lecture notes chem4012 lecture 3 2013 2014
TRANSCRIPT
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CHAPTER THREE
Preliminary and Primary Treatment processes-
Part-2
Primary & Secondary Clarifiers1
PART A
Primary Clarifiers
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Lesson objectives:
1. Describe Type II settling, use of column settling
test for analysis
2. Go over design parameters and values
3. Do an example design
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Primary treatment--
Clarification/Settling/Sedimentation
Purpose--remove readily settleable suspended solids and floating
material
Removal range: 50-70% of SS (typically 60%)25-40% of BOD5(typically 33%--recall that only a
portion of total BOD5is particulaterest is soluble,
which doesnt settle) (viewgraph Fig 5-46)
Process based on gravity settling (presented previously)--
particles removed in primary clarifier are dilute, heterogeneous,
tend to flocculate
Form of settling = Type 2 (flocculant settling), though type 1
analysis also used (as in grit chambers--M&E Example 5-6)4
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Background 1/2
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Background 2/2
Can analyze Type 2 settling by column tests (will discuss briefly), but design of
primary clarifiers for WW is primarily done using an empirical approach such that
vs, vhconsiderations are integrated into empirical parameters7
Summary of Settling Test 1/3
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Summary of Settling Test 2/3
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Summary of Settling Test 3/3
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Example 5-7 1/3
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Example 5-7 2/3
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0.14 x
0.19 x
0.54x
12.6
12.35
62.9
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Example 5-7 3/3
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0.14 x
0.19 x
0.54 x
6.65
1.4
24.3
100-34.5
34.5
2.2
Design parameters 1/4
a) Overflow rate = surface loading rate = Q/A
Recall that in Type 1 settling, OFR = vsc= critical settling velocity
In Type 2 settling, a single vsc does not exist because particlesflocculate as they settle
b) Hydraulic detention time or = V/Q = AH/Q
Temperature may be a factor (often not)--if it is, use safety
factor as shown in Fig 5-48 (viewgraph)
c) Scour velocity v scour=> vhhorizontal fluid velocity (eq 5-46)
Not explicity determined for design--as with OFR, nonuniform
particles make analysis difficult
vhset by OFR and (set surface area and depth)14
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Design parameters 2/4
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Design parameters 3/4
d) Weir loading rate (brief aside to weir design)
Weir is typically used to control or monitor effluent flow from basin
to exit channel or pipe (launder)--usu. Placed as far as possible frominlet flow
e.g. Coors primary clarifier
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State Guidelines 1/2
Section 5.13.1 GeneralInlets--dissipate flow, distribute evenly and prevent short
circuiting
Scum removal required
Multiple units required except when downstream processes
sufficiently reliable to meet effluent standards
Mechanical sludge collection and removal required
Flow control required
5.13.2 Primary clarifier design
OFR = 800 gal/ft2-d (33 m/d) at Qavg
= 1 to 4 hr at Qavg
Weir loading rate 10,000 gal/ft-d (125 m2/d) at Qavg
H 7 ft (2.25 m)19
State Guidelines 2/2For example, lets look at center feed circular clarifiers (commonly used)
M&E Fig 5-41
Center well distributes flow
R.O.T.: well diameter = 15-20% of tank Dwell depth = 3-8 ft (1-2.5 m)
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Design process for primary clarifiers 1/4
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Design process for primary clarifiers2/4
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Design process for primary clarifiers3/4
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Design process for primary clarifiers4/4
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Clarifier design 1/4
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Clarifier design 2/4
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Clarifier design 3/4
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Clarifier design 3/4
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Clarifier design 4/4
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PART B
Secondary Clarification
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Lesson objectives:
Quantify settling for design purposes by 3 methods:
1. Single batch column test
2. Solids flux analysis
3. State point analysis (not covered here)
Define other descriptors used to characterize Type 3
settling
Design example
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Secondary clarification
Purpose: to remove settleable solids from activated sludge, thicken
sludge
Usually clarifier area required for thickening > area required for
settling--measure time required for both by column tests
Three experimental methods to design secondary clarifiers:
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Single batch column settling test
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Area Requirement Based On Single-
Batch Test 3/4
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Area Requirement Based On Single-
Batch Test 4/4
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Example-5-8
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Example 5-8 2/4
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Example 5-8 3/4
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Example 5-8 4/4
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Analysis for Design
Based on initial
interface settling
velocity (ZSV)
Single curve can be
used for design
analysis butWe will concentrate
on Solids Flux
analysis using
multiple curves. 43
Solids Flux
Rate at which solids pass through a given area
mass per unit area per unit time
e.g., lb/ft2-day or kg/m2-hrCalculated as the product of concentration and
velocity
e.g., lb/ft3x ft/day = lb/ft2-day
There are two solids flux components in a
mechanical clarifier
Gravity or Batch solids flux, SF gUnderflow flux, SFu
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Clarifier Definition Sketch*
(after M&E Fig. 8-34)
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Gravity Flux
Due to zone settling under the influence of gravity alone (as
in batch column test)
Ci= initial solids concentration in batch column test
Vi= initial interface settling velocity or ZSV at that
concentration
Example: Ci= 2000 mg/L (or g/m3), Vi= 0.1m/min
SFg
= 2000 g/m3x 1 kg/1000 g x 0.1 m/min = 0.2 kg/m2-min
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Making a Solids Flux Plot 1/3
Determine gravity flux at several Ciby doing
multiple batch column tests
Plot SFgv. C
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Making a Solids Flux Plot 2/3
Plot underflow flux for a chosen Ub
Straight line from origin with slope = Ub
Slope on these axes (flux/concentration) has
velocity units
Plot total flux as the sum of the components (or
not)
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Making a Solids Flux Plot 3/3
(M&E Figure 8-36)
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Solids Flux Plot Useful* Version 1/2
Plot gravity flux as before
Plot underflow flux line as follows:
Straight line with slope = -Ub
Tangent to gravity flux curve
Easier to draw
No need to calculate or plot total flux
Easier to read
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*Knowing how to draw and interpret this version can be useful on homeworks, exams, termproblems, and future jobs.
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Solids Flux Plot Useful* Version 2/2
(M&E Figure 8-37)
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Reading the Plot 1/3
Gravity flux, SFg
Solids flux due to Type 3 (gravity) sedimentation alone. The
vertical distance from the x-axis to the flux curve Limiting Flux, SFL
The maximum total flux that can pass downward through the
clarifier. The yintercept of the underflow line.
Underflow flux, SFu
The solids flux due to removal of solids and water from the
bottom of the clarifier. The vertical distance from the flux curve
to the horizontal extension of SFL
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Reading the Plot 2/3
Underflow rate, Ub
The downward velocity induced by removal of solids and water
from the bottom of the clarifier. The negative slope of the underflow
line.
Limiting Concentration, CL
The solids concentration that occurs in the sludge blanket where
zone settling occurs. The x-coordinate of the point of tangency
between the underflow line and the flux curve.
Underflow concentration, Cu
The solids concentration that occurs at the point of withdrawal from
the clarifier. The x-intercept of the underflow line.
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Reading the Plot 3/3
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Clarifier Area from SFL It is the maximum total flux that can be transmitted downward
So it can be used to calculate clarifier area needed for thickening
(Type 3 sedimentation)
but we will do it slightly differently later
Co= solids concentration in incoming flow (Q + Qu)
Example: Co= 0.5 lb/ft3(~8000 mg/L), Q + Qu= 3 mgd(million gal/day), SFL= 40 lb/ft
2-day
A = 0.5 lb/ft3x 3e6 gal/day x 1 ft3/7.48 gal / 40 lb/ft2-day
A = 5000 ft2(40 ft. diameter circular clarifier)
57Everyone knows how to calculate the required area for clarification (Type 2 sed) from
overflow rate, right?
Just in Case Area for Clarification
(Type 2 sedimentation)
Lab test data analysis gives minimum required
detention time.
Settling depth/detention time gives maximum surface
settling rate or overflow rate (OR)
Inflow rate/OR gives minimum area
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Example 8-112/5
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Example 8-113/5
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Example 8-114/5
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Example 8-115/5
Primary/Secondary Sedimentation
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Design of Primary Sedimentation
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State guidelines
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For conventional AS,
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