smith kyla m 201106 msc thesis

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CHARACTERIZATION OF ACTIVATED CARBON FOR

TASTE AND ODOUR CONTROL

by

Kyla Miriam Smith

A thesis submitted in conformity with the requirementsfor the degree of Master of Applied Science

Graduate Department of Civil EngineeringUniversity of Toronto

Copyright by Kyla Miriam Smith (2011)


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CHARACTERIZATION OF ACTIVATED CARBON FOR TASTE AND

ODOUR CONTROL

Kyla Smith

Master of Applied Science

Graduate Department of Civil EngineeringUniversity of Toronto

2011

ABSTRACT

Iodine number, BET surface area, taste and odour compound isotherms, and trace capacity

number tests were used to rank five different granular activated carbons according to

thermodynamic adsorption performance. These tests were compared to expected activated carbon

service life and loading results of rapid small-scale column tests (RSSCTs) run with water from

two lake sources spiked with geosmin and 2-methylisoborneol (MIB). Trace capacity number,

used to specifically identify high adsorption energy sites on activated carbon, was hypothesized

to be correlated to geosmin/MIB breakthrough and loading performance of different activated

carbons. This study found no such clear correlation. However, when only bituminous coal

activated carbons were considered, correlations to MIB breakthrough were strengthened. Natural

organic matter (NOM) adversely affected adsorption, resulting in decreased RSSCT throughput

to breakthrough in surface water with higher total organic carbon (TOC). Methods for improving

characterization tests and RSSCTs when NOM is present are discussed.


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ACKNOWLEDGMENTS

I would like to thank my supervisor, Ron Hofmann, for his guidance, support and for providing

the opportunity for me to be a part of the Drinking Water Research Group. Thank you also to

Susan Andrews for assisting with data analysis, troubleshooting and for providing helpful

feedback.

I am grateful to Jinwook Kim for countless hours of help and advice in the lab, Fariba Amiri and

Russell DSouza for assistance in the lab and Sean OToole for returning many (many!) times to

help fix the GCMS. Giovanni Buzzeo, Alan McClenaghan and Joel Babbin were extremely

helpful in construction and in hauling barrels of water. I would also like to thank Tom Hartig,Dave McNamara and their colleagues at Calgon for their advice and direction. Thank you Laura

Meteer, Aaron Wood and Richard Jones for helping to coordinate visits for water collection at

the Georgina Water Treatment Plant and the Ajax Water Supply Plant.

This work was partially funded by the Natural Sciences and Engineering Research Council of

Canada, Calgon Carbon Corporation, the Region of York and the Region of Durham.

To the DWRG team, I really could not have asked for a better group of people to work alongside.

Special thanks to those who helped me move thousands of kilos of water from the 1 stfloor to the

4thand to Scott, Juan and Bryony who enthusiastically agreed to go on field trips to various water

treatment plants.

To Heather, Jon, Sarah, Emma and Juan: thank you for helping these final months in the lab, for

your support and feedback in writing and data analysis and for your day-to-day encouragement.

Finally, to my incredible, supportive family and friends, I am so grateful for your love and

encouragement.


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TABLE OF CONTENTS

ABSTRACT................................................................................................................................................................II

ACKNOWLEDGMENTS........................................................................................................................................ III

TABLE OF CONTENTS......................................................................................................................................... IVLIST OF TABLES.................................................................................................................................................... VI

LIST OF FIGURES.................................................................................................................................................VII

GLOSSARY........................................................................................................................................................... VIII

1 INTRODUCTION AND RESEARCH OBJECTIVES...................................................................................1

1.1 BACKGROUND ............................................................................................................................................11.2 RESEARCH OBJECTIVES ..............................................................................................................................21.3 DESCRIPTION OF CHAPTERS........................................................................................................................31.4 REFERENCES...............................................................................................................................................3

2 LITERATURE REVIEW..................................................................................................................................4

2.1 GRANULAR ACTIVATED CARBON IN WATER TREATMENT .........................................................................42.1.1 BACKGROUND ............................................................................................................................................42.1.2 PRODUCTION OF ACTIVATED CARBON .......................................................................................................42.1.3 ADSORPTION MECHANISMS.........................................................................................................................52.1.4 USES FOR ACTIVATED CARBON..................................................................................................................82.2 TASTE AND ODOUR ISSUES.........................................................................................................................92.2.1 TASTE AND ODOUR COMPOUNDS ...............................................................................................................92.2.2 ODOUR THRESHOLD CONCENTRATION.....................................................................................................112.2.3 COMPETITION WITHNOM........................................................................................................................112.3 ACTIVATED CARBON CHARACTERIZATION FOR MICROPOLLUTANT CONTROL.........................................122.3.1 DETERMINATION OF PHYSICAL PROPERTIES.............................................................................................122.3.1.1 DISTRIBUTION OF ENERGY SITES..............................................................................................................152.3.2 DETERMINATION OF ADSORPTION CAPACITY...........................................................................................16

2.3.3 RAPID SMALL-SCALE COLUMN TESTS......................................................................................................182.3.3.1 ASSUMPTIONS MADE WITH THE RSSCT...................................................................................................232.3.3.2 CONSTANT OR PROPORTIONAL DIFFUSIVITY? ..........................................................................................252.3.4 THE ACCELERATED COLUMN TEST ..........................................................................................................262.4 CURRENT SELECTION METHOD FOR PURCHASING CARBONS ...................................................................292.5 REFERENCES.............................................................................................................................................31

3 ASSESSMENT OF ACTIVATED CARBON CHARACTERIZATION TESTS FOR TASTE ANDODOUR CONTROL.................................................................................................................................................36

ABSTRACT ...............................................................................................................................................................363.1 INTRODUCTION.........................................................................................................................................373.2 EXPERIMENTAL ........................................................................................................................................403.3 RESULTS AND DISCUSSION .......................................................................................................................393.4 SUMMARY AND CONCLUSIONS .................................................................................................................54

3.5 REFERENCES.............................................................................................................................................56

4 THE EFFECTS OF COMPETITIVE ADSORPTION BETWEEN T&O COMPOUNDS AND NOM ONCHARACTERIZATION TESTS AND RSSCTS...................................................................................................59

ABSTRACT ...............................................................................................................................................................594.1 INTRODUCTION.........................................................................................................................................604.2 SELECTION OF THERMODYNAMIC CHARACTERIZATION TESTS FOR PREDICTING TASTE AND ODOUR

CONTROL..................................................................................................................................................614.3 OPTIMIZING RSSCTS FOR PREDICTING ADSORPTION OF TASTE AND ODOUR COMPOUNDS .....................664.4 SUMMARY AND CONCLUSION...................................................................................................................71


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4.5 REFERENCES.............................................................................................................................................73

5 SUMMARY AND RECOMMENDATIONS.................................................................................................75

5.1 SUMMARY ................................................................................................................................................755.2 CONCLUSIONS ..........................................................................................................................................755.3 RECOMMENDATIONS ................................................................................................................................765.4

REFERENCES.............................................................................................................................................76

A: DEFINITIONS.................................................................................................................................................77

B: MATERIALS AND METHODS (CHAPTER 3) ..........................................................................................79

B.1 DESCRIPTION OF ACTIVATED CARBONS ...................................................................................................79B.2 ACTIVATED CARBON PREPARATION.........................................................................................................81B.3 SAMPLE WATER PREPARATION ................................................................................................................83B.4 TASTE AND ODOUR COMPOUND PREPARATION........................................................................................84B.5 ACTIVATED CARBON PHYSICAL CHARACTERISTICS.................................................................................86B.5.1 APPARENT DENSITY .................................................................................................................................86B.6 ACTIVATED CARBON ACTIVITY INDICES ..................................................................................................88B.6.1 IODINENUMBER TEST ..............................................................................................................................88B.6.2 TRACE CAPACITYNUMBER......................................................................................................................92

B.6.3 TRACE CAPACITYNUMBER GAS PHASE ...................................................................................................93B.7 TOTAL ORGANIC CARBON ........................................................................................................................96B.8 RAPID SMALL-SCALE COLUMN TESTS......................................................................................................98B.9 TASTE AND ODOUR COMPOUND ANALYSIS ............................................................................................107B.10 QUALITY CONTROL TESTS......................................................................................................................111B.11 DATA ANALYSIS.....................................................................................................................................112

C: ADDITIONAL RESULTS (CHAPTER 3) ..................................................................................................115

C.1 TRACE CAPACITYNUMBER TEST ...........................................................................................................115C.2 TRACE CAPACITYNUMBER GAS PHASE TEST ........................................................................................116C.3 PORE SIZE DISTRIBUTION .......................................................................................................................117C.4 MIBAND GEOSMIN BREAKTHROUGH AND LOADING RESULTS ..............................................................118C.5 CORRELATION TESTS ..............................................................................................................................126

C.6 RAW

DATA

.............................................................................................................................................128D: EXPERIMENTAL DESIGN.........................................................................................................................130

E: QAQC .............................................................................................................................................................132


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LIST OF TABLES

TABLE 2.1 CHEMICAL AND PHYSICAL CHARACTERISTICS OF GEOSMIN AND MIB(PIRBAZARIET AL.,1992)................10TABLE 2.2 COMPARISON OF CALCULATIONS USED FOR RSSCTAND ACT ..................................................................27TABLE 3.1 PROPERTIES OF FIVE ACTIVATED CARBONS USED IN THIS STUDY................................................................34TABLE 3.2 SMALL-COLUMN RSSCTPARAMETERS FOR ALL FIVE CARBONS ................................................................37TABLE 3.3 EXAMINATION OF CORRELATION BETWEEN CHARACTERIZATION TESTS (LEAST-SQUARES LINEAR

REGRESSION) ......................................................................................................................................................40TABLE 3.4 BED VOLUMES TO MIBBREAKTHROUGH (20%OF C0),RANKING IN PARENTHESES...................................45TABLE 3.5 COMPARISON OF MIBBREAKTHROUGH (20%OF C0)TO CHARACTERIZATION RESULTS (R

2VALUES,+/-

INDICATES POSITIVE OR NEGATIVE SLOPE IN LINEAR REGRESSION ANALYSIS) ....................................................47TABLE 3.6 ADSORPTIONAAND TRANSPORTBPORE VOLUMES FOR FIVE CARBONS (CALGON,2009)..............................47TABLE 3.7 COMPARISON OF LOADING (AT 50,000BED VOLUMES)AND CHARACTERIZATION RESULTS (R2VALUES,+/-

INDICATES POSITIVE OR NEGATIVE SLOPE IN LINEAR REGRESSION ANALYSIS) ....................................................51TABLE 3.8 COMPARISON OF TOCLOADING (AT 50,000BED VOLUMES)AND CHARACTERIZATION RESULTS (R2

VALUES,+/-INDICATES POSITIVE OR NEGATIVE SLOPE IN LINEAR REGRESSION ANALYSIS).................................54


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LIST OF FIGURES

FIGURE 2.1 ACTIVATED CARBONS INTERNAL STRUCTURE:MULTIPLE LAYERS OF GRAPHITE PLATES IN RANDOM

ARRANGEMENT .....................................................................................................................................................5FIGURE 2.2 MASS TRANSFER ZONE THROUGH AN ACTIVATED CARBON COLUMN ...........................................................7FIGURE 2.3 GEORGINA WATER TREATMENT PLANT GACCONTACTOR..........................................................................8FIGURE 3.1 ELEMENTS OF RSSCTSET-UP:SAMPLING PORTS [A],FLOATING LID [B],FULL RSSCTSET-UP [C] .........37FIGURE 3.2 RSSCTSYSTEM SCHEMATIC,TOTAL OF 6GACCOLUMNS IN RSSCTSET-UP ...........................................38FIGURE 3.3 PARALLEL COLUMNS (CARBON A)SHOWING REPRODUCIBLE MIBBREAKTHROUGH CURVES...................42FIGURE 3.4 COMPARISON OF DUPLICATED COLUMN RESULTS FOR GEOSMIN BREAKTHROUGH....................................42FIGURE 3.5 MIBBREAKTHROUGH CURVE FOR CARBON BWITH GOMPERTZ CURVE FIT ..............................................43FIGURE 3.6 EXAMPLE OF MIBBREAKTHROUGH CURVES,LAKE SIMCOE.....................................................................44FIGURE 3.7EXAMPLE OF GEOSMIN BREAKTHROUGH CURVES,LAKE SIMCOE ..............................................................44FIGURE 3.8 COMPARISON OF MIBBREAKTHROUGH USING CARBON BFROM TWO SOURCE WATERS ..........................45FIGURE 3.9 COMPARISON OF IODINE NUMBERS TO MIBBREAKTHROUGH RESULTS FOR CARBONS IN FOUR WATERS...48FIGURE 3.10 COMPARISON OF TCNVALUES TO MIBBREAKTHROUGH RESULTS FOR CARBONS IN FOUR WATERS ......48FIGURE 3.11 MIBLOADING CAPACITY,LAKE SIMCOE ................................................................................................50FIGURE 3.12 LOADING CAPACITY OF FIVE CARBONS AT 50,000BED VOLUMES IN TWO SOURCE WATERS ....................50FIGURE 3.13 TOCBREAKTHROUGH CURVES,LAKE SIMCOE.......................................................................................52FIGURE 3.14 TOCLOADING FOR ALL FIVE CARBONS,LAKE SIMCOE ...........................................................................53


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GLOSSARY

Roman Letters

angstrom (equivalent to 10-10metre or 0.1 nanometer)C0,i initial bulk phase concentration

Dg,i combined solute distribution parameter (dimensionless)

Dgp,i pore solute distribution parameter (dimensionless)

Dgs,i surface solute distribution parameter (dimensionless)

Ds,i surface diffusivity

Dp,i pore diffusivity

Edp,i

pore diffusion modulus (dimensionless)

Eds,i surface diffusion modulus (dimensionless)

Ki Freundlich isotherm capacity constant

kf,i film transfer coefficient

l/ni Freundlich isotherm intensity constant (dimensionless)

L length of fixed bed

Q flow rate

R carbon particle size (mm)

Re Reynolds number (dimensionless)

Sc Schmidt number (dimensionless)

Sti Stanton number (dimensionless)

t real or elapsed time

vi interstitial velocity: v/

v approach velocity

X defines the dependence of the intraparticle diffusion coefficient on particle

sizeGreek Letters

void fraction (dimensionless)

density

fluid residence time in packed bed, empty bed contact time (EBCT)

viscosity of water


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Terms

Adsorbate substance that is being adsorbed

Adsorbent solid material to which compound (or adsorbate) is being adsorbed

(i.e., activated carbon)

Acronyms

DFPSDM dispersed-flow pore-surface-diffusion model

EBCT empty bed contact time (min)

GAC granular activated carbon

LC large column

RSSCT rapid small-scale column test

SC small column


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GOVERNING EQUATIONS OF THE RSSCT

SC

SC

SCVelocity

LengthEBCT

RSSCTs

LC

SC

SC,S

LC,S

X

LC

SC

LC

SC

t

t

D

D

R

R

EBCT

EBCT

0

2

X

LC

SC

LC,p

SC,p

R

R

D

D

X

LC

SC

LC,S

SC,S

R

R

D

D

RSSCT Assuming Constant Diffusivity (X=0)

LC

SC

LC

SC

LC

SC

t

t

R

R

EBCT

EBCT

2

SC

LC

LC

SC

R

R

v

v

RSSCT Assuming Proportional Diffusivity (X=1)

LC

SC

LC

SC

LC

SC

t

t

R

R

EBCT

EBCT

LC

min,SC

SC

LC

LC

SC

Re

Re

R

R

v

v


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1 INTRODUCTIONANDRESEARCHOBJECTIVES

1.1 BACKGROUND

Granular activated carbon (GAC) is often used to remove natural organic matter, colour, and

micropollutants during drinking water treatment (AWWA, 2006). In the Great Lakes region, in

Canada and the United States, seasonal taste and odour episodes caused by geosmin and 2-

methylisoborneol (MIB) drive utilities to invest millions of dollars on activated carbon

contactors or filter caps to minimize consumer complaints of adverse tastes and/or smells in their

water.

Appropriate selection of GAC for taste and odour control remains a challenge as specific

information about the GACs adsorption performance specific to a utilitys water source is often

limited. GAC adsorption performance is affected by several factors including the organic and

inorganic chemical composition of the specific natural water being treated and the target

compounds to be removed, as well as the physical and chemical properties of the activated

carbon (Karanfil, 2006).

Often utilities will select a carbon on the basis of traditional carbon characterization tests, such as

iodine number, that may not be representative of taste and odour compound adsorption (Chen et

al., 1997). More appropriate information and characterization tests would be useful to reliably

and accurately predict adsorption performance for geosmin and MIB removal for a particular

water treatment utility. Conducting pilot or even rapid small-scale column tests can be time-

consuming and costly. This study aimed to gain an understanding of whether simpler laboratory-

scale characterization tests, or combinations of tests, can provide accurate predictions of GAC

adsorption performance.

In this study, five characterization tests were used to rank five different activated carbons

according to adsorption performance. The results of these tests were then compared to the results

of kinetic carbon column tests (rapid small-scale column tests RSSCTs) run with two different

lake waters. The intent was to determine if any of the carbon characterization tests provided


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useful predictions of GAC effectiveness and service life for taste and odour control. By running

RSSCTs, potentially confounding but real life parameters of performance such as competitive

adsorption and characteristics of the source water were taken into account. Equipped with

information on which characterization tests provide the most accurate information on adsorption

performance, utilities could better ensure that the most appropriate GAC is chosen for their

source water.

1.2 RESEARCH OBJECTIVES

The overall objective of this research was to compare various carbon characterization tests used

to determine GAC adsorption capacity for geosmin and MIB with test results from kinetic, rapid

small-scale column tests (RSSCTs). Adsorption characterization tests and RSSCTs were

conducted to:

(1)Characterize the adsorption capacity of five commonly used types of GAC according to

BET surface area, iodine number, geosmin and MIB isotherms, trace capacity number

(TCN) and the trace capacity number gas phase (TCNG).

(2)Characterize the five GACs according to MIB, geosmin and TOC breakthrough and

loading capacity according to results from RSSCTs.

(3)Investigate the relationship between the various measured carbon characteristics (iodine

number, surface area, etc.) and geosmin/MIB removal in RSSCTs.


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1.3 DESCRIPTION OF CHAPTERS

Chapter 2 Literature review/background information on activated carbon characterization

methods and taste and odour issues.

Chapter 3 Evaluation of characterization tests for selecting carbon and a comparison of these

test results to adsorption performance with RSSCTs.

Chapter 4 Literature review and recommendations regarding the effect of competitive

adsorption between NOM and taste and odour compounds for adsorption sites of GAC.

Chapter 5 Summary of significant findings and recommendations for future research.

1.4 REFERENCES

American Water Works Association (2006) AWWA Standard: Granular Activated Carbon,ANSI/AWWA B604-05. American Water Works Association Research Foundation, Denver,CO.

Chen G., Dussert B.W. and Suffet I.H. (1997) Evaluation of granular activated

carbons for removal of methylisoborneol to below odor threshold concentration in drinkingwater. Water Research31(5), 1155-1163.

Karanfil T. (2006) Activated carbon adsorption in drinking water treatment. Activated CarbonSurfaces in Environmental Remediation. Elsevier Ltd.,345-373.


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2 LITERATUREREVIEW

2.1 GRANULAR ACTIVATED CARBON IN WATER TREATMENT

Granular activated carbon has a very large internal surface area (>500 m2/g) making it suitable

for the adsorption of a variety of contaminants during water treatment. The adsorption process

onto carbon is by no means a new water treatment technology with records showing carbon

being used for water treatment as early as 2,000 B.C. (Baker, 1949). Today, activated carbon

with its unique high adsorptive capacity is used worldwide for various applications. Within the

United States (US), 80 % of the total demand for activated carbon is for liquid-phase

applications, 55 % of which is used for the removal of water contaminants (Marsh andRodriguez-Reinoso, 2006).

2.1.1 BACKGROUND

Activated carbon in water treatment is used in both powder and granular form. Powdered

activated carbon (PAC) (particles < 0.05 mm) is added to the water in batches and left for a

specified contact time before being removed by flocculation, sedimentation and/or filtration

(Sontheimer et al., 1988). Granular activated carbon (GAC) (particles 0.3 - 3 mm) is placed inthe water treatment train as a fixed bed adsorber or as a cap on a granular media filter. GACs

higher initial cost over PAC is usually justified as GAC contactors are simpler processes to

operate, more efficient in the use of the carbon, and are capable of being reused (Herzing et al.,

1977). This research focused solely on the use of GAC.

2.1.2 PRODUCTION OF ACTIVATED CARBON

Activated carbon can be produced from almost any carbonaceous material. Common materialsused include bituminous coal, lignite coal, coconut shells and wood. These raw materials are

fired in the absence of oxygen in a step referred to as carbonization. The activation process that

follows carbonization is a carefully controlled process in which the carbon is heated at extremely

high temperatures (315 - 925C) in the presence of carbon dioxide or steam. This activation

process disrupts the orderly arrangement of the graphitic plates of the carbon creating a vast


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network of pores of different shapes and sizes throughout the cross-linked graphitic crystallite

planes (Figure 2.1). As a result, activated carbon exhibits an extremely large surface area (greater

than 500 m2/g) and the capacity to adsorb dissolved organic material.

Figure 2.1 Activated carbons internal structure: multiple layersof graphite plates in random arrangement(Image: Calgon Carbon Corporation)

2.1.3 ADSORPTION MECHANISMS

Adsorption of trace contaminants may be due to various combinations of chemical, electrostatic

and physical interactions (Karanfil, 2006). A carbons adsorption capacity and surface chemistryare two main factors affecting its capacity to remove a given micropollutant. Adsorption capacity

is usually attributed to a carbons internal pore volume (Considine et al., 2001). The activated

carbons surface chemistry is also important as the adsorption of the pollutant is preceded by the

adsorbate displacing the water to reach the surface of the carbon (Considine et al., 2001,

Pendleton et al., 1997). There are two broad adsorption mechanisms: physiosorption and

chemisorption.

In water treatment, physiosorption or physical adsorption is the principle mechanism for the

removal of organics (MWH, 2005). There are three main interactions that should be considered

when examining physical adsorption onto an activated carbon: adsorbate-water interactions,

adsorbate-carbon surface interactions and water-carbon surface interactions. Adsorption capacity

is determined by the strength of the adsorbate-carbon surface interactions compared to the other

interactions. Adsorbate-carbon surface interactions depend on both physical and chemical factors


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(Karanfil, 2006). Physical factors such as size distribution of pores across the activated carbon

and adsorbate molecular dimensions, will determine the accessible surface area available for

adsorption (Karanfil, 2006).

The principle attractive force between the adsorbate and the activated carbon is the dispersion

force or London-van der Waals force. Adsorbate molecules are attracted by van der Waals forces

and attach themselves to the surface of the activated carbon. As van der Waals forces are directly

related to the polarizability of the adsorbate and the adsorbent, a stronger attraction between

adsorbate and the activated carbon surface will exist with increasing polarizability and size.

Therefore, larger and more non-polar compounds will adsorb more easily and strongly to

activated carbon. Physical adsorption is a reversible process and thus desorption of adsorbate

compounds must always be considered.

When examining chemisorption or chemical adsorption, the type of reaction occurring on the

surface of the carbon, the molecular structure of the adsorbate and the chemistry of the solution

all affect the adsorbate-carbon surface interactions. Chemical adsorption and physical adsorption

are sometimes difficult to differentiate. Chemical adsorption occurs as the reaction between the

adsorbate and the surface of the activated carbon forming a covalent or an ionic bond. The

charged surface attracts opposite charges and repels like charges as stated by Coulombs law

(MWH, 2005).

The specific adsorption mechanisms for the two major taste and odour compounds, 2-

methylisoborneol (MIB) and geosmin, are still being explored in the research, however, several

hypotheses have been made. The main mechanism generally agreed upon in the literature for the

adsorption of micropollutants is that of physical adsorption in micropores. Within the

micropores, opposing pore walls are close enough together to create a site of overlapping

adsorption forces and the pore size is similar to the molecular size of the adsorbates being

targeted for removal. Several studies also show that contaminants with low solubility and a

molecular size and shape similar to those of the pore sites available in the activated carbon are

the most readily adsorbed (AWWARF, 2007). Newcombe et al.(1997) speculated that the most

likely adsorption mechanism for MIB is hydrophobic attraction to the carbon surface or through

a specific mechanism involving the alcohol functional group. Other studies on the removal of


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organic compounds agree with the removal mechanism being hydrophobic interactions

(AWWARF, 2007).

Mass Transfer Zone

In a downflow activated carbon filter, as the adsorbate enters the top of the bed, it continues to

saturate the bed beginning at the inlet, but follows a distinctive concentration profile as it

continues down the bed. The area in the activated carbon bed where adsorption is occurring is

known as the mass transfer zone (MTZ). The MTZ is the length of bed required for the adsorbate

to be transferred from solution into the carbon. Once the front of the MTZ reaches the effluent,

breakthrough of the adsorbate has occurred (Figure 2.2). The concentration found in the effluent

will continue to increase until it approaches the influent concentration at which point the

activated carbon bed is considered to be exhausted or spent. The term breakthrough is usuallydefined according to the treatment objective. In drinking water applications, the odour threshold

concentration (see Section 2.2.2), or concentration at which consumers detect a particular

compound, is the most apt way with which to define breakthrough. When consumer complaints

begin to be reported due to taste or odour in their drinking water, the treatment objectives of the

activated carbon are no longer being met.

Figure 2.2 Mass transfer zone through an activated carbon column(adapted from Vermeulen, 1958)

(C0: influent concentration, C: Effluent concentration, tb: time to breakthrough, te: time to bed exhaustion)

1

0

C/C0

Used/spent carbon

Unused carbon

tb teOperating time, t


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2.1.4 USES FOR ACTIVATED CARBON

In the modern era, the earliest applications of activated carbon were for industrial use in

decolourization of sugar in the late 18th century. Activated carbon went on to be used for the

treatment of polluted air, recycling of solvents and in the purification of by-products fromchemical, pharmaceutical and food manufacturing processes (Sontheimer et al., 1988). In 1920,

activated carbon increasingly became popular as part of water treatment processes, mainly for

the removal of taste and odour compounds. The highly controlled and precise activation process

used to produce activated carbon is capable of producing specific carbons with different

properties appropriate for a wide variety of applications.

A very common application for activated carbon in drinking water treatment today is for the

control of taste and odour episodes. Some utilities apply seasonal control measures such as the

use of PAC; however, many have installed granular activated carbon contactors that remain

operational all year round. A properly designed and operated GAC contactor can be operated for

several years to reduce T&O compound concentrations (Figure 2.3). Length of use depends on

source water characteristics including the presence of NOM which compete for adsorption sites

with taste and odour target compounds (Newcombe et al., 1997; Pelekani and Snoeyink, 1999),

the activated carbon chosen (Chudyk et al., 1979; Lalezary et al., 1986; Newcombe et al., 2002)

and the flow rate.

Figure 2.3 Georgina water treatment plant GAC contactor


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2.2 TASTE AND ODOUR ISSUES

The aesthetics of drinking water has a large influence on consumer perception of their drinking

water. A utility ultimately seeks to make the water potable and palatable. It is speculated that

taste and odour episodes will continue to increase with the presence of zebra mussels clarifyingwater in the Great Lakes, leading to increased temperatures and resulting improved conditions

for the growth of algae (Anderson and Quartermaine, 1998). More frequent complaints in certain

municipalities may therefore be expected from consumers due to taste and odour in their

drinking water. Consumers may also perceive a risk to their health due to taste and/or odour in

their water, resulting in a loss of consumer confidence.

A survey completed of 377 water utilities in Canada and the US by the American Water Works

Association (AWWA) stated that, fiscal resources spent by water utilities to control taste and

odor problems averages $67,800, representing an average of 4.5 percent of their total budget

(Suffet et al., 1996). Tools to predict adsorption capacity, not only for organic compounds in

general, but specifically for taste and odour, are therefore needed to allow utilities to make the

most cost-effective choice.

2.2.1 TASTE AND ODOUR COMPOUNDS

The known causes of taste and odour (T&O) are summarized in a drinking water taste and odour

wheel by Suffet et al.(1999). Of particular interest for the Great Lakes region in Canada and the

US are the two naturally occurring compounds which produce earthy-musty odours in water,

geosmin and 2-methylisoborneol (MIB). While other compounds also cause earthy-musty odours

(2-isopropyl-3-methoxy pyrazine (IPMP), 2-isobutyl-3-methoxy pyrazine (IBMP), and 2,3,6-

trichloroanisole (TCA)), geosmin and MIB are considered the major compounds in the earthy-

musty category (Lalezary et al., 1986) and are the most commonly identified (Rao et al., 2003).

Physical and chemical characteristics of geosmin and MIB are shown in Table 2.1.


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Table 2.1 Chemical and physical characteristics of geosmin and MIB (Pirbazari et al., 1992)

Name Geosmin 2-Methylisoborneol (MIB)Molecularstructure*

Molecularformula

C12H22O C11H20O

Molecular weight 182 g/mol 168 g/molKow 3.70 3.13

*(Image source for structures: National Library of Medicine, ChemIDplus Advanced, http://chem.sis.nlm.nih.gov/chemidplus/)

Geosmin and MIB are produced naturally by planktonic and benthic algae, most commonly

cyanobacteria, fungi, bacteria and actinomycetes (Lloyd et al., 1998). Specifically, geosmin is

produced by blue-green algae such as Oscillatoria simplicissima and Anabaena scheremetievi

and MIB is a product of certain blue-green algae (Oscillatoria curviceps and Oscillatoria tenius)

and Actinomycetes (Herzing, 1977). Both compounds are considered semi-volatile and produce

an earthy, musty odour in drinking water. Taste and odour episodes are a seasonal issue in theGreat Lakes region, with studies from Lake Ontario, Canada showing that episodes occur in the

summer months due to high water temperatures creating algae blooms and bacterial growth in

the lake (Rao et al., 2003; Ridal et al.,2001). Raoet al.(2003) reported that geosmin production

peaks annually but is not always found to be at offensive levels.

Both geosmin and MIB are low molecular weight, tertiary alcohols. MIB has a hydrocarbon

skeleton containing one hydroxyl group, making it relatively hydrophobic (Considine et al.,

2001; Pendleton et al., 1997). MIB has a molecular weight of 168 g/mol (Newcombe et al.,

2002a) and is roughly spherical in shape with a diameter of 0.6 nm (Pendleton et al., 1997).

1,2,7,7-tetramethyl-2-norborneol, or geosmin, which directly translates to earthy smell, has a

molecular weight of 182 g/mol.


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MIB has generally been found to be the least adsorbable compound to activated carbon of the

five earthy-musty compounds listed above and is therefore often chosen as the model compound

in activated carbon taste and odour studies (Chen et al., 1997). Other studies have shown that

MIB is readily adsorbed by microporous carbon unless competing with compounds for

adsorption (Newcombe et al., 2002b). Adsorption of MIB has also been found to be more

affected than geosmin by the presence of humic acid and glycolic acid. Sugiura et al. (1997)

hypothesized this was due to the difference of pore size required by geosmin and acids for

adsorption as well as the difference in molecular structure of the two musty odour compounds.

2.2.2 ODOUR THRESHOLD CONCENTRATION

The odour threshold concentration (OTC) of a compound is the concentration at which

consumers can detect that compound in their water. Both geosmin and MIB may be detected by

humans at extremely low concentrations. A recommended reduction of MIB to below 10 ng/L

was given by Chen et al. (1997). Although the OTC for both these compounds varies slightly

across the literature, thresholds as low as 9 ng/L and 4 ng/L have been reported for MIB and

geosmin, respectively (Kim et al., 1997; Pirbazari et al., 1993).

2.2.3 COMPETITION WITH NOM

Commercial activated carbons are generally designed for drinking water treatment applicationsto deal with the removal of small molecular weight hydrophobic organic contaminants and not

specifically the removal of dissolved organic material (DOM) (Dastgheib et al., 2004). Although

its primary use may not be to remove DOM, competition of DOM with targeted compounds

necessitates the understanding of the competition effects that exist between these compounds.

Adsorption capacity for trace contaminants has been reported to be reduced due to competition

with natural organic matter (NOM) in numerous studies. The extent to which competitive

adsorption has an effect is dependent on the initial concentration of the trace contaminant (Najm

et al., 1991), the molecular structures of the NOM and the trace contaminants (Herzing et al.,

1977; Newcombe et al., 1997, 2002b) and the type of activated carbon (Chen et al., 1997;

Pelekani and Snoeyink, 1999).


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Competition of trace contaminants with NOM for adsorption sites will be described in more

detail in Chapter 4.

2.3 ACTIVATED CARBON CHARACTERIZATION FOR MICROPOLLUTANT

CONTROL

In order to choose the most appropriate activated carbon for micropollutant removal, a clear

understanding of its adsorption capacity is required. Carbon characterization for adsorption

capacity is conducted using three main parameters: physical properties, activity indices (isotherm

tests) and kinetic tests. Ideally, a combination of all of these parameters should be used to enable

a utility to make a more accurate and educated choice when purchasing an activated carbon.

When it comes time for a utility to purchase an activated carbon, the carbon manufacturer willgenerally provide information on the adsorption capacity of the carbon. An activated carbons

adsorption capacity is commonly described first by the carbons physical characteristics and

secondly by using simple lab-scale tests such as the iodine number or the tannin number. Studies

have shown, however, that these tests are not always reliable in predicting activated carbon

performance. Although they serve as a good starting point to narrow down the choice of

activated carbon, more precise tests catered to taste and odour adsorption capacity are needed.

Chen et al. (1997) suggests the use of isotherm tests using the compound of interest (i.e.,

geosmin or MIB) in organic pure water1 noting that they are the true representation of the

inherent adsorption potential of a particular activated carbon. Beyond isotherm tests, additional

tests need to be conducted in order to consider kinetics within the activated carbon bed and the

influence of characteristics from the natural source water that will vary from season to season.

Rapid small-scale column tests are an example of a bench-scale test used to design and evaluate

full-scale granular activated carbon contactors.

2.3.1 DETERMINATION OF PHYSICAL PROPERTIES

The physical properties of an activated carbon contribute information needed to understand a

carbons adsorption capacity. Activated carbons porous nature is the key component

contributing to its ability to adsorb large quantities of organics (Sontheimer et al., 1988). When

examining the porosity of an activated carbon, pore size, pore shape, pore volume, pore size

1Organic pure water was obtained by glass-distilling Milli-Qwater.


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distribution, and surface area of the carbon are all important parameters to consider (Sontheimer

et al.,1988).

The adsorption process takes place in four steps: bulk solution transport, film diffusion transport,

pore and surface transport and adsorption (Hand et al., 1983). These steps occur on the outer

surface of the adsorbent as well as within the carbons pore structure. An understanding of the

pore size distribution provides key information on the adsorption process and, subsequently, how

a particular activated carbon will perform in adsorbing an adsorbate of interest.

Pores within an activated carbon are split into four size categories by the Union of Pure and

Applied Chemistry (IUPAC): macropores (500 ), mesopores (20 - 500 ), secondary

micropores (7 - 20 ) and primary micropores (7 ) (Lastoskie et al., 1993). Although there isvery little adsorption that occurs in the macropores, this region is very important in the diffusion

process (Sontheimer et al., 1988). The surface area in macro- and mesopores is very small and

thus the amount of material adsorbed on these sites is considered negligible. There is an inverse

relationship between pore size and surface area. Thus, a larger number of small pores for a given

pore volume will yield a larger surface area for the activated carbon (MWH, 2005). The majority

of the internal surface area of an activated carbon is within the micropores. As a result, most of

the adsorption of organic compounds occurs in the micropores (Karanfil, 2006).

Several tests exist to measure the pore volume and surface area of an activated carbon. These

tests involve exposing the activated carbon to a certain amount of adsorbate (in liquid or gas

form) and measuring the quantity of adsorbate that is taken up by the carbon. Various dosages of

carbon are exposed to the quantity of adsorbate until equilibrium is reached (at a constant

temperature) indicating what is referred to as the adsorption isotherm for that adsorbate. Mercury

porosimetry is often used to evaluate the pore volume and surface area distribution in the

macropore and mesopore range. Nitrogen isotherms (at liquid temperature, 77K) are commonly

used to determine mesopore and micropore volumes. Brunauer, Emmett and Teller (1938)

developed an isotherm method that is still commonly used to indicate the specific surface area of

an activated carbon, the BET surface area.


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Adsorption Isotherms

At equilibrium, an adsorption phase concentration, or amount of adsorbate (mg) per gram of

adsorbent, can be calculated using Equation 2-1; where qe is the adsorbent phase concentration

after equilibrium (mg adsorbate/g adsorbent), C0 is the initial concentration of the adsorbate

(mg/L), Ce is the equilibrium concentration of the adsorbate after adsorption has occurred

(mg/L), V is the volume of liquid in the reactor and m is the mass of the adsorbent (g). Three

main theories exist to determine adsorption capacity of adsorbents using this adsorption phase

concentration and the data from the isotherm test: Freundlich, Brunauer Emmett and Teller

(BET) and Langmuir theories.

m

)VC(Cq e0

e

(2-1)

The Freundlich isotherm is most commonly used to describe adsorption capacity of activated

carbon in the water treatment context. The Freundlich isotherm is an appropriate empirical

equation as it describes heterogeneous adsorbents, or adsorbents with varying site energies, such

as activated carbon (MWH, 2005). The Freundlich isotherm is the following (Equation 2-2):

nefe CKq

m

x 1 (2-2)

where x/m is the mass of the adsorbate (mg) adsorbed per unit mass of the adsorbent (g) after

equilibrium, Kf is the Freundlich capacity factor ((mg adsorbate/g adsorbent) x (L water/mgadsorbate)1/n), 1/n is the Freundlich adsorption intensity parameter (unitless) and the other terms

are as defined above. The Freundlich intensity parameter varies widely for each adsorbate being

considered and must be determined for each compound being studied.

The BET theory describes the adsorption of gases onto a solid surface with the assumption that

adsorption occurs in multiple layers. The BET theory accounts for multiple layers in which

adsorption occurs, however, it maintains that site energy is the same for the first layer and equal

to free energy of precipitation for subsequent layers. This differs from the Langmuir model

which assumes that adsorption site energy is the same for all sites and that the largest capacity

occurs on one monolayer. This assumption makes the Langmuir equation invalid for activated

carbon adsorption measurement as activated carbon, as previously mentioned, has a wide range

of pore sizes that will continue to adsorb organics even as the adsorbate concentration increases.


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Pore Size Distribution

Pore size distribution in carbon is an important property influencing the adsorption process

(Pelekani and Snoeyink, 1999). The pore size distribution provides information on the fraction of

total pore volume that will be available for adsorption by an adsorbate of a certain size. Pore size

distribution has a large impact on competitive adsorption (Pelekani and Snoeyink, 1999).

Newcombe et al. (1997) studied the pore size distribution of an activated carbon using adsorption

of nitrogen (77K) and BET plot and noted that the adsorption of different size fractions of natural

organic matter (NOM) had significant effects on the surface area and pore volume distributions

available to MIB. When NOM consisted of small compounds similar to MIB, competitive

adsorption competition was greatest due to direct competition between NOM and MIB for the

pore sites.

Additional Important Physical Characteristics

Other physical characteristics such as apparent density, moisture content, hardness and abrasion

number are also important. The apparent density is defined as the mass of carbon per unit volume

of carbon bed, including the pore volume (MWH, 2005). Apparent density enables the packed

density of a carbon bed to be determined. Activated carbons with higher density are generally

preferred. Hardness and abrasion number are both important parameters to consider to minimize

costs due to loss of carbon from carbon contactors. Both parameters are also indicators of a

carbons ability to withstand frequent backwashing and repeated handling during regeneration.

2.3.1.1 DISTRIBUTION OF ENERGY SITES

Adsorption energy will vary within the porous structure of an activated carbon. Adsorption

energy is determined by the amount of surface area with which an adsorbate comes in contact.

Adsorption energy for micropollutants is greatest in micropores because multiple contact points

exist between the adsorbate and the activated carbon surface allowing for multiple surface forces

to overlap resulting in increased adsorption forces (Pelekani and Snoeyink, 1999; Karanfil,

2006). Within the carbon the larger pore sizes are considered lower energy pores. If the graphitic

plates within the carbon are further apart, the adsorption energy decreases. If the plates are too

far apart, then no adsorption energy exists, however, the space, known also as transport pores,

provides avenues for the adsorbates to enter the wide variety of adsorption pore structures (CCC,

2002).


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Different activated carbons will display different distributions of adsorption energy, each

providing varying proportions of high versus low energy pores. The raw material and the

activation process play a large role in producing the activated carbon best suited for specific

applications. If a particular pore does not have the required amount of pore energy to remove a

specific compound, the compound will not be adsorbed in that location. Compounds requiring

less energy may adsorb to lower energy adsorption sites. Therefore, if the purpose of an activated

carbon application is to remove larger and more easily adsorbed compounds (i.e., low solubility,

high molecular weight) an activated carbon with more low energy pores would be selected.

Easily adsorbed compounds can adsorb in all of the adsorption pore structure, in both high and

low energy sites. Generally, carbons with high overall surface area will be best suited for

removal of these compounds.

A specific consideration would include if the intended use for the activated carbon involves

removal of compounds more difficult to adsorb (i.e., high solubility, low molecular weight) or

compounds at trace concentration levels, a carbon with more high energy pores would be

required. Studies have shown that compounds are likely to adsorb in a pore approximately the

same size as the adsorbate due to more contact points with the carbon and a resulting more

favourable adsorption energy (Newcombe et al., 1997). Lower energy pores would not be

utilized by these compounds.

2.3.2 DETERMINATION OF ADSORPTION CAPACITY

As described in Section 2.3.1, adsorption isotherms are helpful in determining the adsorption

capacity of an activated carbon. However, adsorption isotherms, as thermodynamic tests, will

only provide a guideline to what type of carbon is needed for a particular purpose, i.e., taste and

odour control.

Iodine number

Iodine number is the most common indicator of activated carbon adsorption capacity provided to

utilities by carbon manufacturers. The iodine number is the measure of iodine (I2) adsorbed from

a 0.1 N solution by a gram of activated carbon when the residual concentration of the solution is


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0.02 N. The iodine number measures total energy sites in an activated carbon rather than

specifying between low and high energy sites.

Additional Conventional Equilibrium Adsorption Capacity Tests

Other adsorption isotherms are conducted to provide information on the amount of a certain

surrogate that will be adsorbed to the activated carbon. Surrogates include phenol, butane,

molasses and tannic acid. The phenol test is the amount of carbon required to reduce phenol

concentration from 200 g/m3to 20 g/m3. Phenol has a high aqueous solubility and is a potential

pollutant in certain source waters. A butane activity test can be conducted to determine the

micropore volume of an activated carbon. It is a measure of the ability of a carbon to remove

butane from dry air. The molasses test reveals the amount of carbon necessary to decolourize a

standard molasses. Due to the large molecular weight of the colour producing substances in themolasses, this test is not as applicable to the drinking water industry but rather is relevant for the

sugar processing industry and decolourization purposes. The tannin value test measures the

concentration of GAC (in mg/L) that is required to reduce the standard tannic acid concentration

from 20 mg/L to 2 mg/L (AWWA, 2006). These surrogates, each with differing molecular sizes,

provide information on the pore sites with corresponding sizes that exist within the carbon.

Trace Capacity Number (TCN) test

In order to determine the adsorption capacity for trace contaminants, a more appropriately sized

surrogate is necessary. The Trace Capacity Number (TCN) test is performed to measure the trace

adsorption capacity of carbon. The TCN method has been verified for coconut carbons and

bituminous coal based virgin, reactivated, and calcined carbons (CCC, 1999). The trace capacity

number test is conducted in both liquid (TCN) and gas phase (TCNG) using acetoxime solution

and tetrafluoromethane gas, respectively, as indicator compounds to determine adsorption

capacity.

The liquid phase TCN test is included in the appendix of the AWWA Granular Activated Carbon

Standard (ANSI/AWWA B604-05) as a surrogate adsorption capacity test to the iodine number

test for GAC (AWWA, 2006). The method involves three different known weights of carbons

being treated with a standard acetoxime solution for a specific contact time. The trace capacity

number is the mass (mg) of acetoxime adsorbed onto 1 mL of activated carbon at a 30 mg/L


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residual concentration. This value indicates the trace adsorption capacity of the carbon being

tested.

The gas phase TCN test or TCNG is defined as the ratio (in g/100mL) of the mass of

tetrafluoromethane (CF4) adsorbed by a volume of activated carbon sample when the carbon is

saturated with CF4vapour. The TCNG method is modified based on the butane number method

as described in ASTM D5742-95.

2.3.3 RAPID SMALL-SCALE COLUMN TESTS

Several studies have noted that conventional equilibrium tests such as iodine and tannin numbers

are inconsistent at predicting adsorption capacity of a carbon (Chen et al., 1997; Sontheimer et

al., 1988). Adsorption isotherms such as the Freundlich isotherm test using the contaminant of

interest were found to be a better indicator of performance. These adsorption isotherms,

however, are thermodynamic tests and do not provide any information on the kinetics of

adsorption. Information on adsorption kinetics is required to compile a more complete

understanding of GAC performance. Kinetic bench-scale or pilot tests are needed to ensure that

the carbon chosen best suits the utilities source water.

Rapid small-scale column tests (RSSCTs) are kinetic tests used extensively to help in the designand evaluation of full-scale GAC adsorption processes. There are three main advantages in using

a RSSCT: (1) a RSSCT can be completed in a fraction of the time it would be required to do a

pilot study, (2) extensive isotherm or kinetic studies are not needed to predict full-scale

performance from a RSSCT and (3) only a small volume of water is needed for a RSSCT,

allowing the test to be completed easily in a laboratory setting (Crittenden, 1987).

Several articles over the past 20 years have described the design and the successful

implementation of the RSSCT. This section serves to show the development and discussion of

the RSSCT in the literature.

A mathematical model of the adsorption process in a packed media bed was developed prior to

the RSSCT. It is called the dispersed-flow, pore-surface-diffusion model (DFPSDM) and was

referenced by Crittenden et al. (1987). The DFPSDM was used in scaling down the full-scale


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adsorber as it contains many of the mechanisms that occur in fixed-bed adsorption. The

DFPSDM maintains that the adsorption process is a function of (1) advective flow, (2) axial

dispersion and diffusion, (3) liquid-phase mass transfer resistance, (4) local adsorption

equilibrium at the exterior surface of the carbon, (5) surface diffusion, (6) pore diffusion, and (7)

competitive equilibrium of solutes upon the carbon surface.

Work by Berrigan (1985) reviewed the DFPSDM and noted the presence of six dimensionless

groups in the governing equations of the model. It was proposed that if these 6 groups were kept

constant between the small and large carbon columns, there would be exact similitude between

the two columns. Caveats to the RSSCT are that backwashing effects are not considered in the

model, the scaling procedure is based on the DFPSDM and therefore will only work in situations

where the DFPSDM applies and finally, the effect of biological activity within the carbon bed isignored (Crittenden, 1987). RSSCTs also are generally run using a single batch of water and

therefore will not take into account variations of water quality (i.e., seasonal or climate event

related) at full-scale.

The six dimensionless groups are:

1. Surface solute distribution parameter, Dgs,i

i

iea

isC

qDg

,0

,,

1

(2-3)

))((

)1)()((

ionconcentratphasebulkinitialfractionvoid

fractionvoidqFreundlichdensitycarbon

2. Pore solute distribution parameter, Dgp,i

1,

p

ipDg (2-4)

)()1)((

fractionvoid

fractionvoidfractionpore

Note: Void fraction is equal to the pore fraction plus the empty space between the carbon

particles in the carbon bed.


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3. Modified Stanton number, Sti

R

kSt

i,f

i

1 (2-5)

)fractionvoid)(radiusparticlecarbon(

)fractionvoid)(bedintimeresidencefluid)(tcoefficientransferfilm(

1

4. Pore diffusion modulus, Edp,i

2R

DgDEd

i,pi,p

i,p

(2-6)

2)radiusparticlecarbon(

)bedintimeresidencefluid)(parameterondistributisolutepore)(ydiffusivitpore(

5. Surface diffusion modulus, Eds,i

2R

DgDEd

i,si,s

i,s

(2-7)

2)radiusparticlecarbon(

)bedintimeresidencefluid)(parameterondistributisolutesurface)(ydiffusivitsurface(

6. Peclet number, Pei,D

i

S

iDe

vL

Pe (2-8)

tydispersiviaxial

velocity)erstitial)(intlengthbed(

The Stanton number represents film transfer effects, the Peclet number represents dispersive

effects, and intraparticle diffusion is represented by the Surface and Pore Diffusion moduli.

Berrigan (1985) notes that in most fixed-bed adsorption processes, the rate of film transfer

(shown by the Stanton number) is rarely the limiting mass transfer mechanism. The surface

diffusion modulus, however, is often an important consideration with mass transport often being

controlled by surface diffusion. Berrigan states that there is only one set of circumstances where

all six dimensionless parameters can be maintained equal between the large and small columns.

For this to occur, surface diffusivity, DS, must be independent of carbon particle size (i.e.,

constant diffusivity). The dimensionless parameters are used to create the following two


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governing equations that must be satisfied to ensure similitude between the large and small

columns:

LC

SC

SC,S

LC,S

LC

SC

LC

SC

t

t

D

D

R

R

EBCT

EBCT

2

(2-9)

and with DS,LC= DS,SCthis equation reduces to:

LC

SC

LC

SC

LC

SC

t

t

R

R

EBCT

EBCT

2

(2-10)

For the case of constant diffusivity, to ensure similitude in terms of dispersion and film transfer

effects (St and Pe), the following equation is used:

SC

LC

LC

SC

R

R

v

v

(2-11)

Combining the above 2 equations, it is determined that the small column length is dictated by

carbon particle sizes:

LC

SC

LC

SC vR

Rv

LC

LC

SC

SC EBCT

R

REBCT

2

Since,VelocityApproach

LengthEBCT

LC

LC

LC

SC

SC

SC

VelocityApproach

Length

R

R

VelocityApproach

Length2

LC

LC

SC

LC

LC

LC

SC

SCVelocityApproach

VelocityApproachR

RLength

R

RLength

2

The above equation reduces to: LCLC

SC

SC LengthR

RLength

(2-12)

Crittenden et al. (1987) argue that intraparticle diffusivity normally controls the adsorption

process. Therefore, Equation 2-11 may be ignored to an extent without compromising the results


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in an effort to have more control over the calculated small column length. The limitation is that

the Reynolds number (2-14) for the small column should remain in the mechanical dispersion

region, which evidence suggests corresponds to a minimum value of approximately 1 (Crittenden

et al., 1991). As such, Equation 2-11 can be modified to yield a recommended minimum small

column superficial velocity:

LC

min

SC

LC

LCSC

SC

LC

LCRe

Re

R

Rvv

R

Rv

1 (2-13)

LCiLC

dvRe (2-14)

Where,

d = diameter of particles (m)i= interstitial velocity (m/s) [approach velocity/porosity of bed]= density of the fluid (kg/ m)= dynamic viscosity of fluid (Pas or kg /ms)LC = large column

In the more general circumstance where surface diffusivity may vary with particle size

(proportional diffusivity), one cannot simultaneously satisfy all six dimensionless parameters. In

this case, Crittenden et al.(1987) suggest ensuring solely that similitude in terms of intraparticle

diffusivity is maintained (i.e., Equation 2-9). Efforts to maintain similar Stanton and Pecletnumbers are abandoned.

Another important parameter in the RSSCT is one which allows the comparison of breakthrough

curves, regardless of bed size and is termed bed volumes (BV) (Equation 2-15):

EBCT

t

V

VBV

F

W (2-15)

Where VWis the volume of water treated, VFis the volume of the carbon filter bed, t is the time

(min) over which the water passed through the bed and EBCT is the empty bed contact time

(min).


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2.3.3.1 ASSUMPTIONS MADE WITH THE RSSCT

Constant Diffusivity

To obtain the RSSCT equation:LC

SC

LC

SC

LC

SC

t

t

R

R

EBCT

EBCT

2

the following assumptions are made:

1. Constant surface diffusivity (surface diffusivity of GAC is identical in both full-scale

and RSSCT systems)

2. The solute distribution parameters are the same for carbons in both small and large

columns, implying that the following properties are identical:

a. Equilibrium capacity

b. Bed void fractions

c. Carbon particle densities

d. Influent concentrations

3. Pore diffusivities and surface diffusivities are identical in large and small columns.

4. TheLC

SC

t

tratio indicates the time saved using a RSSCT and is determined by the amount

of reduction of the large column GAC particles.

EquationSC

LC

LC

SC

R

R

v

v ensures that Reynolds number of the small-scale process is the same as the

large scale process (with SCv and LCv being the superficial velocities, or loading rates of the

small and large columns, respectively).

Proportional Diffusivity

The EPA Bench- and Pilot-Scale manual highlights that several researchers have had positive

results scaling their column tests, especially for removal of NOM as measured by TOC and

UV254, assuming proportional diffusivity (McGuire et al., 1989; Summers and Crittenden; 1989

Summers et al., 1992; Wallace et al., 1988).

To obtain the RSSCT equation for proportional diffusivity:LC

SC

LC

SC

LC

SC

t

t

R

R

EBCT

EBCT

the

following assumptions are made:


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1. Surface diffusivity is linearly dependent on/proportional to carbon particle size

2. Surface diffusion is the controlling process

3. Similitude for St and Pe numbers is abandoned

Example Calculation Using Proportional Diffusivity

For Carbon A,

100 x 325 mesh screens used for RSSCT Geometric Mean Particle Size is 0.0890mm.

EBCTSC= 0.39 minEBCTLC= 7.5 minRSC= 0.089 mmRLC= 1.6 mm (8 x 30 mesh)

LC

SC

LC

SC

LC

SC

tt

RR

EBCTEBCT

And,

VLC= 4.1 gpm/ft2

RSC= 0.089 mmRLC= 1.6 mmReSC,min= 1 (*as suggested in Crittenden et al., 1991)

ReLC= 11648

LC

SC

SC

LC

LC

SC

R

R

V

V

Re

Re min,

11648

1

0890

61

14 2

mm.

mm.

ft/gpm.

VSC = 0.0015gpm/ft2= 2.45 L/s/m2

Hydraulic loading rate of small-scale column (VSC)is 2.45 L/s/m2.

As stated in Crittenden et al.(1991) the minimum column-diameter to particle size ratio shouldbe 50 to avoid channelling within the column. Therefore,

Column diameter = 0.46cm = 0.046mmParticle size (small-scale) = 0.089mm

Ratio 0.046:0.089is greater than 50, thus minimizing the wall effect.


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2.3.3.2 CONSTANT OR PROPORTIONAL DIFFUSIVITY?

In the event where it is unknown as to whether surface diffusivity is independent (constant

diffusivity) or dependent (proportional diffusivity) on carbon particle size, isotherm tests can be

conducted as described by Hand et al.(1983) to determine the relationship.

In order to determine the appropriate scaling factor for the mini-column tests, batch tests using

carbon of different sizes along with the adsorbent of interest, in the water matrix of interest, are

performed, with a plot of C/Co versus t/(RSC)2 and t/(RLC)

2 examined. In the plot, for a given

C/Co, the average ratio of t/(RSC)2to t/(RLC)

2is equal to the ratio of Ds,SCto Ds,LC. The ratio could

then be used in Equation 2-14 to obtain the governing equation for the mini-column tests.

LC

SC

SC,S

LC,S

LC

SC

LC

SC

t

t

D

D

R

R

EBCT

EBCT

0

(2-16)

Where,R = carbon particle sizet = real or elapsed timeC/C0= effluent concentration divided by influent concentrationDS= surface diffusion coefficientSC = small-scale

LC = large scale

In the case of proportional diffusivity, with similitude for St and Pe numbers abandoned, it is

only Equation 2-14 which governs the design. This means that different combinations of

superficial velocity, vSC, and column length, LSC, can be selected, so long as the ratio of LSC/vSC

is equal to EBCTSC. Crittenden et al. (1987) advise, however, that to ensure that dispersion

effects remain negligible, velocities and lengths need to be selected according to Equation 2-13.

Studies conducted to date have found that when NOM is present PD-designed columns bestpredicted NOM breakthrough curves (Summers et al., 1989). The CD-designed columns

predicted earlier breakthrough as particle size decreased (EPA, 1996).


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2.3.4 THE ACCELERATED COLUMN TEST

The ACT and RSSCT are very similar. The ACT test is the in-house small-scale column test

used by Calgon Carbon Corporation to predict activated carbon performance. The principal

difference lies in the exponent that is used in the scaling factor equations. This exponent is

derived experimentally rather than by making assumptions about the system having constant or

proportional diffusivity.

The governing equation for the ACT relates the length of the mass transfer zones to the mean

carbon diameter:

SC

LC

SC

LC

R

R

MTZ

MTZ

(2-17)

Calgon Carbon Corporation (CCC) determines the alpha factor experimentally by adsorbing a

solution of acetoxime in three identical micro-column tests where the columns contain the same

mass of carbon but with different mean particle diameters. The alpha factor CCC consistently

found in their tests was approximately 1.1 under most drinking water conditions.

The MTZ is defined here as the amount of time between 1 % and 50 % breakthrough of

acetoxime. A plot of ln(MTZ) versus ln(R) yields the alpha factor as the slope. Note that any

percent breakthrough can be used for the determination of the alpha factor, as long as it is

consistent among the three micro-columns. Equation 2-15 therefore is similar to the RSSCT

Equation 2-9, where the ratio of MTZs is equivalent to the ratio of treatment times, tSCand tLC:

RSSCT equation:LC

SC

SC,S

LC,S

LC

SC

LC

SC

t

t

D

D

R

R

EBCT

EBCT

2

Re-written ACT equation:

LC

SC

LC

SC

LC

SC

R

R

t

t

MTZ

MTZ (2-18)

As mentioned above, Calgon reports that the alpha factor is normally 1.1. The derivations of the

RSSCT suggest that the scaling factor, if controlled by intraparticle diffusivity, can be between 2

(constant diffusivity) and 1 (diffusivity is exactly linearly proportional to particle size). It is


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perhaps logical that the true situation should fall between 1 and 2: i.e., 1.1. No information is

available, however, to support the claim that 1.1 is almost always the appropriate scale factor.

With the ACT, to select the length of the columns, the following equation is used:

LC

SC

lC

LC

SC

SC

LC

SC

R

R

v

L

v

L

EBCT

EBCT (2-19)

Calgon recommends that similar approach velocities be used for the small- and large-columns,

reducing Equation 2-19 to:

LC

SC

LCSCR

RLL (2-20)

By keeping the large- and small-column approach velocities equal, the ACT violates the only

circumstance reported by Berrigan (1985) where all 6 dimensionless parameters can be similar

between large and small columns: i.e., constant diffusivity in which the ratio of approach

velocities must equal the ratio of carbon particle sizes (Equation 2-11). However, by selecting an

alpha factor not equal to 2, the ACT automatically rejects constant diffusivity. The RSSCT also

violates this condition in proportional diffusivity designs where the approach velocities are

arbitrarily ignored provided that the corresponding Reynolds Numbers remain in the mechanical

dispersion regime (Equation 2-13).

A Comparison of the RSSCT and the ACT

A summary comparison of the RSSCT and ACT is given in Table 2.2.

Table 2.2 Comparison of calculations used for RSSCT and ACTEquations RSSCT ACT

Column Length

LC

sc

LCSCR

RLL

where, 12

LC

sc

LCSCR

RLL

where, = 1.1

Flow Rate (Q)Q = vSCx areaSC

where, vSC=calculated superficialvelocity in the small column

Q = vLCx areaSCwhere, vLC= the given loading

rate/approach velocity of the largecolumn

VelocitySC ( v SC)

LC

minSC,

LC

SC

LC

SCRe

Rev

R

Rv v SC=

SC

SC

A

Q


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To determine the volume of water needed for a RSSCT, two derivations are possible:

Derivation 1:

T

SCSCSC tQVol where, VolSCis the minimum volume of influent water needed and Q is theflow rate and tTis the total run time and where the subscript SC indicates small column.

Substitute, Q = VA into the above equation

T

SCSCSCSC tAvVol (2-21)

Since,LC

SC

LC

SC

LC

SC

R

R

EBCT

EBCT

t

t

LC

SC

LCSCR

Rtt

Therefore, Equation (2-21) becomes:

LC

SC

LCSCSCSCR

RtAvVol

AndLC

min,SC

SC

LC

LC

SC

Re

Re

R

R

v

v substituted into the above equation gives:

LCSCLC

min,SC

LC

LC

SC

LCSC

LC

min,SC

SC

LC

LCSC tARe

Rev

R

RtA

Re

Re

R

RvVol

Similarly,

Derivation 2:

Volume of Water Needed = BV x Volume of Carbon (m3)

= BV x LSCx AreaSC

Recall,SC

LC

LC

SC

R

R

v

v and LC

SC

LC

SC vR

Rv

SCLC

min,SC

LC

SC

LC

SCSCSCSC EBCTRe

Rev

R

RAreaEBCTvArea

BV

NeededVolume


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LC

SC

LC

LC

min,SC

LC

SC

LC

SCR

REBCT

Re

Rev

R

RArea

BV

NeededVolume

As seen above, volume is only a function of the small column area since the other components

(EBCTLCand vLC) are fixed.

In contrast, for ACT:

11.

LC

SC

LCLCSCSCLCSCR

REBCTvAreaEBCTvArea

BV

NeededVolume

As seen above, with the ACT protocol, the volume will change with a change in carbon sizes.

EBCT reduces to Length/Approach Velocity (V) as shown below:

SC

SC

SCSC

SCSC

SC

SC

SCV

L

AV

AL

Q

VolumeEBCT

Surface Loading Rate is the same as approach velocity (m/hr).

A

QdingRateSurfaceLoa

LC

SC

SC,S

LC,S

LC

SC

LC

SC

t

t

D

D

R

R

EBCT

EBCT

In an ACT, velocities are the same for small and large scale columns. Therefore, the above

equation becomes:

LC

SC

LC

LC

SC

SC

LC

SC

R

R

V

L

V

L

EBCT

EBCT LC

LC

SC

SC LR

RL

2.4 CURRENT SELECTION METHOD FOR PURCHASING CARBONS

The following section serves to explain how utilities are assisted in choosing an activated carbon

for drinking water treatment. Most of the information was obtained through communication with

Calgon Carbon Corporation, a major manufacturer of activated carbon.


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Filtrasorb is Calgon Carbon Corporations main product line available to drinking water

treatment utilities. When assisting a utility in the selection of an activated carbon, mesh size is

one influencing factor. For example, F300 (8 x 30 mesh) may be chosen over F400 (12 x 40

mesh) as a coarser mesh size allows for deeper carbon beds without as much headloss resulting

in a longer service life. Therefore, utilities installing carbon for total organic carbon (TOC) and

disinfection by-product (DBP) removal tend to choose a coarser carbon product.

For TOC removal, carbons with high iodine numbers are generally preferred due to larger overall

adsorption capacity for organics. A carbon that has been activated for longer will have less

micropores and a higher iodine number but a lower trace capacity number. Another Calgon

product, F600, has a low iodine number but a high TCN value. Reactivation of a carbon will also

contribute to a decrease in micropores, thus lowering the TCN value. Reactivation therefore isconsidered a better choice for utilities using carbon for TOC removal as the carbon structure is

opened up, resulting in larger pores in the reactivated product.

Other parameters considered are hardness and abrasion values. During backwash, carbons with

lower hardness and abrasion values will generate fine particles. These fine particles will increase

headloss in the carbon bed and result in a loss of carbon mass. Ash content is also an important

parameter to consider as this is additional carbon mass that is not needed and reduces the overall

efficiency of the carbon on a per mass basis. It also increases the cost effectiveness of the

activated carbon on a per mass basis.


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2.5 REFERENCES

American Water Works Association (2006) AWWA Standard: Granular Activated Carbon,ANSI/AWWA B604-05. American Water Works Association Research Foundation, Denver,

CO.

American Water Works Association Research Foundation (AWWARF) (2007) Removal ofEDCs and pharmaceuticals in drinking and reuse treatment processes. American Water WorkAssociation Research Foundation, Denver, CO.

Anderson B.C. and Quartermaine L-K. (1998) Tastes and odors in Kingstons municipaldrinking water: A case study of the problem and appropriate solutions.Journal of Great LakesResearch 24(4), 859-867.

Baker M.N. (1948) The Quest for Pure Water. American Water Works Association Inc., New

York.

Berrigan J.K. (1985) Scale-up of rapid small-scale adsorption tests to fixed-scale adsorbers:Theoretical and experimental basis. Masters Thesis from Michigan Technological University.

Brunauer S., Emmett P.H. and Teller E. (1938) Adsorption of gases in multimolecular layers.Journal of the American Chemical Society 60, 309-19.

Calgon Carbon Corporation (CCC) (1999) Determination of trace capacity number (TM-79).Standard operating procedure for trace capacity number test method provided by Calgon.Calgon Carbon Corporation, Pittsburgh, PA.

Chen G., Dussert B.W. and Suffet I.H. (1997) Evaluation of granular activatedcarbons for removal of methylisoborneol to below odor threshold concentration in drinkingwater. Water Research31(5), 1155-1163.

Chudyk W.A., Snoeyink V.L., Beckmann D., Temperly T.J. (1979) Activated carbon versusresin adsorption of 2-methylisoborneol and chloroform. Journal of American Water WorksAssociation71(9), 529-538.

Considine R., Denoyel R., Pendleton P., Schumann R. and Wong S.H. (2001) The influence ofsurface chemistry on activated carbon adsorption of 2-methylisoborneol from aqueous

solution.Colloids and Surfaces A: Physicochemical and Engineering Aspects 179(13), 271-280.

Crittenden J.C., Berrigan J.K. and Hand D.W. (1986) Design of rapid small-scale adsorptiontests for a constant diffusivity.Journal of Water Pollution ControlFederation 58(4), 312-9.

Crittenden J. C., Berrigan J. K. and Hand D.W. (1987) Design of rapid fixed-bed adsorption testsfor non-constant diffusivities.Journal of EnvironmentalEngineering113(2), 243-259.


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Crittenden J.C., Reddy P.S., Arora H., Trynoski J., Hand D.W., Perram D.L. and Summers R.S.(1991) Prediction of GAC performance with RSSCTs. Journal of American Water WorksAssociation 83(1), 77-87.

Dastgheib S.A., Karanfil T. and Cheng W. (2004) Tailoring activated carbons for enhanced

removal of natural organic matter from natural waters. Carbon42, 547-557.

Gillogly T. E. T., Snoeyink V. L., Vogel J. C., Wilson C. M. and Royal E. P. (1999) DeterminingGAC bed life.Journal of American Water Works Association91(8), 98-110.

Hand D.W., Crittenden J.C., ASCE M. and Thacker W.E. (1983) User-oriented batch reactorsolutions to the homogeneous surface diffusion model.Journal of Environmental Engineering109(1), 82-101.

Herzing D., Snoeyink V. and Wood N. (1977) Activated carbon adsorption of the odorouscompounds 2-methylisoborneol and geosmin. Journal of American Water Works Association

69(4), 223-228.Huang C., Benschoten J.E.V. and Jensen J.N. (1996) Adsorption kinetics of MIB and geosmin.

Journal of American Water Works Association22,116-128.

Karanfil T. (2006) Activated carbon adsorption in drinking water treatment. Activated CarbonSurfaces in Environmental Remediation.Elsevier Ltd,345-373.

Karanfil T. and Kilduff J. (1999) Role of granular activated carbon surface chemistry on theadsorption of organic compounds 1. Priority Pollutants. Environmental Science andTechnology33(18), 3217-3224.

Karanfil T., Kitis M., Kilduff J. E. and Wigton A. (1999) Role of granular activated carbonsurface chemistry on the adsorption of organic compounds 2. Natural organic matter.Environmental Science and technology33(18), 3225-3233.

Khiari D. and Watson S. (2007) Tastes and odours in drinking water: Where are we today?Water Science and Technology 55(5), 365-366.

Kim Y., Lee Y., Gee C. and Choi E. (1997) Treatment of taste and odor causing substances indrinking water. Water Science and Technology35(8), 29-36.

Lalezary S., Pirbazari M., Dale M., Tanaka T. and McGuire M. (1988) Optimising the removalof geosmin and 2-methylisoborneol by powdered activated carbon. Journal of AmericanWater Works Association80(3), 73-80.

Lalezary S., Pirbazari M. and McGuire M. (1986) Evaluating activated carbons for removing lowconcentrations of taste-and-odor producing organics. Journal of American Water WorksAssociation78(11), 76-82.

Lastoskie C., Gubbins K. E. and Quirke N. (1993) Pore size distribution analysis of microporouscarbons: A density functional theory approach.Journal of Physical Chemistry97,4786-4796.


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Lloyd S.W., Lea J.M., Zimba P.V. and Grimm C.C. (1998) Rapid analysis of geosmin and 2-

methylisoborneol in water using solid phase micro extraction procedures. Water Research32(7), 2140-2146.

Marsh H. and Rodrguez-Reinoso F. (2006) Activated carbon. Elsevier, Amsterdam, Boston.Permanent link: http://simplelink.library.utoronto.c

smith kyla m 201106 msc thesis

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