Lecture Notes CHEM4012 Lecture 3 2013 2014

8/13/2019 Lecture Notes CHEM4012 Lecture 3 2013 2014

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CHEM4012- Ahmed Al-Dallal/2013-2014 1

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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5

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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-----------------------

%


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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

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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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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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Clarifier design 1/4

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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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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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(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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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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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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Lecture Notes CHEM4012 Lecture 3 2013 2014

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