Septic tank
| A septic tank partially installed in the ground |
| | The same tank before installation |
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A
septic tank also known as a
septic system is a small scale
sewage treatment system common in areas with no connection to main sewerage pipes. In
North America approximately 25% of the population relies on septic tanks; this can include suburbs and small towns as well as rural areas. In Europe they are generally limited to rural areas only.
The term "septic" refers to the
anaerobic bacterial environment that develops in the tank and which decomposes or mineralises the waste discharged into the tank. The alterative to a septic tank is to use an aeration system.
#Excessive dumping of cooking oils and grease can fill up the upper portion of the septic tank and can cause the inlet drains to block. Oils and grease are often difficult to degrade and can cause odour problems and difficulties with the period emptying.#Flushing non-biodegradable hygiene products will rapidly fill or clog a septic tank and these materials should not be disposed of in this way.#The use of waste macerators or grinders for disposal of waste food will cause a rapid overload of the system and early failure.#Certain chemicals may damage the working of a septic tank, especially pesticides, herbicides, materials with high concentrations of bleach or caustic soda (lye) or any other inorganic materials such as paints, solvents etc. #Roots from trees and shrubbery growing above the tank or the drain field may clog and or rupture them. #Playgrounds and storage buildings may cause damage to a tank and the drainage field. In addition, covering the drainage field with an impervious surface or hard-standing will seriously affect its efficiency.
Some pollutants, especially
sulfates, under the anaerobic conditions of septic tanks, are reduced to
hydrogen sulfide, a
pungent and
toxic gas. Likewise,
nitrates and
organic nitrogen compounds are reduced to
ammonia. Because of the anaerobic conditions,
fermentation processes take place, which ultimately generate
carbon dioxide and
methane, both of which are known greenhouse gases.
Because of the fermentation processes the contents of a septic tank are anoxic with a low redox potential which keeps
phosphate in a soluble and thus mobilised form. Because phosphate can be the limiting nutrient for plant growth in many eco-systems, the discharge from a septic tank into the environment can trigger prolific plant growth including algal blooms which can also include blooms of potentially toxic
cyanobacteria,
Soil capacity to retain phosphorus is large compared with the load through a normal residential septic tank. An exception occurs when septic drain fields are located in sandy or coarser soils on property adjoining a water body. Because of limited particle surface area, these soils can become saturated with phosphate. Phosphate will progress beyond the treatment area, posing a threat of eutrophication to surface waters.
Many pathogenic bacteria can survive in septic tanks for a very long time, owing to the anaerobic conditions. Depending on soil conditions, pathogenic bacteria can leach into groundwater and surface waters. In North America, this is the case with many lakeshore communities.
In areas with high population density,
groundwater pollution levels often exceed acceptable limits. Some small towns are facing the costs of building a very expensive centralized wastewater treatment system because of this problem, owing to the high cost of extended collection systems. Too often, the efficient and economical alternative of a properly designed de-centralised wastewater treatment plant is not considered.
To slow the pollution, building moratoriums and limits on the splitting of property are often imposed. Ensuring existing septic tanks are functioning properly is also helpful for a limited time, but it is not the solution. Once polluted, groundwater is very slow to clean - thus urgent action is appropriate.
Erma Bombeck's book
The Grass is Always Greener Over the Septic Tank (ISBN 0070064504) is technically incorrect. The grass is greener over the
leach field, which is better watered and has more nutrients than the surrounding land. However, it is not unusual to find better growth over the septic tank itself as well, particularly the end nearest the leach field.
In
Cockney Rhyming Slang (common in Britain and
Australia) "septic tank" or, more commonly, simply "septic" or "
seppo" is used as a
pejorative word for an American, based on a rhyme for "
Yank".
The term
Honey wagon is a euphemism originally referring to a horse drawn wagon carrying a tank of liquid manure or sewage.
Today a tanker truck equipped with a
sludge pump to empty septic and conservancy tanks is also euphemistically known as a
Honey sucker.
*
Backwater*
Bioreactor*
Cesspit*
Latrine*
Outhouse *
Perc test*
Pit toilet*
Sewage treatment*
Sewer *
Waste disposal*
EPA on septic tanks
