In his history of early Rome, Dionysius of Halicarnassus credits the success of the empire not the heroics of Aeneas, republican rectitude, or the virtues of the emperor, but to unglorious feats of civil engineering:
The extraordinary greatness of the Roman Empire manifests itself above all in three things: the aqueducts, the paved roads, and the construction of the drains.
Roman Antiquities, 3.67.5
The citizens of the great cities of the Roman Empire viewed public amenities like baths, fountains, latrines and sewers as indices of civilization that distinguished them from barbarians. Those amenities were dependent on an abundant and regular supply of clean water, usually in excess of local supplies. Water from distant elevated regions were brought into the city via aquaeductus (aquae = waters; ductus = led, directed), or aqueduct. At itThe aqueduct system consisted mainly of underground pipes and tunnels, but occasionally to maintain the correct elevation and grade, or to ovecome a topographical obstacle like a ravine or valley, the water had to be carried above ground over bridges.
The overland aqueduct spans are the product of two building technologies invented by the Romans—the arch and concrete. Unlike the Greek post and lintel system, the arch supported much greater weights and spanned longer distance. Whereas the Greeks essentially piled cut stone into stacks to make walls and columns, the Roman mixed rubble with quicklime and water into concrete, which could be moulded into whatever shaped the tensile strength of the aggregate would bear. The combination of the arch and concrete made the gigantic structures of the empire possible, including the aqueduct, which is a sequence of arches, or arcade, made of concrete covered with stone or brick revetment. The aqueduct system, in turn, quadrupled the water supply into Rome, enabling its rapid expansion and causing its population to rise due to the health benefits of fresh, running water.
Water traveled along a deep trench at the top of the arcades, which was often covered to prevent evaporation and contamination. Deep valleys and ravines could be spanned and rivers traversed by bridges composed of superimposed arcades, without fear of wind sheer or currents. Some of the more elaborate spans, such as the Pont-du Gard in Provence have roads and walkways added at different levels allowing them to serve as roadway bridges as well as aqueducts. In the city, the aqueducts were incorporated into the city walls, the arch openings serving as gateways. The attic story of the Porta Maggiore in Rome carried water from two separate aqueducts, the Aqua Anio Vetus and the Aqua Claudia, both of which originated near Agosta, some 67km away. The Aqua Claudia, begun by Caligula and finished by Claudius in A.D. 47 delivered 185,000 cubic meters of water everyday, 20% of the city’s supply.
When the water reached the city it emptied into a castellum, a large cistern on a hill; from there ceramic or lead pipes directed the water to the various access points. In ancient Rome, water was free and no one had to walk more than 150 feet to the nearest public fountain. Waste waters from latrines and baths and rain water emptied into the cloaca or underground sewer system which emptied into the Tiber at a point just south of they city. The 260 miles of aqueduct supplying Rome were overseen by the curator aquarum, appointed by the Emperor.