pj4 design of sedimentation units

8/6/2019 PJ4 Design of Sedimentation Units

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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 specialcircumstances demand, full treatment consist of preliminary treatment, primarysedimentation, 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 higher standards or effluent arerequired. Full treatment also incorporates sludge treatment on most schemes.

b. Primary Sedimentation

Preliminary treatment will have removed gross floating solids, grit and, if special provisionshave been made, grease and oil. the first stage of full treatment is to remove up to 75% ofthe remaining suspended solids in the sewage to reduce the strength of the liquid passingto the biological treatment process. Primary sedimentation must be efficient if thefollowing biological process is to work effectively and percolating filters are not to becomeblocked or choked.

Certain sections of the treatment industry believe that the use of primary settlement tanksis unnecessary. Some managers of treatment plant consider that if sewage is adequatelyscreened and macerated it can be passed direct for biological treatment if an activatedsludge plant is used. This view is held strongly in some parts of the USA and a large

number of plants of this type are operating. A serious disadvantage of this treatmentmethod is the large quantity of entirely activated sludge which is produced. This is moredifficult to treat than crude sludge which is discharged from primary sedimentation units.

A much higher load is also passed to the activated sludge units. If a trade waste isdischarged to the treatment works which is toxic to the purifying bacteria in the biologicalsections no treatment will be given to the sewage. If primary sedimentation tanks areincorporated within a scheme a measure of treatment is given to the sewage even if thebiological 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 fromsewage. Any liquid which contains heavy solid particles will become clarified if allowed tostand in a tank.

The solids settle out and form a sludge at the bottom of the tank from where they can beremoved. It is undoubtedly true that the efficient operation and maintenance ofsedimentation tanks will enable an adequately sized biological plant to provide satisfactorytreatment and give rise to a highly efficient works. Efficiently designed sedimentationtanks should effect a reduction in suspended solids of up to 75%. There is no reason whythis figure should not be reached unless a high percentage of colloidal matter is present inthe sewage. In addition to the removal of suspended solids, a reduction in biochemicaloxygen demand of about 35% will also be achieved. The following table gives a typicalappreciation of the settleable elements in the treatment stages.

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Description of Discharge Percentage of annualaggregate flow

Percentage ofaggregate time fordischarge

Flows exceeding Formula A

Storm Tanks (3-6) x D.W.F

Primary settlement tanks3 x D.W.F.

2.4

6.6

91

1

9

100

2. Theory of Continuous Flow Sedimentation

Because flow in such an apparently simple unit as a sedimentation tank is extremelycomplex, any theory of sedimentation 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 onStokes law for the smaller particles, larger particles being assumed to be less affected by

viscosity.

A detailed description of Hazens theory is unnecessary, the important conclusion beingthat depth had little effect on sedimentation, and the smallest size of particle that could besettled depended on the surface area of the tank. As the larger particles tend to settlefirst, the smallest size of particle which can be settled is inversely proportional to thepercentage removal of suspended solids and hence is an indication of the efficiency ofremoval.

Hazens conclusion can be explained as follows:

To achieve a particular degree of solids removal the time of detention of a parcel ofsewage must be such that all particles below a certain size can fall to the bottom of thetank after entering at top water level:

The trajectory of the particle is shown in the simplified diagram above.

Time of detention =SV

d

Q

lbd=

which simplifies to: VS =lb

Qbl = area, A

therefore VS =

A

Q

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From which it can be seen that, for a high percentage removal VS will be small and henceA 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 bythe diagram above, and hence design is based on general rules formulated fromexperience with existing tanks and on empirical conclusions.

Actual flow conditions arising in tanks take the form of currents and eddies, the effects ofwhich tend to reduce the effective capacity of the tank and to scour the previously settledsludge.

3. Design Procedure

3.1 GeneralAs has been shown in our simplified example, settling efficiency is partly dependant uponsurface area, and the theoretical concept of upward flow is used to assess the area, eventhough in the actual design, flow is predominantly horizontal. Theoretically the upwardvelocity relates to the settlement velocity of the smallest particles to be settled, i.e. V3 inthe simple example previously discussed. Upward velocities used for designs are at peakflow, i.e. 3 x dwf.

A typical upward velocity is 1.25 m/hr, from which a surface loading rate can becalculated. In this instance the surface loading rate would be:

1.25 x 24 m3/m2/day = 30m3/m2/day

From the surface loading rate, the tank area can be determined.

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In practice however, study has shown that the settling behaviour of organic sewageparticles is not only dependant on surface loading as Hazens classical theory indicates butis 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 timerelated and it has been shown that rapid settling takes place in the first hour followed by aperiod of more gradual clarification. Fluctuations in influent concentration will not affecteffluent 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 economicallysound and this is the figure generally adopted in the UK although some designers usedetention periods in the range 1 - 1 hours for primary tanks prior to aeration tanks.Although detention periods should always be referred to in terms of capacity at themaximum flowrate they are sometimes given in terms of dry weather flows: as the peakflow for full treatment is approximately 3 dwf the detention may be referred to as 6 hours atdwf 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 endand overflows at the other. In a circular tank sewage enters at the centre andoverflows at the perimeter.

The merits and details of each type of tank will be discussed later. The length/breadthratio for a rectangular tank is generally taken as between 3 and 4.

The third parameter is sedimentation tank design is the weir overflow rate calculated bydividing 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 150m3/m/day and 300 m3/m/day, though the IWPC Manual on British Practice allows up to450 m3/m/day. Thus the design procedure can be summarised as follows:

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GIVEN:

MAXIMUM RATE OF FLOW & TANK TYPE

NO

YES

YES

PROCEED WITH DETAILED DESIGN

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DecideRetention Time

Calculate Volume

Decide SurfaceLoading Rate

Calculate Area

Decide on No. ofTanks

Decide onCriticalProportion

Dimensions

CalculateDimensions of Tanks

Amend, orChange no. ofTanks

Are DimensionsReasonable

Check Weir OverflowRate


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The Sedimentation tanks discussed previously are tanks in which the flow is nominallyhorizontal. This type utilises upward flow such that particles of sewage whose settlingvelocities are less than the upward velocity would be carried upwards, and would meetlarger particles settling. The settling particles coalesce with the rising particles and theresulting flocculants solids continue to settle, entrapping other rising particles being

carried upwards.

The upward velocity referred to in connection with horizontal flow tanks is a theoreticalconcept, but in the case of the upward flow type tank is the actual velocity of flow.

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

A =Vs

Q

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Table 1 Circular sedimentation tanks: diameter and surface loading related topopulation equivalents.

Tank

dia. (m)

Population equivalents (x 103) for surface loadings of:

1.5 m3/m2.h 2.0 m3/m2.h

810

12.515

17.520

22.525

27.530

2.43.85.98.511.515.119.023.528.533.9

3.25.07.911.315.420.125.431.438.045.2

Table 2 Circular sedimentation tanks: sidewall depths

Internaltankdia. (m)

Sidewall depth (incl. 600 mm freeboard) for 2 h retention and 1.5 m3/m2. h surfaceloading

Floor gradient

1 in 2 1 in 5 1 in 10 1 in 50

Actual Preferred Actual Preferred Actual Preferred Actual Preferred

8.010.012.5

15.017.520.022.525.027.530.0

2.932.772.57

2.352.141.93----

3.03.02.5

2.52.02.0----

3.333.273.19

3.103.022.932.852.772.682.60

3.53.53.0

3.03.03.03.03.02.52.5

3.473.433.40

3.353.313.273.223.183.143.10

3.53.53.5

3.53.53.53.03.03.03.0

3.553.533.50

3.484.483.473.453.433.423.40

3.53.53.5

3.53.53.53.53.53.53.5

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pj4 design of sedimentation units

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