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

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

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

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

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)

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

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]

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)

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

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

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.

Type III Settling Determination of Gravity Flux

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)

Type III Settling Determination of Bulk Flux

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

Type III Settling Determination of Total Flux

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

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

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)

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

Figure 8-36, Metcalf & Eddy

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.

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

Step 2: Gravity Flux

Step 3: Generate Gravity Flux Graph

Step 4: Find the Limiting Flux

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

Step 5: Thickening Area

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

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