Plant-like organisms which live in water. Algae problems can appear in water that is not disinfected properly. However there are kinds of algae that can stay alive at high chlorine concentrations. At high temperatures there are more algae problems than at low temperatures.


Chemical product that prevents algae growth and destroys the algae already present.


A nitrogen compound, which causes chloramine in reaction with chlorine.

Backwashing of filter

Letting water go through filter backwards to clean filter elements and filter material.This is a normal maintenance procedure for cleaning the sandfilters and some diatomaceous earth filters.


Very small organisms that get into the water via swimmers and dirt. Without sufficient disinfection a pool turns into a breeding place for bacteria. Many kinds of bacteria can cause an illness or infection.

Bathing load

The number of people that use a pool or a certain quantity of water within a certain period of time.

Break point chlorination

This is the point at which all the chlorine is oxidised or destroyed and only free chlorine is left to kill bacteria. This point is reached when the free chlorine value is about 10 times higher than the combined chlorine value.

Calcium carbonate

Calcium carbonate badly dissolves in water. It can precipitate and therefore cause alkalinity. Scaling of the pool sides and equipment might occur.

Calcium hardness

The calcium hardness is the total of dissolved calcium ions, which are present in the water. The more calcium ions the higher the calcium hardness.

Calcium hypochlorite

A quickly soluble, non-stabilised inorganic chlorine product, especially suitable for use in soft water.


The most used chemical substance for disinfection of pool water. There are two kinds, namely organic (stabilised) chlorine combinations (less sensitive for UV radiation of the sun) and inorganic chlorine products that are broken down faster by UV radiation.

Chloramines (combined chlorine)

A substance formed when pollution (like nitrogenous substances and ammonium)reacts to chlorine. Chloramines are responsible for the chlorine smell and irritation of skin and eyes. Chloramines have a very limited disinfection power in comparison to free chlorine.

Chlorine stabiliser

See cyanuric acid

Cyanuric acid

A chemical compound that can be added to pool water to decrease the breakdown of chlorine by the UV radiation of the sun. Cyanuric acid should only be used in combination with inorganic chlorine products like calcium hypochlorite. With organic chlorine products cyanuric acid functions as carrier for the chlorine.

DPD 1 tablets

Test tablets to measure the free chlorine value or total bromine value.

Filter material

Sand, diatomaceous earth or other fine material, which is used to filter undissolved pollution out of the water.

Free chlorine

The concentration of chlorine which is available to kill algae and bacteria.

Hard water

The hardness of the water is the sum of dissolved ions and magnesium ions which are present in the water. These ions are responsible for phenomenons like scaling.The more calcium and magnesium ions in the water, the harder the water. The amount of ions present can be measured and is expressed in mmol/l (quantity of minimolecules per litre) also often expressed in English degrees (°E). The calculation factor is 7; or 1 mmol/l = 7 °E.

0 to 5 °E very soft water
5 to 10 °E soft water
10 to 15 °E medium hardness
15 to 22.5 °E very hard water
22.5 to 37.5 °E hard water

You may check the hardness of the water in your region with your local water company.


An inorganic (non-stabilised) family of chlorine compounds, which are used as chlorine carriers in various forms. The family exists of: calcium hypochlorite, lithium hypochlorite and sodium hypochlorite.

Langelier index

See saturation index

Magnesium hardness

The concentration of magnesium salts in the water



Neutralisation of chlorine

Chlorine can be neutralised with natrium thiosulphate or hydrogen peroxide.


A chemical process to remove unwanted combination out of the water.


The pH-value of the water shows whether the water is acid or alkaline. The pH-scale goes from 0 - 14. With a pH of 7.0 the water is neutral (not acid nor alkaline). Water with a pH lower than 7.0 is acid and water with a pH higher than 7.0 is alkaline.

Phenol red tablets

Test tablets for measuring the pH-value of the water


Parts per million or mg/litre or g/m3. A measure unit, which shows how much water per unit, should be dosed or measured.

Saturation index (S.I.)

A numerical value, which shows whether the water is in balance or not. Factors, which are taken into account, are the pH-value, the total alkalinity, the calcium hardness, the total dissolved solids (TDS) and the temperature.The ideal values are between -0.3 and +0.3.

Shock treatment

See super chlorination

Soft water

Water which hardly contains ANY dissolved calcium- or magnesium salts. Click here for more information!

Stabilised chlorine

Organic chlorine compound which contains cyanuric acid. The two most important chlorine compounds are sodium dichlor isocyanurate (Melpool 55/G and Melpool 63/G) and trichlor isocyanuric acid (Melpool 90/G, 90/20, 90/200, MF/200). They owe their popularity to the fact that much less chlorine is necessary, because of the presence of cyanuric acid.


Superchlorination or a shock treatment is a procedure which is followed in order to change the chloramines (combined chlorine) into free chlorine. A shock treatment is suitable for various causes:

Destruction of bacteria and other micro-organisms that have built up a resistance against the usual amounts of chlorine
Prevention of algae growth. Destruction of unwanted chloramines (the creators of chlorine smell and eyeirritation)
Super chlorination should be done with a quickly soluble disinfection product. For superchlorination you can use melpool 70/G or melpool 63/G.

Total alkalinity

The total alkalinity is the total quantity of alkaline substances like bicarbonates and hydroxides. The notion TA is often confused with the pH-value.

TDS (mineralisation)

TDS stands for Total Dissolved Solids like metals, minerals and dirt. A high TDS value (> 1500 ppm) can decrease the effectiveness of the disinfection.

Trichlor isocyanuric acid

A slow dissolving organic (stabilised) chlorine compound containing ± 90% active chlorine. Trichlor isocyanuric acid is available as powder, granular and tablets.

Water balance

Water is in balance when it is neither corrosive nor alkaline. The concept of the water balance is based on the fact, that water has a need for specific minerals. These minerals can be considered as food for the water. If there is not enough to eat, the water gets hungry or "corrosive", with too much food the water gets overfed (saturated) and it gets alkaline.