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SEPTIC TANK: STEPS TO DESIGN
The design of septic tank in our homes and office are important because it leads to good health.
Septic tank is a chamber made of concrete, fiberglass, PVC or plastic, through which domestic wastewater (sewage) flows for primary treatment. Septic is the anaerobic bacterial environment that develops in the tank which decomposes or mineralizes the waste discharged into the tank. The tanks has to be design to retain sewage for 24hrs. During which certain biological decomposition by the action of anaerobic bacteria takes place.
Design Process
The capability of septic tank is mainly based on the number of peoples who use the tank and interval of sludge removal. The sludge has to be remove every 2 years but it depends on the users. The tank is build with concrete blocks that contains cement mortar and the foundation floor made of cement concrete in 1:2:4 ratio.
When collecting and eliminating dirt, the floor of septic tank is supported with a slope of 1:10 to 1:20 in the direction of the inlet side. This means that the floor of the outlet side will be on the higher elevation than the floor at inlet side.
STEPS
i) Length, Width and Depth of Septic Tank
The various items should be know before embarking on the project.
Width = 750mm(min)
Length = 2 to 4 times width
Depth = 1000 to 1300mm. (min below water level) + 300 to 450mm free board
Maximum depth = 1800mm + 450 mm free board
Capacity = 1 cubic meter (10 cubic feet) minimum
ii) Detention period
Detention period of 24hrs (mostly) considered in septic tank design. The rate of flow of effluent must be equal to the rate of flow of influent.
iii) Inlet and outlet pipes
Inlet pipes use an elbow or T pipe of 100 mm diameter for a depth of 250-600mm below the liquid level. While the outlet pipe use an an elbow or T type of 100 mm diameter for a depth of 250-500 mm below the liquid level. Pipes may be of stone ware or asbestos and are submerged.
iv) Baffle Walls of Septic Tank
For small tanks, RCC hanging type scum baffle walls are provided in septic tanks. Baffle walls are provided near the inlet. It is optional near the outlet.
A distance of L/5 is placed for the inlet baffle wall, where L is the length of the wall. The baffle wall will extended 150mm above to scum level and 400-700mm below it.
Scum being light, generally floats at the water level in the tank. Thickness of the wall varies from 50mm to 100mm. The large tanks lower portion are having holes for flow of sludge.
v) Roofing Slab of Septic Tank
RCC slab of thickness of 75-100mm is use to cover the top of the septic tank. The RCC slab thickness should depend on the size of the tank. Circular manholes of 500mm clear diameter are provided for inspection and desludging. Rectangular opening has a clear size of 600 by 450mm.
vi) Ventilation Pipe
For outlet of foul gases and ventilation purpose cast iron or asbestos pipe of 50-100mm diameter is provided. It should extend 2m (min) above ground level. Top of the ventilation pipe is provided with a mosquito proof wire mess or cowl.
Suitable Sizes of Septic Tank Based on Number of Persons
| No of users | Length L in m | Breath B in m | Liquid Depth in m | Liquid Capacity to be provided in m3 | Free Board in m | Sludge to be removed(m3) | Interval Cleaning |
| 5 | 1.5 | 0.75 | 1.01.05 | 1.121.18 | 0.30.3 | 0.360.72 | 1 year2 year |
| 10 | 2.0 | 0.9 | 1.01.4 | 1.82.0 | 0.30.3 | 0.721.44 | 1 year2 year |
| 15 | 2.0 | 0.9 | 1.32.0 | 2.33.6 | 0.30.3 | 1.082.16 | 1 year2 year |
| 20 | 2.3 | 1.1 | 1.31.8 | 3.34.6 | 0.30.3 | 1.442.88 | 1 year2 year |
| 50 | 4.0 | 1.4 | 1.32.0 | 7.311.2 | 0.30.3 | 3.607.20 | 1 year2 year |
For more information, go through the following article on GambetaNews.
It’s good that you mentioned that the thickness of RCC slabs covering your septic tanks are designed depending on how large your tank is. This is something that my grandpa would need to keep in mind when he gets his mountainside cottage’s septic tank designed. He’d want the slab to be thick enough to prevent backups and odors from escaping, but not too thick for the septic tank cleaners to find it hard to open. I didn’t know that the septic tank slab cover is different from the manhole cover, which is used for desludging purposes.