Below is a step by step description of the potable water treatment process for which there are many variations. The basic guiding principle is to have several barriers for pathogens so that if one fails the other steps can catch the culprit.

The first step in the potable water treatment process is Surface water or Well water pumping. In some dry climate areas, desalinated Water is used as a feed source for potable water such as in Saudi Arabia and Australia. In case such a clean source of feed water is used, then many of the following steps may not be needed.

Next is Aeration for removal of any undesired gases such as H2S or for oxidation of Ferrous metal ions into ferric. Aeration is needed for Well Water sources as obviously surface water is already aerated.

Coagulation/Softening/Flocculation/Settling or DAF (Dissolved Air Floatation) for removal of SS (Suspended Solids organic and inorganic in nature). These processes happen in clarifiers. A coagulant is dosed which helps suspended colloidal particles to clump together forming flocs that settle much faster. Softening occurs when lime and/or soda ash is added to the water and this reacts with the hardness in the water. Flocculation can be assisted by dosing polymers known as flocculants which stick to many flocs at the same time. It is possible to soften the water using an ion Exchange process such as MIEX (Magnetic Ion Exchange) which is used in Perth, Australia.

It is also possible to replace the clarification step using Ultra Filtration which can remove Suspended Solids and many bacteria and viruses.

Next is Ozonation - Contact Tank (1st level Disinfection). This step Removes bacteria/viruses and also oxidizes taste/odor molecules that may have escaped from the clarification process. Sometimes certain bacteria that occur in nature produce taste and odor molecules that are a nuisance to the consumer. These small molecules need to be removed.

Next is Filtration using slow fine sand filters or AC (Active Carbon) filters that act as Bio filters (Biofilters assimilate N and P in the water and make it harder for bacteria downstream to thrive). The same principle is used in biofilters for fish aquariums.

Next Dose SBS to remove residual O3 and store in tank/basin.

Next is Chlorination (2nd Level of Disinfection)

Then Store in Underground Basin

Next UV lamps (3rd Level Disinfection for Cryptosporidium or Giardia cysts) zap out any pathogens that may have escaped the previous steps.

Add Fluoride for tooth protection.

Add Phosphate for corrosion inhibition of old Lead or Carbon Steel pipes

Pump to distribution Network (through underground basins into water towers)

Dose residual Chlorine to distributed water in order to have a residual chlorine concentration at the tap (Point of Use). This step is needed to protect the end user in case there are leaks in the distribution network. It is important to note that the Dutch and some other developed countries produce and distribute safe drinking water without using Chlorine. For more details on this read the following article on the TU Delft website.

Sludge from Clarification and Filter backwash is dewatered, the water is recycled and the solids are sent to landfill.

Some of the most prominent water utilites in the world are:

United Utilites in the Uk serving Greater Manchester

Anglian Water in the UK

Southern Water in the UK

Severn Trent in the UK

Thames Water in the UK

Veolia in France

Suez in France

Gelsenwasser AG in Germany

Mainova AG in Germany