The Romans constructed numerous aqueducts in order to bring water from distant sources into their cities and towns, supplying public baths, latrines, fountains and private households. Waste water was removed by complex sewage systems and released into nearby bodies of water, keeping the towns clean and free from effluent. Aqueducts also provided water for mining operations, milling, farms and gardens.Aqueducts moved water through gravity alone, being constructed along a slight downward gradient within conduits of stone, brick or concrete. Most were buried beneath the ground, and followed its contours; obstructing peaks were circumvented or, less often, tunnelled through. Where valleys or lowlands intervened, the conduit was carried on bridgework, or its contents fed into high-pressure lead, ceramic or stone pipes and siphoned across. Most aqueduct systems included sedimentation tanks, sluices and distribution tanks to regulate the supply at need.Rome's first aqueduct supplied a water fountain sited at the city's cattle market. By the third century AD, the city had eleven aqueducts, sustaining a population of over a million in a water-extravagant economy; most of the water supplied the city's many public baths. Cities and municipalities throughout the Roman Empire emulated this model, and funded aqueducts as objects of public interest and civic pride, ""an expensive yet necessary luxury to which all could, and did, aspire."":)Most Roman aqueducts proved reliable, and durable; some were maintained into the early modern era, and a few are still partly in use. Methods of aqueduct surveying and construction are noted by Vitruvius in his work De Architectura (1st century BC). The general Frontinus gives more detail in his official report on the problems, uses and abuses of Imperial Rome's public water supply. Notable examples of aqueduct architecture include the supporting piers of the Aqueduct of Segovia, and the aqueduct-fed cisterns of Constantinople.