Overall, however, the system that was already in ancient societies in the Near East, Asia, and Rome largely did not develop further until the modern Industrial Age in the late 18th and 19th centuries. The main innovation that developed was the introduction of mechanical pumps. Large sewer systems could not be created with better tunneling technologies and pumping stations could be used to move water irrespective of the terrain slope. Cities during this time became unhealthy as populations expanded and migrated to cities. By the late 19th century, chemical treatment was used to treat waste water, mostly in the form of using chlorine to treat water. Sand filtration was also used to clean water, with the introduction of sand filtration to London in 1829.
The world's first modern sewer system was in London. It was developed to have 450 miles of sewers that used large tunnels that applied a combination of gravity and pumping stations to move waste water. This enabled London to handle its very rapidly growing population, reversing the many problems, such as typhoid, the city had with disease outbreak in the earlier part of the 19th century. Paris in the late 19th century also witnessed large-scale expansion. Similar to London, the mixture of waste water with drinking water made typhoid and other epidemics widespread. The city responded by building 600 kilometres of aqueducts that brought potable spring water into the city, while underground sewers were built that drained waste, with dirty water used to help flush sewage away from the city. Other cities in North American and Europe built similar systems during the 19th century. By the 1880s, flush toilets, known as water closets, began to be marketed and installed just as large sewer systems became increasingly established in urban centers in Europe and North America. By the early 20th century, chemical treatment of sewage became more established.
In the 20th century, sewage treatment plants began to be established with the innovation of using sludge or natural bacteria as part of the waste water treatment process. In this case, tanks or pools of waste water would be treated with bacteria to help breakdown waste. Chemical treatment was also