What is a Septic tank?
A septic tank is a tank where sewage is stored before being sent to a treatment plant, where solids settle out and bacteria convert waste into a liquid that can then be safely released. In undeveloped areas, septic tanks are used to dispose of night soil. It’s best to have enough water available for flow of the night soil from the latrines to the septic tank, and for the proper function of the septic tank.
The size of a septic tank is engineered so that the sewage is contained in the tank for twenty-four hours. This allows for biological decomposition of anaerobic bacteria which liquefies and breaks down the night soil, leaving behind a small quantity of soil called sludge. This settles at the bottom of the tank. A clear liquid called effluent then flows out of the tank, which is disposed through absorption in the soil soak pit if there is no municipal drainage system in the area.
If there is a municipal drainage line in that area, the effluent will be discharged into the drain. Disinfecting agents like bleaching powder or phenyl should never be used in cleaning latrines as they can kill the bacteria growth in the septic tank as a result of the biological decomposition which is disrupted.
Septic Tank Design
The capacity of septic tank is relevant to the number of people using it as well as interval of sludge removal. Generally sludge is removed every two years. The capacity of a septic tank is between 130 liters – 70 liters per head. For example, if it’s a small number of users, 130 liters per head is acceptable. A septic tank is usually surrounded by a brick wall in which cement mortar, about 20cm thick.
Both the interior and exterior of the wall and the top of the floor is plastered with at least 12mm thick cement. The inside corners of septic tank are rounded. Water-proofing agents like Impermo, Cem-seal or Accoproof are added to the mortar at 2% of the cement weight.
Water-proofing agent is added in the same proportion to the concrete for the floor of the tank. To maximize efficiency of sludge collection and removal, the floor of septic tank is constructed with a slope of 1:10 – 1:20 towards the inlet side. This makes the floor of the outlet side on the higher elevation compared to the floor at the inlet side.
Septic Tank Components
Inlet and outlet pipes
An elbow or T pipe is submerged below the liquid level. These pipes are made of stone ware or asbestos.
Baffle Walls of Septic Tank
RCC hanging type scum baffle walls are built in to septic tanks near the inlet.
Roofing Slab of Septic Tank
The top of the septic tank has an RCC slab.
For the release of foul gases and ventilation, cast iron or asbestos pipes with mosquito proof wire mess or cowl are at the top of the tank.
And there you have it! that’s how septic tanks are designed.