# Design of septic tank | Design of septic tank calculation

Design of septic tank is necessary to manage the waste excreta by human beings in a well-planned manner.

In civil engineering, the design of septic tank play important as well as useful knowledge to be with civil engineers. I have attached an example of the design of septic tank for ten users as an example by solving numerically.

### Purpose of a septic tank

1. Deposition of settling solids in sewage by sedimentation.
2. Partial or complete digestion of sludge prior to disposal of sludge.
3. Storage of sludge and scum accumulated in between successive cleanings.

### Construction of a septic tank

Septic tank is generally constructed by using the following materials and requirements of the component of a septic tank.

1. Septic tank should be watertight and constructed of bricks masonry, stone masonry, or sometimes concrete.
2. It is rectangular in plan and length is 2- 4 times of breadth.
3. Generally, two compartments are provided with a partition wall at a distance of (2/3)rd of the inlet.
4. The direct current between inlet and outlet is prevented using pipe tees at inlet and outlet.
5. The baffle wall with the opening is constructed at some distance away from an outlet.
6. The outlet pipe is kept 15cm lower than the inlet pipe.
7. A vent pipe is provided for the escape of gases.
8. The floor of the tank should be of cement concrete and slope towards the sludge outlet.

## Components of Septic Tank

#### Baffle Wall

The baffle wall is generally at the mid of the septic tank to divide the septic tank sludge and water into two portions.

It may be or may not be in the septic tank according to size and design of septic tank.

#### Slab

The slab is covering of septic tank to protect unwanted smell to expose into the atmosphere directly. It also prevents to external materials to enter into it.

#### Ventilation Pipe

This pipe in septic tank allows bad smell from septic tank at high level from the building to be protected from unsafe environment around septic tank.

#### Inlet and Outlet pipe

This pipe is provided at the upper portion of septic tank toward the source of sewage. It may be of concrete or plastic materials.

Its diameter is about 100 mm and kept about 200 mm to 600 mm below the water level. It has an elbow or T shape you can see in the picture above.

#### Cover

The cover of the septic tank allows cleaning the wastage after it filled. We don’t have to uncover the full slab of the septic tank.

#### Free Board

The freeboard is the gap between an upper slab of the septic tank and liquid level. It should be provided to prevent the sludge to touch the slab before it filled.

## Processes take place to design septic tank

1. Sewage inflow
2. Maximum sewage flows into a septic tank is based upon a number of plumbing fixtures and discharging simultaneously.
3. The estimated number of fixtures serving 50 people for residential colonies up to 300 for boarding schools is estimated from peak discharge tables.
4. Sedimentation
5. For a standard temperature of 25⁰C, 10 liters per minute flow rate and 0.92 m2 surface area are required.
6. For 0.05 mm size particle removal, more than 75% sewage of specific gravity 1.2 mm and a minimum depth of 25-30 cm is required.
7. Sludge digestion
8. The suspended solids entering the may be taken as 70 gm per day per capita.
9. Assume 60% of solid removed along with fresh sludge with 5% solid content.
10. The volume of fresh sludge may be taken as 0.0083 m3 / capita/day.
11. The digestion period for average sludge is 63 days.
12. Considering the volume required for sludge and scum accumulated, the septic tank is designed for the detention period of 24 to 48 hours.

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### Working of the septic tank

• Sludge is withdrawn either yearly or half-yearly.
• This process is carried out once in 2 to 3 years.
• Disinfectants, soap, and water are not allowed in the tank.
• Septic tank produces septic action by anaerobic bacteria.
• Lighter materials such as scum float and heavier particles sink as sludge.
• Gases escape through vent paced at building top height and effluent is discharged to soak pit.
• Seeding should be done before the proper functioning of the septic tank.

• Easy to construct and no skill supervision is required.
• Once installed, they give a long carefree service.
• Sludge, effluent, and scum can be disposed of without causing a serious nuisance.
• It can remove about 90% BOD and 80% suspended matter.
• There is no maintenance problem as there are no moving parts.
• The cost is less and within the reach of private households.

• Leakage of gases through the top of the septic tank may cause bad smell and air pollution.
• The size required large and uneconomical when serving more persons.
• Working in a septic tank is unpredictable and non-uniform.
• Septic tank requires a large volume of flushing water.
• The soak ways can overflow if not designed, built and operated properly.

### Design criteria for septic tank

1. Volume determination
2. The volume of sewage settlement

V₁ = N * Q * T

Where N is the number of the user.

Q is the rate of water supply.

T is the detention time ( 1 to 3 days)

IF not given take T = 1 day

Q = 120 litre/ day; For continuous water supply

Q = 80 litre/ day ; For intermittent water supply

• Volume for sludge of digested

V₂  = 0.0425 m3/ person = Number of users * 0.0425 m3

• Volume for storage of digested sludge

The volume for storage of digested sludge depends upon the periodic cleaning periodic of the septic tank.

Volume required for storage of digested sludge

To determine the required volume all of above determined volume should be added.

Total volume required (V) = V₁ + V₂ + V₃

Other considerations

1. L/B = 2 to 4
2. Minimum depth = 1 m ( effective ) and free board = 30 to 45 cm
3. Minimum width = 0.75 m
4. Minimum effective volume = 1m3

Let us take an example for design of septic tank for 10 users with numerical.

### Example:

Q) Design a septic tank for 10 users. The rate of sewage flow is 100 lpcd. Assume sludge is cleaned from the septic tank once in 3 years.

Solution;

Given that,

N = 10

Q = 100 lpcd * 10 = 1000 lit/day

T = 3 years

=>  Cds (Volume required for storage of digested sludge) = 0.085 m3/person

For design of septic tank

V = V₁ + V₂ + V₃

Now,

V₁ = Q * t

Assume t = 1 day

V₁ = 1000* 1 = 1000 lit = 1 m3

V₂ = R * N

= 0.0425 * 10

= 0.425

V₃ = Cds * N

= 0.085 * 10

= 0.85 m3

Substituting these values in above equation, we get

V = V₁ + V₂ + V₃

= 1 + 0.4245 + 0.85

= 2.275 m3

Assume, Effective depth = 1.2 m

Plan area of septic tank

As = ( V / d )

= 2.275/1.2 = 1.896 m2

Take, L/B = 3

L = 3B

As = L*B = 3B2 ( According to design criteria and assumption, given above)

= 0.795 m    say 0.8 m

L = 3 *B = 3 * 0.8

= 2.4 m

Take, free board = 0.30 m

Overall depth, H = 1.2 + 0.3

= 1.5 m

Hence, provide a septic tank of size

2.4m * 0.8m * 1.5m) for 10 users.

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