How a Drain, Waste & Vent (DWV) System Works
In every home is a drain, waste & vent system, also called DWV. This drainage system is designed to carry away waste water while preventing the flow of sewer gases into your home. Some homes have a gray water system that has a drain line for sinks, showers, dishwasher and laundry and a separate black water line for toilets. The gray water can be used for irrigation and toilet flushes.
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Water flows down hill. The whole DWV system is built on this principle and so most drain systems are gravity flow systems. Each drain in your home travels down to a larger branch drain. All the branch drains connect to a waste stack, a vertical pipe that carries water to the main drain or sewer line. A large pipe leaves your home and leads to the city sewer or your own septic system. All of this is downhill, all the way to the sewer or septic. In some cases, a home's sewer line will be below the level of the city sewer, or a basement bathroom is below the home's main sewer line and so pump equipment must be used to move the waste out to the main sewer or septic.
All fixtures and appliances have a trap. A trap keeps a few ounces of water at a low spot in the pipe to seal the drain. This plug of water prevents sewer gas from traveling through the pipes and into your home. An example of a trap is the "P-trap" found under a sink. The water runs through the drain and through the trap. The last few ounces remain behind in that trap. It fills the pipe and so prevents sewer gas from getting into your home. Toilets have there own trap built right into the fixture.
Along with all the drain and waste pipes, there is a system of vent pipes integrated into the drain system. These vent pipes allow sewer gas to be vented out above your home where it can quickly mix with the air and dissipate. The vents also serve the important function of preventing a vacuum or siphon from occurring. Because water traveling through the pipes would create a siphon effect, the water in the traps would be pulled along leaving the traps empty, allowing sewer gas to enter the home. Also, if a vacuum occurs, the draining water slows down much like how water gurgles out when poured from a bottle. This slower moving water results in the greater likelihood of clogging. A vent system is necessary for the proper operation of a drain and waste system. Each fixture must be properly vented and so a vent line branches off of the drain line, to either join up to the main vent or vents directly up through the roof.
The gravity system works well, unless something impedes the flow of draining water. Swiftly moving water carries waste away. However, if water cannot move swiftly, then the whole system starts to fail. First waste builds up, further slowing water flow until nothing is moving. The initial problem that results in this cascade effect may be simply inappropriate waste being sent down the drain. It can result from dips developing in the drain line as the result of earth settling. Deterioration of the drain pipes can result in rough surfaces that snag toilet paper, which builds up, eventually clogging the line. Tree roots grow through pipe fittings and also snag waste. Another problem is putting grease, sand, coffee grounds and other "coagulants" down the drain. While coagulants may not be a plumbing term, it conjures the image of exactly what is happening. Things like sand and grease tend to rest on the bottom of the pipe and aren't easily washed away. They just coagulate and build up over time.
The DWV system is really pretty simple. It has some important refinements and the system has to be put together well, with attention to details. However, it should not be intimidating to you because in the end, it is a bunch of pipes running downhill.