1. CHEE 370 Waste Treatment Processes
Lecture #12
Type III Settling Cont’d

2. Type II
(Flocculent)
Type I
(Discrete)
Type III
(Zone)

3. Type III: Hindered or Zone Settling
The particles settle as a collective “zone” or layer
All particles in the layer have the same settling velocity, regardless of the particle size

4. Type III Settling Designing Secondary Clarifiers
Calculate the area required for:
Clarification - removal of solids from liquid
Thickening - concentrating the suspension to provide a concentrated underflow
The larger of the two areas is used as the basis for designing the clarifier
Design with a factor of safety of 1.75

5. Type III Settling Clarifier Design
To obtain data for design, settling tests are repeated for different initial concentrations
Typical range is X = 1 to 15 kg/m3
Plot solids/liquid interface height versus time for each initial X
Estimate the zone settling velocity for each initial X from the slope
vs = f(X)

6. Type III Settling Determination of Gravity Flux
Determination of the Zone Settling Velocity
Figure 8-35a, Metcalf & Eddy

7. Type III Settling Secondary Clarifier Design
Calculate the area required for clarification
Where:
vo = initial zone settling velocity at the feed concentration (X), [m/h], (function of X)
Ac = surface area for clarification [m2]
Qe = overflow rate of clarified liquid [m3/h]

8. Type III Settling Secondary Clarifier Design
Calculate the area required for thickening
Find the gravity mass flux
Where:
Gg = gravity flux [M/L2•T] (kg/(m2•h))
vi = settling velocity at solids concentration Xi [L/T] (m/h)
Xi = local concentration of solids [M/L3] (kg/m3)

9. Type III Settling Determination of Gravity Flux
Step A
Determination of the Zone Settling Velocity
Figure 8-35a, Metcalf & Eddy

10. Type III Settling Determination of Gravity Flux
Step B
Settling Velocity (from Step A)
vs Concentration
Step C
Gravity Flux vs Concentration
Figure 8-35, Metcalf & Eddy

11. Practice Problem
A clarifier associated with an activated sludge process receives 2.64 m3/min of effluent from an activated sludge bioreactor at a solids concentration of 2000 mg/L. Using the following data, determine the area of an ideal clarifier operating at an underflow velocity of 9.14 m/d.

12. Type III Settling Determination of Gravity Flux

13. Type III Settling Secondary Clarifier Design
Calculate the area required for thickening
Find the bulk mass flux due to underflow pumping
Where:
ub = bulk downward velocity of the solids [L/T] (m/h)
Qu = underflow flowrate [L3/h] (m3/h)
A = surface area of settling tank [L2] (m2)

14. Type III Settling Determination of Bulk Flux

15. Type III Settling Secondary Clarifier Design
Calculate the area required for thickening
Find the total mass flux
Plot G, Gg, Gu

16. Type III Settling Determination of Total Flux

17. Limiting Flux
Identify GL - the limiting solid flux - from the minimum on the total flux curve
GL ~ 56 kg/m2•day
XL ~ 3.3 kg/m3

18. Type III Settling Secondary Clarifier Design
Identify which area is greater:
Area for clarification
Area for thickening
Use the larger area to size the clarifier
Adesign = 1.75*Acalculated
For an ideal clarifier, L = 4W

19. Type III Settling Secondary Clarifier Design
Alternative Design Strategy:
Design a clarifier for a specific underflow solids concentration Xu (rather than being given ub)
Find the limiting flux (GL ) from the plot of gravity flux (Gg) versus solids concentration (X)

20. Type III Settling Secondary Clarifier Design
Draw a tangent line to the Gg curve from Xu on the x-axis to the y-axis
The intersection with y-axis is GL
XL can be found from the x-axis at the point of intersection between the tangent line and Gg curve
The underflow velocity (ub) is given by the slope of the tangent line

21. Figure 8-36, Metcalf & Eddy

22. Practice Problem
A clarifier associated with an activated sludge process receives 2.64 m3/min of effluent from an activated sludge bioreactor at a solids concentration of 2000 mg/L. Using the following data, determine the area of an ideal clarifier if the underflow solids concentration is 6000 mg/L.

23. Step 1: Clarifier Area Solids applied = 2000 mg/L, Xu = 6000 mg/L
From chart, vo=1.37 m/h at an influent solids concentration of 2000 mg/L
Solve for Qe using a mass balance and substitution
(1) Q = Qe+Qu
(2) XQ = XeQe + XuQu
A  89 m2

24. Step 2: Gravity Flux

25. Step 3: Generate Gravity Flux Graph

26. Step 4: Find the Limiting Flux

28. What happens if X > XL?
The clarifier cannot handle influent solids concentrations that are greater than XL
If it is expected that X > XL under certain processing conditions, adjustments have to be made:
Design a larger clarifier area to handle the increased solids
Decrease the flow rate into the clarifier during the peak loading hours to ensure that X < XL

29. Step 5: Thickening Area

30. Considering Clarification A  89 m2 Considering Thickening: A  131 m2
thickening area determines the design
A = 1.75 x (131)  229 m2
For an ideal clarifier:
L = 4W  A = 4W2
Clarifier dimensions
W  7.57 m, L  30.3 m

31. Type III Settling Secondary Clarifier Design
Secondary clarifiers are generally designed based on the peak loading values
If the peaks are of short duration, it is possible to use an average value for the solids loading
Deeper tanks facilitate greater flexibility in terms of operation and offer a larger margin of safety to manage changes in the AS process