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 treatment, 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 higher 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, grease 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 following 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 consider that if sewage is adequately screened 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 operating. 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 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 is discharged 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 areincorporated 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 also be achieved. The following table gives a typical appreciation of the settleable elements in the treatment stages.

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Description of Discharge Percentage of annual aggregate flow

Percentage of aggregate time for discharge

Flows exceeding Formula ‘A’

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

Primary settlement tanks 3 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 extremely complex, 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 on Stoke’s law for the smaller particles, larger particles being assumed to be less affected by viscosity.

A detailed 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 tend 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 =

which simplifies to: VS = bl = area, A

therefore VS =

2

From which it can be seen that, for a high percentage removal VS 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 formulated 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 the tank and to scour the previously settled sludge.

3. Design Procedure

3.1 GeneralAs 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. V3 in the simple example previously discussed. Upward velocities 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 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 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 related and 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 - 1½ hours for primary tanks prior to aeration tanks. 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 as 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 overflow 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 m3/m/day and 300 m3/m/day, though the IWPC Manual on British Practice allows up to 450 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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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

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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 solids continue to settle, entrapping other rising particles being 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 velocity of flow.

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

A =

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

Tank dia. (m)

Population equivalents (x 103) for surface loadings of:

1.5 m3/m2.h 2.0 m3/m2.h8

1012.515

17.520

22.525

27.530

2.43.85.98.5

11.515.119.023.528.533.9

3.25.07.9

11.315.420.125.431.438.045.2

Table 2 Circular sedimentation tanks: sidewall depths

Internal tank dia. (m)

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

Floor gradient1 in 2 1 in 5 1 in 10 1 in 50

Actual Preferred Actual Preferred Actual Preferred Actual Preferred8.010.012.515.017.520.022.525.027.530.0

2.932.772.572.352.141.93----

3.03.02.52.52.02.0----

3.333.273.193.103.022.932.852.772.682.60

3.53.53.03.03.03.03.03.02.52.5

3.473.433.403.353.313.273.223.183.143.10

3.53.53.53.53.53.53.03.03.03.0

3.553.533.503.484.483.473.453.433.423.40

3.53.53.53.53.53.53.53.53.53.5

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