PRIMARY SEDIMENTATION
Location in the wastewater treatment plants
Removal
-
40 – 70 % of TSS
30 – 40 % of BOD
Contents
-
Flow coming in from sewers only
Incoming flow + return sludge
Collection of suspended solids:
-
Mechanical scraper
Skimmer (oil & grease)
Types:
-
Horizontal Flow
Solids Contact
Inclined Surface
Horizontal Flow
-
Rectangular
Square
Circular
Selection depends on
-
Size of installation
Regulations
Site condition
Engineering preference
Comparison between rectangular and circular clarifiers:
Advantages: for rectangular clarifier
-
Less land space for multiple units.
Make use of common wall.
Easier to cover if necessary.
Less short circuiting.
Lower losses at inlet and outlet.
Less power consumption for sludge collection.
Longer travel distance.
Disadvantages:
-
Possible dead zones at corners.
Sensitive to flow surges.
Needs multiple weirs to maintain low weir loading.
Design Factors
-
Overflow rate, OFR
Detention time
Weir loading rate
Shape and dimension
Inlet and outlet arrangements
Sludge removal system
To minimize the following:
-
Eddy currents induced by incoming flow
Surface current due to wind
Vertical convection current induced by temperature difference
Density current effect
Currents induced by scrapers
Design OFR
Flow
Range (m3 / m2 . d)
Typical (m3 / m2 . d)
Average Flow
(No AS return)
30 – 50
40
Peak Flow
(No AS return)
70 - 130
100
Average Flow
(AS return)
25 – 35
30
Peak Flow
(AS return)
45 - 80
60
Detention Time:
1 to 4 hours
Weir Loading Rate:
Average Flow < 1 MGD
Average Flow > 1 MGD
W.L. rate 124 m3 / m . d
W.L. rate 185 m3 / m . d
Dimensions
Clarifier
Range
Common Value
10 - 100
25 - 60
L:W
1-7
4-5
L:D
4 - 25
7 - 18
2.5 – 5.0
3.5
3 - 24
6 - 10
Diameter (m)
3 - 60
10 – 40
Side-water depth (m)
3-6
4
Rectangular
Length (m)
Side-water depth (m)
Width (m)
Circular