The growth of cities and the (1)______ of industries in the nineteenth and twentieth centuries brought increasing pollution of the rivers and waterways. As a consequence, water purification treatments (2)______ become more and more important as a means of providing safe (3)_______.
The most widely used method of water purification is water filtration plants (4)______ remove impurities from the water by passing it (5)_____ sand, and in some systems, treating the water with elements such (6)______ chlorine.
Filtration systems (7)_____ in Western Europe. The first treatment plants were built in 1829 to purify the River Thames in England.
In 1854, the (8)_____ for water treatment plants became recognized and the need for pure water a matter of public concern when a London cholera epidemic was (9)_____ to a public well. Most cities, first in England, then in the U.S and later throughout the world, developed filtration plants to avoid the cholera outbreaks that frequently ravaged (10)______ communities.