Final answer:
The Indus Valley Civilization was the first to have sewer systems in their cities, with citywide drainage systems and indoor toilets connected to the network.
Step-by-step explanation:
The first civilization to have a sewer system in their cities was the Indus Valley civilization. This ancient society had highly developed water engineering and citywide drainage systems. They managed to create an efficient and sanitary urban environment comparable to modern cities. Their cities, such as Harappa and Mohenjo-Daro, had homes with indoor plumbing, and their streets had sophisticated drainage systems. Various features including wells for water supply, standard-sized mud bricks for construction, and a grid-like layout of the cities reflect their advanced approach to urban planning and public hygiene.
Their sewer systems served a population as large as forty thousand residents in each city and featured indoor toilets that connected to a drainage network. The existence of these systems indicates a high level of labor specialization, coordination, and strong social organization within the Indus Valley civilization, although they appeared to have existed without a powerful centralized state.