Unit operation

1. EXPERT LECTURE
ON
UNIT OPERATION
Delivered BY:
DR. N. J. MISTRY, PROFESSOR
Civil Engineering Department
Sardar Vallabhbhai National Institute of Technology, Surat - 7

2. UNIT OPERATIONS
Screening
Mixing
Flocculation
Sedimentation
Floatation
Elutriation
Vacuum Filtration
Heat Transfer and Drying
PRELIMINARY TREATMENT SYSTEM
Approach Channel
Screen Chamber
Grit Chamber
Skimming Tank )Oil and Grease Traps)
PRIMARY TREATMENT SYSTEM
Above +PST
Reduces 60 – 70 % of Settleable Suspended Solids and
30 – 32 % of Organic Suspended Solids

3. BAR SCREEN
Clarifier

4. EQUALIZATION TANK
Plot for Computation of Volume of an Equalization Basin

5. SCREEN CHAMBER
Screen Type Size of Clear Opening
Coarse Screen More than 50 mm
Medium Screen 25 – 50 mm
Fine Screen 10 – 25 mm
DESIGN CONSIDERATIONS
Where
sin v θ
h
4/3
b
W β L
úûh = é
ù
´ êë
hL = head loss (m)
β = bar shape factor
W = width of bar facing the flow
b = clear spacing between the bars
hV = velocity head of flow approaching the rack (m)
q = inclination of the bars
Head loss is also computed by the expression
hL = 0.0729 (v2 – v2h)
Where hL = head loss (m)
v = velocity of flow through screens, m/s
vh = velocity of flow before screens, m/s (approach velocity)

6. GRIT CHAMBER
Design Criteria
Flow maximum
Peaking factor 2.5 – 3.0
Detention time, t 30 – 90 s (usually 60 s)
Flow through velocity, vh 0.2 – 0.4 m/s (usually 0.3 m/s)
Settling velocity, vs 0.016 – n0.021 m/s for particles of 0.2 mm
diameter
0.01 – 0.015 m/s for particles of 0.15 mm diameter
and specific gravity of 2.65
Liquid depth, D 1 – 1.5 m
Length, L 3 – 25 m
Quantity of grits 0.022 - 0.075 m3/1000 m3 of flow
Overflow rate 1200 – 1700 m3/m2-d (at Qpeak) (1200 – 1700)

7. GRIT CHAMBER

8. DETERMINING SETTLING VELOCITY
Where
4( ρ - ρ
)d
= s w
´
s ρ
g = acceleration due to gravity (usually assumed as 9.81 m/s2
Pw = density of water (usually assumed as 1000 kg/m3
Ps = density of solid particles
(normally of specific gravity 2.65 = 2.65 x 1000 = 2650 kg/m3
d = diameter of particles (usually assumed as 0.2 mm = 0.2 x 10-3 m
CD = coefficient of drag force depends on flow conditions
(or Reynolds number, Re)
g
3C
V
D w

9. SKIMMING TANK (OIL AND GREASE TRAP)
Detention time 3 – 30 min.
Surface area 250 m2/m3 flow
PST as a Load Reducing Unit for Secondary Treatment System

10. SKIMMING TANK

11. PRIMARY SEDIMENTATION TANK
Dimensions
Rectangular Tank: Range Typical
Length (m) 15 – 100 30
Width (m) 3 – 30 10
Depth (m) 2.5 – 5 4
Circular Tank:
Diameter (m) 3 – 60 30
Depth (m) 3 – 5 4
Bottom slope, mm/mm 0.02 – 0.05 0.03

12. DETENTION TIME (Hydraulic Retention Time, HRT)
V = Q x t
t = V
(days)
Q
24(hours)
t = V ´
Q
Where
t = detention time in days
V = volume of basin in m3
Q = flow of wastewater in m3/d
Schematic Diagram for Concept of Short-circuit of Flow

13. FLOW THROUGH VELOCITY OR HORIZONTAL VELOCITY, Vh
Flow through velocity, V = distance travelled =
l
h time taken
t
The volume of the tank, V = L x B x D
The cross-sectional area of the tank, A= B x D
Xvolume of tank (liquid)
time
The Flow Rate, Q = V =
t
Q B D L
= ´ ´  = ´ ´
= ´
Q
= =
L
t
V
h
=
L Vh t
Q
V
h
V
orAx
x
A
Vh
h
X
Q A
( V L B D)
t
= ´

14. SURFACE LOADING RATE (SLR) OR OVERFLOW RATE (OFR)
)
SLR = flow (m d
/ )
2
3
surface area (m
= settling velocity (m/d)
Therefore, numerically SLR = Vs
WEIR LOADING RATE (WLR)
Weir Overflow Rate flow rate (m /d) 3
m /m d
3
= = -
total weir length (m)
Depth of Tank D = Vs x t (Vs = SLR)
Total depth D = net depth + free board + sludge zone depth
= D + 0.3 + D1
= D+0.3+10% of D

15. THANK YOU FOR
YOUR ATTENTION
75