Lesson 14:

Sedimentation and Flotation


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:




For a circular clarifier you will first use the following equation to determine surface area:

Surface Area, ft2 = 0.785 x (diameter, ft)2


The setting tank is 150 feet in diameter and the flow to the unit is 5.5 MGD. What is the surface loading rate in gallons/day/ft2?

Surface Area, ft2 = 0.785 x (150 ft)2

Surface Area, ft2 = 0.785 x 22,500 ft2

Surface Area, ft2 = 17,663 ft2


Now that we know the surface area we can calculate the surface loading rate in gallons/day/ft2.




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?

*When dealing with a circular tank you will use the following formula to determine the weir length:

Length of circular weir = Π (pi) x weir diameter





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. 


Part 1 of your Assignment: Complete the math worksheet for this lesson and return to instructor via email, fax or mail.. Each question is worth 25 points.

Part 2 of your Assignment: Complete Assignment 14 on Sedimentation and Flotation. You may do the Assignment online to get credit or print it out and send it to the instructor. (50 points)




Answer the questions in the Lesson 14 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.