Raw water, after being collected at the four reservoirs, is transferred through the aqueducts to the four Water Treatment Plants. There, it is treated, disinfected and then distributed to the citizens of Athens.

The four Water Treatment Plants are the following:

Galatsi WTP
Aharnon WTP
Polydendri WTP
Aspropyrgos WTP

Raw water arriving at the WTP is untreated. It contains different impurities (e.g. branches, leaves, sludge etc) as well as bacteria and microorganisms not visible by naked eye.
In the WTP the following steps are followed:

1st step: Chlorine addition and initial disinfection by pre-chlorination. Microbes in the water are killed facilitating its subsequent treatment.

2nd step: Addition of aluminum sulphate and clarification. The solution of aluminum sulphate facilitates the particles in the water to stick together (flocs) and to settle out as heavier. This procedure is called flocculation - coagulation. It is a two-step procedure, the difference being on the strength of the water mixing caused by mechanical (mixers) and hydraulic means.

3rd step: Sedimentation tank.
In this tank, water is still and the particles (flocs) settle on the bottom of the tank. The water here is cleaned about 80%.

4th step: Cleaning filters (sand filters). The very light particles and the colloids (20%) which cannot settle are withheld on special sand filters from which water comes out clean enough to be potable.

If prechlorination is not satisfactory, additional chlorine is added in the water at its exit from the closed water reservoirs and before it enters the water distribution network.