Waste disposal became a major concern as societies moved from nomadic cultures to building more permanent settlements. Odors, nuisance, and health risks primarily from infectious diseases became apparent. The advent of the water carriage system was the major development toward removal of human waste from the residential areas. Now the human waste could be diluted with large quantities of water, carried through underground pipes to a centralized location outside the population centers and disposed of in a large and suitable water body. As the population grew with increased urbanization and concentration in major population centers, discharging untreated wastewater in natural waters created serious water pollution problems. Using the water for drinking with some or no treatment still caused serious health risk. By the mid-nineteenth century, the engineers, public health officials, and the public were searching for treatment methods. How to treat the wastewater before discharging continued to be an unsolved problem until early to the mid-twentieth century.