Preliminary and Primary Wastewater Treatment
Theoretical depth of flow (D) = 1.4 m
Required detention time (T) = = 1400 / 23.6 = 59.32 s (say 60 s)
Velocity of Flow (V) = 0.30 m/s
Theoretical length of channel (L) = V.T = 0.30 x 60 = 18.0 m
Flow (Q) = 50 MLD = 50000 / 86400 = 0.5787 m3/s
Area of flow (A) = = 0.5787 / 0.30 =1.929 m2
Width of the Channel = = 1.929 / 1.4 = 1.377 m (say 1.4 m)
Whetted Perimeter (P) = (1.4 + 1.4 + 1.4) = 4.2 m
Using Manning’s equation:
, Where, S = Channel slope, m/m
Using this equation, S was calculated to be 1.02 x 10-4
Thus, Design Length (L’) = 1.5.(18.0) = 27.0 m
Design Depth (D’) = (0.35 + 1.4 + 0.25) = 2.0 m
Design Width (W) = 1.4 m
Reason 1: Provision of lower depth, without increasing the cross-sectional area would increase horizontal flow velocity through the tank, which may lead to scouring and consequent re-suspension of settled particles.
Reason 2: Wind currents affect shallow settling tanks more. The turbulence resulting from such wind currents may hinder settling efficiency.
Reason 3: The settling theory, which suggests that settling efficiency in a settling basin is not affected by depth, assumes that particles settle discretely. However, during sedimentation, may organic particles in wastewater flocculate, forming bigger particles. The settling efficiency for flocculent suspensions in a sedimentation tank improves with depth.