**Preliminary and Primary Wastewater
Treatment**

__SOLUTION__

- Design a grit chamber of rectangular cross-section. Following information is provided:
Design Flow (Q): 50 MLD; Settling velocity of the
smallest particle to be removed completely is 0.0236 m/s; Specific Gravity
of particles (S
_{s}): 2.65; Horizontal mean flow velocity (V): 0.30 m/s; Theoretical depth (D): 1.4 m; Calculate the dimensions and slope (mm/m) of the grit chamber. While designing the actual grit chamber, add 25 percent to the depth for grit collection, and 0.25m freeboard. Also add 50 percent to the theoretically calculated length. Value of Manning’s ‘n’ is 0.020. Assuming that the volume of grit in the wastewater is 0.15 m^{3}/ML, and 100 percent grit removal in the grit chamber, calculate the total mass and mass of dry grit produced per day. The collected grit has a porosity of 0.5.

**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 m^{3}/s

Area of flow (A) = _{}= 0.5787 / 0.30 =1.929 m^{2}

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**

- Give three reasons of why even though, theoretically speaking,
depth has no effect on the particle removal efficiency in a primary
settling tank used in wastewater treatment, a depth of 3-5 m is provided
in practice.

**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.