indoor pools. Too high a level causes turbidity. Keep in mind that concentrations above 100 ppm are labeled toxic by WHO. Cyanuric acid is often incorporated in other products. The content can rise unintentionally and unnoticed. The optimal value depends on the desired free chlorine concentration FC: 5 to 10 times more. So if FC is 3 ppm then CYA should be between 15 and 30 ppm. If that ratio is not correct, FC loses much of its effect. That is why CYA is so important. Too high a concentration causes a considerably higher risk of infections, it limits the algae, virus and bactericidal effect of the free chlorine.
- < 5 * FC: Add cyanuric acid.
- > 10 * FC: Replace part of the swimming pool water with fresh water.
The acidity. A pH of 7 is neutral, lower than 7 is an acidic environment and more than 7 is a base environment. Together with free chlorine, this is the most important factor. A too low pH endangers health because free chlorine is less effective and causes chlorine compounds that cause illness. That is why the limits are regulated by law in many countries. A Danish study states that a pH range of 7.2 and 7.4 is optimal.
- < 7.2: This requires absolute action. Add sodium carbonate or sodium bicarbonate. See also Total Alkalinity.
- > 7.4: Slightly less urgent and easier to solve: Add hydrochloric acid. Do not let the pH exceed value 7.8.
The total alkalinity is the ability to buffer. What is a buffer? A buffer solution is able to keep the pH, the acidity, stable when acid (rainwater) or base (chlorine production) is added. Sodium bicarbonate is ideally suited to give the swimming pool water a buffering effect. An extremely important aspect.
- < 80 ppm: Add sodium bicarbonate.
- > 300 ppm: Add hydrochloric acid.
The calcium hardness of the water. Calcium and magnesium salts make the water hard. We are talking about lime dissolved in the water.
- < 90 ppm: Increase the hardness. Metals and plaster are now susceptible to erosion. The hardness can be increased by adding calcium chloride.
- > 300 ppm: Reduce the hardness. Cloudy water and limescale are the consequences of too high a hardness. And limescale is again a basis for algae growth.
- If the fill water is soft, part of the water can be replaced. Rainwater is extremely soft and you can of course play with it.
- If the filling water is also too hard, the problem is more complex, see here.
To measure salinity, good and cheap meters are available through Asian sites. Because salt is the basis of your swimming pool, this is a value that must be properly monitored. The value, which changes due to rain and added chemicals, will be fairly stable.
You can also measure the pH or acidity with test strips. Precisely because the pH is so enormously important, a meter is a good investment. You can also get a good meter from Asian sites. You can consider keeping pH test strips with a range around 7 next to them for control.
They come in many designs, they measure things like iron content, and so on. They come in many qualities, some give a rough indication, others have a more precise range. You also see this often reflected in the purchase price. Strips that only measure one value are generally more accurate. A good multi-test strip measures at least the following:
- “Free Chlorine”
- “pH”, the acidity.
- “Total Alkalinity”, the ability to neutralize acids and bases or, as chemists say, the ability to buffer.
- “Calcium Hardness” or the hardness of the water.
- “Cyanuric Acid”, protection of free chlorine against UV.
To a Perfect Pool in 3 Steps
You now have an image of all parameters. If your swimming pool is reasonably in order, then of course you can experiment. But let’s assume there is a considerable imbalance. Then it is wise to first perform the steps below in a laboratory consisting of a container that contains 10 litres of swimming pool water. You will test and measure there. Make sure you have a good kitchen scale. Add chemicals and see if it also delivers the desired result. If you are satisfied then you convert the used masses to the large swimming pool.
Step 1: The water base
Before you arrange things like free chlorine, the water base must first be adjusted. This involves arranging:
- Calcium content – hardness
- Cyanuric acid content
Suppose the salinity is 6000 ppm and you want 4000 ppm, cyanuric acid content is 100 ppm and you want 50 ppm. To regulate salinity, one third of the water must be replaced with fresh water. However, to control the cyanuric acid content, half of the water needs to be replaced, after which the salinity is only 3000 and you have to add salt again.
Then of course there is the hardness. Depending on the hardness of the fill water, you replace water or use a flocculant.
Now the basics of everything are perfect, of course you first did this in your laboratory. Measure all values well as a check.
This is the time to realize that you only add calcium compounds, cyanuric acid and cyanuric acid compounds and salt as it is really necessary because these substances do not simply disappear from the pool. And again, if it is not on the package what is in it then you should not put it in your pool. For example, some chlorine products also contain cyanuric acid. Time for the next step.
Step 2: pH and alkalinity
The two influence each other. Too little alkalinity makes the swimming pool vulnerable to major pH changes. At the same time, the pH must be controlled really well. In your 10 litre laboratory you can best start with sodium bicarbonate where you test both the pH and alkalinity but give priority to the pH.
Step 3: free chlorine
Free chlorine does not affect the other substances in the pool so this is the final step. In principle, free chlorine is able to keep everything perfectly hygienic because all conditions such as hardness, pH and cyanuric acid level is in order. If algae do arise, you can increase the free chlorine. In persistent cases, it is an option to first shock, raising free chlorine temporarily in a range of 5 to 15 ppm or even more. In addition to the maximum capacity of the chlorine generator, adding pure chlorine is an option. Sodium hypo chlorite is an option too but raises salinity, all other stuff: Don’t! If that is not sufficient, as a last resort, use Poly(Dimethylamine-Co-Epichlorohydrin).