WTO Coagulation and Flocculation 3 — Questions and Answers
Question 1: Cationic polymers work as coagulant aids primarily by:
- Raising the pH of the water
- Neutralizing the negative surface charge of particles (Correct answer)
- Increasing the dissolved oxygen in the flocculation basin
- Oxidizing organic color compounds
Correct answer: Neutralizing the negative surface charge of particles
Cationic (positively charged) polymers neutralize the negative charge on suspended particles, reducing repulsion and promoting aggregation.
Question 2: What does a 'Gt' value represent in flocculation design?
- The ratio of settled turbidity to raw turbidity
- The product of velocity gradient and detention time, indicating total mixing energy (Correct answer)
- The coagulant dose in mg/L multiplied by contact time
- The flow rate divided by basin volume
Correct answer: The product of velocity gradient and detention time, indicating total mixing energy
Gt (dimensionless camp number) is the product of velocity gradient G and detention time t, representing the total collision opportunity for particles in a flocculation basin.
Question 3: Which coagulant is most commonly used in conventional US water treatment plants?
- Ferric chloride
- Polyaluminum chloride (PACl)
- Alum (aluminum sulfate) (Correct answer)
- Sodium aluminate
Correct answer: Alum (aluminum sulfate)
Alum (aluminum sulfate) is the most widely used coagulant in US water treatment due to its effectiveness, availability, and low cost.
Question 4: High natural organic matter (NOM) in source water affects coagulation by:
- Increasing the optimal coagulant dose needed (Correct answer)
- Decreasing the required coagulant dose
- Eliminating the need for pH adjustment
- Reducing the formation of metal hydroxide floc
Correct answer: Increasing the optimal coagulant dose needed
NOM consumes coagulant through reactions with organic molecules, increasing the required dose to achieve effective particle destabilization.
Question 5: In a tapered flocculation basin, mixing energy is designed to:
- Increase from inlet to outlet to break apart large flocs
- Decrease from inlet to outlet to allow gentle floc growth (Correct answer)
- Remain constant throughout all compartments
- Alternate between high and low energy in each compartment
Correct answer: Decrease from inlet to outlet to allow gentle floc growth
Tapered flocculation decreases mixing intensity from inlet to outlet so that initially formed floc particles grow gently without being sheared apart.
Question 6: What is the typical detention time for a flocculation basin in conventional water treatment?
- 30 seconds to 2 minutes
- 20 to 40 minutes (Correct answer)
- 2 to 4 hours
- 8 to 12 hours
Correct answer: 20 to 40 minutes
Flocculation basins are typically designed with 20–40 minutes of detention time to allow adequate particle collisions and floc growth before sedimentation.
Question 7: Which water quality parameter most directly indicates whether coagulation was effective before sedimentation?
- Residual chlorine
- Settled water turbidity or floc size (Correct answer)
- Dissolved oxygen
- Total dissolved solids
Correct answer: Settled water turbidity or floc size
Settled water turbidity and visual floc size and density directly indicate how well coagulation and flocculation prepared particles for removal by sedimentation.
WTO Coagulation and Flocculation 3
Cationic polymers work as coagulant aids primarily by: