By what year was virtually all (>90%) of the Urban population in the US served by drinking water treatment plants?

The mid-20th century marked a significant transformation in public health, particularly in relation to drinking water standards and treatment. By 1940, over 90% of the urban population in the United States began to be served by drinking water treatment plants. This was largely due to advancements in technology and the recognition of the importance of clean drinking water for preventing waterborne diseases.

The establishment of water treatment infrastructure during this time was a response to outbreaks of diseases like cholera and typhoid fever, which highlighted the need for safe drinking water. Local municipalities started implementing chlorination, filtration, and other treatment processes to ensure the water was safe for consumption.

After 1940, while the percentage of urban populations served by treatment plants continued to rise, it was already at a critical threshold by this year. Subsequently, as the infrastructure was developed and improved throughout the 1950s and 1960s, it became increasingly common for more populations, including those in rural areas, to gain access to treated drinking water.

Therefore, the pivotal year when virtually all (>90%) of the urban population began to receive water from treatment facilities is 1940.