Chemical disinfection by chlorine.
The standard method for keeping swimming pool water clean is chemical disinfection, mostly with chlorine. This can be added to water in the form of gas, liquid, powder or tablets. However, it can also be obtained by electrolysis in the pool's water cycle, often referred to as "salt water pool".
Chlorine and other disinfectants are toxic and aim to kill all life in the water. Bacteria and algae have a very large surface and a fast metabolism and therefore die oﬀ very quickly. But our skin, eyes and mucous membranes are also attacked.
In order for the chlorine to develop its deadly eﬀect, the pH value must be adjusted in the range of 7.2-7.6. This is done automatically in the saltwater pool by adding hydrochloric acid.
The chlorine reacts For example, with the nitrogen from sun creams, skin particles and urine to form carcinogenic compounds.
Especially at high temperatures and heavy use, the disinfecting eﬀect is often too weak to reliably prevent algae. Algicides, usually copper-based, so even more poison, need to be added. After a Saharan rain, the water becomes cloudy and the dust is so ﬁne that it cannot be caught by the ﬁlter. A chemical ﬂocculant must now be used.
You can no longer see the nutrients, but the impurities stay in the water and accumulate there.
By the way, chlorine is an odorless gas. The typical “smell of chlorine” in swimming pools is caused by the reaction products of the chlorine.
Since all these reaction products and toxins accumulate in the pool water, the water has to be partially replaced constantly. Public pools therefore have to exchange 5% of the total water volume every day!
Chemically disinfected water doesn't mean clean water, it just means DEAD WATER. As soon as the disinfectant is missing or the pH value shifts outside of the necessary range, algae and bacteria start to multiply exponentially, the water turns green and shows its degree of pollution.
The picture shows the same pool 7 days before (blue, transparent) and after failure of the disinfection (green and cloudy).