Environmental Engineering - II (Sessional)

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