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.
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.
