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Whether your home is on a sewer or septic system, the systems within your home are essentially the same. Drainage systems do not depend on pressure, as supply systems do. Instead, waste matter leaves your house because the drainage pipes all pitch, or angle, downward. Gravity pulls the waste along. The sewer line continues this downward flow to a sewage treatment facility or a septic tank.
While the system sounds simple, there's more to it, including vents, traps, and clean outs. The vents sticking up from the roof of your house allow air to enter the drainpipes. If there were no air supply coming from the vents, wastewater would not flow out properly and the water in the traps would need to be siphoned away.
Traps are vital components of the drainage system. You can see a trap under every sink. It is the curved or S-shape section of pipe under a drain. Water flows from the basin with enough force to go through the trap and out through the drainpipe, but enough water stays in the trap afterward to form a seal that prevents sewer gas from backing up into your home. Every fixture must have a trap. Toilets are self-trapped and don't require an additional trap at the drain. Bathtubs frequently have drum traps, not only to form a seal against sewer gas but also to collect hair and dirt in order to prevent clogged drains. Some kitchen sinks have grease traps to collect grease that might otherwise cause clogging. Because grease and hair are generally the causes of drain clogs, traps often have clean-out plugs that give you easier access to remove or break up any blockage.
Since a drainage system involves all of these components, it is usually referred to as the DWV: the drain-waste-vent system. If water is to flow out freely and waste is to exit properly, all components of the DWV must be present and in good working order. Examine the pipes in the basement or crawl space under your house to help you understand the system better.