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Septic Tank Design, Process, Detention Period | With Example

In rural areas along with ponds and lagoons Septic tanks are introduced as a lo cost sanitation.

Define Septic Tank

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.

Detention Period of Septic Tank

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.

How Septic tank Works?

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.

Septic Tank Design

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.

Septic Tank Design Calculations

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.

  • Width of tank= Given in the Table No 1 = minimum 0.75m
  • Hight of tank= 1.2 to 1.8m (without free board)
  • Free board= from 0.3 to 0.45m, should not be less than 0.3m
  • Total Depth= height + free board
  • volume of tank= (Sewage flow x Detention Time) + (Sludge accumulation rate x cleaning period)= minimum 1cubic meter.
Number of PeoplesLength L in mWidth B in mLiquid Depth D in m (min)Liquid Capacity to be Provided in m3Free Board in m (min)Sludge to be Removed in m3Interval of Cleaning in months
5150.751.00
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
1020.91.00
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
1520.91.00
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
202.31.11.00
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
5041.41.00
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
Table No 1: Dimensions of Septic Tank as per Number of Persons

Septic Tank Design Example:

Academic Question:

Design of Septic Tank for 150 Peoples. Water allowance is 120 liters per capita per day.

Given Data:

Total Peoples= 150

Water allowance= 120 liters/capita/day

Step1: Calculation of Total Tank Capacity

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

Step2: Septic tank Dimensions

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

Septic tank Design on Site

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

Conclusion

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.

Video on Septic Tank Design

FAQs on Septic Tank Design

What is Septic Tank?

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.

How do someone know what size septic tank to get?

Here are Table Dimensions of Septic Tank as per Number of Persons

How is septic tank design is calculated?

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.

What is standard size of septic tank?

Septic tank minimum size is 750mm width and 1500mm length(2xwidth) and height about 1200mm. Standard minimum capacity of septic tank is 1cubic meter.

PPT on Septic Tank Design

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