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1、IIP a r tWATER AND WASTESIn the United States, each person generates almost 75,700 liters (20,000 gallons) of sewage each year. Fruits, vegetables, grains, milk products, and meats de- rived from nutrients in the soil are brought into cities, to be later flushed out as sewage. Some communities disch
2、arge 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 treatmen
3、t 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 cant finish the natural cycle by return- ing the nutri
4、ents back to the soil, and end up with in- creasing 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
5、, consuming much of the oxygen in the water in the process. With- out 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 pro- duce quality dec
6、lines. Artificial fertilizers are applied to replace the wasted natural fertilizers. Designers can step into this process when they make decisions about how wastes will be generated and han- dled by the buildings they design. Sewage treatment is expensive for the community, and becomes a critical is
7、- sue for building owners where private or on-site sewage treatment is required. In a geographically isolated com- munity, like Marthas Vineyard off the Massachusetts coast, restaurants have been forced out of business by the high cost of pumping out their septic tanks. One lo- cal businessman calcu
8、lates 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, Dees Harbor Caf was closed after its septic sys- tem failed, and the owner lost her life savings. Even in less remote locations, dependence on a sep
9、tic 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 pri- vate sewage treatment systems depend on the number of fixtures served a
10、nd the permeability of the soil as de-1 1C h a p t e rTreating and Recycling Water55termined by a percolation test. Rural building sites are often rejected for lack of suitable sewage disposal.RURAL SEWAGE TREATMENTIn times past, rural wastes ended up in a cesspool, a porous underground container of
11、 stone or brick, which allowed sewage to seep into the surrounding soil. Cess- pools did not remove disease-causing organisms. Within a short time, the surrounding soil became clogged with solids, and the sewage overflowed onto the surface of the ground and backed up into fixtures inside the buildin
12、g. Cesspools have mostly been replaced by septic sys- tems (Fig. 11-1). A typical septic system consists of a sep- tic tank, a distribution box, and a leach field of perfo- rated drainpipes buried in shallow, gravel-filled trenches. Septic tanks are nonporous tanks of precast concrete, steel, fiberg
13、lass, or polyethylene that hold sewage for a period of days while the sewage decomposes anaerobi- cally. Anaerobic digestion produces methane gas and odor. During this time, the sewage separates into a clear, relatively harmless effluent and a small amount of min- eral matter that settles to the bot
14、tom. Soaps and slow-to-degrade fats and oils float to the top of the tank to form a layer of scum. Inlet and outlet baffles in the tank prevent the surface scum from flowing out. The liquid moves through a submerged opening in the middle of the tank to a second chamber. Here finer solids con- tinue
15、to sink, and less scum forms. This part of the pro- cess is known as primary treatment. When the effluent leaves the septic tank, it is about 70 percent purified. The longer sewage stays in the tank, the less polluted is the effluent. If the building and its occupants practice water conservation, le
16、ss water and wastes flow through the septic tank, the effluent stays in the tank longer before being flushed out, and it emerges cleaner. Every few years, the sludge is pumped out of the septic tank and is hauled away and processed to a harmless state at a remote plant. The methane gas and sewage odor stay in the tank. Each time sewage flows into the tank, an equal vol- ume of nitrate-rich water flows out and is distributed into the leach field, which provides secondary treatment. There t
