Lesson 4:

Primary Treatment



In this lesson we will answer the following questions:



Reading Assignment

Read the online lecture, also reference Chapter 4 in your textbook, Operation of Wastewater Treatment Plants Vol. 1





After the pretreatment the next step in the process is to let the flow slow way down so that anything else that will settle by gravity can do so. This process is known as primary sedimentation. What matters is how fast the flow is allowed to go through the tank. This velocity is usually kept around 1 ft/min or 0.3 m/min, which is alot slower than the flow going through the grit channel. At this average velocity the detention time of a primary sedimentaiton tank is often around two hours. The material that settles to the bottom of a primary sedimentation tank is scraped to a s ump and removed by a pump. This watery mixture of solids is called primary sludge.




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.

Not nearly as many of the suspended solids are removed by primary sedimentation. Only about half of the suspended solids are big enough to settle by gravity. The smaller suspended solids are big enough to be trapped on the filter paper but stay suspended in the water no matter how long the sample is allowed to settle. Generally speaking, the primary sedimentation will remove between 50% and 70% of the suspended solids. A larger percentage of the suspended solids can be removed if chemicals are added to help with settling.

Many of the settling solids in the water contribute to the BOD. With the removal of the settlable solids, the BOD will also be reduced. The difference in the BOD concentration from raw influent to primary effluent ranges from 25% to 35%. The Primary Sedimentation Tank's expected performance removals include:



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.  
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.  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.   

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

One of the most important things to remember is how much of each type of solids and BOD can be removed from the wastewater by using primary sedimentation.


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:



Hydraulic Detention Time

Based on these removals it's safe to say that the biggest purpose of a Primary Sedimentation Tank is to remove settleable solids. This is accomplished by slowing the flow down to about 1 ft/min. The flow at this velocity will stay in the primary tank from 1.5 to 2.5 hours. The length of time the water stays in the tank is called the hydraulic detention time.


The influent flow rate to a Primary Sedimentation Tank is 1.36 MGD. The tank is 75 ft in length, 18 ft wide, and has a water depth of 12 ft. What is the detention time of the tank in hours?

Tank Volume = 121,176 gal



Detention Time = 2.1 hours




Surface Loading Rate

An imporant calculation to monitor is the surface loading rate, or surface overflow rate. This is the amount of flow going into a tank compared to the size of the water surface of the tank. It is believed that by keeping this value inside a certain range the removal of settleable solids will be maintained. The recommended range of values are from 800 to 1000 gpd/ft2.




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.





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?





Sludge Removal

How fast the sludge is removed from the primary tanks is largely dependent on what kind of solids handling equipment is being used. If the sludge is being pumped to a digester, thicker primary sludge would be better. A sludge concentration of 5% in this type of situation is reasonable. However, if the first solids handling unit is a gravity thickener, fresher solids in a thinner sludge (0.5 to 1.0%) is essential.


Primary sludge is pumped to a gravity thickener at a rate of 480 gpm. The sludge concentration is 1.4%. How many pounds of solids are pumped to the thickener daily?

Start first with the pounds of sludge (water and solids) going to the thickener:


We want to know how many pounds of solids this contains. The sludge is 1.4% solids. From this we can calculate:




Primary Effluent

By this point in the treatment process, we've removed the large debris, the grit and the settleable materials. What is left is what is referred to as the primary effluent. Primary effluent is usually cloudy and frequently grey in color. It has a lot of dissolved food and other chemicals still in it.

Two of the most important chemicals left to remove are phosphorus and ammonia. On one hand we don't want too much of these two to go out with the effluent, but if there isn't enough of either one of these in the raw effluent, the process won't work right. Both phosphorus and ammonia are needed by the carbonaceous "bugs" by serving as nutrients. The "bugs" need these nutrients the same way people need vitamins. If there is too much of these chemicals leaving the plant in the effluent, it will cause tremendous algae growth. Excess algae will use up oxygen.

Depending on where your plant is located, the treatment process may be concerned about treating just one of these. Phosphorus limits are most important to plants that discharge to lakes, whereas plants that are more inland are more concerned about ammonia concentration because of its bad effect on ground water.

Now that we know we don't want too much phosphorus and ammonia in the water, how do we reduce their concentrations in the water? The next step is called Secondary Treatment. In biological plants, the "bugs" are used to eat the dissolved food and chemicals. Different types of units can be used, but the principle is the same: the microscopic "bugs" eat the food and then we just get rid of some of the bugs. In the process, the BOD is lowered as is the concentration of phosphorus. Under certain conditions, the ammonia concentrations may also be reduced.






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. 




Operations of Wastewater Treatment Plants, California State University, Sacramento




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