Pool water treatment

Fig. 2 Example of a pool installation

The raw water flowing out of the overflow channel and, if applicable, the ground absorbers present is first collected in an overflow tank. From here, the water treatment typically begins, alternatively, in one of the following process combinations:

In most process variants, a flocculant is metered into the raw water which binds dirt particles by clumping so that these can be filtered out more easily. In addition, the phosphate, which algae require for their growth, is retained as flocculant in the filter as a sparingly soluble aluminum or iron phosphate, thus preventing algae growth. In some cases powder activated carbon is added before flocculation. This binds non-polar impurities contained in the water, e.g. Trihalomethanes (these impurities are formed as disinfectant by-products in the process).


In the next stage, the particles contained in the water (flocked micro particles, hair, activated carbon, etc.) are filtered out with a single or multilayer filter. The requirements for the filter quartz - e.g. AQUAGRAN® from Euroquarz - are laid down in DIN 19643. Without effective filtering by AQUAGRAN® no clean water treatment can take place.

In some process variants, the filtrate is still pretreated by the actual disinfecting stage either by running through a filter with grain-activated carbon or by treating it with ultraviolet radiation. In the first case, it is again a matter of removing nonpolar impurities. The UV irradiation is a supplementary disinfecting step, which leads to the possibility of working with lower chlorine concentrations during chlorination.


The water, so far known as filtrate, is now disinfected (see disinfection).


No matter which cleaning method is selected, the pH value must be adjusted before reintroduction into the pool. According to DIN 19643, the value should be at least pH 6.5, the upper value being dependent on whether and which flocculants are used and whether it is sea or fresh water.

The heating of the water is achieved by flowing wholly or partly through heat exchangers. Normally, this happens before chlorination. The heated water flows back into the water circuit via a return flow. The water circuit now starts from the beginning.


An alternative technique prepares the water with so-called pre-coat filters. The filter medium used is a triple layer of powder particles which are floated onto a frame covered with gauze. The first one is a layer of powdered diatomaceous earth or pearlite. Just like the activated carbon, these substances have an extremely large internal surface area of about 900 m² per g. This is followed by a layer of activated carbon powder, and finally a layer of diatomaceous earth or pearlite is added. Application to the gauze (which is tensioned by means of an internal support frame) is effected by suspending the respective medium and then taking the filter into the maintenance mode. Because of the flow conditions, uniform layers of the filter media are formed on the gauze - the so-called "pre-coat" process. Each layer is applied individually. This type of filter is so fine and intense that you can do without "flocculation".


In some cases, an acidified powder activated carbon is used. This activated carbon variant has the following advantages:


  • ·        No dusts during filling (because the powder activated carbon is moistened by the acid)
  • ·        The pH value is automatically lowered
  • ·        Because of the low pH-value absolutely free of germs


If the pre-coat filter is clogged, the filter is emptied and the gauze washed with water. The contaminated sludge on diatomaceous earth or perlite and activated charcoal is extended. Now the process begins anew.


In most cases the disinfection of the water is made with chlorine. This is done by chlorine, sodium hypochlorite or calcium hypochlorite, in order to achieve a depletion effect in the pool.


To dissolve the chlorine into the water, there are a number of different apparatuses and methods, which are not described here. There are several process engineering possibilities with apparatuses which require a lower chlorine concentration. Chlorine is still the state of the art, because it kills germs and bacteria as safely and efficiently as no other substance. Normally, the concentration of "free" chlorine to be targeted is 0.3-0.6 mg / L. The free chlorine is the fraction of the chlorine which has not yet reacted.

Unfortunately, the chlorine reacts not only with the microorganisms, but also compounds with the unwanted solids contained in the water (see above). A series of so-called disinfectant by-products are formed, which must be removed from the water. It is often these disinfectant byproducts which produce the "typical" chlorine odor in the swimming pool.


After disinfection, e.g. by chlorine the filtrate can be called "pure water".