SEDIMENTATION

Sedimentation is a physical water treatment process using gravity to remove suspended solids

from water. Solid particles entrained by the turbulence of moving water may be removed naturally by

sedimentation in the still water of lakes and oceans. Settling basins are ponds constructed for the

purpose of removing entrained solids by sedimentation. Clarifiers are settling basins built with

mechanical means for continuous removal of solids being deposited by sedimentation.

The most common form of sedimentation follows coagulation and flocculation and precedes

filtration.  This type of sedimentation requires chemical addition (in the coagulation/flocculation step)

and removes the resulting floc from the water.  Sedimentation at this stage in the treatment process

should remove 90% of the suspended particles from the water, including bacteria.  The purpose of

sedimentation here is to decrease the concentration of suspended particles in the water, reducing the

load on the filters.  

Sedimentation can also occur as part of the pre-treatment process, where it is known as pre

sedimentation.  Pre sedimentation can also be called plain sedimentation because the process depends

merely on gravity and includes no coagulation and flocculation. Without coagulation or flocculation,

plain sedimentation can remove only coarse suspended matter which will settle rapidly out of the

water without the addition of chemicals.  This type of sedimentation typically takes place in a

reservoir, grit basin, debris dam, or sand trap at the beginning of the treatment process. 

While sedimentation following coagulation/flocculation is meant to remove most of the

suspended particles in the water before the water reaches the filters, pre sedimentation removes most

of the sediment in the water during the pre-treatment stage.  So pre sedimentation will reduce the load

on the coagulation/flocculation basin and on the sedimentation chamber, as well as reducing the

volume of coagulant chemicals required to treat the water.  In addition, pre sedimentation basins are

useful because raw water entering the plant from a reservoir is usually more uniform in quality than

water entering the plant without such a holding basin.

There a four types of sedimentation behaviours. Types 1 sedimentation is discrete settling.

These types will settle of silt in intake and other headwork before coagulation. Besides, it also

softening precipitates in separates softening basin. This type was characterized by particle that settles

discretely at a constant setting velocity. It will settle as individual particles and do not flocculate or

stick to other particles during settling. The type 2 sedimentation is flocculent settling. This type is

being use in settling flocculated water in sedimentation basin. This type was characterized by particles

that flocculate during sedimentation. Since they flocculate, their size is constantly changing; therefore

the settling gravity is changing. Usually it occurs in alum or iron coagulation in primary sedimentation

and in settling tanks in tricking filtration. For the third types of sedimentation behavior which is zone

settling it will be apply in meddle portion of gravity thickener. These particles are at high

concentration. It also tends to settle as a mass and a distinct clear zone and sludge zone are present.

Usually, these types occur in lime-softening sedimentation, activated sludge sedimentation and sludge

thickeners. For the last type which is compression settling it settling near bottom of gravity thickener.

The sedimentation of suspensions with solids concentration is so high. The particle is in contact with

another and further sedimentation can occur by compression of the mass.

For the designing of the sedimentation basin, that has three types of shape which is include

rectangular, circular and square. In water treatment plant that we are design at Pekan Kuala Nerang,

we choose rectangular settling basin. The selection of the shape of our water treatment plant is based

on the size of installation, the available land and site condition and preferences and experience of

operators and/or design engineers. By using rectangular basins, we get more advantages such occupy

less spaces by common walls, less short circuiting due to near plug flow condition and less power

requirement. However, this type also proposes disadvantages. The example of the disadvantages is

restricted in width by collection equipment, require multiple rows of weir, high maintenance of

sprockets, chain and it sensitive to flow surges.

Hence, in order to design a good water treatment plant, we are followed influent and effluent

structures of sedimentation basin. For influent structure, the distribution flow needs to be evenly

across the width. The transport well developed floc properly. Then, the head loss must be as small as

possible and lastly we integrate with flocculation basin.

For effluent structures, we use effluent weir plates with notches or orifices. Besides use

collection through and central channel and also provide uniform distribution over the large area. The

lifting of particles should be minimized and lastly we low the weir loading rate.

SEDIMENTATION UNIT

The water undergo the coagulation and flocculation process before its come to the

sedimentation process. Coagulation and flocculation can be optimized through jar testing to create

particles that more readily settle. Sedimentation is a process used to separate the settleable solids

from water through gravity settling.

Concept of sedimentation

There are two important terms to understand in sedimentation zone design which are the

particles (floc) settling velocity, Vs and the overflow rate and the velocity at which the tank is

designed to operate Vo or u. The particles removed is independent of the depth of the

sedimentation tank. As long as Vs is greater than Vo , the particles will settle downward and be

removed from the bottom of the tank regardless of the depth.

In our water treatment plant, the inclined surface or tube settler is used in the

sedimentation process. The figure 1 shows the flow of water in inclined surface method of

sedimentation.There are several basic features of inclined surface or tube settler method:

• It is using inclined tube.

• The depth of basin is divided into numerous channels.

• The depth of fall of particles is reduced.

• The settling time required is reduced.

• Flow is laminar.

• Provide large surface area and low hydraulic loading.

• Insignificant wind effect.

Figure 1 Flow of water in inclined surface of sedimentation.

Shallow-depth sedimentation theory

In our water treatment plant, the tube settler is used in sedimentation. Tube settlers are a

practical application of the shallow-depth sedimentation theory that was first proposed by A.

Hazen in 1904. Figure 2 helps to illustrate the basic concept of shallow-depth sedimentation. In an

idealized plug-flow rectangular basin with length Lο and depth Hο, all discrete particles with

settling velocities greater than or equal to Vο carried at a water flow velocity of V will be

removed in the basin. Particles with settling velocities (Vs) less than Vο will be removed at a

lesser efficiency proportional to Vs/Vo.

Figure 2 The basic concept of shallow-depth sedimentation.

If the settling depth were reduced to H, then all particles with settling velocity Vs would

be removed in the basin. If the settling depth were not reduced, a basin with length mush greater

than Lo would be required to capture these slower settling particles. Consequently, this theory

illustrate how decreasing settling depth can be substituted for basin length to achieve the same

particle removal efficiency at a given hydraulic longitudinal velocity. However, practical

application of this theory had the potential to reduce both sedimentation basin size and

construction costs.

The 60 tube are used in the water treatment plant to reduce clarifier size. With tube⁰

settlers , overflow rates can generally be increased two to three times over conventional

clarification overflow rates as a result of the reduced settling time inside the tubes. Tube are

normally placed at the exit of the rectangular basins. The advantage of this type of sedimentation

method is to allow the gravity sludge removal and create laminar flow conditions. Figure 3

illustrates how the effective settling zone is reduced to inches inside the tubes and how sludge

slides by gravity to conventional sludge collection equipment below the tube installation.

Figure 3 illustrates how the effective settling zone.

Tube Settler Systems

Flow through tube settling clarifiers

Tube settlers increase the settling capacity of rectangular sedimentation basins by reducing

the vertical distance a floc particle must settle before agglomerating to form larger particles. The

clarifiers are fitted with tubes, installed on an incline. Each functions as a small, shallow settling

basin in its own right. Water enters into and is then directed upwards through the inclined tubes or

between inclined plates, like a series of very shallow sedimentation basins stacked on top of each

other.

Figure 4 The tube settler system

Figure 5 Cross-sectional of sedimentation basin

The floc has a small distance to settle down to the bottom of each tube or plate. It then

flows downwards, due to gravity before it collects in a sludge area at the bottom of the basin prior

to removal. These clarifiers offer a very high ratio of settling area per unit volume of water. They

increase sedimentation efficiency and in a smaller volume of basin means less space is needed.

The figure 4 below shows the tube settler systems.

Figure 6 The Flow through tube settling clarifiers

It is flexible since it is in the form of hexagonal cellular honeycomb. It is made of PVC

with a smooth surface, sludge forming in the cells slides ad very easily falls to the lower part.

Tube settlers use multiple tubular channels sloped at an angle of 60 and adjacent to each other,⁰

which combine to form an increased effective settling area. The figure 7 shows the standard

hexagonal tube settler. This provide for a particle settling depth that is significantly less than the

settling depth of a conventional clarifier, reducing settling times. Tube settlers capture the settle

able fine floc the escapes the clarification zone beneath the tube settlers and allow the larger floc

to travel to the tank bottom in a more settle able form. The tube settler’s channel collects solids

into a compact mass which promotes the solids to slide down the tube channel. The floc solids

next will be go through sludge treatment process.

The figure 8 below shows the differences between the conventional settling and by using

tube settlers.

Shape of tube settlers Module of tube settlers

Top view of tube settlers

Dimensions: W: 250mm, W1: 100mm, W2: 25mm, H1: 60mmThickness: 1.2mmWeight (dry): 89kg/mᵌMaterial: PVCMaximum operation temperature: 55 C⁰Standard module length: 900-2000mmVertical module height: 750-1750mm

Figure 7 Standard tube settlers.

We use the tube settler system in our sedimentation process because the tube settlers offer

an inexpensive method of upgrading existing water treatment plant clarifiers and sedimentation

basins to improve performance. They can also improve the performance of existing settling basins

by reducing the solids loading on downstream filters. It is made of lightweight PVC, tube settlers

can be easily supported with minimal structures that often incorporate the effluent trough

supports.

Figure 8 The differences between the conventional settling and by using tube settlers.

The figure 9 below illustrates the tube settler system of installation. The basin is

constructed and support before the tube settler is arranged to fix the basin. The standard frame is

used to support it to be 60 inclined.⁰

Figure 9 Tube settler installation process.

The basins equipped with tube settlers can operate at 2 to 4 times the normal rate of basins

without tube settlers. It is possible to cut coagulant dosage by up to half while maintaining a lower

influent turbidity to the treatment plant filters. Less filter backwashing equates to significant

operating cost savings for both water and electricity. New installations using tube settlers can be

designed smaller because of increased flow capability.

Flows of existing water treatment plants can be increased through the addition of tube

settlers. Tube settlers increase allowable flow capacity by expanding settling capacity and

increasing the solids removal rate in settling tanks. Like any type of equipment, tube settlers will

require periodic cleaning and maintenance. Basin walkway design or a protective covering above

the tube settlers should be provided.

Design of Sedimentation

Total Flow Rate, Q = 293366.70 m3/day

=293.4 x 103 m3

day x

1day24 hr

= 12225 m3/hr

Number of basins = 4

Flow rate for each tank, Qtank = Flowrate , QNumber of basin

= 12225 m3/hr4

= 3056.25 m3/hr

Length, L = 50 m

Width, W = 30.5 m

Depth, D = 5 m

Volume of basin, V = Length x Width x Depth

= 50 m x 30.5 m x 5 m

= 7625 m3

Detention time of each clarifier, t = Volumeof basin ,VQ

= 7625 m3

3056.25m3

hr

= 2.495 hr

Flow rate for each tank, Qtank =3056.25 m

3

hr x

24 hr1day

= 73350 m3/day

Area of surface of each basin, As = Length x Width

= 50 m x 30.5 m

= 1525 m2

Overall flow rate, u = Flowrate ,Qtank

Area of surface of eachbasin , A s

=73350

m3

day1525 m2

= 48.1 m3/m2.day

Side Surface Area of each basin, A = Width x Depth

= 30.5 m x 5 m

= 152.5 m2

Horizontal Velocity, v = Flowrate , Qtank

Side Surface Area of eachbasin, A

=73350

m3

day152.5 m2

= 480.98 m3/m2.day

There are 3 rows of weir plates along the length of the tank with 0.9m wide central

effluent collection channel.

Row of weirs plate = 3

Length of weir, Lw = Row of weirs plate x (Width - 0.9 m)

= 3 x (30.5 m - 0.9 m)

= 88.8 m

Weir Loading rate, uw = Flowrate , Qtank

Lengthof weir , Lw

=73350

m3

day88.8 m

= 826 m3/m.day

V-notch weirs

Figure 5-6: V-Notch Sharp Crested Weir

The discharge over an unsubmerged V-Notch sharp-crested weir is defined as:

(5.38)

Where Q = Discharge over weir (m3/sec., ft3/sec.)C = Coefficient of discharge (C = 0.58 typically used for a 90° V-notch

weir)Q = Angle of notch (degrees)H = Head above bottom of notch (m, ft)

Table 5-1: V-Notch Weir Coefficient of Discharge

Head (dfeet)Weir Angle (degrees)

22.5 30 45 60 90 120

0.5 .611 .605 .596 .590 .584 .581

1.0 .593 .590 .583 .580 .576 .575

1.5 .586 .583 .578 .575 .572 .672

2.0 .583 .580 .576 .573 .571 .571

2.5 .580 .578 .574 .572 .570 .570

3.0 .579 .577 .574 .571 .570 .570

Design of V-Notch Weir

Flow rate for each tank, Qtank =3056.25 m

3

hr x

24 hr1day

= 73350 m3/day

Water depth over the notch = 4 cm

= 0.04 m

Coefficient of discharges, C = 0.58

Gravitational forces, g = 9.81 m/s2

Degree of V-notch weir, ∅ = 90 ⁰

Head above bottom of notch, H = 0.04 m

Weir discharges, q = 815

C√2 g [ tan(∅2 )] H52

= 815

x 0.58x √2x 9.81[ tan( 902 )]0.04

52

= 0.000438 m3/s

To determine the number of 90 ⁰ V-notch

Flow rate for each tank, Qtank =3056.25 m

3

hr x

1hr60 min

x 1min60 s

= 0.849 m3/s

Number of Notch, Nnotch = Flowrate ,Q tank

Weir discharge , q

= 0.849 m3 /s0.000438 m3/s

= 1938.36

≈ 1938 notches

1 tank contain 12 weirs plates, therefore 1 weirs plates contains:-

1 weirs plates = 193812

= 161.5 ≈ 162