How did Public Sanitation Develop?
With the beginning of settled life, a new problem arose as people began to live in one place throughout the year. That problem was public sanitation. With increased population, the need to adequately remove human waste and maintain relatively clean water supplies became an increasing challenge. By prehistory, this challenge was addressed in societies, with increasing sophistication as cities grew and became more complex.
Early History of Sanitation
Sanitation is evident in the earliest settled societies. In the Indus Valley, Mesopotamia, and Iran, evidence exists for the use of sewers and clay pipes for removing human waste. With ceramics being present in societies by 7500-7000 BCE, this technology became utilized for making clay pipes could safely transport waste. By the Neolithic in the Near East in the 7th and 6th millennia, vertical shafts were used for waste disposal and wells had begun to be utilized within villages for getting freshwater.
In the 3rd millennium BCE, clay pipes now extended into sewer systems within structures. In the Indus, urban planning, included public sewers, are evident at sites such as Lothal. Mohenjo-daro in the Indus may have developed the first toilets with seats and shaped as something similar to Western-style toilets today.  In Mesopotamia, houses would have vertical shafts that would send waste far below, keeping clean and waste water separated. Alternative horizontal pipe systems and drains moved waste and waste water out of structures and into larger drains or cesspits. By the second millennium BCE, toilets could be found in Southern Europe, Southeast Asia, Near East, Central Asia, India, China, and Southeast Asia. The Minoan city of Knossos in Crete had, by this time, developed a system that brought clean water, on the one hand, and an alternative system that took waste water away from the city, including developing a storm sewage canals for overflow during heavy rains. 
Late Antiquity of Sanitation
One of the most elaborate ancient sewer systems was the Cloaca Maxima, which was a large-scale sewer system built for Rome. It was likely constructed already in the Etruscan period and became more elaborate during the Roman period. The Roman covered what was an initially open-air sewage system into a closed system. The underground system connected public baths and latrines. The system of aqueducts that connected to Rome, where there were seven aqueducts by the 1st century CE, brought freshwater supplies to the city. The elaborate waste removal and freshwater supply to Rome enabled it to grow substantially in size, reaching over one million people, making it the first city to likely reach that size. The Romans took sanitation seriously, as they realized the health benefits. Outposts and other towns throughout the empire utilized aqueducts, wells, latrines, and sewage systems to keep areas clean. (Figure 1)
In the Near East and Central Asia, qanat systems, or underground water channels, brought safe drinking water to urban places as well as bringing water to fields. Cities also utilized natural small streams and created artificial channels to wash out waste water away from them. The Qin and Han dynasties utilized plumping systems within their cities that developed sewer systems that were already around by the Bronze Age in China. 
In the Medieval Period, cities began to utilize small open canals along streets to move waste water out of cities. Sometimes natural stream were utilized to move waste away from a city, although this had the detrimental result of polluting water downstream. By the late Medieval period, however, cities were developing more sanitary policies and becoming administered by central municipalities that managed issues of waste water and the provision of drinking water. public latrines were constructed, although by the 16th centuries flush toilets were also utilized at least by the royalty.
Interestingly, in Japan and China, there was a different solution to the problem of human waste. In this case, there was a collection system where human waste was used to fertilize agricultural fields. In Western states, this did not develop because of the dependence on cattle, while in Japan and China cattle were not as integrated into the agricultural process.
By the 16th and 17th centuries, it was determined that potassium nitrate needed for gunpowder could come from human waste. European cities soon used their public latrines as a way to collect materials that would then be used to create potassium nitrate used for gunpowder. This industry helped to create a way to recycle and remove waste from public areas, helping cities to become more health and grow by the late Medieval and early modern periods .
Overall, however, the system that was already in ancient societies in the Near East, Asia, and Rome largely did not develop to be very different until the modern Industrial Age by the 19th century. One of the main innovations that developed sanitation was the introduction of mechanical pumps. Large sewer systems could now be created with better tunneling technologies that could ignore gravity and pumping stations could be used to move water irrespective of the terrain across a city. As cities during this time became unhealthy as populations expanded and people migrated to cities as the Industrial Revolution developed, this became an important technical development. 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 introduced to clean water, with the introduction of sand filtration to London in 1829 .
The world's first modern sewer system was in London (Figure 2). 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 kilometers 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 activated sludge or natural bacteria and protozoa 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 utilized.
The history of sanitation parallels the history of cities and urban life. Places of high density population only grew to new levels when developments in sanitation were possible. The first innovations in prehistory and early historical periods led to developments in late antiquity, such as that seen in Rome, the Near East, Central Asia, and East Asia. However, after this period, relatively modest development occurred. This resulted in urban populations rarely surpassing one million prior to 1800. It was only with the development of modern sewage systems, pumps, chemical treatments, modern toilets, filtration, and activated sludge that sanitation greatly improved and now allows our modern cities to be possible. This has now led us to the point where modern cities can grow to far greater size and human population has subsequently greatly expanded.
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- For more on early sanitation, see: Mitchell, P. D. (Ed.). (2015). Sanitation, latrines and intestinal parasites in past populations. Farnham, Surrey ; Burlington, VT: Ashgate.
- For more on Indus sanitation systems, see: Angelakēs, A. N. (Ed.). (2014). Evolution of sanitation and wastewater technologies through the centuries. London: IWA Publ.
- For more on Bronze Age sanitation, see: Eslamian, S. (Ed.). (2014). Handbook of engineering hydrology. Environmental hydrology and water management. Boca Raton: Taylor & Francis.
- For more on Rome's ancient water works and sewage system, see: Casson, L., & Casson, L. (1998). Everyday life in ancient Rome (Rev. and expanded ed). Baltimore: The Johns Hopkins University Press.
- For more on water and sanitation systems in late antiquity, see: Liebeschuetz, J. H. W. G. (2015). East and West in late antiquity: invasion, settlement, ethnogenesis and conflicts of religion. Leiden: Brill.
- For more on Medieval sanitation, see: Gies, F., & Gies, J. (1999). Daily life in medieval times: a vivid, detailed account of birth, marriage, and death; food, clothing, and housing; love and labor, in the Middle Ages. New York: Black Dog & Leventhal Publishers, pg. 138.
- For more on using human waste as fertilizer, see: King, F. H. Farmers of forty centuries: or, Permanent agriculture in China, Korea, and Japan. Miami, FL: HardPress Publishing.
- For more on the use of human waste for gunpowder, see: Cressy, D. (2013). Saltpeter: the mother of gunpowder (1st ed). Oxford: Oxford University Press.
- For more on sanitation's development in the 19th century, see: Juuti, P., Katko, T. S., & Vuorinen, H. S. (Eds.). (2007). Environmental history of water: global views on community water supply and sanitation. London: IWA Pub.
- For more on how cities transformed as sanitation improved, see: McGranahan, G. (Ed.). (2001). The citizens at risk: from urban sanitation to sustainable cities. London ; Sterling, VA: Earthscan.
- For more on modern sanitation systems, see: Jenkins, D., Wanner, J., IWA Conference Activated Sludge - 100 Years and Counting, & International Water Association (Eds.). (2014). Activated sludge - 100 years and counting: [papers delivered at the Conference “Activated Sludge ... 100 Years and Counting!” held in Essen, Germany, June 12th to 14th, 2014]. London: IWA Publ.