Early peoples had no need of engineering works to supply their water. Hunters and nomadscamped near natural sources of fresh water, and populations were so sparse that pollution of the watersupply was not a serious problem. After community life developed and agricultural villages becameurban centres, the problem of supplying water became important for inhabitants of a city, as well as forirrigation of the farms surrounding the city. Irrigation works were known in prehistoric times, and before2000 BC the rulers of Babylonia and Egypt constructed systems of dams and canals to impound theflood waters of the Euphrates and Nile rivers, controlling floods and providing irrigation waterthroughout the dry season. Such irrigation canals also supplied water for domestic purposes. The firstpeople to consider the sanitation of their water supply were the ancient Romans, who constructed a vastsystem of aqueducts to bring the clean waters of the Apennine Mountains into the city and built basinsand filters along these mains to ensure the clarity of the water. The construction of such extensivewater-supply systems declined when the Roman Empire disintegrated, and for several centuries localsprings and wells formed the main source of domestic and industrial water.The invention of the force pump in England in the middle of the 16th century greatlyextended the possibilities of development of water-supply systems. In London, the first pumpingwaterworks was completed in 1562; it pumped river water to a reservoir about 37 m above the level 4of the River Thames and from the reservoir the water was distributed by gravity, through lead pipes,to buildings in the vicinity.Increased per-capita demand has coincided with water shortages in many countries. SoutheastEngland, for example, receives only 14 per cent of Britain's rainfall, has 30 per cent of itspopulation, and has experienced declining winter rainfall since the 1980s.In recent years a great deal of interest has been shown in the conversion of seawater to freshwater to provide drinking water for very dry areas, such as the Middle East. Several differentprocesses, including distillation, electrodialysis, reverse osmosis, and direct-freeze evaporation, havebeen developed for this purpose. Some of these processes have been used in large facilities in theUnited States. Although these processes are successful, the cost of treating seawater is much higherthan that for treating fresh water.
đang được dịch, vui lòng đợi..