Septic Tanks

Looking after a septic tank requires careful attention, plus knowledge of the basic principles.

Most households in Britain produce huge quantities of wastewater. This comes not only from the lavatories, but also from baths, showers and basins, kitchen sinks, dishwashers and washing machines. For most people, their wastewater simply goes down the drains and into the underground sewage network. But many people in rural areas live outside the reach of any sewage system. They have to deal with the sewage themselves, and the most common solution is a septic tank.


Septic tanks are a comparatively recent innovation: the first was developed in France in the 1880s. The process essentially involves the settling of wastewater in a tank, to separate it from solid matter. Relatively clear wastewater is then leached into the ground by means of porous pipes and a soakaway. The wastewater in a septic tank is primarily organic material: bacterial activity helps to break down the solids in the tank, reducing its volume, and over time bacteria also renders harmless the waste water in the soakaway. (The word 'septic' derives from the Greek septos, meaning decayed.) Septic tanks have to be carefully managed by the householder, but no matter how carefully, at some point the solids will build up, and - once year or perhaps once every two years - the solids will have to be pumped out by a contractor and the tank cleaned.

The tank

In the past the tanks were made of brick and rectangular, but these days they tend to be round, prefabricated and made of heavy duty plastic or fibreglass. Pipes, laid at a shallow gradient, lead down from the house, feeding the wastewater (the influent) into the tank. Some solids will float (notably oils from soaps, and fats from kitchen washing-up) forming an oily crust at the surface; this is called scum. Some solids will sink to the bottom; this is called sludge. In between these two layers of solids is the 'clear water' - in fact not so clear at all, but clouded with suspended solids. The tank may have a secondary compartment behind a slotted screen that keeps out the scum and sludge but lets pass the clear water. This means that when new influent comes into the tank, it displaces only clear water to the next stage, not solids. Alternatively, this function may be served by a secondary tank, connected to the first with a pipe. The influent should remain in the tank for at least 24 hours, if not three days, before heading to the soakaway.

The soakaway

The clear water feeds into the soakaway (also known as the leach field, or drain field). One pipe, or a set of pipes (often in a herring-bone pattern), made of a porous or perforated material, leads out at a shallow angle into an underground 'percolation trench', typically filled with gravel or stone chippings. From here the effluent will simply soak away into the soil. The length of the pipes will depend on the amount of wastewater they have to deal with, and the absorbency of the surrounding soil (which has to be tested carefully prior to construction).


The size of the tank and soakaway has to be carefully judged according to the number of people using the house. There are formulae for estimating this. If the system is too small, the soakaway will be unable to cope, and wastewater will back up. Solids are then likely to clog up the pipes and the soakaway, debilitating the entire system; when a septic tank goes wrong, it stinks.

Cleaning and clearing

The scum and sludge build up over time, and have to be periodically cleared out; otherwise they will enter and clog up the pipes and soakaway, which can be costly to repair. The task of clearing out the solids, or 'desludging', is usually carried by a contractor who comes with a tanker (known euphemistically as a 'honey wagon'), and attaches pipes to the tank to remove the sludge and scum by suction. This is also an opportunity to clean the tank and the pipes.

Dos and don'ts

  • Septic tanks have to be regularly monitored to check that the solids are not building up too much.
  • The bacteria in the system are vital to its function. Detergents, bleach and other chemical cleaning fluids are harmful to them, and their use in the house should be kept to a bare minimum (there are similar products that are bacteria-friendly).
  • For the same reasons, minimise also the amount of kitchen fat and grease that is poured down the sink.
  • Effluent coming out of a septic tank system must never be discharged into a watercourse, (unless sanctioned by Environment Agency). To do so is to commit a serious crime.
  • Rainwater goes into sewage systems in cities, but should not go into a septic tank, where it can overwhelm the system.
  • Do not consider installing a septic tank in an area that has a high water table or is prone to flooding.


Septic tank construction in the UK is governed by the British Standards Code of Practice for the Design of Small Sewage Treatment Works and Cesspits, BS 6297: 1983, and subsequent building regulations.

What is a cesspit?

A cesspit (or cesspool) is simply a large sealed holding tank for sewage. It does not treat the sewage; rather, the cesspit simply fills up, and has to be emptied on a regular basis by a contractor (such as once every six weeks).

Comments on this article

'Segun Alawode 16 January, 2013

Thank you for the lectures.I am a fresh graduate of Architecture who reside in Nigeria.My question is,can you simply go a bit.more practical!!!By analyzing all the theories in diagrams for better understanding.
Kind regards,
'Segun Alawode.

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