Septic tank

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.

Description

A septic tank generally consists of a tank of between 1000 and 1500 gallons which is connected to the inlet wastewater pipe at one end and to a septic drain field at the other. These pipe connections are generally made via a T pipe which allows liquid entry and egress without disturbing any crust on the surface. Today the design of the tank usually incorporates two chambers, each of which is equipped with a manhole cover, which are separated by means of a dividing wall which has openings located about midway between the floor and roof of the tank. Wastewater enters the first chamber of the tank, allowing solids to settle and scum to float. The settled solids are anaerobically digested reducing the volume of solids. The liquid component flows through the dividing wall into the second chamber where further settlement takes place with the excess liquid then drains in a relatively clear condition from the outlet into the leach field, also referred to as a drain field, or seepage field, depending upon locality. The remaining impurities decompose in the soil and the water is eliminated through percolation into the soil, and eventually taken up through the root system of plants or added to the groundwater. A piping network, often constructed in a stone filled trench, distributes the wastewater throughout the field with multiple drainage holes in the network. The size of the leach field is proportional to the volume of wastewater and inversely proportional to the porosity of the drainage field. The entire septic system can operate by gravity alone, or where topographic considerations require, with inclusion of a lift pump.

An Imhoff tank is a two stage septic system where the sludge is digested in a separate tank. This avoids mixing digested sludge with incoming sewage. Also some septic tank designs have a second stage where the effluent from the anaerobic first stage is aerated, before it drains into the seepage field.

Waste that is not decomposed by the anaerobic digestion eventually has to be removed from the septic tank or else the septic tank fills up and undecomposed wastewater discharges directly to the drainage field. Not only is this bad for the environment, but if the sludge overflows the septic tank into the leach field, it may damage the leach field piping requiring expensive repairs. How often the septic tank has to be emptied depends on the volume of the tank relative to the input of solids, the amount of indigestible solids and the ambient temperature (as anaerobic digestion occurs more efficiently at higher temperatures). In general it is rare for a septic tank system to require emptying more than four times a year, and by careful management many users can reduce emptying to once per year or even less. When emptying a tank, a small residue of about 10% of the sludge should be allowed to remain in the tank to ensure that anaerobic decomposition is rapidly re-started when the tank re-fills.

A properly designed and normally operating septic system is odor free and besides periodic inspection and pumping of the septic tank should last for decades with no maintenance.

A well designed and maintained concrete, fiberglass or plastic tank should last about 50 years.

Potential problems

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 cleaning 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.

Environmental Issues

Some pollutants, especially sulfates, are reduced under the anaerobic conditions of septic tanks, in this case to smelly and toxic hydrogen sulfide. Likewise, nitrates and organic nitrogen compounds are reduced to ammonia. Because of the anaerobic conditions, fermentation processes take place, which generate ultimately carbon dioxide and methane, both of which are known greenhouse gases.

Because of the fermentation processes, the pH of the content of septic tanks keeps phosphate in soluble, i.e. mobilised form. The discharge of the content of a septic tank into the environment can trigger algal blooms, including blooms of potentially toxic cyanobacteria, because phosphate is often the limiting nutrient for alagal growth in many eco-systems.

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)-- and the common expression that is the same as the title-- 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.

"Septic tank" in slang

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.

References

Sewerage | Toilets | Waste | Waste management | Waste treatment technology

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Description

Potential problems

Environmental Issues

"Septic tank" in slang

See also

References