Septic Tanks - What are they?

Before we talk about septic tanks, it is worth knowing why we need a septic tank. As we all know, the septic tank receives what we all do not want; that is sewage. Although we don't need sewage, if the matter is left unattended or not given the due priority that it deserves, the mayhem that can cause may keep you worried until you dispose it safely.

Often the disposal of sewage is a real challenge to the house builders and all of us have to face the problem of building of our own house sooner or later. The building of houses keeping provision for safe disposal of sewage is a very important aspect in building. It is human nature that we tend to think more about what happens first than what happens last.

Sewage disposal is often given the least priority and in most of the cases we want to do something for it and forget about it. However if the thing is not done at once in a way it demands that can make you and your environment polluted. Therefore it is extremely useful to think about disposal of sewage in an environmentally friendly manner in either new house building and or maintenance of the existing houses. This will keep you relaxed and saves lot of money in maintaining the services later on.


Sewage is wastewater of a community. It may be purely domestic origin or it may contain some industrial, commercial or agricultural waste water as well.

Wastewater = Domestic sewage + industrial/ commercial/ institutional sewage

Domestic Sewage

Domestic sewage is the collection of human body waste (faeces + urine [Black water]) and wastewater resulting from personal washing, laundry, food preparation and cleaning of kitchen utensils (Grey water).

Contrary to the domestic sewage, industrial and commercial sewage can contain harmful substances to the human health both in short term and in long term. Heavy metals from factories and radioactive substances from hospitals can be cited as examples.

Why Treat Sewage?

Treatment of sewage is essential,

  1. To prevent the pollution of;
    • Ground water
    • Surface water
    • Marine environment
    • Flora & fauna
    • General environment
  2. To reduce the spread of communicable diseases caused by the pathogenic organisms in the sewage such as Cholera, typhoid, diarrhea, intestinal worms etc.

The recent out break of Dengue, Malaria, and Japanese Encephalitis etc are examples.

Method of Treatment

The method of treatment depends on quality of sewage to be treated. Domestic sewage is the simplest form of sewage and that contribute to the largest volume in a municipal sewerage scheme. Sri Lankan households are not connected to public sewer systems in most of the areas due to unavailability of such service and therefore we are compelled to treat the sewage at the point of generation.

Design Aspects

Generally 95% of the water used in a premises end up in sewer. The following table gives an indication how much sewage is generated by a small family

No of inmates Average Unit discharge Litres/ person Total volume in litres
2 160 320
4 160 640
6 160 960
8 160 1,320

Usually the effective volume of a septic tank shall be three times the daily inflow and as such a family of 6 shall have a tank of nearly 3 m3 capacity - A septic tank shall be completely water tight and shall have ventilation pipes extended beyond the roof level of the building. The tank shall be a structurally sound to withstand the internal and external forces and shall have access for maintenance and cleaning purposes. Municipal laws prohibit the discharge of wastewater in to the open drains.

Soakage Pits

Soakage pits are used to soak septic tank effluent in to the surrounding soil. They do not provide any direct treatment and are based on the principle that the effluent gets treated as it passes through the surrounding soil before entering the ground water table or other water body. Therefore following shall be noted in building and using soakage pits. Soakage pits shall be

  • at least 18 m away from a well or other drinking water source
  • at least 5 m from the nearest building
  • at least 10 - 20 m from any other soakage pit
  • at least 1.5 m shall be kept between the bottom of the tank to the seasonal ground water table
  • Adequate contact area with the surrounding soil to absorb the effluent in to the soil. In clayey soil, larger pits will be needed.
  • Adequate openings shall be left in the walls of the pit to have the contact with the surrounding soil

If a soakage pit cannot be constructed due to above reasons there are following measures in place of a soakage pit

  • Seepage beds
  • Seepage trenches
  • Anaerobic bio filters


The NWSDB has a model of a septic tank and a soakage pit on display and following diagram is illustrative of the model

septic tank