The Galatsi WTP was constructed by Ulen and Co in the years 1925 - 1931. The facility production capacity was expanded with the construction of two plant extensions in 1952 and 1964. The WTP occupies an area of approximately 25 hectares and is comprised of two hydraulically independent units (water levels at entrance of flocculation tanks +162 m and +163 m respectively). The unit with the +162 m level is referred to as "the old unit". It includes facilities built in 1931 as well as the 1952 extension. Its water treatment capacity is 180.000 m3/day. The unit with the +163 m water level is referred to as "the new unit". It primarily consists of works constructed with the 1964 extension and its daily water treatment capacity is 360.000 m3. The filtration system consists of double gravity sand filters, of American design, with the Wagner-type drainage system, with constant flow Simplex "S" type hydraulic regulators and a hydro-pneumatic, and valve operating system.

The water is disinfected primarily with chlorine and clarified using aluminum sulphate. The Galatsi WTP supplies the portion of the distribution network that serves the city center of Athens and the Municipality of Piraeus with a daily total water treatment capacity of 540.000 m3. This will increase to 600.000 m3 with the completion of the Renovation and Modernization project, which is underway.