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Septic systems are locally buried waste treatment systems. Historians believe that the first use of septic systems was in France during the 1850s. By the mid-1880s, two-chambered septic systems were introduced in the United States. Today, from a quarter to one-third of all American households use a septic system.
Septic systems are usually composed of a watertight tank that has been built to receive and to hold toilet and kitchen sink material. Septic tanks are usually made of concrete, plastic, or fiberglass; tanks made from these materials can have a 50-year life span. Steel is not recommended because it rusts quickly and gives an operational life of 10 or perhaps 20 years. When a toilet is flushed into a septic tank, it receives the raw wastewater from a pipe that flows into the two-chambered watertight tank. The size of septic tanks varies depending upon local code requirements. However, a tank with a capacity of 1,500 liquid gallons is common for a family of four in a single dwelling.
Most septic system tanks have two semi-compartments or chambers. The two open chambers improve the tank’s solid removal efficiency. When the raw wastewater arrives in the first chamber of the septic tank, it separates the settleable materials and retains them. Any floatable solids suspend in the raw wastewater. The settleable solids settle to the bottom of the tank where they become part of a layer of sludge. A scum layer is formed from grease and other light materials. Eventually, both the scum and the organic solids in the sludge are liquefied by bacteria, which break them down into water-soluble fatty acids. In between the scum layer and the sludge layer is a soup of water-soluble materials. It contains urea (from urine) and household chemicals (soaps, detergents, creams, toothpaste). Baffles in the design of the tank help to prevent its solid materials flowing into the second chamber.
The operational component of the septic system is bacteria. The flushed wastewater in the first chamber contains microbes excreted from human digestive tracts. Human digestive tracts contain bacteria that participate in the digestive process. These bacteria do not require oxygen (anaerobic). Some of them are health hazards like Escherichia coli (E. coli) bacteria. Anaerobic and some aerobic bacteria in the buried tank convert the solid wastes into liquids that can then be allowed to filter through the ground. Gasses are formed in the liquefaction process, but these are vented through the building’s plumbing vent stack. These gases are usually methane and hydrogen sulfide, although other gases may be present. The second chamber receives its load next; it is usually semi-clarified wastewater. The finer materials in the wastewater in the second chamber then settle to the bottom. The liquefied waste leaches into a drain field where it is absorbed by the ground. Regular scheduled pumping of the tank prevents build up of sludge and extends the life of a system. When toilets do not flush or there is a backup in the system, there is a problem.
A low level of bacteria often causes septic system problems. Soaps, high organic material, bleach, or other household chemicals may kill bacteria. Simply flushing too much water into the system interferes with its smooth operation. The solution to most septic system problems is to restore the bacteria level so that the bacterial “eating” process can continue to break down proteins, starches, carbohydrates, animal and vegetable fat, oils, cellulose, and other materials. There are a wide variety of commercially prepared septic bacterial powers or liquids available that can be flushed or drained into a septic system. Most of the time this will restore the flow in the system, however, it may also degrade the leach field and shorten the life of the system.
The drain field must be far enough above the water table that septic system effluent does not leach into it. Also, layers of impermeable clay or other soils can interfere with the efficient work of the system if they are too close to the level of the tank. However, if the soil has a high percolation rate and no impermeable layer of soil exists, then the system may permit septic effluent to contaminate the water table. This is extremely important if water is drawn from a well that is in the general area of the septic system. Floods or severe drought can interfere with the normal and successful operation of the drain field. Other factors affecting leaching in the drain field are the pH of the soil, soil moisture, and its natural ecology of microbes that are competitors for effluent bacteria. Metals from pipes or from the ground may affect the operation and life of a septic system.
In recent years there has been a growing awareness that the rate of failure in septic systems (or “on site wastewater treatment facilities”) is far higher than had previously been estimated. The leakage of sewage into groundwater allows a range of inorganic and organic chemicals to migrate through aquifers to streams and riparian areas, causing extensive environmental damage and health risks for human populations. More management and more direct regulation throughout communities across the United States may be needed. The widespread distribution of these facilities, however, along with the difficulty of assessing their condition and the potential expenses for homeowners, make this a problem with little immediate prospect for resolution.
Bibliography:
- H. Swartz et al., “Steroid Estrogens, Nonylphenol Ethoxylate Metabolites, and Other Wastewater Contaminants in Groundwater Affected by a Residential Septic System on Cape Cod, MA,” Environmental Science & Technology (v.40/16, 2006);
- S. Environmental Protection Agency, Wastewater Treatment Systems Manual (EPA, 2002);
- D. Wickham et al., “Statewide Empirical Modeling of Bacterial Contamination of Surface Waters,” Journal of The American Water Resources Association (v.42/3, 2006);
- Dodge Woodson, Builder’s Guide to Wells and Septic Systems (McGraw-Hill Professional, 1997).