A SEWER SYSTEM provides underground drainage and waste treatment for an entire city. By removing contaminated water and accumulated refuse, it dramatically reduces the spread of disease among the population.
Cities with a Sewer System may grow beyond size 12, as the infrastructure can support a larger, healthier population in sanitary conditions.
The great sewer of Rome, the Cloaca Maxima, was constructed in the 6th century BC and remains in partial use today. Roman cities that invested in sanitation infrastructure consistently outlasted those that did not, their populations swelling while neighbours were ravaged by plague and famine.