<Table> <Tr> <Td> </Td> <Td> The examples and perspective in this article may not represent a worldwide view of the subject . You may improve this article, discuss the issue on the talk page, or create a new article, as appropriate . (July 2015) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> The examples and perspective in this article may not represent a worldwide view of the subject . You may improve this article, discuss the issue on the talk page, or create a new article, as appropriate . (July 2015) (Learn how and when to remove this template message) </Td> </Tr> <P> In modern plumbing, a drain - waste - vent (or DWV) is part of a system that removes sewage and greywater from a building, and regulates air pressure in the waste - system pipes to aid free flow . Waste is produced at fixtures such as toilets, sinks, and showers, and exits the fixtures through a trap, a dipped section of pipe that always contains water . </P> <P> DWV systems maintain neutral air pressure in the drains, allowing free flow of water and sewage down drains and through waste pipes by gravity . It is critical that a sufficient downward slope be maintained throughout, to keep liquids and entrained solids flowing freely towards the main drain from the building . In relatively rare situations, a downward slope out of a building to the sewer cannot be created, and a special collection pit and grinding lift "sewage ejector" pump are needed . By contrast, potable water supply systems operate under pressure to distribute water up through buildings, and do not require a continuous downward slope in their piping . </P>

What is the purpose of a vent in a plumbing system