The document summarizes the design of a septic tank. It calculates the required volumes for sedimentation (1.8 m3), sludge digestion (1.06 m3), and sludge storage (15 m3). The total volume is determined to be 1.8 + 1.06 + 1.4 * 15 = 27.24 m3. The suitable dimensions of the septic tank are determined to be 6 m x 4 m x 1.5 m, with the first compartment being 4 m x 4 m x 1.5 m and the second being 2 m x 4 m x 1.5 m. Diagrams of the plan and elevation views of the septic tank are also provided.

1. Department of CEN, PCIU
Engineering Materials Sessional
CEN 200
Design of septic tank
Md. Mehedi Hassan Masum
Lecturer
Department of Civil Engineering
Port City International University

2. 2
Introduction Result & Discussion Conclusion
Department of CEN, PCIU
Design of Septic Tank:
Assume wastewater flow, q = 90 lpcd
No of people = 50
Sedimentation:
Minimum hydraulic retention time th = 1.5 − 0.3 × 𝑙𝑜𝑔(𝑝 × 𝑞)
= 1.5 − 0.3 × 𝑙𝑜𝑔(50 × 90)
= 0.4 𝑑𝑎𝑦𝑠
The volume required for sedimentation is given by,
Vh = 10
− 3 × 𝑃 × 𝑞 × 𝑡ℎ
= 10
− 3 × 50 × 90 × 0.4
= 1.8 𝑚3

3. 3
Introduction Result & Discussion Conclusion
Department of CEN, PCIU
Sludge Digestion:
Assume, design temperature = 25∙C
td = (1.035)35-T
= (1.035)35-25
= 42.3 days
Volume required for sludge digestion = 0.5𝑝 × 10 − 3 × 𝑡 𝑑
= 0.5 × 50 × 10
− 3 × 42.3
= 1.06 𝑚3

4. 4
Introduction Result & Discussion Conclusion
Department of CEN, PCIU
Sludge Storage:
Assume sludge accumulation rate = 0.06 m3/person/year
Desludging interval = 5 years
Vsl = 𝐶 × 𝑃 × 𝑁
= 0.06 × 50 ×× 5
= 15 𝑚3
Total Volume of Septic Tank = 𝑉ℎ + 𝑉𝑑 + 1.4 × 𝑉𝑠𝑙
= 1.8 + 1.06 + 1.4 × 15

5. 5
Introduction Result & Discussion Conclusion
Department of CEN, PCIU
Effective Depth of Septic Tank:
Assume, A = 24m2
Maximum Depth of Sludge, dsl = (Vsl/A)
= 15/24 = 0.625 m
Submerged Scum Depth, dss = 0.4 × 𝑉𝑠𝑙/𝐴
= 0.4 × 0.625 = 0.25 m
Total clear space = 0.075+0.3
= 0.375m

6. 6
Introduction Result & Discussion Conclusion
Department of CEN, PCIU
Depth of Sedimentation = Vh/A
= 1.8/24
= 0.075m <0.375m
Therefore, total effective depth = 0.375+0.625+0.25
= 1.25 m
= 1.5 m
Suitable Dimension of Septic Tank =6 𝑚 × 4 𝑚 × 1.5 𝑚
1st Compartment = 4 𝑚 × 4 𝑚 × 1.5 𝑚
2nd Compartment = 2 𝑚 × 4 𝑚 × 1.5 𝑚

7. 7
Introduction Result & Discussion Conclusion
Department of CEN, PCIU
Figure: Plan of the septic tank (6 m×4 m×1.5 m)

8. 8
Introduction Result & Discussion Conclusion
Department of CEN, PCIU
Figure: Elevation section of the septic tank (6 m×4 m×1.5 m)

9. Department of CEN, PCIU
researchgate.net/profile/Mehedi_Masum
For further query feel free to contact:
mehedi.ce.cuet@gmail.com

10. Department of CEN, PCIU