Primary Sedimentation

trextemperΜηχανική

22 Φεβ 2014 (πριν από 3 χρόνια και 4 μήνες)

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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


C
ollection

of suspended solids
:

-

Mechanical scraper

-

Skimmer (oil & greas
e)


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 collecti
on.

-

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 outle
t 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 O
FR


Flow

Range
(
m
3

/ m
2

. d
)

Typical (m
3

/ m
2

. d)

Average Flow (No AS return)

30






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Detention Ti
me:

1 to 4 hours


Weir Loading Rate:

Average Flow < 1 MGD

W.L. rate 124 m
3

/ m . d

Average Flow > 1 MGD

W.L. rate 185 m
3

/ m . d


Dimensions


Clarifier

Range

Common Value

Rectangular



Length (m)

10
-

100

25
-

60

L : W

1
-

7

4
-

5

L : D

4
-

25

7
-

18

Side
-
water depth (m)

2.5


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-




-



Circular



Diameter (m)

3
-

60

10




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-
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-

6

4