DESIGN OF SEDIMENTATION UNITS SEDIMENTATION TANK DESIGN 1. General Considerations a. Full Treatment

trextemperMécanique

22 févr. 2014 (il y a 3 années et 1 mois)

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1

DESIGN OF SEDIMENTATION UNITS


SEDIMENTATION TANK DESIGN


1.

General Considerations



a.

Full Treatment



Flows up to 3 x DWF and infiltration are normally given full treatment. Unless special

circumstances demand, full treatment consist of
preliminary

t
reatment, primary

sedimentation, single
-
stage biological treatment and secondary sedimentation. Double
-

stage biological treatment,
recirculation

etc., are introduced if the wastes so require.

Tertiary treatment is included as an additional stage if hi
gher standards or
effluent

are

required. Full treatment also incorporates sludge treatment on most schemes.



b.

Primary Sedimentation



Preliminary treatment will have removed gross floating solids, grit and, if special provisions

have been made, greas
e and oil. the first stage of full treatment is to remove up to 75% of

the remaining suspended solids in the sewage to reduce the

strength of the liquid passing

to the biological treatment process. Primary sedimentation must be efficient if the

follow
ing biological process is to work effectively and percolating filters are not to become

blocked or choked.



Certain sections of the treatment industry believe that the use of primary settlement tanks

is unnecessary. Some managers of treatment plant con
sider that

if sewage is adequately

s
creened and macerated it can be passed direct for biological treatment if an activated

sludge plant is used. This view is held strongly in some parts of the USA and a large


number of plants of this type are operatin
g.

A serious disadvantage of this treatment

method is the large quantity of entirely activated sludge which is produced. This is more

difficult to treat than crud
e sludge which is
discharged from primary sedimentation units. A

much higher load
i
s
al
so

passed
to
the activated sludge units. If a trade waste is

di
scharged to the treatment works which is toxic to

the purifying bacteria in the biological

sections no treatment will be given to the sewage. If primary sedimentation tanks are


incorporate
d within a scheme a measure of treatment is

given to the sewage even if the

biological process has

been put out of action by the discharge of a toxic waste.



Settlement is the cheapest and most satisfactory way of removing suspended solids from

sewage.
Any liquid which contains heavy solid particles will become clarified if allowed to

stand in a tank.



The solids settle out and form a sludge at the bottom of the tank from where they

can be

removed. It is undoubtedly true that the efficient operation
and maintenance of

sedimentation tanks will enable an adequately sized biological plant to provide satisfactory

treatment and give rise to a highly efficient works. Efficiently designed sedimentation

tanks should effect a reduction in suspended solids
of up to 75%. There is no reason why

this figure should not be reached

unless a high percentage of colloidal matter is present in

the sewage. In addition

to the removal of suspended solids, a reduction in biochemical

oxygen demand of about 35% will al
so be achieved. The following table gives a typical

appreciation of the settleable elements in the treatment stages.







2

Description of Discharge

Percentage of annual
aggregate flow

Percentage of
aggregate time for
discharge

Flows exceeding Formula ‘A’


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1


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2.

Theory of Continuous Flow Sedimentation



Because flow in such an apparently simple unit as a sedimentation tank is extremely

complex, any theory of sedimentati
on is bound to be based on a grossly simplified model if

complex analysis is to be avoided.

A. Hazen proposed a theory of settlement based on

Stoke’s law for the smaller particles, larger particles being assumed to be less affected by

viscosity.



A det
ailed description of Hazen’s theory is unnecessary, the important conclusion being

that depth had little effect on sedimentation, and the smallest size of particle that could be

settled depended on the surface area of the tank. As the larger particles t
end to settle

first, the smallest size of particle which can be settled is inversely proportional to the

percentage removal of suspended solids and hence is an indication of the efficiency of

removal.



Hazen’s conclusion can be explained as follows:



To achieve a particular degree of solids removal the time of detention of a parcel of

sewage must be such that all particles below a certain size can fall to the bottom of the

tank after entering at top water level:





The trajectory of the particle is

shown in the simplified diagram above.



Time of detention =

S
V
d
Q
l
b
d




which simplifies to:


V
S

=

l
b
Q

bl = area, A


therefore

V
S

=

A
Q



3


From which it can be seen that, for a high percentage removal

V
S

will be small and hence

A must be large.



A similar conclusion can be reached where the influence enters other than at the surface.



The flow pattern in a sedimentation tank is much more involved than that suggested by

the diagram above, and hence
design is based on general rules formulate
d from

experience with

existing tanks and on empirical conclusions.



Actual flow conditions arising in tanks take the form of currents and eddies, the effects of

which tend to reduce the effective capacity of th
e tank and to scour the previously settled

sludge.




3.

Design Procedure



3.1

General


As has been shown in our simplified example,

settling efficiency is partly
dependant upon

surface area, and the theoretical concept of upward flow is used to assess

the area, even

though in the actual design, flow is predominantly horizontal. Theoretically the upward

velocity relates to the settlement velocity of

the smallest particles to be settled, i.e. V
3

in

the simple example previously discussed. Upward ve
locities used for designs are at peak

flow, i.e. 3 x dwf.




A typical upward velocity is 1.25 m/hr, from which a surface loading rate can be



calculated. In this instance the surface loading rate would be:





1.25 x 24 m
3
/m
2
/day


=

30
m
3
/m
2
/day



From

the surface
loading

rate, the tank area can be determined.


4


In practice however, study has shown that the settling behaviour of organic sewage

particles is not only dependant on surface loading as Hazens classical theory indicates but

is also dependant
on detention period. The particles agglomerate (called flocculation)

during the sedimentation process by chance collision, particle attraction (
because

of

differential rates of settling) and electro
-
molecular forces.

These processes are time

rela
ted a
nd it has been shown that
rapid settling takes place in the first hour followed by a

period of more gradual clarification. Fluctuations in influent concentration will not affect

effluent quality in a tank where detention period is a dominant parameter.


Detention periods in excess of two hours at the maximum rate of flow is not economically
sound and this is the figure generally adopted in the UK although some designers use
detention periods in the range 1
-

1
½

hours for primary tanks prior to aeration t
anks.
Although detention periods should always be referred to in terms of capacity at the
maximum flowrate they are sometimes given in terms of dry weather flows:
as the peak
flow for full treatment is approximately 3 dwf the detention may be referred to a
s 6 hours at
dwf i.e. 2 hours x 3 dwf = 6 dwf.



3.2

Circular and Rectangular Tanks



In a rectangular tank the horizontal velocity is ‘linear’ as sewage enters at one end


and overflows at the other. In a circular tank sewage enters at the centre and



overflows at the perimeter.



The merits and details of each type of tank will be discussed later. The

length/breadth

ratio for a rectangular tank is generally taken as between 3 and 4.



The third parameter is sedimentation tank design is the weir over
flow rate calculated by

dividing maximum treatment flow by total weir length. Too high a weir overflow rate may

result in solids being carried over. The weir overflow rate is

generally kept between 150

m
3
/m/day and 300 m
3
/m/day, though the IWPC Manual

on British Practice allows up to

450 m
3
/m/day. Thus the design procedure can be summarised as follows:
























5

GIVEN:


MAXIMUM RATE OF FLOW
&
TANK TYPE








































NO













YES


















YES


PROCEED WITH DETAILED DESIGN





Decide
Retention Time

Calculate Volume

Decide Surface
Loading Rate

Calculate Area

Decide on No. of
Tanks

Decide on
Critical
Proportion
Dimensions

Calculate
Dimensions of Tanks

Amend, or
Change no. of
Tanks

Are Dimensions
Reasonable

Check Weir Overflow
Rate


6




7





The Sedimentation tanks discussed previously are tanks in which the flow is nominally

horizontal. This type utilises upward flow such that particles of sewage whose settling

velocities
are less than the upward velocity
would

be carried
upwards, and would meet

larger particles settling. The settling particles coalesce with the rising particles and the

resulting flocculants
s
ol
i
ds continue to settle, entrapping other rising particles bei
ng carried

upwards.



The upward velocity referred to in connection with horizontal flow tanks is a theoretical

concept, but in the case of the upward flow type tank is the actual

v
elocity of flow.



If the settling velocity of the smallest particle to
be settled is known (Vs) then the

area of

tank is




A =

Vs
Q














8


Table 1

Circular sedimentation tanks: diameter and surface loading related to




population equivalents.





Tank

dia. (m)

Population equivalents (x 10
3
) for
surface loadings of:


1.5 m
3
/m
2
.h

2.0 m
3
/m
2
.h

8

10

12.5

15

17.5

20

22.5

25

27.5

30

2.4

3.8

5.9

8.5

11.5

15.1

19.0

23.5

28.5

33.9

3.2

5.0

7.9

11.3

15.4

20.1

25.4

31.4

38.0

45.2



Table 2

Circular sedimentation tanks: sidewall depths



Internal
tank

dia
. (m)

Sidewall depth (incl. 600 mm freeboard) for 2 h retention and 1.5 m
3
/m
2
. h surface
loading

Floor gradient

1 in 2

1 in 5

1 in 10

1 in 50

Actual

Preferred

Actual

Preferred

Actual

Preferred

Actual

Preferred

8.0

10.0

12.5

15.0

17.5

20.0

22.5

25.0

27.5

30.0

2.93

2.77

2.57

2.35

2.14

1.93

-

-

-

-

3.0

3.0

2.5

2.5

2.0

2.0

-

-

-

-

3.33

3.27

3.19

3.10

3.02

2.93

2.85

2.77

2.68

2.60

3.5

3.5

3.0

3.0

3.0

3.0

3.0

3.0

2.5

2.5

3.47

3.43

3.40

3.35

3.31

3.27

3.22

3.18

3.14

3.10

3.5

3.5

3.5

3.5

3.5

3.5

3.0

3.0

3.0

3.0

3.55

3.53

3.50

3.48

4.48

3.47

3.45

3.43

3.42

3.40

3.5

3.5

3.5

3.5

3.5

3.5

3.5

3.5

3.5

3.5