Preliminary and Primary Wastewater
Treatment
SOLUTION
Solution:
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
Solution:
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.