Rural Sewage Treatment

In times past, rural wastes ended up in a cesspool, a porous underground container of stone or brick, which allowed sewage to seep into the surrounding soil. Cesspools 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 building.

Cesspools have mostly been replaced by septic systems (Fig. 11-1). A typical septic system consists of a septic tank, a distribution box, and a leach field of perforated drainpipes buried in shallow, gravel-filled trenches. Septic tanks are nonporous tanks of precast concrete, steel, fiberglass, 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 mineral matter that settles to the bottom. 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 continue to sink, and less scum forms. This part of the process 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, less 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 volume of nitrate-rich water flows out and is distributed into the leach field, which provides secondary treatment. There the water is absorbed and evaporates. Nitrate-hungry microbes in the soil consume the potentially poisonous nitrates. In the process, plant food is manufactured in the form of nitrogen.

Nothing that can kill bacteria should ever be flushed down the drain into a septic system. Paints, varnishes, thinners, waste oil, photographic solutions, and pesti

Leaching Field cides can disrupt the anaerobic digestion. Coffee grounds, dental floss, disposable diapers, cat litter, sanitary napkins and tampons, cigarette butts, condoms, gauze bandages, paper towels, and fat and grease add to the sludge layer in the bottom of the tank. Some systems include a grease trap in the line between the house and the septic tank, which should be cleaned out twice a year.

Trained professionals must clean the tank at regular intervals. As the sludge and scum accumulate, there is less room for the bacteria that do their work, and the system becomes less effective. If the scum escapes through the outlet baffle into the leach field, it clogs the earthen walls of the trenches and decreases the necessary absorption. Most tanks are cleaned every two to four years.

Most septic systems eventually fail, usually in the secondary treatment phase. If the septic tank or the soil in the leaching field is not porous enough, or if the system is installed too near a well or body of water, or beside a steep slope, the system can malfunction and contaminate water or soil. Most communities have strict regulations requiring soil testing and construction and design techniques for installing septic tanks. If the site can't support the septic tank, the building can't be built.

Aerobic (with oxygen) treatment units (ATUs) can replace septic tanks in troubled systems. By rejuvenating existing drainfields, they can extend the system's life. Air is bubbled through the sewage or the sewage is stirred, facilitating aerobic digestion. After about one day, the effluent moves to the settling chamber where the remaining solids settle and are filtered out. Because aerobic digestion is faster than anaerobic digestion, the tank can be smaller. However, the process is energy intensive and requires more maintenance. The effluent then moves on to secondary treatment.

Secondary treatment can use a number of different techniques, with varying impact on the building site. Disposal fields are relatively inexpensive, and do not require that the soil be very porous or that the water table be very deep below the surface. Drainlines of perforated pipe or agricultural tile separated by small openings are located in shallow trenches on a bed of gravel and covered with more gravel. The effluent runs out of these lines and through the gravel, until it seeps into the earth. The gravel's spaces hold the liquid until it is absorbed.

Buried sand filters that use sand, crushed glass, mineral tailings, or bottom ash are also used for secondary treatment. They are applied where the groundwater level is high, or in areas of exposed bedrock or poor soil. A large site area is required, but the ground surface can become a lawn or other nonpaved surface. Buried sand filters can be a remedy for failed disposal fields.

Seepage pits are a form of secondary treatment appropriate for very porous soil and a low water table only. Seepage pits can also be used as dry wells to distribute runoff from pavement gradually.

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