The cisterns were not only places for storing the city's water but were also "purgatori" — places for purifying the water. Water entered the reservoir from the Acquedotto Leopoldino through layers of gravel and carbon, a method of filtration which was later replaced by a more modern chlorine treatment system. The later removal of the gravel filtering system meant that the cistern could be used to its maximum capacity to store 10,000 cubic metres of water. This reservoir and purification plant is still in use today.