WTO Coagulation and Flocculation 2 ā Questions and Answers
Question 1: What is the primary purpose of a jar test in water treatment?
- To measure dissolved oxygen levels
- To determine the optimal coagulant dose and pH (Correct answer)
- To test the filtration rate of settled water
- To measure turbidity after disinfection
Correct answer: To determine the optimal coagulant dose and pH
The jar test simulates the coagulation-flocculation process at bench scale to find the best coagulant dose and pH before full-scale application.
Question 2: Zeta potential is used in water treatment to measure:
- The pH of the coagulated water
- The surface charge of suspended particles (Correct answer)
- The velocity of water through a flocculation basin
- The concentration of dissolved minerals
Correct answer: The surface charge of suspended particles
Zeta potential quantifies the electrokinetic charge on colloidal particles, indicating how well coagulation has neutralized particle charges.
Question 3: Which of the following best describes 'sweep flocculation'?
- Using high mixer speeds to break apart large flocs
- Entrapping particles in a precipitate of metal hydroxide (Correct answer)
- Adding polymer to increase floc density
- Lowering pH to dissolve organic particles
Correct answer: Entrapping particles in a precipitate of metal hydroxide
Sweep flocculation occurs when excess metal coagulant forms a hydroxide precipitate that sweeps particles out of suspension as it settles.
Question 4: Ferric sulfate as a coagulant is most advantageous over alum in water with:
- High pH and low turbidity (Correct answer)
- Low temperature and low alkalinity
- High dissolved oxygen and neutral pH
- Low color and high hardness
Correct answer: High pH and low turbidity
Ferric sulfate performs better than alum at higher pH values because ferric hydroxide is less soluble over a wider pH range.
Question 5: What effect does low water temperature have on flocculation?
- It speeds up floc formation by increasing particle collisions
- It improves floc density and settleability
- It slows floc formation because water viscosity increases (Correct answer)
- It has no measurable effect on flocculation kinetics
Correct answer: It slows floc formation because water viscosity increases
Cold water is more viscous, which reduces particle collision frequency and slows floc growth, often requiring longer detention times or higher coagulant doses.
Question 6: A flocculation basin's G value (velocity gradient) is typically maintained in what range?
- 1ā10 sā»Ā¹
- 10ā75 sā»Ā¹ (Correct answer)
- 200ā400 sā»Ā¹
- 500ā1000 sā»Ā¹
Correct answer: 10ā75 sā»Ā¹
Flocculation basins typically use a G value of 10ā75 sā»Ā¹ to promote gentle particle collisions that build floc without shearing it apart.
Question 7: Why is rapid mixing applied immediately after coagulant addition?
- To allow coagulant to settle before flocculation begins
- To disperse the coagulant uniformly throughout the water before hydrolysis products form (Correct answer)
- To raise water temperature for better chemical reactions
- To reduce the pH before floc formation
Correct answer: To disperse the coagulant uniformly throughout the water before hydrolysis products form
Rapid mixing must occur within milliseconds of coagulant addition to ensure uniform dispersion before metal hydrolysis products precipitate unevenly.
WTO Coagulation and Flocculation 2
What is the primary purpose of a jar test in water treatment?