THE SEPTIC TANK
The septic tank is used to treat the sewage from individual buildings. The sewage will pass through sewer pipes from toilets to the septic tank at the house.
The septic tank is a sealed round or rectangular container that is used to break down the sewage so that it becomes effluent through the action of bacteria living on the waste matter.
A household septic tank usually consists single rectangular concrete tank with a dividing wall in it. The septic tank is constructed close to the house where it is going to be used.
Septic tanks are always divided into two sections. Normally, the first is approximately twice the size of the second tank. In rectangular tanks, there is a brick wall constructed crosswise to provide the division. An opening is provided in the dividing wall below the level of the sewage level to allow effluent to pass from the first to the second section.
Rectangular tanks usually have concrete bottoms slabs and lids, but some may have metal lids. The lids can be lifted off during de-sludging or maintenance.
How a septic tank works
A septic tank must be filled with water before it is used. It helps to start the treatment of the sewage by the bacteria. The sewage treated by the bacteria turns the waste matter into effluent (wastewater) and a solid substance called sludge. The top surface of the material in the septic tank gets covered by a hard crust known as a scum blanket. This blanket acts as an air sealkeeping oxygen away from the sewage. The lack of oxygen helps in the breakdown of the sewage by the bacteria.
The sludge collects at the bottom of the tanks. Ultimately, there will be too much sludge in the tank and it must be pumped out or removed, and disposed of properly.
The advantage of having two tanks in a rectangular tank is most of the sludge stays in the first tank or section during the operation. In the second tank or section, the sewage undergoes further treatment to remove solid matter. The effluent then flows to the soakage pit. However, this effluent still contains a considerable amount of germs and parasites.
Soakage Pits
Soakage pits are used to soak septic tank effluent into 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 groundwater table or other water bodies.
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 groundwater table,
- adequate contact area with the surrounding soil to absorb the effluent into the soil. In clayey soil, larger pits will be needed.
- Adequate openings shall be left in the walls of the pit to have contact with the surrounding soil.
