1

Department of Civil Engineering-I.I.T. Delhi

CEL 212: Environmental Engineering Second Semester 2012-13

Sedimentation Solution

Q1. Determine the settling velocity of a spherical particle with diameter of 100 micron and a specific

gravity of 2.3 in water at 25degC?

Hint: See lecture notes; Solution of Q4

Q2. Define following: (i) Type 1 suspension; (ii) flocculating particles (iii) dilute suspensions, and (iv)

concentrated suspensions.

Hint: See lecture notes

Q3. What is the basic difference between settling of inert suspended particles and biological solids (i.e.,

biomass)? How would the settling pattern change if I have a sample with 100 mg/L inert suspended

particles and 800 mg/L biological solids?

Hint: See lecture notes;

Q4. Determine the settling velocity of a spherical particle with diameter of 200 micron and a specific

gravity of 2.3 in water at 25degC? Comment on settling behaviour of this type of particles.

[8+2=10 points]

2

Q5. For a flocculants suspension, determine the removal efficiency for a basin 10 ft deep with an

overflow rate V0 equal to 10 ft/h, using the laboratory settling data? Which parameters need to be varied

to increase removal efficiency? [15 points]

3

Q6. A settling column analysis is run on a type-I suspension. The settling column is 2 m tall and the

initial concentration of the well-mixed sample is 650 mg/L. Results of the analysis are below:

Time, min. 0 58 77 91 114 154 250

conc. remaining, mg/L 650 560 415 325 215 130 52

What is the theoretical efficiency of the settling basin that receives this suspension if the loading rate is

2.4×10-2

m/min? Which parameters need to be varied to increase removal efficiency? [5 points]

4

Q7. Can 100 nm nanoparticles in wastewater be removed using sedimentation tank in a municipal

wastewater treatment? [10 points]

Hint: See lecture notes; no as it will require long time (>4 hours), so practically no.

Q8. Revisit Example 4-2 (Peavy et al., Text Book). Consider a scenario where we have four discrete

particles in beginning (settling velocity=0.00017 m/min.). After settling for Z1 depth, two particles come

closer and start settling together (now we have only two discrete particles settling from depth Z1). Further

settling of two discrete particles (approximated for modelling purposes) for Z2 (Z2>Z1) depth from the

top of the column, all particles come closer and started settling together as a single particle only (now we

have only one discrete particle settling starting from depth Z2). Model the settling process of these

particles. Is it possible to model removal of these particles in the settling column and calculate overall

removal? What additional information you would like to have, if any?

Hint: As per discussion in class

Q9. Using the data from Q6, determine the theoretical efficiency of a settling basin with a surface area of

500 m2 and 14400 m

3/d? Given that vi = 0.04 x, where x is remaining fraction of suspended particles. [5

points]

Hint: As per solution of Q4

Q10. Revisit Example 4-2 (Peavy et al.,Text Book). Using the given data and developed curve, if

percentage overall removal is 80% what is the loading rate in the tank? (unit: m3/m2-d). [Hint: Need to do

trial-and-error to get overall flow rate value.] [5 points]

Q11. Revisit Example 4-3 (Peavy et al.,Text Book). Using the given data, calculate overall removal from

the settling basin at 2 m instead of at 3m depth? Comment on removals observed at 2m and 3 m settling

basin depths.

5

Hint: As per solution of Q5

Q12. Coagulation is generally used to collect all particles together with the help of chemicals and

polymer and then these collected particles are removed together in the settling process. Should I use this

process to remove discrete or flocculent particles? Why? Explain removal of these particles during the

coagulation process using alum chemistry.

Hint: See lecture notes

Q13. Solve Example 4-6 (Peavy et al.,Text Book). Subsequently, solve problem 4-34 (Peavy et al., Text

Book).