A sewer system is an underground network of pipes that carries sewage and wastewater from homes and businesses to a treatment plant. There are two types of sewer systems. Modern sanitary sewer systems are designed to be a separate network of pipes and infrastructure from those that manage stormwater; these are known as separate sanitary sewers. Some municipalities still have combined sewers that manage both sanitary sewage and stormwater in the same pipes; these are known as combined sewers.