SEDIMENTATION

 

Sedimentation, or clarification, is the process of letting suspended material settle by gravity.

Suspended material may be particles, such as clay or silts, originally present in the source water.

More commonly, suspended material or floc is created from material in the water and the

chemical used in coagulation or in other treatment processes, such as lime softening.

Sedimentation is accomplished by decreasing the velocity of the water being treated to a point below which the particles will no longer remain in suspension. When the velocity no longer supports the transport of the particles, gravity will remove them from the flow.

 

 

 

FACTORS AFFECTING SEDIMENTATION

Several factors affect the separation of settleable solids from water. Some of the more common types of factors to consider are:

 

 

 

 

PARTICLE SIZE

 

The size and type of particles to be removed have a significant effect on the operation of the sedimentation tank. Because of their density, sand or silt can be removed very easily. The velocity of the water-flow channel can be slowed to less than one foot per second, and most of the gravel and grit will be removed by simple gravitational forces. In contrast, colloidal material, small particles that stay in suspension and make the water seem cloudy, will not settle until the material is coagulated and flocculated by the addition of a chemical, such as an iron salt or aluminum sulfate.

The shape of the particle also affects its settling characteristics. A round particle, for example, will settle much more readily than a particle that has ragged or irregular edges.

 

All particles tend to have a slight electrical charge. Particles with the same charge tend to repel each other. This repelling action keeps the particles from congregating into floc and settling.

 

 

 

 

WATER TEMPERATURE

Another factor to consider in the operation of a sedimentation basin is the temperature of the water being treated. When the temperature decreases, the rate of settling becomes slower. The result is that as the water cools, the detention time in the sedimentation tanks must increase. As the temperature decreases, the operator must make changes to the coagulant dosage to compensate for the decreased settling rate. In most cases temperature does not have a significant effect on treatment. A water treatment plant has the highest flow demand in the summer when the temperatures are the highest and the settling rates the best. When the water is colder, the flow in the plant is at its lowest and, in most cases, the detention time in the plant is increased so the floc has time to settle out in the sedimentation basins.

 

 

 

 

CURRENTS

 

Several types of water currents may occur in the sedimentation basin:

 

Density currents caused by the weight of the solids in the tank, the concentration of solids and temperature of the water in the tank.

Eddy currents produced by the flow of the water coming into the tank and leaving the tank.

 

The currents can be beneficial in that they promote flocculation of the particles. However, water currents also tend to distribute the floc unevenly throughout the tank; as a result, it does not settle out at an even rate.

Some of the water current problems can be reduced by the proper design of the tank. Installation of baffles helps prevent currents from short circuiting the tank.

 

 

 

 

SEDIMENTATION BASIN ZONES

 

Sedimentation basins have 4 zones

1. The Inlet zone,

2. The Settling zone,

3. The Sludge zone, and

4. The Outlet zone.

Each zone should provide a smooth transition between the zone before and the zone after.

 Zones in Rectangular Sedimentation Basin

Zones in Rectangular Sedimentation Basin

Each and every zone has its own unique purpose. All zones are in a rectangular sedimentation basin.

Zones in a Circular Sedimentation Basin

Zones in a Circular Sedimentation Basin

In a square or circular basin (clarifier), water typically enters the basin from the center rather than from one end and flows out to outlets located around the edges of the basin. But the four zones can still be found within the clarifier the above figure.

 

 

Inlet Zone

The two primary purposes of the inlet zone of a sedimentation basin are to distribute the water and to control the water’s velocity as it enters the basin. In addition, inlet devices act to prevent turbulence of the water. The incoming flow in a sedimentation basin must be evenly distributed across the width of the basin to prevent short-circuiting. Short-circuiting is a problematic circumstance in which water bypasses the normal flow path through the basin and reaches the outlet in less than the normal detention time. In addition to preventing short-circuiting, inlets control the velocity of the incoming flow. If the water velocity is greater than 0.15 m/ sec, then floc in the water will break up due to agitation of the water. Breakup of floc in the sedimentation basin will make settling much less efficient.

Inlet arrangement for a rectangular basin

Inlet arrangement for a rectangular basin

 

 

The inlet of rectangular basin is shown in the figure above. The stilling wall, also known as a perforated baffle wall, spans the entire basin from top to bottom and from side to side. Water leaves the inlet and enters the settling zone of the sedimentation basin by flowing through the holes evenly spaced across the stilling wall.

The second type of inlet allows water to enter the basin by first flowing through the holes evenly spaced across the bottom of the channel and then by flowing under the baffle in front of the channel.
The combination of channel and baffle serves to evenly distribute the incoming water.

 

 

Settling Zone

After passing through the inlet zone, water enters the settling zone where water velocity is greatly reduced. This is where the bulk of settling occurs and this zone will make up the largest volume of the sedimentation basin. For optimal performance, the settling zone requires a slow, even flow of water. The settling zone may be simply a large area of open water.

 

 

Outlet Zone

The outlet zone controls the amount of water flowing out of the sedimentation basin. Like the inlet zone, the outlet zone is designed to prevent short-circuiting of water in the basin. In addition, a good outlet will ensure that only well-settled water leaves the basin and enters the filter. The outlet in the form of overflow weir can also be used to control the water level in the basin. The best quality water is usually found at the very top of the sedimentation basin, so outlets are usually designed to skim this water off the sedimentation basin.

 Outlet arrangemenfin rectangular basin

Outlet arrangement in rectangular basin

A typical outlet zone begins with a baffle in front of the effluent. This baffle prevents floating material from escaping the sedimentation basin and clogging the filters. After the baffle, the effluent structure, which usually consists of a launder, weirs, and effluent piping, is located. A typical effluent structure is shown in the figure.
The primary component of the effluent structure is the effluent launder, a trough which collects the water flowing out of the sedimentation basin and directs it to the effluent piping. The sides of a launder typically have weirs attached. Weirs are walls preventing water from flowing uncontrolled into the launder. The weirs serve to skim the water evenly off the tank.

 

Finger weirs in rectangular basin

Finger weirs in rectangular basin

 

 

A weir usually has notches, holes, or slits along its length. These holes allow water to flow into the weir. The most common type is the V -shaped notch shown on the picture above which allows only the top few centimeters of water to flow out of the sedimentation basin. Conversely, the weir may have slits cut vertically along its length, an arrangement which allows for more variation of operational water level in the sedimentation basin.
Water flows over or through the holes in the weirs and into the launder. Then the launder channels the water to the outlet pipe. This pipe carries water away from the sedimentation basin and to the next step in the treatment process. The effluent structure may be located at the end of a rectangular sedimentation basin or around the edges of a circular clarifier. Alternatively, the effluent may consist of finger weirs an arrangement of launders which extend out into the settling basin as shown below.

 

 

Sludge Zone

The sludge zone is found across the bottom of the sedimentation basin where the sludge is collected temporarily. Velocity in this zone should be very slow to prevent re-suspension of sludge.
A drain at the bottom of the basin allows the sludge to be easily removed from the tank. The tank bottom should slope toward the drains to further facilitate sludge removal. In some plants, sludge removal is achieved continuously using automated equipment. In other plants, sludge must be removed manually.

Related Topics:

1.     Flocculation Basin Flocculation is the operation in which the coagulated water must...

2.     Sedimendation  Purpose Of  Sedimentation Sedimentation is a unit operation to settle...

 

 

 

 

 


SELECTION OF BASIN

There are many sedimentation basin shapes. They can be rectangular, circular, and square.

 

 


 

 

Rectangular Basins

 

Rectangular basins are commonly found in large-scale

water treatment plants. Rectangular tanks are popular

as they tend to have:

High tolerance to shock overload

Predictable performance

Cost effectiveness due to lower construction cost

Lower maintenance

Minimal short circuiting

 

 

 

 

 


Circular and Square Basins

 

Circular basins are frequently referred to as clarifiers. These basins share some of the performance advantages of the rectangular basins, but are generally more prone to short circuiting and particle removal problems. For square tanks the design engineer must be certain that some type of sludge removal equipment for the corners is installed.

 

 

 

 

 

 

 

 

HIGH RATE SETTLERS

 

High rate tube settlers are designed to improve the characteristics of the rectangular basin and to increase flow through the tank. The tube settlers consist of a series of tubes that are installed at a 60 degree angle to the surface of the tank. The flow is directed up through the settlers. Particles have a tendency to flow at an angle different than the water and to contact the tube at some point before reaching the top of the tube. After particles have been removed from the flow and collected on the tubes, they tend to slide down the tube and back into the sludge zone.

 

 

 

 

 

 

 

 

SOLIDS CONTACT UNITS

 

A solids contact unit combines the coagulation, flocculation, and sedimentation basin in one unit. These units are also called upflow clarifiers or sludge-blanket clarifiers. The solids contact unit is used primarily in the lime-soda ash process to settle out the floc formed during water softening. Flow is usually in an upward direction through a sludge blanket or slurry of flocculated suspended solids.

 

 

 

 

 

Sedimentation Basin Design and Problems

Factors Influencing Efficiency

Floc Characteristics

To a large extent, a sedimentation basin's efficiency will depend on the efficiency of the preceding coagulation/flocculation process.  The size, shape, and density of the floc entering the sedimentation basin will all influence how well the floc settles out of the water.  Floc which is too small or too large, is irregularly shaped, or has a low density will not tend to settle out in the sedimentation basin. 

Even if the coagulation/flocculation process is very efficient, floc can disintegrate on its way to or in the sedimentation basin.  Previously formed floc will disintegrate if the water velocity is too high, if there are sharp bends in the pipe at the inlet...

Sharp bends break up floc.


if water is discharged above the sedimentation basin water level...

Water discharged above basin water level causes floc breakup.

or if throttle valves are used. 

 

 

Short-circuiting

Another major cause of inefficiency in the sedimentation basin is short-circuiting, which occurs when water bypasses the normal flow path through the basin and reaches the outlet in less than the normal detention time.  The picture below shows a basin in which the water is flowing primarily through the left half of the basin.  (Flowing water is shown as green blobs.)  An efficient sedimentation basin would have water flowing through the entire basin, rather than through just one area.

Short circuiting.

 

 


When water in the sedimentation basin short-circuits, floc does not have enough time to settle out of the water, influencing the economy of the plant and the quality of the treated water.

Short-circuiting in a sedimentation basin can be detected in a variety of ways.  If areas of water in the basin do not appear to be circulating, or if sludge buildup on the bottom of the basin is uneven, then tests may be called for.  Floats or dyes can be released at the inlet of the basin to determine currents.

A variety of factors can cause short-circuiting in a sedimentation basin.  Basin shape and design, along with design of the inlet and outlet, can cause short-circuiting.  You may remember from the last lesson that a long, thin sedimentation basin is less likely to short-circuit than is a short broad one.  Uneven distribution of flow either at the inlet or outlet can also cause short-circuiting.  If the weir at the outlet is not level or if some of the notches clog, flow will be uneven and will cause short-circuiting. 

In addition to the design of the basin, characteristics of the water can also cause short-circuiting.  Differences of temperature can cause stratification of the water - separation of water into bands of different temperature.  Incoming water will tend to flow through the band of water which corresponds to its own temperature, and will not spread throughout the rest of the basin. 

 

Short-circuiting due to temperature stratification.


(We should note that temperature can cause other problems with sedimentation as well.  Cold water prevents floc from settling, so that longer settling times or larger doses of coagulant chemicals are needed.)

 

 

Other Problems

Gases in the water may cause floating scum, which can carry over into the filters.  Sprinkling water on the on the scum may cause the scum to settle, but it is usually a  better practice to find and fix the source of the problem.  Gases in the sedimentation basin are usually caused by water being introduced in the pump or by a leak in the raw water line. 
 
Another sedimentation basin problem is algal growth.  If sedimentation basins have sufficient sunlight, algae will grow on the walls of the basin.  These algae can break loose and clog the filter.  Algae are best treated with shock chlorination, a method of feeding 5-10 ppm of chlorine into the raw water or of sprinkling HTH around the basin walls just before the plant is shut down for a few hours.  The chlorine will kill the algae while the chlorinated water sits in the tank.

A few other factors can also influence sedimentation basin efficiency.  Intermittent operation of the basin can cause settling problems.  Also, design problems such as excessive surface loading or weir loading can cause problems.  We will discuss surface and weir loading in the second half of this lesson. 

 

 

Testing

Inefficient operation of the sedimentation basin generally results in floc, scum, or algae carryover from the sedimentation basin to the filter.  This carryover will result in clogging of filters and sometimes in degradation of the finished water quality.  In order to prevent these problems, the operator should monitor the water entering, leaving, and in the sedimentation basin.

The turbidity of water entering and leaving the sedimentation basin should be tested regularly.  This test is a direct measure of the efficiency of the sedimentation process in removing suspended particles from water. 

The temperature of water entering the sedimentation basin should also be tested.  Temperature changes, as you remember, can cause short-circuiting and can prevent floc settling.  However, temperature changes are usually gradual and seldom cause sudden changes in the sedimentation process.

Finally, the operator should perform a visual survey of the sedimentation basin.  He or she should note sludge and floc conditions, settling, and the clarity of the water in the outlet.  If floc is being carried over the weirs, this condition is usually evident to the trained operator and can be corrected. 

 

 

Designing a Rectangular Sedimentation Tank

Introduction

Designing a rectangular sedimentation tank is similar in many ways to designing a flocculation chamber.  However, water in a sedimentation basin is not agitated, so the velocity gradient is not a factor in the calculations.  Instead, two additional characteristics are important in designing a sedimentation basin. 

The overflow rate (also known as the surface loading or the surface overflow rate) is equal to the settling velocity of the smallest particle which the basin will remove.  Surface loading is calculated by dividing the flow by the surface area of the tank.  Overflow rate should usually be less than 1,000 gal/day-ft.2 

The weir loading is another important factor in sedimentation basin efficiency.  Weir loading, also known as weir overflow rate, is the number of gallons of water passing over a foot of weir per day.  The standard weir overflow rate is 10,000 to 14,000 gpd/ft and should be less than 20,000 gpd/ft.  Longer weirs allow more water to flow out of the sedimentation basin without exceeding the recommended water velocity.

 

 

Specifications

The sedimentation basin we will design in this lesson will be a rectangular sedimentation basin with the following specifications:




Diagram of sedimentation basin.

  • Rectangular basin
  • Depth: 7-16 ft
  • Width: 10-50 ft
  • Length: 4 × width
  • Influent baffle to reduce flow momentum
  • Slope of bottom toward sludge hopper >1%
  • Continuous sludge removal with a scraper velocity <15 ft/min
  • Detention time: 4-8 hours
  • Flow through velocity: <0.5 ft/min
  • Overflow rate: 500-1,000 gal/day-ft2
  • Weir loading: 15,000-20,000 gal/day-ft

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Overview of Calculations

We will determine the surface area, dimensions, and volume of the sedimentation tank as well as the weir length.  The calculations are as follows:

1.      Divide flow into at least two tanks.

2.      Calculate the required surface area.

3.      Calculate the required volume.

4.      Calculate the tank depth.

5.      Calculate the tank width and length.

6.      Check flow through velocity.

7.      If velocity is too high, repeat calculations with more tanks.

8.      Calculate the weir length.

 

1. Divide the Flow

The flow should be divided into at least two tanks and the flow through each tank should be calculated using the formula shown below:

Qc = Q / n

Where:

Qc =  flow in one tank
Q   =  total flow
 n   =  number of tanks


We will consider a treatment plant with a flow of 1.5 MGD.  We will divide the flow into three tanks, so the flow in one tank will be:

Qc = (1.5 MGD) / 3

Qc = 0.5 MGD

 

 

2. Surface Area

Next, the required tank surface area is calculated.  We will base this surface area on an overflow rate of 500 gal/day-ft2 in order to design the most efficient sedimentation basin. 

The surface area is calculated using the following formula:

A = Qc / O.R.

Where:

A = surface area, ft2
Qc = flow, gal/day
O.R. = overflow rate, gal/day-ft2



In our example, the surface area of one tank is calculated as follows:


A = (500,000 gal/day) / (500 gal/day-ft2)

A = 1,000 ft2


(Notice that we converted the flow from 0.5 MGD to 500,000 gal/day before beginning our calculations.)

 

 

3. Volume

The tank volume is calculated just as it was for flocculation basins and flash mix chambers, by multiplying flow by detention time.  The optimal detention time for sedimentation basins depends on whether sludge removal is automatic or manual.  When sludge removal is manual, detention time should be 6 hours.  We will consider a tank with automatic sludge removal, so the detention time should be 4 hours.

The volume of one of our tanks is calculated as follows:

V = Q t

V = (500,000 gal/day) (4 hr) (1 day/24 hr) (1 ft3/7.48 gal)

V = 11,141 ft3


(Notice the conversions between days and hours and between cubic feet and gallons.)

 

 

4. Depth

The tank's depth is calculated as follows:

d = V / A

Where:

d = depth, ft
V = volume, ft3
A = surface area, ft2



For our example, the depth is calculated to be:


d = (11,141 ft3) / (1,000 ft2)

d = 11.1 ft


The specifications note that the depth should be between 7 and 16 feet.  Our calculated depth is within the recommended range.  If the depth was too great, we would begin our calculations again, using a larger number of tanks.  If the depth was too shallow, we would use a smaller number of tanks. 

 

 

5. Width and Length

You will remember that the volume of a rectangular solid is calculated as follows:

V = L W d

Where:

V = volume
L = length
W = width
d = depth



For our tank, the length has been defined as follows:


L = 4 W



Combining these two formulas, we get the following formula used to calculate the width of our tank:


Calculating width.



In the case of our example, the tank width is calculated as follows:

Calculations

W = 15.8 ft



The length is calculated as:

L = 4 (15.8 ft)

L = 63.2 ft

 

 

6 and 7. Flow Through Velocity

Checking the flow through velocity is done just as it was for the flocculation basin.  First, the cross-sectional area of the tank is calculated:

Ax = W d

Ax = (15.8 ft) (11.1 ft)

Ax = 175.4 ft2


Then the flow through velocity of the tank is calculated (with a conversion from gallons to cubic feet and from days to minutes):


V = Qc / Ax

V = (0.0000928 ft3-day/gal-min) (500,000 gal/day) / (175.4 ft2)

V = 0.26 ft/min



The velocity for our example is less than 0.5 ft/min, so it is acceptable.  As a result, we do not need to repeat our calculations.

 

 

8. Weir Length

The final step is to calculate the required length of weir.  We will assume a weir loading of 15,000 gal/day-ft and use the following equation to calculate the weir length:

Lw = Qc / W.L

Where:

Lw = weir length, ft
Qc = flow in one tank, gal/day
W.L. = weir loading, gal/day-ft



So, in our example, the weir length is calculated as follows:


Lw = (500,000 gal/day) / (15,000 gal/day-ft)

Lw = 33.3 ft


The weir length should be 33.3 ft. 

 

 

Conclusions

Our plant should build a sedimentation tank which is 11.1 feet deep, 15.8 feet wide, and 63.2 feet long.  This tank will have a surface area of 1,000 ft2 and a volume of 11,141 ft3.  The flow through velocity will be 0.26 ft/min.  The weir length will be 33.3 ft. 

 



Review

Sedimentation basin efficiency is influenced by floc characteristics, water temperature, short-circuiting, gases in the water, algal growth on tank walls, intermittent tank operation, surface loading, and weir loading.  To ensure optimal performance, the operator should test turbidity and temperature of the water and should visually survey the basin 

Design of a sedimentation basin involves the following steps:

·  Divide flow into at least two tanks.

·  Calculate the required surface area.

·  Calculate the required volume.

·  Calculate the tank depth.

·  Calculate the tank width and length.

·  Check flow through velocity.

·  If velocity is too high, repeat calculations with more tanks.

·  Calculate the weir length.





New Formulas Used

To calculate tank surface area:


A = Qc / O.R.



To calculate tank depth:


d = V / A



To calculate width of a rectangular tank where length is four times the width:


Calculating width.

 

To calculate length of a rectangular tank where length is four times the width:

L = 4 W


To calculate flow through velocity:

V = Qc / Ax



To calculate weir length:

Lw = Qc / W.L