Lesson 3:

Sedimentation and Flotation


In this lesson we will learn the following:
  • How does sedimentation fit into the water treatment process?
  • What zones are present in a sedimentation basin?
  • How is sedimentation sludge disposed of?



Reading Assignment

There are no other reading assignments.


Primary Sedimentation


Sedimentation is a treatment process in which the velocity of the water is lowered below the suspension velocity and the suspended particles settle out of the water due to gravity.  The process is also known as settling or clarification

Settled solids are removed as sludge, and floating solids are removed as scum in a clarifier. Wastewater leaves the sedimentation tank over an effluent weir to the next step of treatment. The efficiency or performance of the process is controlled by: detention time, temperature, tank design, and condition of the equipment.

Most water treatment plants include sedimentation in their treatment processes.  However, sedimentation may not be necessary in low turbidity water of less than 10 NTU.  In this case, coagulation and flocculation are used to produce pinpoint (very small) floc which is removed from the water in the filters. 



Location in the Treatment Process

In most municipal wastewater treatment plants, the treatment unit that immediately follows the grit channel is the sedimentation and flotation unit. This unit may be called a settling tank, sedimentation tank, or clarifier. The most common name is primary clarifier, since it helps to clarify or clear up the wastewater. 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 pretreatment process, where it is known as presedimentation .  Presedimentation can also be called plain sedimentation because the process depends merely on gravity and includes no coagulation and flocculation.  Without coagulation/flocculation, plain sedimentation can remove only coarse suspended matter (such as grit) 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, presedimentation removes most of the sediment in the water during the pretreatment stage.  So presedimentation 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, presedimentation 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. 

The rest of this lesson will be concerned with sedimentation following coagulation and flocculation.  We will consider types of sedimentation basins and parts of a typical sedimentation basin, as well as the disposal of sludge.



Types of Basins

Three common types of sedimentation basins are shown below:

Rectangular sedimentation basin.

Rectangular basins are the simplest design, allowing water to flow horizontally through a long tank.  This type of basin is usually found in large-scale water treatment plants.  Rectangular basins have a variety of advantages - predictability, cost-effectiveness, and low maintenance.  In addition, rectangular basins are the least likely to short-circuit, especially if the length is at least twice the width.  A disadvantage of rectangular basins is the large amount of land area required.

Double-deck basin.

Double-deck rectangular basins are essentially two rectangular sedimentation basins stacked one atop the other.  This type of basin conserves land area, but has higher operation and maintenance costs than a one-level rectangular basin. 


Square or circular sedimentation basins with horizontal flow are often known as clarifiers .  This type of basin is likely to have short-circuiting problems. 

A fourth type of sedimentation basin is more complex.  Solids-contact clarifiers , also known as upflow solids-contact clarifiers or upflow sludge-blanket clarifiers combine coagulation, flocculation, and sedimentation within a single basin.  Solids-contact clarifiers are often found in packaged plants and in cold climates where sedimentation must occur indoors.  This type of clarifier is also often used in softening operations. 


The sedimentation and flotation units commonly found are primary clarifiers, secondary clarifiers, combined sedimentation-digestion units, flotation units, and Imhoff tanks. The most important function of the primary clarifier is to remove as much settleable and floatable material as possible. Removal of organic settleable solids is very important because they cause a high demand for oxygen (BOD) in receiving waters or subsequent biological treatment units in the treatment plant.

If the horizontal velocity of water is slowed to a rate of 1.0 to 2.0 feet of travel per minute most particles with a specific gravity of 1.05 (5% more than water) will settle to the bottom of the container. Specific gravity of water is 1.000 at 4.0°C or 39°F; it weighs 8.34 pounds per gallon. There are many factors that will influence settling characteristics in a particular clarifier. A few of the more common ones are temperature, short circuits, detention time, weir overflow rate, surface loading rate, and solids loading. Water expands as temperature increases (above 4°C) or contracts as temperature decreases (down to 4°C). Below 4°C, the opposite is true. In general, as water temperature increases, the settling rate of particles increases; as temperature decreases, so does the settling rate. Molecules of water react to temperature changes. They are closer together when liquid temperature is lower; thus density increases and water becomes heavier per given volume because there is more of it in the same space. As water becomes more dense, the density difference between water and solid particles becomes less; there the particles settle more slowly.





All sedimentation basins have four zones - the inlet zone, the settling zone, the sludge zone, and the outlet zone.  Each zone should provide a smooth transition between the zone before and the zone after.  In addition, each zone has its own unique purpose.

Zones can be seen most easily in a rectangular sedimentation basin, such as the one shown below:

Zones in a sedimentation basin.

In a 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:



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.  We will discuss short-circuiting in the next lesson.

In addition to preventing short-circuiting, inlets control the velocity of the incoming flow.  If the water velocity is greater than 0.5 ft/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. 

Two types of inlets are shown below.  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.

From: Water Works Operator's Manual

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 floc 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 expanse of open water.  But in some cases, tube settlers and lamella plates, such as those shown below, are included in the settling zone. 

Settling tubes.
Tube settlers and lamella plates -
Water flows up through slanted tubes or along slanted plates. 
Floc settles out in the tubes or plates and drifts back down into the
lower portions of the sedimentation basin.  Clarified water passes
through the tubes or between the plates and then flows out of the basin.

Tube settlers and lamella plates increase the settling efficiency and speed in sedimentation basins.  Each tube or plate functions as a miniature sedimentation basin, greatly increasing the settling area.  Tube settlers and lamella plates are very useful in plants where site area is limited, in packaged plants, or to increase the capacity of shallow basins. 



Outlet Zone

The outlet zone controls the water flowing out of the sedimentation basin - both the amount of water leaving the basin and the location in the basin from which the outflowing water is drawn.  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 can also be used to control the water level in the basin. 

Outlets are designed to ensure that the water flowing out of the sedimentation basin has the minimum amount of floc suspended in it.  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. 

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 comes the effluent structure, which usually consists of a launder, weirs, and effluent piping.  A typical effluent structure is shown below:

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. 

A weir usually has notches, holes, or slits along its length.  These holes allow water to flow into the weir.  The most common type of hole is the V-shaped notch shown on the picture above which allows only the top inch or so 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 , or effluent 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.

Finger weirs.



Sludge Zone

The sludge zone is found across the bottom of the sedimentation basin where the sludge collects temporarily.  Velocity in this zone should be very slow to prevent resuspension 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.  If removed manually, the basin should be cleaned at least twice per year, or more often if excessive sludge buildup occurs.  It is best to clean the sedimentation basin when water demand is low, usually in April and October.  Many plants have at least two sedimentation basins so that water can continue to be treated while one basin is being cleaned, maintained, and inspected. 

If sludge is not removed from the sedimentation basin often enough, the effective (useable) volume of the tank will decrease, reducing the efficiency of sedimentation.  In addition, the sludge built up on the bottom of the tank may become septic , meaning that it has begun to decay anaerobically.  Septic sludge may result in taste and odor problems or may float to the top of the water and become scum.  Sludge may also become resuspended in the water and be carried over to the filters. 



Flotation Processes

After primary sedimentation, wastewater always contains some suspended solids that neither settle nor float to the surface and therefore remain in the liquid as it passes through the clarifier. Dissolved solids will, of course, travel through the clarifiers because they are unaffected by these units. Colloids and emulsions are two other forms of solids that are very difficult to remove. Colloids are very small, finely divided solids (particulates that do not dissolve) that remain dispersed in liquid for a long time due to their small size and electrical charge. Colloids are usually less than 200 millimicrons in size, and generally will not settle readily. Organic colloids exert a high oxygen demand, so their removal is desirable. The typical percentage of suspended solids in the primary clarifier is 40-60%.




Sludge Disposal

Sludge Composition

The sludge which is found in the bottom of a sedimentation tank is primarily composed of water.  The solids in the sludge are mainly excess coagulant, such as alum.  Alum sludge has a solids concentration of only about 1% when automatically removed from the basin, or about 2% if manually removed.  (The greater solids concentration of manually removed sludge is due to a small amount of gravity thickening.)

Many options exist for disposal of sedimentation sludge.  Here we will discuss disposal in streams, sanitary sewers, lagoons, and landfills.



Disposal in Streams and Sewers

In the past, sludge and backwash water was typically released into streams and other bodies of water.  However, this practice is becoming much less common and is now well regulated.  Backwash water and sludge can only be released into streams if a discharge permit has been granted by the NPDES.  The NPDES further requires extensive daily monitoring of the water quality when the sludge is being discharged. 

Alternatively, sludge may be piped directly to the sanitary sewer.  However, this disposal option also has its disadvantages.  Sludge can cause sewer blockages.  In addition, fees charged by the wastewater treatment plant can be expensive.  Releasing sludge into the sewage lines in large batches makes treatment of the wastewater problematic, so sludge is typically released slowly over a long time period, which requires a large holding tank at the water treatment plant. 



Thickening Sludge

Most of the other alternatives require transporting sludge away from the treatment plant.  Sludge is typically dried before it is trucked away since the greater volume of wet sludge makes it much more expensive to transport.  This drying process is known as dewatering or thickening .  Alum sludge is difficult to thicken, but a variety of devices have been developed to thicken the sludge, some of which are explained below. 

In many cases, the sludge is treated by the addition of polymers to aid in the dewatering process.  Alternatively, the sludge can be heated or frozen and thawed to increase the solids concentration.  Treating the sludge to aid in thickening is known as conditioning the sludge.

Once the sludge has been conditioned, it may be thickened in a lagoon, drying bed, or one of several other devices. 

Lagoons, which are small-volume storage ponds, are the simplest device used to thicken sludge. These lagoons are filled with sludge and the solids are allowed to settled due to gravity to the bottom of the lagoon while the clear water is pumped off the top.

After a few months, gravity and evaporation will have reduced the sludge to a 30-50% solid state.  The sludge can then be covered with soil and left on site, or may be trucked to a landfill off-site.

Drying beds are often used to thicken sludge destined for a landfill more quickly than the sludge would be thickened in a lagoon.

A drying bed is similar in design to a sand filter, with a layer of sand underlain by a layer of gravel.  The sludge is applied to the top of the sand and the water percolates down through the sand and gravel and is drained away.  When the sludge is sufficiently dry, it is carefully removed from the top of the sand and is trucked to a landfill.  When sludge is preconditioned with chemicals, dewatering in a drying bed may take only a few days or weeks.

Other processes used to thicken sludge include filter presses, belt filter presses, centrifuges, and vacuum filters.   These processes result in sludge with a solids content ranging from 30 to 50%.




Any enclosed area, such as a wet well for a pump, may contain accumulated poisonous, asphyxiating, or explosive gases if ventilation is not proper. The most common of these gases are hydrogen sulfide, which causes a "rotten egg" odor; Chlorine, which causes death by suffocation and formation of acid in the lungs; Carbon dioxide, which can cause asphyxiation by displacing oxygen in a poorly ventilated area; Carbon monoxide, which interferes with the blood's ability to carry oxygen; and methane.




Process Control Calculations

As with many other wastewater treatment plant unit processes, process control calculations aid in determining the performance of the sedimentation process. Process control calculations are used in the sedimentation process to determine:



Surface Loading Rate

The surface loading rate is the number of gallons of wastewater passing over 1 square foot of tank per day. This can be used to compare actual conditions with design. (Plant designs generally use a surface loading rate of 300 to 1,200 gallons/day/ft2.) To view more examples, read on page 136 in your text.



The flow into a clarifier is 4.0 MGD in a tank 90 feet long and 35 feet wide. What is the surface loading rate?


You will have to convert 4.0 MGD to gal/day, which is 4,000,000 gpd. Then plug in the numbers:




Weir Overflow Rate

The weir overflow rate is the amount of water leaving the settling tank per linear foot of weir. The results of this calculation can be compared with design. Normally weir overflow rates of 10,000 to 20,000 gal/day/ft are used in the design of a settling tank.

To view more examples, read page 136 in your textbook.



The circular settling tank is 90 feet in diameter and has a weir along its circumference. This effluent flow is 2.55 MGD. What is the weir overflow rate in gal/day/foot?



Sludge Pumping

For the plant operator, for process control purposes, knowing the amount of sludge pumped each day is important. This information is not only important for proper operation of the sedimentation process, but for sludge treatment processes as well. Such information includes accurate data regarding the quantity of solids and volatile solids removed from the sedimentation tank.


The sludge pump operates 15 minutes per hour. The pump delivers 30 gallons/minute of sludge. Laboratory tests indicate that the sludge is 5.1% solids and 69% volatile matter. How many pounds of volatile matter are transferred from the settling tank to the digester?



Sedimentation Expected Performance

Primary sedimentation clarifiers can be expected to remove:

Settleable solids 90-95%
Total Suspended Solids 40-60%
BOD5 25-35%





Sedimentation is a process used to remove suspended particles from water using gravity.  Sedimentation may take place in a presedimentation device such as a reservoir, grit basin, debris dam, or sand trap; or sedimentation may follow coagulation/flocculation and take place in a rectangular sedimentation basin, a double-deck rectangular basin, a clarifier, or a solids-contact clarifier. 

Sedimentation basins typically have four zones - the inlet zone which controls the distribution and velocity of inflowing water, the settling zone in which the bulk of settling takes place, the outlet zone which controls the outflowing water, and the sludge zone in which the sludge collects.  Sludge may be disposed of a in sewer or stream or may be conditioned and then thickened in a lagoon, drying bed, filter press, belt filter press, centrifuge, or vacuum filter before being transported to a landfill. 




Answer the questions in the Lesson 3 quiz .  When you have gotten all the answers correct, print the page and either mail or fax it to the instructor.  You may also take the quiz online and submit your grade directly into the database for grading purposes.