Sewers require observation as well as regular maintenance and cleaning; without this, pipes can clog -- and sewer systems can shut down. In order to have quick access to various areas of the sewer system, manholes are installed throughout cities. These holes are blocked with heavy, metal manhole covers to prevent people from falling into them accidentally. Sewers require ventilation to maintain normal air pressure; manholes allow the circulation of air through the system.
The earliest sewer pipes scientists discovered were made of clay; these pipes are dated to 4000 BCE. The Romans also used lead pipes; which we now know were not safe for this purpose. Early Americans made pipes from any material available, including wood and stone. Today's modern pipes for sewer systems are made from PVC, a type of plastic.
Almost all of the water that runs through American sewage systems is recycled and re-used, due to the limited amount of water on this planet. In order to re-use water it must be processed and sanitized. First the water is allowed to settle, so most pollution settles to the bottom. Then the water is mixed with oxygen and microorganisms that eat and digest the bacteria that remains in the water.
Waste water is collected by drainage systems in order to prevent flooding and to prevent pollutants from infiltrating the water supply. A separate sewage system is used in most new cities. Separate sewage systems involve sanitary sewers that only transport sanitary water; Storm drains collect and transport ground water. In a combined system, a combined sewer is used, mixing ground water with the sanitary water system. As sewer systems are updated, separate systems are installed.