In modern plumbing, a drain-waste-vent (or DWV) is a system that removes sewage and greywater from a building and vents the gases produced by said waste. 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. All fixtures must contain traps to prevent gases from backing up into the house. Through traps, all fixtures are connected to waste lines, which in turn take the waste to a soil stack, which extends from the building drain at its lowest point to above and out of the roof. Through the building drain, the waste is removed from the building and taken to a sewage line, which leads to a septic system or a public sewer. Cesspits are generally prohibited in developed areas.
The venting system, or plumbing vents, consists of pipes leading from fixtures to the outdoors, usually via the roof. Vents provide for relief of sewer gases, admission of oxygen for aerobic sewage digestion, and maintenance of the trap water seals which prevent sewer gases from entering the building. Every fixture is required to have an internal or external trap; double trapping is prohibited by plumbing codes. With exceptions, every plumbing fixture must have an attached vent. The top of stacks must be vented too, via a stack vent, which is sometimes called a stink pipe[1].
DWV systems maintain neutral air pressure in the drains, allowing flow of water and sewage down drains and through waste pipes by gravity. As such, it is critical that a downward slope be maintained throughout. 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 comparison, potable water supply systems operate under pressure to distribute water up through buildings.
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Purpose
A sewer pipe is normally at neutral air pressure compared to the surrounding atmosphere. When a column of waste water flows through a pipe, it compresses air in the pipe, creating a positive pressure that must be released or it will push back on the waste stream and downstream traps' water seals. As the column of water passes, air must flow in behind the waste stream or negative pressure (suction) results. The extent of these pressure fluctuations is determined by the fluid volume of the waste discharge.
Excessive negative air pressure, behind a 'slug' of water that is draining, can siphon water from trap seals at plumbing fixtures. Generally, a toilet outlet has the shortest trap seal, making it most vulnerable to being emptied by induced siphonage. An empty trap can allow noxious sewer gasses to enter a building.
On the other hand, if the air pressure within the drain becomes suddenly higher than ambient, this positive transient could cause waste water to be pushed into the fixture, breaking the trap seal, with dire hygiene and health consequences if too forceful. Tall buildings of three or more stories are particularly susceptible to this problem. Vent stacks are put in parallel to waste stacks to allow proper venting in tall buildings.
Venting mechanisms
To prevent the problems of high pressure in a drain system, sewer pipes will usually vent via one of two mechanisms.
Venting to atmosphere
Most residential buildings' drainage systems in North America are vented directly through the buildings' roofs. The DWV pipe is typically ABS or PVC DWV-rated plastic pipe equipped with a flashing to prevent water entering the buildings. Older homes may use copper, iron, lead or clay pipes, in rough order of increasing antiquity.
Under many older building codes, a vent stack, a pipe leading to the main roof vent, is required to be within a five foot radius of the draining fixture (sink, toilet, shower stall, etc.). To allow only one vent stack, and thus one roof protrusion as permitted by local building code, sub-vents may be tied together and exit a common vent stack.
A blocked vent is a relatively common problem caused by anything from leaves, to dead squirrels, to ice dams in very cold weather. Symptoms range from bubbles in the toilet bowl when it is flushed, to slow drainage, and all the way to siphoned (empty) traps and sewer gases entering the building. When a fixture trap is venting properly, a "sucking" sound can often be heard as the fixture empties out.
Air admittance valve
Air admittance valves (AAVs) are pressure-activated, one-way mechanical vents, used in a plumbing system to eliminate the need for conventional pipe venting and roof penetrations. A discharge of wastewater causes the AAV to open, releasing the vacuum and allowing air to enter plumbing vent pipe for proper drainage. Otherwise, the valve remains closed, preventing the escape of sewer gas and maintaining the trap seal. Using AAVs can significantly reduce the amount of venting materials needed in a plumbing system, increase plumbing labor efficiency, allow greater flexibility in the layout of plumbing fixtures, and reduce long-term roof maintenance problems associated with conventional vent stack roofing penetrations.
While some state and local building departments prohibit AAVs, the International Residential and International Plumbing Codes allow it to be used in place of a vent-through-the-roof. AAV's are certified to reliably open and close a minimum of 500,000 times, (approximately 30 years of use) with no emanation of sewer gas; and some manufacturers claim their units are tested for up to 1.5 million cycles, or at least 80 years of use. Air Admittance Valves have been effectively used in Europe for more than two decades. U.S. manufacturers offer warranties that range from 20 years to lifetime.
Fittings
Drainage and venting systems require not only pipe, but also many fittings which add considerably to their cost of construction. But fittings such as "clean-outs" enhance the maintainability of the systems.
Municipal water and waste water treatment
One of the primary responsibilities of local governments is to provide potable water and treatment of sewage. The potable water side is called the WTP or Water Treatment Plant; and the waste/sewage removal system is called the WWTP Waste Water Treatment Plant; A municipal system's WTP treats water and brings it to home in compliance with federal, state, and local regulations governing water quality. The WWTP provides for the removal and treatment of waste water; the Waste water treatment facility or natural waste site (Usually a body of water or river) is typically as far away from inhabited areas as practical. Where a building is unable to connect to a municipal sewage system, the Waste/Sewage treatment is accomplished by means of a septic system and periodic hauling-off of the solids produced.
References
- ^ Gee, Steve (30 July 2007). "Cat burglar's crude escape". The Daily Telegraph. http://www.news.com.au/dailytelegraph/story/0,22049,22157566-5006009,00.html. Retrieved 29 January 2009.
See also
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