Podcast
Questions and Answers
What is the primary mechanism by which flocculation enhances particle collision?
What is the primary mechanism by which flocculation enhances particle collision?
- Reducing the electrostatic repulsion between particles
- Increasing the temperature of the water
- Adding chemicals that neutralize the pH
- Introducing turbulence to the water (correct)
The product of which two parameters is most often used to characterize the performance of flocculators?
The product of which two parameters is most often used to characterize the performance of flocculators?
- Water temperature and pH
- Velocity gradient and detention time (correct)
- Turbidity and alkalinity
- Dissolved oxygen and conductivity
What range of Gt values is generally considered optimal for flocculation?
What range of Gt values is generally considered optimal for flocculation?
- 10 to 100
- 1,000,000 to 10,000,000
- 10,000 to 100,000 (correct)
- 100 to 1,000
How does a high G value influence floc formation, and why is it important to taper the velocity gradient?
How does a high G value influence floc formation, and why is it important to taper the velocity gradient?
What proportion of the influent G values should the effluent G values be set to, according to the text?
What proportion of the influent G values should the effluent G values be set to, according to the text?
Why is it important to exercise caution in the design of the outlet of the flocculator and the inlet of the sedimentation tank?
Why is it important to exercise caution in the design of the outlet of the flocculator and the inlet of the sedimentation tank?
What materials are paddles commonly made of for creating turbulence in flocculators?
What materials are paddles commonly made of for creating turbulence in flocculators?
What is the primary advantage of baffled flocculation basins?
What is the primary advantage of baffled flocculation basins?
According to the slides, what is the definition of velocity gradient in the context of flocculation?
According to the slides, what is the definition of velocity gradient in the context of flocculation?
What is the significance of the detention time (T) in the context of hydraulic residence time, and how is it calculated?
What is the significance of the detention time (T) in the context of hydraulic residence time, and how is it calculated?
What does 'Vp' represent in the context of water power and flocculation?
What does 'Vp' represent in the context of water power and flocculation?
In the formula for drag force on a paddle, $D = C_D A_p \rho V_p^2 / 2$, what does $A_p$ represent?
In the formula for drag force on a paddle, $D = C_D A_p \rho V_p^2 / 2$, what does $A_p$ represent?
According to the provided design example, a water treatment plant uses an alum dosage of 40mg/l with flocculation. What is the significance of this alum dosage?
According to the provided design example, a water treatment plant uses an alum dosage of 40mg/l with flocculation. What is the significance of this alum dosage?
The design example specifies a Gt value of $4 \times 10^4$ for optimal flocculation. How does this value guide the design of the flocculator?
The design example specifies a Gt value of $4 \times 10^4$ for optimal flocculation. How does this value guide the design of the flocculator?
In the solution provided, the volume of the flocculator is calculated based on an assumed G value. What is the relationship between the G value and the required volume of the flocculator?
In the solution provided, the volume of the flocculator is calculated based on an assumed G value. What is the relationship between the G value and the required volume of the flocculator?
The solution assumes a depth of 5m for the flocculator and calculates the length and width accordingly. What is the primary consideration in choosing these dimensions?
The solution assumes a depth of 5m for the flocculator and calculates the length and width accordingly. What is the primary consideration in choosing these dimensions?
Given the power requirement calculation $G = P/(V \cdot \mu)$, how does an increase in water viscosity ($\mu$) affect the power required for flocculation, assuming G and V are constant?
Given the power requirement calculation $G = P/(V \cdot \mu)$, how does an increase in water viscosity ($\mu$) affect the power required for flocculation, assuming G and V are constant?
The solution divides the flocculator into three partitions with different G values (G1, G2, G3). What is the purpose of using varying G values in differentpartitions?
The solution divides the flocculator into three partitions with different G values (G1, G2, G3). What is the purpose of using varying G values in differentpartitions?
The 'design example' concludes with calculating the paddle wheel speed. What is the primary factor that determines the appropriate paddle wheel speed for flocculation?
The 'design example' concludes with calculating the paddle wheel speed. What is the primary factor that determines the appropriate paddle wheel speed for flocculation?
In the context of flocculation, if the velocity of the paddle ($V_p$) is increased, what is the expected effect on the drag force (D) on the paddle, according to the formula $D = C_D A_p \rho V_p^2 / 2$?
In the context of flocculation, if the velocity of the paddle ($V_p$) is increased, what is the expected effect on the drag force (D) on the paddle, according to the formula $D = C_D A_p \rho V_p^2 / 2$?
Flashcards
Flocculation
Flocculation
The process in which particle collision is enhanced through turbulence to form larger flocs.
Gt Value
Gt Value
A parameter for flocculators, representing the product of velocity gradient (G) and detention time (t).
Tapered Velocity Gradient
Tapered Velocity Gradient
Refers to gradually reducing the velocity gradient in flocculation tanks to prevent breaking up of formed flocs.
Baffled Flocculation Basins
Baffled Flocculation Basins
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Velocity Gradient (G)
Velocity Gradient (G)
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Hydraulic Residence Time (T)
Hydraulic Residence Time (T)
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Power Input to Water (P)
Power Input to Water (P)
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Drag Force (D)
Drag Force (D)
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Study Notes
- Flocculation is a process enhancing particle collision through turbulence.
- A common parameter for flocculators is the product of the velocity gradient and the detention time of the mixing tank (Gt).
- Gt values typically range from 10^4 to 10^5.
- High G values can create small, compact flocs, while low values can lead to large, less dense flocs.
- Tapering the velocity gradient values along the flocculator tank is desirable.
- Effluent G values are set to be half that of the influent.
- Larger flocs settle better in sedimentation tanks, emphasizing caution in flocculator outlet and sedimentation tank inlet design to avoid high velocity gradients.
- Paddles using redwood and aluminum blades are used to create turbulence in flocculators; traditionally, baffles served the same purpose.
- Baffled flocculation basins prevent short circuiting, but their mixing intensity depends on the flow rate.
Velocity Gradient
- For mechanical flocculators, the formula to compute the velocity gradient is G=√(P/Vμ).
- G = velocity gradient (s⁻¹)
- P = power input to water (W or Nm/s)
- V = volume of water mixed (m³)
- μ = viscosity (N.s/m²)
- w = specific weight (N/m³)
- H = friction loss through the tank (m)
- Velocity gradient is the absolute velocity difference between two fluid particles divided by the distance between them
- It's measured in s⁻¹
- Can be computed in baffled flocculators and outlet/inlet ports with a specific formula.
- If T is the detention time where T = V/Q
- Velocity gradient in ports should not exceed that within the flocculator.
Hydraulic Residence Time
- For time-dependent reactions, fluid particle residence time in the reactor affects reaction completion.
- In ideal reactors, the average time in the reactor (theoretical or hydraulic detention time) is defined as T=V/Q.
- T: theoretical detention time in seconds
- V: volume of fluid in reactor in m³
- Q: flow rate into reactor in m³/s
- Given desired detention time and flow rate, the liquid volume can be calculated to achieve the design detention time.
Water Power and Drag Force
- The power input to water is given by P = D * Vp
- D = drag force on paddles in N
- Vp = paddle tip velocity relative to water (approx. 75% of actual speed) in m/s
- Drag force on the paddle is D = CD * Ap * p * (Vp^2)/2
- CD = drag coefficient equal to 1.8 for the paddles
- Ap = combined area of slats, not exceeding 40% of the total area encompassed by the paddle
Design Example: Water Treatment Plant
- Plant processes 50,000 m³/day of water, with alum dosage of 40mg/l and Gt value of 4x10⁴.
- Water temperature is 15°C.
Solution
- Monthly Alum Dosage: 60,000 kg per month
- Flocculator Volume: 771.6 m³
- Assuming a G value of 30 s⁻¹ and Gt = 4 x 10⁴, detention time (T) is 22.2 minutes, deduced from formula T = 4x104/(30x60 s/min) Volume (V) calculation of 771.6 m³, showing as V = QT = 50,000 m³/day x 4x104 / (30 x 86400 s/day).
- Assume a 5m depth, so the length is 15m and the volume is
- The computed width equals 10.3 m
- The Power requirement G = P/(V.μ)
- P = G2Vμ
- G in the three partition to be as follows:
- G₁ = 40 /S
- G₂ = 30 /S
- G3 = 20 /S
- At 15°C μ = 1.139×10-3 N.s/m²
Solution for power and speed
- P₁ = 0.47 Kw
- P₂ = 0.26 Kw
- P3 = 0.12 Kw
- P = D.Vp = CDAp p Vp³/2
- assume Vp = 0.67 m/s x 0.75 = 0.5m/s
- ρ = 999.1 kg/m3
- P₁ = 468.7 N.m/s = 1.8 x Ap x 999.1 x 0.5³/2
- Ap = 2.5 x 4 x 3 W = 2 x 468.7 / (999.1 x 1.8 x 0.5²)
- W = Ap/30 = 0.14 m
- Vp = distance travelled in one rev x w (angular velocity ) = π D m/rev x w
- W₁ = Vp/ (π D) = (0.5/0.75)x60 s/min/(3.14X4.2m) = 3.05 rev/min
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