Soil stack (in green) is connected to the sewer at the bottom and vented at the top, while each plumbing fixture has its own trap (shown in white) 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.