A septic tank can be used to treat the sewage from individual buildings at the building itself or for the whole community, at the lagoon. The sewage will pass through sewer pipes to the septic tank either at the house or at the lagoon.

The septic tank is a sealed round or rectangular container which is used to break down the sewage so that it becomes effluent through the action of bacteria living on the waste matter.

9.1 Septic tank design

A household septic tank usually consists of two round concrete tanks with lids placed close to each other. They are connected by a pipe. This type of septic tank is designed to be used by up to 10 people. Round tanks are constructed (built) at a factory and transported to the site (place) where they are to be used.
Fig.  2.34: A round septic tank system
Fig. 2.34: A round septic tank system.

A septic tank can also be a single rectangular concrete tank with a dividing wall in it. A rectangular septic tank is designed to be used by more than 10 people and is often used for sewage treatment at a lagoon. The tank is constructed on the site where it is to be used.
Fig.  2.35: A rectangular septic tank system.
Fig. 2.35: A rectangular septic tank system.

Septic tanks are always divided into two sections, the first being twice the size of the second. In round septic tanks, the separation into two tanks provides this division. In rectangular tanks the dividing wall provides the division. This wall will have a hole in it below the level of the sewage to allow effluent to pass from the first to the second section.

Round septic tanks have concrete bottoms and lids. Rectangular tanks usually have concrete bottoms and lids, but some may have metal lids. The lids can be lifted off for maintenance and will have IOs in them.

There are many regulations (rules) which require septic tanks to be constructed, positioned and installed in a particular way. These rules are controlled by local authorities.

It is very important to find out if the regulations are being followed by contractors or anyone else installing (putting in place) new septic tanks in the community. It is a good idea to contact the local EHO to check that the necessary approval has been given to construct and/or install the septic tank disposal system.

If anyone wants to know anything about septic tanks, including the rules relating to their construction, or there are any problems with these tanks in the community, contact the EHO or Environmental Health Practitioner.

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9.2 How a septic tank works

A septic tank must be filled with water before it is used. The water helps start the treatment of the sewage by the bacteria.
The sewage treatment by the bacteria turns the waste matter into effluent (wastewater) and a solid substance called sludge. The effluent gets carried to the leach drain, French drain or lagoon.

The material in the septic tank gets covered by a hard crust known as a scum blanket. This blanket acts as an air seal keeping air away from the sewage. The lack of air helps in the breakdown of the sewage by the bacteria.

The sludge gathers at the bottom of the tanks. Eventually there will be too much sludge in the tank and it must be pumped out and the sludge disposed of correctly.

By having two tanks or a rectangular tank divided into two sections, most of the sludge stays in the first tank or section. In the second tank or section, the sewage undergoes further treatment to remove solid matter.

The effluent is then piped to the effluent disposal system, such as the lagoon.

This water still contains germs and parasites.


The septic tank will need to be checked if there are signs that it is not working properly.

Some signs that a septic tank is not working properly are:
    • The sewage in the toilet or the liquid waste from other fixtures flows away very slowly
    • Liquid waste overflows from the disconnector trap
    • Wet areas are seen at the top of the septic tank
    • There is a strong unpleasant smell near the septic tank
    • The grass around the tank is very green and growing well
In the case of on-site disposal systems, it is important to remember that some of these signs may indicate problems with the leach or French drain. Therefore, these drains will need to be checked at the same time as the septic tanks are checked.

If the septic tank and the leach or French drain need to be pumped out, both should be done at the same time.

9.4 Pumping out septic tanks

Septic tanks should be pumped out every five years to keep the disposal system working properly. However, this may need to be done more often, for example, if they overflow or become blocked.

If there are any signs of a problem with the septic tank (see Section 9.3), it will need to be checked.
The inside parts of the tank system which will need to be checked are:
  • the scum blanket (as it may become too thick and block the inlet pipe)
  • the inlet or outlet pipes (as they may be blocked by solid matter)
  • the sludge (as it may have accumulated so that it fills most of the tank)
  • the tank's bottom, sides or lids (as one or more of these may have been cracked or broken. For example, vehicle movements over septic tanks are likely to damage the lids and sides.)
For the first few times an EHP pumps out a septic tank, it is important to always check with the local EHO or Environmental Health supervisor before any pump-out work is commenced.

These people will provide information on disposal sites and the correct pump-out methods as well as technical help in assessing the inside parts of the septic tank.

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Emptying a septic tank

Before commencing Pump-out

  1. Find out if the community has or can obtain a pump-out tank or tanker. Make sure the sludge pump (pump-out equipment) is available and working.
  2. Locate an appropriate disposal site.
    If a tank or tanker is used, the disposal site must be a place which is suitable for getting rid of the dangerous sludge and effluent and be able to take all the pumped out materials. For example, the site must be well away from water supplies, children's play areas, camp places, rivers and streams, and downwind if possible. Often this place will be a hole dug in a separate part of the community rubbish tip.
    If there is no tank or tanker available, the pumped-out material must be disposed of in a hole near the septic tank.
    The distance between the septic tanks and the disposal hole will depend upon the length of the pump-out hose. The hole must be away from water supplies.
  3. Where possible, remove any tins, bottles, rags, newspaper and other rubbish that may be in the septic tank. This material can either be disposed of in the pump-out hole at the site or at the rubbish tip.
    All sewage material which is to be taken to the tip should be transported in sealed drums.
Fig.  2.36: Removing solid materials.
Fig. 2.36: Removing solid materials.

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When pumping out the septic tank:

  1. Pump out the sludge into the tanker or the hole.
    If using a tanker the sludge can be deposited at the appropriate site away from the community.
    If the disposal site is near the septic tank and the pump-out has commenced, the hole must be guarded at all times even if the pump-out stops for some reason, for example, for a lunch break or because of an equipment breakdown. The sewage must be covered with soil if the pump-out is not finished by the end of the day.
    After covering the sludge with this layer of soil there may still be space in the hole to complete the pump-out the next day.
    When covering the sludge with soil, remember that some time must allowed for the liquid to soak away before putting soil in the hole.
  2. When the job is finished the hole should be filled with a thick layer of soil.
  3. Once the septic tank has been completely emptied, it must be filled with water before it is used again.
Fig.  2.37: Pumping out a septic tank
Fig. 2.37: Pumping out a septic tank.

Note: When pumping out a septic tank using this method, great care must be taken and it is suggested that a local EHO should be consulted.