In rural areas along with ponds and lagoons Septic tanks are introduced as a lo cost sanitation.
A septic tank may be defined as a primary sedimentation tank with a bit longer detention period of about 12 to 36 hours and with provision for digestion of settled sludge. As digestion of settled sludge is carried out by anaerobic decomposition the septic tank is classified as an anaerobic decomposer. And due to anaerobic decomposition of the settled sludge will produce foul gases with a very bad smell and as septic tanks are completely closed there is a need to provide a high vent for the removal of gases.
What is the detention period of septic tank? The detention period of the septic tank generally varies between 12 to 36 hours and for design purposes commonly taken as 24 hours.
A septic tank, a horizontal continuous flow type of sedimentation tank, directly admitting raw sewage, and removing about 60 to 70% of the dissolved matter from it.
The sludge settles at the bottom and the oil and greasy matter float on the surface of the sewage as scum. Both the sludge and scum remain in the septic tank for a period of several months during that period both are decomposed by anaerobic bacteria.
The digested sludge from tank is needs to be removed over a period of 6 to 12 months and must be dried completely before complete disposal at disposal facility.
The liquid effluent from the septic tank is disposed of in Soakpit or in dispersion trenches. Soakpit and dispersion trenches should be sufficiently permeable to easily percolate the effluent in the ground.
Small septic tanks are designed on the basis of average flow and for detention period of 24 hours but for large sized tanks detention period of 12 hours is adopted.
The flow of sewage should be taken as 40 to 70 liters/capita/day, and when sludge is also discharged into the septic tank, flow is taken as 90 to 150 liters/capita/day. The rate of accumulation of sludge is 30 liters/capita/day.
About 70 grams per capita suspended solids enter the septic tank per day. Out of the 60% are removed in the fresh sludge. 70% of that removed solids are volatile suspended matter.
The expected volume of fresh sludge is about 0.00083 square meter per capita per day.
The Septic tank also should have a freeboard of about 30cm which includes scum above the liquid surface. And about 20 to 25 mm should be left for digested sludge for the purpose of seeding.
The Septic tank is rectangular in shape having its length of 2 times or more than 2 times greater than its width and width is having a minimum dimension of 750mm.
For small sized tank the total liquid depth of 120cm is provided while in case of large sized tank liquid depth up to 180cm provided. A free board of about 30 to 45cm is provided, above the level of the liquid for the purpose of fixing of pipes.
An elbow pipe submerged to a depth of about 15 to 25 cm below the level of liquid is provided as the inlet pipe. A single elbow outlet pipe is provided to a depth of at least 15 cm below level of the liquid.
A RCC slab with CI manhole covers are provided to the tank. For tanking out the gases and foul smell a ventilation pipe of 7.5 to 10cm diameter is provided.
| Number of Peoples | Length L in m | Width B in m | Liquid Depth D in m (min) | Liquid Capacity to be Provided in m3 | Free Board in m (min) | Sludge to be Removed in m3 | Interval of Cleaning in months |
|---|---|---|---|---|---|---|---|
| 5 | 15 | 0.75 | 1.00 1.00 1.05 | 1.12 1.12 1.05 | 0.3 0.3 0.3 | 0.18 0.36 0.72 | 6 12 24 |
| 10 | 2 | 0.9 | 1.00 1.00 1.40 | 1.80 1.80 2.52 | 0.3 0.3 0.3 | 0.36 0.72 1.44 | 6 12 24 |
| 15 | 2 | 0.9 | 1.00 1.13 1.18 | 1.80 2.34 3.60 | 0.3 0.3 0.3 | 0.54 1.08 2.16 | 6 12 24 |
| 20 | 2.3 | 1.1 | 1.00 1.30 1.80 | 2.53 3.30 4.55 | 0.3 0.3 0.3 | 0.72 1.44 2.88 | 6 12 24 |
| 50 | 4 | 1.4 | 1.00 1.30 2.00 | 5.60 7.28 11.20 | 0.3 0.3 0.3 | 1.80 3.60 7.20 | 6 12 24 |
Design of Septic Tank for 150 Peoples. Water allowance is 120 liters per capita per day.
Total Peoples= 150
Water allowance= 120 liters/capita/day
The quantity of water supplied = Per Capita Rate x Population
= 120 x 150 = 18000 l/day
Assuming that 80% of water supplied becomes sewage
Now Total flow of Sewage = Total quantity of water supplied x 80%
= 18000 x 80% = 14400 lit/day
Assuming Detention Period = 24 hours
Tank Capacity = (14400/24)liter x 24 hours = 14400 liters.
Now assume that the tank is to be cleared every year. then the sludge storage capacity at the rate of 30 liters per person per year, we have
Volume of sludge deposited= 30 x 150 x 1 = 4500 liters
Therefore,
Total required capacity of tank = Capacity for Sewer + Capacity for Sludge
= 14400 + 4500 = 18900 liters
= 18.9 cumec.
We’ve got volume of tank = 18.9 cumec
Assuming depth of tank as 1.5m, we have i.e overall depth = 1.5 + 0.3(free board) = 1.8
Surface area of tank = (18.9/1.5) = 12.6m2
If the ratio of the length to width is kept as 3:1, we have
3B2 = 12.6
B = 2.05m = 2.1m
Provide width 2.1m and provide length of tank= 6m
Area of cross section provided = 6 x 2.1 = 12.6m3
Hence, septic tank dimensions will be 6m x 2.1 x 1.8m
Take the same question, assume we’ve to design a septic tank for 150 peoples with a water supply of 120 liters per person.
Take 80% efficiency for sewage conversion
Volume of tank = (120/1000)cumec x 150 = 18 cumec
Take liquid depth as 1.5m i.e. Overall depth = 1.5 + 0.3 = 1.8m
Now floor area of tank = 18 / 1.5 = 12 m2
Assume length as 3 times as that of width
So, Area= L x B = 3B x B = 3B2 = 12
Therefore, B = 2m
And L = 3B = 3 x 2 = 6m
Therefore Size of Tank is 6m x 2m x 1.8m
Septic tank is basically a primary sedimentation tank with bit longer detention period of about 12 to 36 hours.
We’ve designed Septic by both theoretical and practical methods and it can be seen that there is not much difference in-between results.
A septic tank may be defined as a primary sedimentation tank with a bit longer detention period of about 12 to 36 hours and with provision for digestion of settled sludge.
Here are Table Dimensions of Septic Tank as per Number of Persons
The Septic tank is rectangular in shape having its length of 2 times or more than 2 times greater than its width and width is having a minimum dimension of 750mm.
For small sized tank the total liquid depth of 120cm is provided while in case of large sized tank liquid depth up to 180cm provided. A free board of about 30 to 45cm is provided, above the level of the liquid for the purpose of fixing of pipes.
About 70 grams per capita suspended solids enter the septic tank per day. Out of the 60% are removed in the fresh sludge. 70% of that removed solids are volatile suspended matter.
Septic tank minimum size is 750mm width and 1500mm length(2xwidth) and height about 1200mm. Standard minimum capacity of septic tank is 1cubic meter.
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