A septic tank is a tank allowing feces to decompose using the allready present microorganisms. Two types can be distinguished:

  • contained: the tank is waterproof (ie used in recreational vehicles) and thus all the flush water and feces remain in the tank; the tank is emptied every two weeks or so and the sludge is moved for processing off-site
  • connected to a drain field (most common approach): the flush water is let to pass into a the drain field and only the feces remains in the tank. The tank is not necessairly waterproof (ie can be punctured, in which case perforated lines are not necessairy), if it is waterproof, then the waterlines connected to it are attached on the bottom of the tank, allowing to pass the water trough unhindered

Construction

The basic design is a big concrete box, buried in the back yard, and connect to all all the house's toilets and sink drain lines (mounted on the front of it). A few hundred feet of perforated drain pipe are layed in shallow channels and the channels are filled with gravel. Cover everything over with topsoil and forget about it. If you really want to you can dump some special 'septic tank bacteria' in it once in a while, or have it pumped out once a year. Most people ignore it - at least until it stops working.

Then you have to pump it all out, possibly even dig up the whole system and replace the field pipe, the tank, the lines, or everything. It is not exactly high technology, and it requires a bit of room (you couldn't have 60 septic fields for sixty houses in a 20 acre subdivision). You can't put them in where the water table is too close to the surface, or too near a stream/pond. I wouldn't call it wasteful. Some would like to make use of the methane generated, or the heat, or the composted solids. Others are happy enough to see it not get dumped into the local ground water.

Applications

This technology is generally used where sewers have not been connected, as sewers are more reliable and present less public health problems.

In the developed world, this technology is mainly used where in remote areas; in poorer communities in the developing world, some form of septic tank may be the standard, even in high density settlements.

Design considerations

  • The outlet should be considered a source of contamination, even though it should be less dangerous and unpleasant than raw sewage. The possibility of leaks must also be considered. Thus the tank and its discharge should be kept a safe distance from any water source 10 m is adequate (Environment Agency Guidelines).
  • The application of a septic tank to a single house leads to relatively high maintenance, a high rate of problems, and greater cost. Sharing a septic tank between a number of houses (where those houses are close) may improve this, provided an effective approach to sharing the maintenance responsibility is found (e.g. sharing costs for a reliable contractor; or giving responsibility to the lowest level of government, if this design is commonly used in an area). However, the Environment Agency in the UK suggest that 15 persons is the maximum population that can be served by a septic tank system (PPG4 Guidelines).

Operation

  • DO NOT USE ANTISEPTICS! Antiseptics must NEVER be used in any toilet or drain leading to the septic tank, as they will kill the anaerobic bacteria, preventing decomposition, and leading to very unpleasant smells and an unsafe discharge.
  • Do not allow the sediment to build up too much, as this will greatly reduce efficiency, increasing the odor, and the level of contamination in the outlet. UK Building Regulations Section H2 recommend an annual emptying interval.
  • Do not allow any rainwater from guttering, yard drains, etc. to enter the septic system. This increases the hydraulic flow rate and flushes excess solids into the drainfield, which can be ruined as a result.
  • Do not use any strong drain cleaners, caustic soda or similar products to clean the drains. These will kill the beneficial anerobic bacteria in the septic tank.

Septic tanks in the developing world

In Indonesia, the term septic tank - tenki septik - is used to describe a very standard method of sewage disposal, which is more basic than the septic tank used in developed countries. It is simply a ring of concrete, in a hole, with an unsealed (i.e. earth) bottom. Thus raw sewage is in direct contact with the earth. Where groundwater comes near the surface (for example Surabaya in East Java) this contributes to serious water contamination problems.

In Indonesia, catfish are sometimes used in septic tanks, as they eat the sewage. See Category:Sanitation#Using catfish to eat sewage.

See also

Interwiki links

External links

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