Vent Piping

The invention of the trap helped to keep sewer gases out of buildings. However, traps were not foolproof. When water moving farther downstream in the system pushes along water in front of it at higher pressures, negative pressures are left behind. The higher pressures could force sewer water through the water in some traps, and lower pressures could siphon (suck) water from other traps, allowing sewer gases to get through (Fig. 10-3).

Figure 10-3 Without a fixture vent.
Fixture vent admits air, maintain air" pressure, releases sewer gases —

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sewer gases

Figure 10-4 With a fixture vent.

Figure 10-4 With a fixture vent.

Vent pipes (Fig. 10-4) are added to the waste piping a short distance downstream from each trap to prevent the pressures that would allow dirty water and sewer gases to get through the traps. Vent pipes run upward, join together, and eventually poke through the roof. Because the roof may be several floors up and the pipes may have to pass through other tenants' spaces, adding vent pipes in new locations can be difficult. The vent pipe allows air to enter the waste pipe and break the siphoning action. Vent pipes also release the gases of decomposition, including methane and hydrogen sulfide, to the atmosphere. By introducing fresh air through the drain and sewer lines, air vents help reduce corrosion and slime growth.

The vent pipes connect an individual plumbing fixture to two treelike configurations of piping. The waste piping collects sewage and leads down to the sewer. The vent piping connects upward with the open air, allowing gases from the waste piping to escape and keeping the air pressure in the system even. This keeps pressure on foul gases so that they can't bubble through the trap water, and gives them a local means of escape to the outdoors.

The vent must run vertically to a point above the spillover line on a sink before running horizontally so that debris won't collect in the vent if the drain clogs. Once the vent rises above the spillover line, it can run horizontally and then join up with other vents to form the vent stack, eventually exiting through the roof.

When all fixtures are on nearly the same level, a separate vertical vent stack standing next to the soil stack is not required. In one-story buildings, the upper extension of the soil stack above the highest horizontal drain connected to the stack becomes a vent called the stack vent. It must extend 31 cm (12 in.) above the roof surface, and should be kept away from vertical surfaces, operable skylights, and roof windows.

When a sink is located in an island, as in some i( kitchen designs, there is no place for the vent line to go up. Instead, a waste line is run to a sump at another location, which is then provided with a trap and vent. A fresh-air vent, also called a fresh air inlet, is a short air pipe connected to the main building drain just before it leaves the building, with a screen over the outdoor end to keep out debris and critters.

Treating and Recycling Water

In the United States, each person generates almost 75,700 liters (20,000 gallons) of sewage each year. Fruits, vegetables, grains, milk products, and meats derived from nutrients in the soil are brought into cities, to be later flushed out as sewage. Some communities discharge bacteria-laden sewage into nearby lakes, rivers, or the ocean. Most cities and towns send the sewage to treatment plants, where the solid matter (sludge) settles out. The remaining liquid is chlorinated to kill bacteria and then dumped into a local waterway.

The sludge is pumped into a treatment tank, where it ferments anaerobically (without oxygen) for several weeks. This kills most of the disease-causing bacteria and precipitates out most minerals. The digested sludge is then chlorinated and pumped into the local waterway.

Waterways can't finish the natural cycle by returning the nutrients back to the soil, and end up with increasing amounts of nutrients. This nutrient-rich water promotes the fast growth of waterweeds and algae. The water becomes choked with plant growth, and the sun is unable to penetrate more than a few inches below the surface. Masses of plants die and decay, consuming much of the oxygen in the water in the process. Without oxygen, fish suffocate and die. The waterway itself begins to die. Over a few decades, it becomes a swamp, then a meadow. Meanwhile, the farmland is gradually drained of nutrients. Farm productivity falls, and produce quality declines. Artificial fertilizers are applied to replace the wasted natural fertilizers.

Designers can step into this process when they make indecisions about how wastes will be generated and handled by the buildings they design. Sewage treatment is expensive for the community, and becomes a critical issue for building owners where private or on-site sewage treatment is required. In a geographically isolated community, like Martha's Vineyard off the Massachusetts coast, restaurants have been forced out of business by the high cost of pumping out their septic tanks. One local businessman calculates that it costs him about one dollar per toilet flush, and if his septic tank fills up, he will have to shut down before it can be pumped. In 1997, Dee's Harbor Café was closed after its septic system failed, and the owner lost her life savings. Even in less remote locations, dependence on a septic tank often limits the size of a restaurant and prohibits expansion.

Sewage disposal systems are designed by sanitary engineers and must be approved and inspected by the health department before use. The type and size of private sewage treatment systems depend on the number of fixtures served and the permeability of the soil as de termined by a percolation test. Rural building sites are often rejected for lack of suitable sewage disposal.

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