Designing A Septic Tank | Septic Tank Construction Methods

The septic tank is the most common small-scale decentralised treatment unit for grey water and blackwater from cistern or pour-flush toilets. It is basically a sedimentation tank. Its shape can be rectangular or cylindrical. Here we will explain the process of making the design for any septic tank.

Design of Septic Tank: The capability of the septic tank is particularly based on the number of peoples who use the tank in addition to the intermission of dirt extraction. typically, dirt needs to be discharged regularly after 2 years. The capability of a tank for storing liquid is taken as 130 liters to 70 liters according to person. For much less number of users, 130 ltr per person is sufficient.

To simplify the method for collection and removal of the dirt, the floor of septic tank is supported with a slope of 1:10 to 1:20 in the direction of the inlet side. It means that floor of the outlet side will be on the higher elevation compared to the floor at inlet facet.

Dimensions of Septic Tank Components
i) Length, Width and Depth of Septic Tank

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: in the procedure of designing the septic tank, detention period should have been 24hrs. The rate of flow of waste matter needs to be equal to the rate of flow of influent.

iii) Inlet and outlet pipes: An elbow or T pipe with 100mm diameter is submerged to a deepness of 250-600mm below the liquid level. For outlet pipe, an elbow or T kind of 100mm diameter pipe is submerged to a deepness of 200-500mm below the liquid level. Choose the pipes which made of stone ware or asbestos.

iv) Baffle walls of Septic Tank: For the tanks small in size, RCC hanging kind scum baffle walls are set in septic tanks. Baffle walls are arranged adjacent to the inlet. it’s far non-compulsory close to the outlet.

The inlet baffle wall is set at a gapping of L/5 from the wall, wherein L denotes the length of the wall. The baffle wall is usually expanded 150mm above to scum level and 400-700mm underneath it.

Since scum is lighter, it commonly floats on the water level within the tank. The density of the wall fluctuates from 50mm to 100mm. Inside the lower part of larger tanks, there are holes for discharging sludge.

v) Roofing Slab of Septic Tank: Primarily based on the size of the tank, RCC slab with density of 75–100 mm is placed at the top of the septic tank. Round manholes with 500mm clear diameter are organized for inspection and desludging. For square opening, clear size is maintained as 600X450mm.

vi) Ventilation Pipe: The cast iron or asbestos pipe having 50-100mm diameter is set up as outlet to discharge polluted gases and for ventilation. The pipes should be expanded 2m (min) over ground level. On the upper part of the ventilation pipe a mosquito proof wire mess or cowl should be placed.

Design of Septic Tank for 20 Users

Liquid capacity of the tank:

@120lts per user = 0.12 X 20 =2.4cum

Take liquid depth as 1.3meter.

Therefore Floor area of the tank = 24/1.3 = 1.85m2

Taking length as 2.5times the breath

L X B=1.85

2.5B X B = 1.85

B = Sqrt(1.85/2.5) = 0.86 say 0.9m

Therefore, the dimension of the tank is 22.5 X 0.9m

Simple, yet effective, septic tanks are part of the solution for our growing concerns about a cleaner environment. Handling wastewater onsite in small quantities is a key to better management of tax revenues for other areas.

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