The 1968 decision to supply Athens with water from Mornos River; the construction of facilities there for water-collection, water-intake and for water-transfer by gravity flow; in combination with the densely populated areas of Athens with its many hills at high altitudes, meant that the new Water Treatment Plant had to be built in Menidi (also known as Aharnon).

The 60 hectares owned by EYDAP in Menidi is where the WTP was constructed. The facility consists of two hydraulically independent water treatment units, with flocculation tanks having water intake levels of +237 m and +238 m. The installation was constructed by the Hellenic Water Company, between 1971 and 1978, however it supplied the network for the first time in June 1974 with water from Yliki Lake. The unit with the +237 m water intake level is referred to as "the old unit". Its initial daily water treatment capacity of 400.000 m3 proved insufficient during periods of high demand, which exceeded 600.000 m3 during the drought of 1987. Thus in 1989, its daily capacity was increased to 600.000 m3, by the construction of additional water-intake pipes leading to the sedimentation tanks. The unit with the +238 m water level is referred to as "the new unit". It began operations in 1992, is completely automated and has a water treatment capacity of 200.000 m3/ day.

The Aharnon WTP primarily supplies the portion of the distribution network that covers the areas of Attica with higher altitudes, however it also provides supplemental quantities of water to the Municipalities of Athens and Piraeus. Its current total water treatment capacity is 800.000 m3/day. With the completion of the Renovation and Modernization project, the Aharnon WTP will have the ability to treat raw water of lower quality with greater efficiency.