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Optimization of combined air stripping and activatedcarbon adsorption for VOC removal from groundwater
Item Type Thesis-Reproduction (electronic); text
Authors Voigt, David Robert,1954-
Publisher The University of Arizona.
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Download date 10/07/2018 10:35:10
Link to Item http://hdl.handle.net/10150/191962
OPTIMIZATION OF COMBINED AIR STRIPPING ANDACTIVATED CARBON ADSORPTION FOR VOC REMOVAL FROM GROUNDWATER
byDavid Robert Voigt
A Thesis Submitted to the Faculty of the
DEPARTMENT OF CIVIL ENGINEERING AND ENGINEERING MECHANICS
In Partial Fulfillment of the RequirementsFor the Degree of
MASTER OF SCIENCEWITH A MAJOR IN CIVIL ENGINEERING
In the Graduate College
THE UNIVERSITY OF ARIZONA
1987
STATEMENT BY THE AUTHOR
This thesis has been submitted in partial fulfillment ofrequirements for an advanced degree at The University of Arizona and isdeposited in the University Library to be made available to borrowersunder rules of the Library.
Brief quotations from this thesis are allowable without specialpermission, provided that accurate acknowledgment of source is made.Requests for permission for extended quotation from or reproduction ofthis manuscript in whole or in part may be granted by the head of themajor department or the Dean of the Graduate College when in his or herjudgment the proposed use of the material is in the interests ofscholarship. In all other instances, however, permission must beobtained from the author.
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below:
G.L. AMY DateAssociate Professor of
Civil Engineering
ACKNOWLEDGMENTS
This thesis and the environmental courses completed represent a
significant personal accomplishment of which I am very proud. My
sincere appreciation to Dr. Gary Amy for his steadfast insistence of
excellence in these academic efforts. Most importantly, my thanks to
my wife and parents for their encouragement and support. Finally, I
owe a word of appreciation to Patricia and Brad for their assistance in
accomplishing the typing and graphics.
111
TABLE OF CONTENTS
Page
LIST OF TABLES v
LIST OF FIGURES vi
ABSTRACT x
INTRODUCTION I
OBJECTIVES 3
LITERATURE REVIEW 4
Air Stripping; Theory, Models and Studies 5GAC; Theory, Models and Studies 15Combining Air Stripping with GAC 17EPA Regulations 20Analysis and Monitoring Techniques 24Case Studies 25
EXPERIMENTAL PROCEDURES 26
Compounds Examined 26Air Stripping Experiments 28Powdered Activated Carbon Experiments 41Analytical Testing 45
RESULTS AND DISCUSSION 47
Air Stripping Experiments 47Activated Carbon Experiments 55
OPTIMIZATION AND ECONOMICS 60
CONCLUSIONS 72
APPENDIX: TABULATIONS AND GRAPHICAL PLOTS OF RESULTS OF THE AIRSTRIPPING EXPERIMENTS 73
REFERENCES 124
iv
LIST OF TABLES
Table Page
1. USEPA Standards For VOCs in Drinking Water 22
2. Properties of Compounds Studied 29
3. Isotherm Data for CT in Presence of TCE and 1,4 DCBon Activated Carbon 42
4. Isotherm Data for TCE in Presence of CT and 1,4 DCBon Activated Carbon 43
5. Isotherm Data for 1,4 DCB in Presence of CT and ICEon Activated Carbon 44
6. Mechanical Specifications for Packing Media 49
7. Sample Concentrations for VOC Experiments 74
8. Percent Removal Relative to Top of Packing 75
9. Incremental Percent Removal Between Sample Ports 76
10. (Cin/Cout) Relative to Top of Packing 77
11. Incremental (Gin/Gout) Between Sample Ports 78
12. Stripping Factors, R 79
13. Number of Transfer Units, NTU, Relative to Top ofPacking 80
14. Number of Transfer Units, NTU, Incremental BetweenSample Ports 81
15. Liquid Mass Transfer Coefficient, K La, Relative toTop of Packing 82
16. Liquid Mass Transfer Coefficient, K La, IncrementalBetween Sample Ports 83
v
LIST OF FIGURES
Figure Page
1. Relationship Between Stripping Factor and NTU 11
2. Diagram of Air Stripping Column and Appurtenances 31
3. Orifice Meter Details and Dimensions 33
4. Orifice Flow Meter, Differential Pressure vs. CFM 34
5. Air Flow Meter and Column Pressure Gauges, ValvePositions, A 36
6. Air Flow Meter and Column Pressure Gauges, ValvePositions, B 37
7. Column Hydraulics for Pall Rings Media 39
8. Column Hydraulics for Tripacks Media 40
9. Experiments 1-5, Pall Rings, CT Removal Relative toTop of Packing vs. A:W 84
10. Experiments 1-5, Pall Rings, TCE Removal Relative toTop of Packing vs. A:W 85
11. Experiments 1-5, Pall Rings, 1,4 DCB RemovalRelative to Top of Packing vs. A:W 86
12. Experiments 1-5, Pall Rings, CT Removal per FootRelative to Top of Packing vs. A:W 87
13. Experiments 1-5, Pall Rings, TCE Removal per FootRelative to Top of Packing vs. A:W 88
14. Experiments 1 - 5, Pall Rings, 1,4 DCB Removal perFoot Relative to Top of Packing vs. A:W 89
15. Experiments 1-5, Pall Rings, CT Removal per Foot,Incremental Between Sample Ports vs. A:W 90
16. Experiments 1-5, Pall Rings, TCE Removal per Foot,Incremental Between Sample Ports vs. A:W 91
v i
vi i
LIST OF FIGURES - Continued
Figure Page
17. Experiments 1-5, Pall Rings, 1,4 DCB Removal perFoot, Incremental Between Sample Ports vs. A:W 92
18. Experiments 1-5, Pall Rings, CT, K La Relative toTop of Packing vs. A:W 93
19. Experiments 1-5, Pall Rings, ICE, KLa Relative toTop of Packing vs. A:W 94
20. Experiments 1-5, Pall Rings, 1,4 DCB, KLa Relativeto Top of Packing vs. A:W 95
21. Experiments 1-5, Pall Rings, CT, KLa IncrementalBetween Sample Ports vs. A:W 96
22. Experiments 1-5, Pall Rings, TCE, K La IncrementalBetween Sample Ports vs. A:W 97
23. Experiments 1-5, Pall Rings, 1,4 DCB, KLa IncrementalBetween Sample Ports vs. A:W 98
24. Experiments 6-9, Pall Rings, CT Removal Relative toTop of Packing vs. A:W 99
25. Experiments 6-9, Pall Rings, TCE Removal Relative toTop of Packing vs. A:W 100
26. Experiments 6-9, Pall Rings, CT Removal per FootRelative to Top of Packing vs. A:W 101
27. Experiments 6-9, Pall Rings, TCE Removal per FootRelative to Top of Packing vs. A:W 102
28. Experiments 6-9, Pall Rings, CT Removal per Foot,Incremental Between Sample Ports vs. A.W 103
29. Experiments 6-9, Pall Rings, TCE Removal per Foot,Incremental Between Sample Ports vs. A:W 104
30. Experiments 6-9, Pall Rings, CT, KLa Relative toTop of Packing vs. A.W 105
31. Experiments 6-9, Pall Rings, ICE, KLa Relative toTop of Packing vs. A-W 106
viii
LIST OF FIGURES - Continued
Figure Page
32. Experiments 6-9, Pall Rings, CT, K La IncrementalBetween Sample Ports vs. A:W 107
33. Experiments 6-9, Pall Rings, ICE, K La IncrementalBetween Sample Ports vs. A:W 108
34. Experiments 10-14, Tripacks, CT Removal Relative toTop of Packing vs. A.W 109
35. Experiments 10-14, Tripacks, TCE Removal Relative toTop of Packing vs A:W 110
36. Experiments 10-14, Tripacks, 1,4 DCB RemovalRelative to Top of Packing vs. A.W 111
37. Experiments 10-14, Tripacks, CT Removal per FootRelative to Top of Packing vs. A.W 112
38. Experiments 10-14, Tripacks, ICE Removal per FootRelative to Top of Packing vs. A.W 113
39. Experiments 10-14, Tripacks, 1,4 DCB Removal perFoot Relative to Top of Packing vs. A.W 114
40. Experiments 10-14, Tripacks, CT Removal per FootIncremental Between Sample Ports vs. A:W 115
41. Experiments 10-14, Tripacks, TCE Removal per FootIncremental Between Sample Ports vs. A:W 116
42. Experiments 10-14, Tripacks, 1,4 DCB Removal perFoot Incremental Between Sample Ports vs. A:W 117
43. Experiments 10-14, Tripacks, CT, Ka, Relativeto Top of Packing vs. A-W 118
44. Experiments 10-14, Tripacks, TCE, KLa, Relativeto Top of Packing vs. A:W 119
45. Experiments 10-14, Tripacks, 1,4 DCB, K La,Relative to Top of Packing vs. A:W 120
46. Experiments 10-14, Tripacks, CT, KLa IncrementalBetween Sample Ports vs. A:W 121
ix
LIST OF FIGURES - Continued
Figure Page
47. Experiments 10-14, Tripacks, TCE, K La IncrementalBetween Sample Ports vs. A:W 122
48. Experiments 10-14, Tripacks, 1,4 DCB, K La IncrementalBetween Sample Ports vs. A:W 123
49. Freundlich Isotherm of CT in Presence of ICE and1,4 DCB on Activated Carbon 57
50. Freundlich Isotherm of TCE in Presence of CT and1,4 DCB on Activated Carbon 58
51. Freundlich Isotherm of 1,4 DCB in Presence of CT andTCE on Activated Carbon 59
52. Packing Depth vs. Percent Removal by Air StrippingPretreatment 67
53. Activated Carbon Use vs. Percent Removal by AirStripping Pretreatment 68
54. Treatment Costs vs. Packing Height, Optimum DesignPoint 70
ABSTRACT
This study examines the combined treatment processes of air
stripping and activated carbon adsorption for removing carbon
tetrachloride, trichloroethylene and 1,4 dichlorobenzene from
groundwater. Air stripping was used as a pretreatment with activated
carbon as a polishing step. Optimization is achieved by determining
the minimum operating and amortization costs for the variables of
packing media depth versus carbon usage rate. A simplified design
example illustrates the method of determining the optimum combination
of treatment processes for the compounds and concentrations examined.
The study included laboratory experiments which examined the air
stripping performance of two different packing media as well as
developing activated carbon isotherms. The transfer unit model was
utilized to estimate the overall liquid mass transfer coefficient (1(0)
for all three compounds at all experimental points.
X
INTRODUCTION
Numerous chemical contaminants have been occurring in drinking
water supplies with increasing frequency during the past several
years. Federal, State, local and private studies have sampled and
identified many hundreds of different chemical compounds in
groundwater. Most are the result of accidental or deliberate
discharges of industrial or commercial chemicals and waste products.
Among these are volatile organic compounds (VOCs) and synthetic organic
compounds (SOCs). Many of these substances have been found to cause
serious adverse health effects in animals and humans and have prompted
the implementation of regulations to control their presence in drinking
water.
Presently, hundreds of drinking water suppliers are faced with
contamination problems which must be addressed. Over recent years,
numerous groundwater pollutants were treated with granular activated
carbon (GAC). However, GAG often proved to be somewhat ineffective as
a reliable and cost effective means of treatment. Frequently VOCs
would achieve early breakthrough, thereby requiring frequent and
expensive carbon replacement or regeneration.
Recently, many researchers and engineers are recognizing the
advantages of using air stripping and activated carbon in a two-stage
treatment process. This system has the ability to effectively treat
groundwaters containing multiple contaminants having diverse
1
2
volatilities and adsorption characteristics. Using air stripping as a
pretreatment step will remove most or all of the highly volatile
compounds as well as varying amounts of the lesser volatile compounds
thus reducing the organic load on the activated carbon. There is also
a wide range of design and operation flexibility with the combined
systems.
This study attempts to demonstrate the applicability of this
system for a selected combination of chemical compounds. Laboratory
experiments were conducted to establish performance of a typical air
stripper and adsorption isotherms were developed to determine the
capacity of a specific activated carbon for the multi-solute solutions.
Current research and implementation of combined air stripping
and carbon adsorption is proving to be an effective technology. The
means by which an optimum combination is determined will remain a
challenge to the engineer. This study suggests one basic approach
together with the development of the necessary preparatory information.
OBJECTIVES
The objective of this study was to demonstrate the advantages of
combining air stripping with activated carbon adsorption for removing
VOCs from groundwater. Optimization is achieved by performing a
comparison of the treatment costs associated with accomplishing a given
level of VOC removal. Laboratory experiments were utilized to
establish air stripping performance and activated carbon capacity for
treating three selected organic contaminants. The compounds were
examined in multi-solute solutions in order to realistically reflect
any competitive effects.
3
LITERATURE REVIEW
The process of air stripping and activated carbon adsorption
have been widely used in recent years, but generally as separate
systems. The water treatment and chemical engineering fields have
generated a vast number of publications and references relating the
both of these processes. Only recently have researchers and industry
begun to report on water treatment systems which combine air stripping
with activated carbon adsorption.
A rather extensive literature search resulted in many reference
articles and bibliographical sources related to VOCs, air stripping and
activated carbon adsorption. Most information is from the American
Water Works Association Journal, Research Foundation reports and
conference papers. Many additional articles were found in other
engineering journals, periodicals and conference proceedings.
Approximately 80 percent of the references were published within
the last five years and the remaining 20 percent were published between
1975 and 1980. Clearly the period since 1980 has been the most
intensive for scientific research and engineering activities in this
field.
It is interesting to note the chronological sequence of events
as reflected in the topics reported in periodical literature.
1975 - Discovery of THMs, SOCs and VOCs in drinking water.
4
5
1978 - EPA proposes use of granular activated carbon for
treatment to meet MCLs.
1980 - Initial publication of research and studies of air
stripping and aeration techniques for treatment of THMs, SOCs, and
VOCs. Most research involved comparisons between GAC and air stripping
in an attempt to demonstrate that air stripping was a better technology
for VOCs, THMs and some SOCs.
1981 - Many case studies are reported which demonstrate
effective use of air stripping for VOC removal from drinking water.
Most researchers do indicate that a series combination of air stripping
and GAG would be effective for removal of multiple contaminants
involving volatile and non-volatile organics.
1983 to 1987 - Research and plant studies present quantitative
data regarding the effectiveness of series operation of air stripping
and GAG treatment. Improvements in air stripper design, packing media
and application are evident.
Air Stripping; Theory, Models and Studies
The basis of air stripping theory is found in the two—film model
for mass transfer between gases and liquids [31]. There are four steps in
the transfer process; 1) diffusion within the bulk liquid, 2) transfer
through the liquid film, 3) transfer through the gas film and,
4) diffusion within the bulk gas. When sufficient mixing is provided
within the bulk gas and bulk liquid phases, the bulk-diffusion factors
are relatively negligible and the rate controlling factor becomes the
transfer through the gas-liquid interface. The degree of solubility of
6
the gas will then dictate whether the gas or liquid film will be rate
controlling. VOCs have relatively low solubilities, thus the mass
transfer across the liquid film is generally rate controlling. The
term used to represent this is the overall liquid mass transfer
coefficient, KL.' The driving force for mass transfer is the
concentration gradient that exists between the bulk gas and liquid.
The overall rate of mass transfer from bulk liquid to bulk gas is
mostly dependent upon the solubility of the VOC, usually expressed in
terms of a Henry's Law constant, and the magnitude of the concentration
gradient produced by the air stripper. The effects of relative
diffusivity of VOCs generally is not important because there is
significant turbulence within the bulk solution and the diffusivity
coefficients, dependent upon molecular size and weight, often do not
differ significantly among VOCs.
Several different design models for air stripping of volatile
organic compounds (VOCs) from water have been developed. The various
equations generally determine the overall mass transfer coefficients
based upon variables such as, compound characteristics, temperature,
packing media, liquid loading rate, air to water ratio, and compound
concentration. Subsequent calculations and characteristic curves are
then utilized to determine packing height, pressure drops, tower
diameter and final air to water ratios.
The chemical engineering field has long used a transfer unit
model for gas absorption system [1]. The efficiency of a given packed
column is represented by the Height of a Transfer Unit (HTU), and the
7
number of transfer units are chosen to achieve a desired transfer ratio
within a certain column height. Chemical engineers generally design
systems that handle concentrated solutions whereas water treatment
concentrations are much more dilute. Consequently, the procedures
developed for the design of gas absorption systems can be simplified
and adapted to water treatment applications.
The kinetic theory of gases states that molecules of dissolved
gases can readily move between the liquid and gas phases.
Consequently, if water contains a volatile compound in excess of its
equilibrium concentration, then the compound will move from the liquid
phase (water) to the gas phase (air) until equilibrium is reached. The
driving force for air striping is derived by maintaining a gradient
between the equilibrium conditions and the operating condition within
the column.
Different compounds will exhibit different degrees of volatility
based upon their Henry's Law constant. Henry's Law constant is the
ratio of the partial pressure of a compound in air to the mole fraction
of the compound in water at equilibrium. Compounds with a high Henry's
Law constant are easier to strip from water than those with a low
Henry's Law constant. Determination of an appropriate Henry's Law
constant for the design model equations is an important step. For most
compounds of concern in domestic water treatment these are generally
accepted values which are widely published. Some researchers have
recently conducted the necessary laboratory experiments to establish or
verify Henry's Law constants [2, 3, 4, 5].
8
The Henry's Law constant (H) is strongly influenced by
temperature. The relationship of H to temperature can be modeled by a
van't Hoff-type relation. If the enthalpy change caused by dissolution
of the compound in water is considered independent of temperature, the
relation takes the following form:
log H --AH° _LRT 1- K
where R = the universal gas constant, 1.987 kcal/kmol, °K; T = absolute
temperature (°K); AH° = change in enthalpy due to dissolution of the
compound in water (kcal/kmol); and K = constant. Henry's Law constants
can thus be adjusted to account for temperature differences that may
exist between literature values, pilot studies or scale-up design.
The transfer unit model (adapted from chemical engineering design
methods) has received extensive use in recent years. Use of the transfer
unit model is found in references [6 - 13]. Kavanaugh and Trussell [6]
present a comprehensive design methodology and quantitative framework for
evaluation of pilot plant studies using the transfer unit model. The
general form of the equation is:
Z = HTU x NTU
where Z = height of packing (m); HTU = height of a transfer unit (m);
NTU = number of transfer units (dimensionless). HTU characterizes the
efficiency of mass transfer from water to air:
L HTU - KLaC
o
9
where L = liquid loading rate (kg . mole/m2 . hour); KLa = overall liquid
phase mass transfer coefficient (hr -1 and Co = molar density of
water (kg . mole/m3 ).
The overall liquid phase mass transfer coefficient, K La is a
function of the specific compounds being removed, packing media, and
liquid loading rate. Recent studies have also found that the air loading
rate may also influence K La [14]. Approximations of KLa can be obtained
from empirical relationships such as the Sherwood-Holloway [15] or
Onda-Takeuchi-Okumoto [16] correlations. However, pilot and lab scale
testing proves to be the most reliable means of determining
K La. Bishop and Cornwell [11] suggest that the transfer unit model be
solved for K La utilizing lab or pilot scale data and a reasonable
estimation of Henry's Law constants.
The number of transfer units, NTU, is a characterization of the
difficulty of removing a solute from the liquid phase. For dilute
solutions and solutes which obey Henry's Law, the expression for NTU can
be solved analytically. The equation for NTU is:
R (Cin/Cout)(R - 1) + 1NTU — lnR - 1 R
where R = stripping factor (dimensionless); C in/Cout = the ratio of
solute influent to effluent concentration (dimensionless). The
stripping factor, R, is defined as:
HGR— Pt L
where H = Henry's Law constant (atm); Pt = the total pressure (atm);
1 0
G - air loading rate (kg moles/m2h); and L - the water loading rate
(kg moles/m2 h).
The relationship between stripping factor, R, and NTU for
various solute removal percentages is illustrated on Figure 1. Note
that R must be greater than 1 to get much removal, but increasing R
beyond 4 or 5 will yield diminishing improvements. The ratio of solute
influent to effluent concentrations, C in/Cout , may be equal to actual
stripper sample concentrations if the objective is to determine KLa or
may represent proposed removal concentrations if the objective is to
design a full-scale stripping column for a given removal efficiency.
Kavanaugh and Trussell [6] also present a design example which
illustrates the calculations needed to evaluate allowable gas flow
pressure drop and tower diameter.
Various other design models have been developed by other
researchers which are not directly derived from the transfer unit model
[2, 3, 11, 17]. Singley et al. [2] present a comprehensive design
approach whereby compound equilibrium constants, H, are derived
experimentally and the mass transfer coefficients are a function of
Reynolds Number, Re. They conclude that the judgement as to whether a
compound can be removed by air stripping must be based upon the Henry's
Law constant and the mass transfer coefficient. Mass transfer
coefficients that were developed were independent of the packing
material and thus could be used with any packing provided the surface
area per unit volume of packing is known. Scale up procedures and
extensive cost analysis information are presented.
30
11
Removal
99.95%
99.90%
99%
90%
25=F-Z
.,(I,4-).r.-= 20=L.w4-0C0LI-154-o
LW.raE== 10
Figure 1. Relationship Between Stripping Factor and NTU.from ref. [2]
12
McCarty et al., [17, 18] present a simple equilibrium model
which suggests that for a given air to water ratio and Henry's Law
constant there will be a corresponding lower concentration of solute in
the effluent at equilibrium. A wide range of compounds were examined
on a full-scale stripping tower.
Reference [3] presents a unique design approach which was used
to plan and operate a full scale air stripping system to remove TCE.
The design equation somewhat resembles the transfer unit model:
ZtA =
RI C 1ln [C2
C1 — -AwHc C 2
KL a 1 RT AHw c
in which Zt = height of packing, m; A = cross-sectional area of tower,
m2 ; L = volumetric liquid flow rate, m3/min; C 1 = the inlet
concentration, ppb; C 2 = the outlet concentration, ppb;
R = the universal gas constant; T = the temperature, °K; Aw = the
volumetric air to water ratio and Hc = Henry's Law constant, m3
atm/mol. Using a generalized flooding and pressure drop diagrams (from
ref 77), the volume of tower packing is calculated:
.5X -
1 (p l/p g ) °As,,
in which X = packing volume, m3 ; )3 1 = density of water, lb/ft3; P g =
density of air, lb/ft 3 . The air loading rate, G, is determined:
G - (YPgf1gc)05
Fia° .2
13
in which Y = gas pressure drop, inches water per foot; g c = gravitational
acceleration, 32.2 ft/sec 2 ; F = packing factor, ft -1 ; and /4= water
viscosity, centipoise. Final calculations determine the tower diameter
and packing depth.
Dyksen et al., [19] presents a summary of packed column pilot
test data from 22 wells with nine different VOCs. Some pertinent
conclusions were;
1) The VOC concentration affected the percent removal rate with
higher VOC concentrations achieving slightly higher removal rates.
2) Higher removal efficiency was obtained if only one VOC was
present in the water.
3) At lower liquid loading rates, the mass transfer
characteristics generally correlated with the Henry's Law constant.
The influence of multiple solutes on stripping rate has been assumed by
most researchers to be negligible; however, no definitive studies have
been conducted. Dyksen also concludes that pilot studies have proved
to be necessary for developing design criteria for full scale treatment
facilities.
At the time when air stripping towers were first being introduced
for use in treating contaminated groundwaters, questions were often
raised concerning the secondary effects of the VOC-laden air that exits
the top of the stripping tower. Umphres et al., [20] briefly review the
use of GAC adsorption for air pollution control. They recommend pilot
studies and steam regeneration of the GAC. Some additional secondary
effects which Umphres also discusses are the problems associated with
14
iron precipitation and removal of free and combined chlorine. It
appears that for typical air stripping facilities significant losses of
either free or combined chlorine will not occur.
Improving stripping tower performance can be approached by a
number of ways other than increasing air to water ratios. Johnson et
al., [21] conducted field studies that focused on raising the
temperature of the water stream in order to remove methyl ethyl ketone,
MEK. A series of batch runs at progressively higher temperatures
produced favorable results. Zanetti et al., [22] elaborate on efficient
liquid distributors and redistributors for improved performance. A
particularly informative paper is presented by Chen [23], although
directed toward the chemical engineering field, does provide the water
treatment design engineer with an overview of some of the materials and
equipment that are available for full-scale stripping towers. Subjects
include support plates, liquid distributors, liquid redistributors,
collectors, packing types and materials, troubleshooting problems and
upgrading existing towers.
Studies investigating the mass transfer of volatile organic
compounds by means of diffused aeration was done by Roberts and Dandliker
[24] and Matter-Muller et al., [25]. Wood et al., [26] and O'Brien and
Stenzel [27] show respectable performance of spray head aeration for a
wide range of VOCs although the operating costs due to increased pumping
pressures are a drawback. Dyksen et al., [28, 29] and Goers and Hubbs
[30] present general comparisons of the various aeration techniques with
a conclusion that packed columns are the most efficient and economical.
15
GAC Theory Models and Studies
Activated carbon adsorption was deemed the best available
technology for VOC removal in the period immediately following the
discovery of toxic organics in water supplies, late 1970s - early
1980s. Although massive volumes of literature exist on the
applications and characteristics of activated carbon and other
synthetic adsorbents some of the most relevant appearing in periodicals
and research reports are presented here. In the experimental work done
for this paper, activated carbon adsorption isotherms were prepared
primarily as a basis of comparison with air stripping and to establish
relative adsorbability of multi-solute solutions on one particular
carbon type. Greater emphasis was placed on analyzing the variables
present with air stripping systems. For this reason the literature
review, results, discussions and conclusion regarding activated carbon
are somewhat general.
The theories associated with activated carbon adsorption are
found in many reference tests [1, 31] and general discussions are also
found in periodical literature [32] and reports [33]. Many different
models have been developed for characterizing single and multi-solute
adsorption processes. McGuire and Suffet [32] present a brief overview
of the following: mass transfer zone model (MTZ), Numeric model
(MADAM, Michigan Adsorption Design and Application Model), Ideal
Adsorbed Solution Theory (IAST), Polamyl Adsorption Potential Theory,
Solvophobic Theory, Brunauer, Emmett, Teller (BET) and net adsorption
energy concept. Weber [34] applied the MADAM model to seven toxic or
16
carcinogenic compounds found commonly in U.S. water supplies and found
the model suitably predicted breakthrough profiles for fixed bed
adsorbers. Crittenden et al., [35] applied the IAST theory to
multi-solute mixtures of six VOCs and used it to predict competitive
effects in waters having unknown background constituents. Crittenden
et al., [36] also found that a simplified version of the MTZ model can
be used to optimize design of fixed bed adsorber systems. Pilot and
full-scale data are presented to demonstrate its validity.
The most widely used models for characterizing single solute
adsorption are the Langmuir and Freundlich equations. The Langmuir
model is based upon the hypothesis of single-layer adsorption. It
assumes that maximum adsorption corresponds to a saturated layer of
solute upon a single-layered adsorbent surface, that the energy of
adsorption is constant, and that there is no transmigration of sorbates
on the sorbent surface. The Freundlich model is an empirically based
curve-fitting approach derived from the Langmuir equation. It is
widely accepted for intermediate solute concentrations where it takes
on the form:
X/M = KCf 1/n
The logarithmic form of the equation gives a straight line plot with
slope 1/n and intercept of Log K for C=1.
log X/M - log K + 1/n log Cf
in which X = Co - C f = the amount of solute adsorbed, C o = the initial
solute concentration, Cf = the final equilibrium solute concentration,
17
M = the mass of sorbate in the reactor, K and 1/n are empirical
constants. The isotherms presented in this report are plotted from the
Freundlich model equation.
Several researchers have examined the removal of organics with
adsorbents other than activated carbon [37-40]. In all cases, however,
GAG was used as a reference for comparison. Snoeyink [37] concluded
that resins are not applicable as a general adsorbent at water
treatment plants because they generally too selective. Snoeyink [41]
does however suggest that two different adsorbents in series may be
used to optimize removal rates, compensate for competitive adsorption
and improve regeneration economy.
Powdered activated carbon, PAC, is also used to remove VOCs,
THMs, and SOCs; however, the method of application is entirely
different from the use of GAC. Since PAC is commonly used for taste,
odor and color removal at water treatment plants its usefulness is
easily extended to include organic contaminants. Research into such
applications are presented by Singley et al., [42], Miller [43] and in
the American Water Works Association Research Foundation, Cooperative
Research Report [33]. McGuire et al., [44, 45] examine the
interactions among different water treatment unit processes in removing
organic contaminants. Several process unit arrangements are presented
which incorporate either PAC or GAC into the treatment scheme.
Combining Air Stripping with GAC
Since early 1980s researchers have recognized the benefits that
could be gained from utilizing a series combination of air stripping
18
with activated carbon adsorption. However, it has only been recent
studies and applications which have proven it to be an effective and
economical system.
The principal advantages of air stripping followed by GAG
adsorption are found in the ability of the system to treat waters
containing multiple contaminants having different volatilities and
adsorption characteristics. The two-stage process will provide added
operational flexibility which can be useful in meeting treatment goals
when influent concentrations change and for optimizing operating costs
such as power and carbon regeneration or replacement.
As a general rule, air stripping is most effective with
low-molecular weight, highly volatile compounds, while activated carbon
adsorption is best with high-molecular weight compounds having low
solubility. Therefore, pretreatment by air stripping will reduce the
organic load on the carbon and may remove compounds competing for
adsorption sites and reduce the problems associated with desorption.
Several references present general discussions of management and
treatment alternatives for dealing with the presence of VOCs in
groundwater supplies [46 - 48, 12, 62]. These references conclude that
some groundwater contamination problems may be adequately treated by
either GAC or air stripping alone; however, the combined processes
usually offer the best solution when multiple contaminants are
involved. The importance of pilot testing is also emphasized.
O'Brien and Stenzel [27, 49] compared the performance and costs
of two different types of air stripping equipment, a conventional
19
packed tower and an induced draft spray head stripper. They point out
that the packed tower stripper is most frequently coupled to pressure
carbon adsorbers whereas the induced draft stripper easily adapts to a
gravity flow carbon filter unit. The results of pilot testing indicate
that the induced draft stripper did not achieve as high removal rates
as the packed tower but the construction and operating costs were
lower.
Amy et al., [50] determined that air stripping followed by
activated carbon adsorption is a viable treatment method for
groundwaters containing certain VOCs and dissolved organic matter. The
study examined the removal of carbon tetrachloride (CT) and
trichloroethylene (TCE) in the presence of dissolved organic carbon
(DOC). They found that the DOC significantly reduced the adsorptive
capacity of the VOCs onto activated carbon. The study clearly
demonstrated that when air stripping is employed as a pretreatment
process, there is a substantial reduction in carbon usage (as compared
to "carbon only" treatment).
They also demonstrated that competitive adsorption is an
important consideration with respect to the activated carbon step.
Most of the competitive adsorption isotherms were based on CT or TCE
concentrations of 40 to 50 ug/L. Slightly higher adsorption capacities
were observed for high as opposed to low initial CT or TCE
concentrations.
The results of the study [50] indicate that, at the
concentrations examined, the removal of TCE will be the controlling
20
factor in the air stripping process while CT removal will control the
carbon adsorption process. They suggest that possible advantages of
coupled air stripping and activated carbon adsorption are: (1) reduced
media depth at a given A:W ratio and liquid rate or (2) the use of a
lower A:W ratio at a given media depth and liquid rate or (3) reduced
liquid rate at a given A:W ratio and media depth.
McKinnon and Dyksen [51] report on the use of GAC and air
stripping for removal of ethers and TCE from a municipal water well in
Rockaway Township, New Jersey. The initial treatment system consisted
of GAG adsorption; however, the ether compounds were achieving
breakthrough within 4 to 6 weeks. A countercurrent packed column air
stripper was added as a pretreatment step to the GAC. In July 1983,
the GAC system was taken offline because of the excellent performance
of the aeration system alone and because of diminishing contaminant
concentrations in the influent.
EPA Regulations
On February 9, 1978, the USEPA published proposed regulations
for the control of organic chemical contaminants in drinking water.
Maximum Contaminant Levels (MCLs) for THMs and VOCs and proposed
monitoring requirements for 59 organic compounds, of which more than
half were VOCs. On March 4, 1982, the EPA published the "National
Revised Primary Drinking Water Regulations, Volatile Synthetic Organic
Chemicals in Drinking Water" which set MCLs for many water contaminants
of concern. On November 13, 1985, EPA promulgated final Recommended
Maximum Contaminant Levels (RMCLs) and proposed MCLs for 8 VOCs. Also
21
proposed in the announcement were monitoring requirements for 51 other
unregulated compounds and proposed MCLs for 26 SOCs which include some
pesticides. The 1986 amendments to the Safe Drinking Water Act (SDWA)
have redefined RMCLs so that they are now known as Maximum Contaminant
Level Goals (MCLGs). In the future MCLs and MCLGs must be proposed
simultaneously and promulgated simultaneously.
July, 1987 the USEPA adopted final drinking water standards for
eight VOCs which will be enforced beginning December 31, 1988 (see
Table 1). The adopted standards are identical to the proposed MCLs set
on November 13, 1985 with the exception of vinyl chloride and 1,4
dichlorobenzene. In the case of 1,4 dichlorobenzene, the USEPA had
originally adopted an MCLG of 750 ug/1 and proposed an MCL of 750 ugh 1
based on a determination that evidence of human carcinogenicity was
inadequate. However, more recent health effects data in April 1987
prompted a change to an MCLG of zero and an MCL of 5 ug/1 based on the
chemical being classified as a probable human carcinogen. The final
MCLG of 75 ugh 1 and MCL of 75 ugh1 are based on a determination that
1,4 dichlorobenzene is a possible human carcinogen. Granular activated
carbon and aeration are designated as the Best Available Technology
(BAT) for meeting the VOC standards.
These eight new VOC standards and all future standards will
apply to non-community water suppliers as well as community systems,
and enforcement fines will be imposed for violations whether willful or
not [52].
Table 1. USEPA Standards for VOCs in Drinking Water
Chemical Final MCL(ugh)
Trichloroethylene 5
Carbon tetrachloride 5
Vinyl Chloride 2
1,2 Dichloroethane 5
Benzene 5
1,4 Dichlorobenzene 75
1,1 Dichloroethylene 7
1,1,1 Trichloroethane 200
22
23
The USEPA will establish initial MCLs for 83 contaminants over
the upcoming three-year period and thereafter MCLs for 25 new
contaminants are to be added to the list every three years.
It is apparent from the above synopsis that there has been much
progress in the regulatory arena; however, a certain amount of
confusion and uncertainty has accompanied it. Water providers and
engineers have been faced with contamination problems which demanded
immediate action; however, the regulatory standards and BAT may not
have been established. Thus many treatment systems have been designed
and implemented to achieve removals based on rather arbitrary target
effluent concentrations.
In 1974-75 the EPA sponsored an 80-city National Organics
Reconnaissance Survey (NORS) to determine the presence of
trihalomethanes (THMs), carbon tetrachloride and 1,2 dichloroethane in
U.S. drinking water supplies. Widespread occurrence of THMs were found
in water treatment plants following the chlorination stage. Very
little evidence of carbon tetrachloride and 1,2 dichloroethane were
found. Subsequently the EPA conducted the National Organics Monitoring
Survey (NOMS) on 113 community water supplies to determine the
frequency of occurrence of 23 selected VOCs, THMs and SOCs. The NOMS
survey also detected the presence of at least 63 additional organic
compounds [53, 54].
On February 9, 1978, the EPA published proposed amendments to
the National Primary Drinking Water Regulations. The regulations
included MCLs for THMs and prescribed granular activated carbon (GAC)
24
treatment for control of SOCs. Significant controversy and
deliberation surrounded the issue for the next few years [55 - 61].
Issues included skepticism of the necessity of GAC treatment,
validation of the toxicological health effects, cost of compliance,
MCLs, and other treatment or management alternatives. By the early
1980s, air stripping had emerged as an effective alternative treatment
method and GAC was gaining acceptance through research and use.
Analysis and Monitoring Techniques
Although the majority of literature regarding VOCs focuses on
treatment system design, modeling and cost effectiveness, etc.,
laboratory sample analysis is also of great significance. Trace
organics analysis has made tremendous advances in recent years in terms
of equipment and procedures. It has become increasingly easier to
classify, isolate, resolve, identify and quantify an ever-widening
array of compounds. Minimum detection limits are gradually improving
with development of new equipment and techniques. As the detection
limit moves lower it will become increasingly difficult to meet MCLs of
zero.
Trussell and Umphres [63], presented a very informative paper in
1978 which summarized the state of the art at that time. In 1981, the
American Water Works Association compiled a collection of 19 papers
dealing with analysis of organics in drinking water [64]. Stevens et
al. [65], utilized GC-FID organics profiles for evaluating treatment
processes designed for removal of VOCs and SOCs. Davenport et al.
[66], suggest the use of a continuous monitoring instrument for general
25
organics. Such a device may be particularly useful for treatment
systems utilizing activated carbon adsorption of highly-colored waters
containing SOCs or VOCs.
Case Studies
A small collection of articles concerning the occurrence and
treatment of groundwater contamination due to VOCs is represented by
references [67 - 73]. Although the majority of these articles refer to
sites located in the Eastern U.S., it is well known that significant
groundwater contamination problems also exist in numerous other places
throughout the country.
It is interesting to note the general trend of treatment
approaches which occurred. Early treatment methods typically used GAC
contactors; however, frequent and expensive carbon regeneration or
replacement soon made GAG an unaffordable method. The solution in some
instances was to add an air stripping tower for pretreatment [70].
Often the GAG could be taken offline completely because the aeration
system was so efficient. Other more recent VOC treatment systems have
simply used air stripping towers alone [68, 69].
EXPERIMENTAL PROCEDURES
The laboratory work that was required for this study consisted
of; air stripping experiments, powdered activated carbon experiments
(isotherms) and analytical testing. The compounds examined included
Carbon tetrachloride (CT), Trichloroethylene (TCE) and para (1,4)
dichlorobenzene (DCB).
Compounds Examined
Carbon tetrachloride (CC1 4 ) is an aliphatic hydrocarbon,
nonpolar, hydrophobic and quite volatile. It has been used primarily
as a solvent, dry-cleaning agent, fire extinguisher agent, grain
fumigant and machinery degreaser in the early 1900s. Current
industrial uses are in the manufacture of refrigerants
(chloro-fluorocarbons-freons). Virtually all CC1 4 involved in
dispersive uses is eventually released to the environment. It has been
estimated that approximately 3.04 million tons of CC1 4 had been emitted
to the environment as of 1975 [21]. In 1975, the National Organic
Reconnaissance Survey (NORS) found 12.5 percent of finished drinking
waters and 4.1 percent of raw waters had CC1 4 present. The occurrence
of CC1 4 in raw waters is generally attributable to covert waste
discharges or spills. Its occurrence in finished waters has also been
traced to CC1 4 contamination of chlorine used in the water treatment
process. Since 1970, carbon tetrachloride has been banned for all
26
27
use in consumer goods in the United States, and in 1978 it was banned
as an aerosol propellant.
Principal toxic effects include central nervous system
depression and cellular necrosis of the liver and kidneys. It is a
suspected mutagen and carcinogen.
Trichloroethylene (C 2HC1 3 ) (TCE) is an aliphatic hydrocarbon,
hydrophobic and moderately volatile. It is commonly considered to be
an "average" compound with respect to removal from water by air
stripping. It has been used primarily as a degreasing solvent in metal
industries and a common ingredient in household products such as spot
removers, rug cleaners, air fresheners, dry cleaning fluids,
refrigerants and inhalation anesthetics. Annual production in 1980 was
reported at 234,000 metric tons; however, stringent regulations have
resulted in its declining use. TCE is probably the most common
contaminant found in groundwater supplies due to leaking storage tanks,
industrial spills, illegal dumping or improper disposal methods.
Groundwater contamination in Tucson, Arizona has been traced to ICE
disposal practices used by aircraft manufacturers in the 1950s. The
disposal method used was considered acceptable at the time but has
proven to be environmentally unsound nearly 30 years later.
TCE is readily absorbed into the bloodstream when ingested and
its metabolites appear to have some moderate bioaccumulative properties.
The National Cancer Institute (NCI) has concluded that ICE is a liver
carcinogen in mice. Cases have been documented of liver and kidney
damage in humans due to acute and chronic exposure to high doses of TCE.
28
Para-Dichlorobenzene (C6H4C1 2 ) (1,4 DCB) is an aromatic
hydrocarbon, hydrophobic and only slightly volatile. DCB is directly
incorporated in a variety of consumer products and widely used as a
reactive intermediate for production of a number of other chemicals.
Its principal uses are as a pesticide in moth repellants,
fumicide-bactericide, odor-suppressor, high pressure lubricant, as a
component in insecticides and germicides and as a precursor in dye
manufacture. Domestic sewage, industrial effluents, spills and wastes
from insecticides, germicides and dye manufacturing are common sources
of 1,4 DCB in the environment. 1,4 DCB has been detected in various
water supplies of cities in the United States and overseas. It has
been determined that 1,4 DCB is degradable by microorganisms,
non-biological factors and sunlight.
The toxicological effects of 1,4 DCB are not well defined;
however, recent USEPA regulatory actions were based on the conclusion
that it is a possible human carcinogen.
Table 2 lists the common properties of the three compounds
studied.
Air Stripping Experiments
The air stripping experiments examined the VOC removal
capability of two different polypropylene packing media; 5/8" pall
rings and 1" tripacks. A lab scale air stripping column, including the
associated control and metering devices, was constructed. The air
stripper was a conventional countercurrent packed column with devices
to measure air and water flow rates as well as air pressure at the base
Table 2.
Parameter
Properties of Compounds Studied
Carbon- Trichloro-tetrachloride ethylene
p-Dichloro-benzene
Molecular Weightg/mol
153.8 131.5 147
Density, g/ml 1.59 1.46 1.46
Boiling Point, °C 77 87 173.4
Solubility, mg/L 800 1,100 49(@ 25°C)
Vapor Pressuremm Hg
91 74 1.0
Henry's Law 1.2 0.49 0.1Constant
Log Kow 2.72 2.29 3.39
Diffusivity (cm2/sec) 0.667 0.681 0.671
29
30
of the column. Figure 2 shows the equipment arrangement and critical
dimensions.
The basic column consisted of a plexiglass tube of five inches
inside diameter, and 72 inches length. At the base of the column was
an internal wire screen which served as a support for the packing media
and provided an inlet well for the air and an outlet sump for the
effluent water. In order to prevent channeling of the water along the
inside of the column walls, four redistribution rings were provided.
The rings were made of plexiglass, five inches outside diameter and
3-3/4 inches inside diameter. They each have a vinyl wiper on the
outside diameter to provide a tight seal against the column and a PVC
skirt on the inside diameter to prevent water from channeling along the
bottom surface of the ring and out to the column wall. The column has
four sampling ports which allow removal of water samples from the
column at the top, bottom and two intermediate depths of the packing.
The influent water was distributed on top of the packing by a PVC
nozzle that produces five small streams of liquid at an approximately
even spacial distribution.
The source of water was from the domestic supply tap in the lab
and the source of air was from the compressed air outlets also
available in the lab. Influent water flow rates were regulated with a
5/8" ball valve and measured with a Wallace and Tiernan rotameter
having a range of 0 to 3.4 gpm. Air flow was regulated with a 1/2"
corporation stop valve and measured with a specially fabricated orifice
meter having a range of 0 to 100 cfm.
LIQUID SAMPLEPORT, INFLUENT
S'I.D. PLEXIGLASCOLUMN
WATER FLOWROTAMETER 141,
PACKING MEDIA
AL-COLUMN PRESSURE
TAPPACKING SUPPORTAND AIR WELL
FROM WATERSUPPLY TAP
WATER FLOW---\CONTROL VALVE
AIR SUPPLYFROM LAS C.A.
' Affs-2.01ACONTROLVALVE
10(--LIQUID DISTRIBUTORNOZZLE
TOP OF PACKING
LIQUID REDISTRIBUTIONRING (TYPICAL)
PERISTALTICPUMP
PUMP SPEEDCONTROLLER
LIQUID SAMPLEPORT, INTERMEDIATE
IN S
STOCK SOLUTIONCONTAINER
*-LIQUID SAMPLE
' PORT, INTERMEDIATE
EFFLUENT WASTE TO DRAIN
31
BOTTOM OFPACKINGLIQUID SAMPLEPORT, EFFLUENT
Figure 2. Diagram of Air Stripping Column and
Appurtenances
32
The orifice meter (for air flow measurement) consisted of a
1-3/4 inch diameter PVC tube with a flat metal orifice plate having a
7/8" diameter orifice (see Figure 3). In order to measure the rate of
air flow, the pressure upstream and downstream (p l and p 2 ) of the
orifice plate are measured and applied to the following equation.
\/I 2g (p l - p 2 )d [ref. 77]w = qd = CYA
1 - B4
where: w - weight of discharge (pounds/second)
q - volume of discharge (cubic feet/second)
d = density of air (0.075 pounds/cubic foot)
B = ratio of throat diameter to pipe diameter
A = area of orifice opening (square feet)
C = coefficient of discharge (0.60)
Y = expansion factor (0.97)
g = gravitational acceleration (32.2 feet/second 2 )
p l = pressure upstream of orifice (pounds/square feet)
p 2= pressure downstream of orifice (pounds/square feet)
The terms C and Y were determined by additional equations in reference
[77]. To facilitate measurement of p l and p2 a differential pressure
manometer gauge was used (Dwyer Model 2002) for the range of 0 to 2
inches of water column. Beyond 2 inches water column it was necessary
to utilize a common water column manometer which would allow
measurement to more than 8 inches of water column. Figure 4 is a plot
of the relationship between p l - p2 and CFM utilizing the
above-described orifice meter and orifice flow equation.
CONCENTRIC ORIFICEORIFICEPLATE
1.70D
ORIFICEPLATE
SECTION A—A, ORIFICE PLATE
A
33
0.32' THK.
AIRFLOW
UPSTREAM PRESSURETAP, HIGH
DOWN STREAMPRESSURE TAP, LOW
21" •
e•
Figure 3. Orifice Meter Details and Dimensions
10
34
5
40
30
9— 2
cu
1.0L
a
. 4
.2
0
CFM = 1.944 V357.15 AP
il t t a t I III(10 20 30 40 50 60 70 80 90 100
Air Flow , (CFM)
Figure 4. Orifice Flow Meter, Differential Pressure vs. CFM.
35
The air pressure within the air well at the bottom of the
packing was measured by utilizing a small diameter tap through the
column wall. The pressure was measured by the differential manometer
gauge for pressures up to 2 inches water column or by the water column
manometer for pressures greater than 2 inches water column. The
measurement of column inlet pressure was coordinated with measurement
of air flow rate by means of a network of pressure tubing and five
control valves. Figures 5 and 6 show the piping schematics and the
valve positions required in order to achieve the appropriate connection
between the primary element and the respective manometer or gauge.
The solutes of concern (CT, TCE and DCB) were introduced into
the influent water stream by means of a concentrated stock feed
solution and a peristaltic pump. The stock feed solution was contained
in a 2 liter aspirator bottle with a rubber membrane interior bladder,
such that it prevented the air from contacting the surface of the stock
solution as it was being withdrawn from the bottom outlet. This helped
to minimize volatilization of the stock solution which could affect the
influent solute concentration. The peristaltic pump (Cole Parmer
7553-10) was equipped with a variable speed controller and was
calibrated at flow rates from 0.2 to 1.0 liters per hour.
Operational flexibility of the air stripper included;
1) Liquid loading rate at 0 to 3.4 gpm
2) Air loading rate 0 to 100 cfm
and 3) Stock solution feed rate 0.2 to 1.0 liters/hour.
36
H I
TYGON TUBING(TYPICAL)
3-WAY VALVE(TYPICAL)
LO 5
MAGNEHELICDIFFERENTIALPRESSUREGAUGE
COLUMN PRESSURETAP
A
WATER COLUMNMANOMETERS
BASE OF COLUMN
AIR FLOW --OP-
ORIFICE METER
Orifice Meter = Magnehelic GaugeColumn Pressure = Water Column Manometer A
Figure 5. Air Flow Meter and Column Pressure Gauges,Valve Positions A.
37
TYGON TUBING(TYPICAL)
3-WAY VALVE
HI (TYPICAL)
LO 5
MAGNEHELICDIFFERENTIALPRESSUREGAUGE
AIR FLOW ---010-
A
WATER COLUMNI /2iMANOMETERS
BASE OF COLUMN
COLUMN PRESSURETAP
ORIFICE METER
Orifice Meter = Water Column Manometer BColumn Pressure = Magnehelic Gauge
Figure 6. Air Flow Meter and Column Pressure GaugesValve Positions B.
38
Each of the two packing media were tested for the hydraulic
characteristics of air flow rate vs. liquid loading rate vs. column
pressure rise. Figures 7 and 8 illustrate the results. The pall rings
exhibited "flooding" at various loading rates; however, the tripacks
would not flood within the operational limits of the air stripper.
After determining the packing hydraulics, a specific range of loading/
flow rates was selected for the test stripping runs. A constant liquid
loading rate of 7.3 gpm/square foot and air to water ratios from 0:1 to
314:1 (cfm:cfm) were used for the pall rings. The tripacks were tested
at a liquid loading rate of 13.9 gpm/square foot and air to water
ratios from 0:1 to 205:1. The rates and ratios were selected within
the optimum hydraulic performance range of each packing media type.
Operation of the air stripping column involved the following
general procedures;
1) Connect all piping for air, water, stock feed, and metering
apparatus.
2) Prepare stock feed solution.
3) Set constant water flow rate as desired.
4) Set stock feed rate (peristaltic pump) as desired.
5) Set air flow rate as desired.
6) Allow system to operate for several minutes to reach
equilibrium.
7) Collect samples at each sample port.
8) Adjust air flow rate to next setpoint and collect additional
samples.
39
1.0
381.
4-)u- 0.8
Air Loading Rate2
(GEM / Ft )
337
0 308271 220
2 0.6
=00
0.4161 95
=o()
0.2
00 5 10 15 20 25
Liquid Loading Rate (GPM / Ft2 )
Figure 7. Column Hydraulics for Pall Rings Media.
381
Air Loading Rate
(GEM / Ft2 )
359
0
0.4
40
Liquid Loading Rate (GPM / Ft2 )
Figure 8. Column Hydraulics for Tripacks Media.
41
Powdered Activated Carbon Experiments
The activated carbon experiments consisted of a series of
"bottle point" tests to establish various adsorption isotherms for CT,
TCE and DCB. The following procedures were used; stock slurry
solutions of powdered activated carbon were prepared in the following
concentrations (mg/ml); 50, 10, 5, 0.5, and 0.1. A commercially
available carbon, Filtrasorb 400 (less than 325 mesh) as manufactured
by the Calgon Corporation was used in the isotherm experiments and is
hereafter referred to as PAC. Filtrasorb 400 is a bituminous coal-
based activated carbon. It has a BET surface area of 1,100 m 2/g, a
particle density of 0.811 g/cm3 , a pore volume of 0.7570 cm3/g and an
average pore diameter of 27.5 angstroms. The appropriate carbon
dosages for the isotherm experiments were obtained by pipetting a given
amount from the stock slurry solutions into 160 ml serum vials.
Adsorption isotherms were conducted for combined solutes (CT,
TCE and DCB) at the concentrations and sorbent doses indicated on
Tables 3, 4 and 5. Two stock solutions of combined CT, ICE and DCB
were prepared such that a 8 ul injection into each of the 160 ml serum
vials would yield the desired initial sorbate concentration. A high
solute concentration of approximately 100, 80 and 70 ugh 1 for CT, TCE
and DCB, respectively, was used for the first phase of adsorption
experiments to simulate non-airstripped water. The second phase of the
experiments used approximate concentrations of 10, 20 and 40 ugh1 for
CT, ICE and DCB, respectively, to simulate pretreated (air stripped)
water. The 160 ml serum vials containing the PAC and solutes were
Table 3. Isotherm Data for CT in Presence of
ICE and 1,4 DCB on Activated Carbon
Dose = M (mg/1) Cf (ug/1) Co - Cf = X (ug/1) X/M (mg/g)
0 7.3
1 7.3
2 5.1 2.2 1.10
5 4.9 2.4 0.48
10 4.1 3.2 0.32
20 2.2 5.1 0.26
33 1.4 5.9 0.18
0 134.0
1 131.6 2.4 2.40
2
5 120.7 13.3 2.66
10 116.9 17.1 1.71
20
33 103.8 30.2 0.92
k = .13
42
1/n = .83
43
Table 4. Isotherm Data for TCE in Presence of
CT and 1,4 DCB on Activated Carbon
Dose = M (mg/1) Cf (ugh) Co - Cf = X (ugh) X/M (mg/g)
0 13.2
1 13.2
2 10.5 2.7 1.35
5 7.9 5.3 1.06
10 4.4 8.8 0.88
20 1.7 11.5 0.58
33 1.1 12.1 0.37
0 125.4
1 121.3 4.1 4.10
2 - -
5 96.6 28.8 5.76
10 82.2 43.2 4.32
20 43.3 82.1 4.10
33 24.7 100.7 3.05
k = .40
1/n = .56
Table 5.
Dose - M (mg/1)
Isotherm Data for 1,4 DCB in Presence of
CT and TCE on Activated Carbon
Cf (ug/1) Co - Cf - X (ug/l) X/M (mg/g)
0 19.8
1 15.2 4.6 4.6
2 8.4 11.4 5.7
5 0 0 0
10 0 0 0
20 0 0 0
33 0 0 0
0 59.1
1 37.4 27.7 27.7
2
5 15.4 43.7 8.7
10 12.0 47.1 4.7
20 11.4 47.7 2.4
33 9.5 49.6 1.5
44
k = .010
l/n = 2.41
45
placed on a gyratory shaker for 24 hours at a constant laboratory
temperature of 25°C. Two bottles without any PAC were prepared in an
identical manner to serve as controls for each isotherm.
The bottles were then each placed into a centrifuge at 1,875 rpm
for a minimum of 15 minutes, causing the PAC to settle to the bottom of
the bottles. Using a gas-tight 60 ml syringe, 120 ml of centrate was
transferred to 120 ml serum vials and immediately capped with teflon
septa. It is estimated that volatilization losses due to the liquid
transfer procedure were less than 5% [40].
Analytical Testing
Analysis of the samples for the air stripping and PAC isotherms
was accomplished by conventional gas chromatography separation using a
liquid/liquid pentane extraction method. A Hewlett Packard 5790A with
a mega bore capillary column (DB-1) and an electron capture detector
(ECD) were connected to an HP 3390A reporting integrator. The
following settings were utilized for isothermal GC operation:
Injector temperature
250°C
Detector temperature
280°C
Carrier gas flow rate
58-60 ml/min
Column pressure
8-12 psi
Typical sample analysis involved a 5 ml injection (via syringe) of
pesticide-grade n-pentane (Fisher Chemical Co.) into the 120 ml serum
vial with 5 ml of solution being displaced through an outlet syringe.
The sample was then shaken for 2 minutes and a 1 ul aliquot of the
pentane phase was removed and injected into the GC. At least one
46
additional injection was also made from each sample for statistical
accuracy.
Prior to sample analysis, calibration solutions were prepared
and injected in order to establish the appropriate response factors for
quantification of chromotographic peaks. A reasonably good linear
response was obtained for the range of concentrations to be examined
for each of the subject compounds. ICE and CT samples were analyzed
separately from the DCB samples due to difficulties in establishing a
single accurate and repeatable separation method for the three
compounds. All air stripping and PAC experiments were conducted using
parallel samples for this reason. CT and ICE were analyzed at an oven
temperature of 27°C and DCB was analyzed at an oven temperature of
100°C. Sample analyses for the air stripping experiments were
conducted immediately following the stripper operation. PAC samples
were analyzed upon completion of the 24 hour contact period and
subsequent liquid transfer procedures.
RESULTS AND DISCUSSION
Air Stripping Experiments
The results of the air stripping experiments reflect only VOC
removal which occurred in the packing media itself and do not measure
or consider VOC removal due to end effects at the influent flow dis-
tributor or effluent air well of the column. Data simulation employs
the transfer unit model for analysis of media-related VOC removal.
Additionally, some comparisons of the liquid and air loading charac-
teristics between the pall rings and tripacks media are also determined.
Experiments 1 through 5 utilized pall rings with influent
concentrations for the VOCs of approximately 40 ugh1 for CT, 50 ug/1 for
TCE and 160 ugh1 for 1,4 DCB. The liquid loading rate was 1 gpm
(.136 gpm/ft 2 ) and air to water ratios ranged from 0 to 314 (vol :vol).
Experiments 6 through 9 utilized pall rings with lower influent VOC
concentrations of approximately 15 ugh1 for CT and 40 ug/1 for TCE. The
results of the 1,4 DCB are not included because of analytical problems
which yielded invalid results. The liquid loading rate was 1 gpm
(.136 gpm/ft 2 ) and air to water ratios ranged from 0 to 314 (vol:vol).
Experiments 10 through 14 utilized 1" size polypropylene tripacks with
influent concentrations for the VOCs of approximately 100 ugh1 for CT,
80 ug/1 for TCE and 70 ugh1 for 1,4 DCB. The liquid loading rate was
1.9 gpm (.259 gpm/ft 2 ) and air to water ratios ranged from 0 to 205
(vol:vol).
47
48
Although two different packing media were utilized, the test
parameters such as liquid loading, A:W and influent VOC concentrations
were not consistent thus direct comparisons of removal efficiency
cannot be made. However, valid comparison can be made with respect to
media air loading resistance, liquid and air loading limits and
relative mass transfer coefficients.
Mechanical specifications for the two packing media per the
manufacturers' literature are shown in Table 6. It is apparent that
the pall rings are a generally smaller size as evidenced by the number
of units per cubic foot. The surface area, void space and unit weight
are comparable. The greatest difference between the two is found in
the packing factors. The packing factor is generally representative of
the pressure drop characteristics for the air flow through the media.
A high packing factor indicates high air flow resistance hence higher
blower costs. The pall rings exhibited a significantly higher pressure
drop than the tripacks at all air/liquid loading rates examined. Refer
to Figures 7 and 8 for graphical plots of the column pressure vs.
liquid loading rate at various air loading rates. Note that it was
possible to achieve "flooding" at all loading rates examined for the
pall rings whereas the tripacks could not be flooded due to geometrical
and hydraulic limitations of the experimental stripping column. In
general, the tripacks media, having a lower packing factor, can handle
greater liquid and air loading rates than the pall rings.
The liquid loading rates used for each packing media were
selected based upon the results of the column hydraulics tests. It was
Table 6. Mechanical Specifications for Packing Media
TypePall Rings Tripacks
Manufacturer Norton Jaeger Tri-Packs, Inc.
Material Polypropylene Polypropylene
Nominal Size (in.) 5/8 1
Surface Area (ft 2/ft3 ) 104 85
Packing Factor (ft -1 ) 97 28
Void Space (%) 87 91
Weight (1b/ft 3 ) 7.25 6.20
Units per cubic foot 6,050 2,600
Cost ($/ft 3 ) NA 37
49
50
desired to use a liquid loading rate which would approach a flooding
condition at the highest air loading rate. This was more relevant for
the pall rings tests than it was for the tripacks; however, Figure 8
shows a pronounced column pressure rise at the highest air flow rate for
the tripacks. The liquid loading rates used are generally comparable to
those used for pilot and full-scale systems [3, 9, 13, 49].
The results of the VOC air stripping experiments are shown on
Tables 7 - 16 and Figures 9 - 48. These Tables and Figures are located
in the Appendix, Tabulations and Graphical Plots of Results of the Air
Stripping Experiments. The data has been analyzed and presented in a
number of ways;
(1) Fraction removed relative to top of packing vs. A:W (Table 8,
Figures 9 - 11, 24, 25, 34 - 36).
(2) Fraction removed per foot of packing relative to the top of
packing vs. A:W (Table 8, Figures 12 - 14, 26, 27, 37 - 39).
(3) Fraction removed per foot of packing for each increment
between sample ports vs. A:W (Table 8, Figures 15 - 17, 28, 29, 40 - 42).
(4) Liquid mass transfer coefficient relative to top of packing
vs. A:W (Table 15, Figures 18 - 20, 30, 31, 43 - 45).
(5) Liquid mass transfer coefficient for each increment between
sample ports vs. A:W (Table 16, Figures 21 - 23, 32, 33, 46 - 48).
Pall Rings - Experiments 1-5
Figures 9 - 11 show that CT achieved the highest removal of the
compounds tested at all A:W ratios. This result was consistent with the
relative volatility of the three compounds. For both CT and ICE most
51
removal occurred at A:W ratios of less than 75:1 with very little
additional removal at higher A:W ratios or packing depths greater than
four feet. The effects of increasing A:W ratios is most evident within
the first two feet of packing, showing a steady increase in removal as
A:W ratios increase for both CT and TCE. As expected, 1,4 DCB shows a
significantly lower removal rate with a leveling-out occurring at A:W
ratios of about 150:1. The effects of increasing packing depth and A:W
ratios are more evident with the 1,4 DCB samples than with CT or TCE.
Figures 12 - 14 more clearly define the effects of packing depth
and A:W on the removal rates. The CT removal is slightly better than TCE
within the first two feet of packing with almost identical removal rate
per foot at packing depths beyond four feet and A:W ratios greater than
75:1. Figures 15 - 17 show removals on an incremental basis between
sample ports. In contrast to Figures 9 - 11, all of the compounds
indicate an increase in removal per foot of packing from A:W ratios of
0:1 to 75:1 and then a gradual decline at the higher A:W ratios. Figures
18 - 20 show the calculated liquid mass transfer coefficient, KLa, rela-
tive to the top of packing, associated with each experiment sample. The
KLa for CT is expectedly higher than TCE or 1,4 DCB. There is a steep in-
crease in K La of CT and TCE for A:W ratios from 0:1 to 75:1 with a gradu-
al increase at higher A:W ratios. The KLa for 1,4 DCB is markedly lower
than CT and ICE but does show a trend similar to that of CT and ICE.
Figures 21 - 23 show the calculated liquid mass transfer
coefficient, KLa, on an incremental basis between sample ports. The
results are very erratic and do not display any identifiable trends.
52
Pall Rings - Experiments 6-9
These experiments examined CT and TCE removal at a lower
concentration range than experiments 1 through 5. Figures 24 and 25
show the fraction removed relative to the top of packing is greater for
CT than TCE at low A:W ratios but are almost identical at higher A:W
ratios. The removal levels off at A:W ratios greater than 75:1 and
packing depths greater than four feet. Figures 26 and 27 show the
fraction removed per foot of packing relative to the top of packing.
The conclusions are generally the same as for Figures 12 - 14.
Figures 28 and 29 show the fraction removed per foot of packing on an
incremental basis between sample ports. The results are generally the
same as for Figures 15 - 17.
Tripacks - Experiments 10-14
Figures 34 - 36 show that CT achieved the highest removal of the
compounds tested at all A:W ratios, as expected. For both CT and TCE
most removal occurred at A:W of less than 40:1. Progressively greater
removal of CT and TCE as A:W ratios increases within the first two feet
of packing. At greater depths the removal levels out. As anticipated,
1,4 DCB shows a significantly lower removal; however, unlike the pall
rings, there is not a leveling out within the A:W ratios tested.
Figures 37 - 39 show the fraction removed per foot of packing
relative to the top of packing. CT achieved notably higher removal
than TCE within the first two feet of packing. A leveling out of CT
and TCE removal is evident for A:W ratios greater than 39:1 and
removals are nearly equal with CT slightly above ICE for packing depths
53
of about four feet. For 1,4 DCB, the data shows a gradual increase in
removals as the A:W ratio and packing depth increase. Figures 40 - 42
show the fraction removed per foot of packing on an incremental basis
between sample ports. The results are rather erratic but all compounds
exhibit an increase in removal rate for A:W ratios from 0:1 to 39:1
with a gradual decrease as A:W ratios further increase.
Overall Comparisons - Experiments 1 - 14
Among the five different result analyses, additional variables
which may be compared are:
(1)Tripacks vs. pall rings
(2) CT vs. ICE vs. DCB removal
(3) High vs. low CT, TCE concentrations
In terms of VOC removal relative to the top of packing, the pall
rings showed consistently greater removal than the tripacks at all A:W
ratios and packing depths. Also there was a greater differential in the
amount of removal versus A:W ratio for tripacks than for pall rings at
packing depths greater than four feet. The 1,4 DCB showed significantly
greater removal with pall rings than tripacks. The pall rings exhibited
a leveling out at higher A:W ratios and packing depths whereas the tri-
packs showed a still-climbing trend. Experiments 1 through 9 established
that both CT and TCE will achieve greater removal rates at high
concentrations than at low concentrations. This is probably due to a
greater concentration gradient (driving force) for film transfer, at the
higher concentration. There is also a greater differential in removal
versus packing depth for low concentration than at high concentration.
54
The evaluation of the results for fraction removed per foot of
packing relative to the top of packing also exhibited the same results
as those described above.
In terms of the fractions of VOC removed incrementally between
sample ports, the data points were very erratic. It does appear,
however, that there is a trend which may be described as a gradual
decline in the removal per foot of packing at the higher A:W ratios.
This was generally observed for all compounds, concentrations, packing
depths and packing types.
The comparison of KLa values between pall rings and tripacks
should consider the difference of packing surface area, a, for each
packing. Pall rings has 104 ft 2/ft3 and tripacks has 85 ft 2/ft3 . Thus
the overall liquid mass transfer coefficient K L may be determined by
factoring out the surface area term, a.
The K La values relative to the top of packing (Figures 18 - 20,
30, 31, 43 - 45) presented many interesting comparisons: there is a
wider difference in the relative KLa value of all three compounds for
the tripacks than for pall rings. Experiments 1 through 9 for CT and
TCE showed that the KLa was generally lower for the low concentrations
than for high concentrations. A consistent upward trend in KLa was
observed as A:W ratio and packing depth increase for all packings,
VOCs, and concentrations. An additional observation shows that for CT,
the tripacks had a higher KLa than pall rings, for TCE, the KLa for
tripacks and pall rings were very similar and for 1,4 DCB the KLa for
pall rings was higher than tripacks. In other words, the tripacks are
55
more effective for highly volatile compounds and the pall rings are
more effective for less volatile compounds.
The KLa values on an incremental basis between sample ports
(Figures 21 - 23, 32, 33, 46 - 48) exhibited very erratic data points.
Some general trends and comparisons are evident however: CT shows a
higher KLa for tripacks than for pall rings at A:W ratios greater than
40:1.
Experiments 6 through 9 show very close KLa values for CT and
TCE (Figures 32 and 33) with a steady upward trend as A:W ratios
increase to about 225:1 and then there is a downward trend to 314:1.
Some of the other results for CT and TCE (Figures 21, 22, 46, 47) shows
similar K La declines at the highest A:W ratios.
Activated Carbon Experiments
The activated carbon experiments consisted of a series of "bottle
point" tests for combined solutes (CT, TCE and DCB) at low and high
relative concentrations. Single solute isotherms were not conducted;
however, the results of other researchers [33, 40, 42, 50] may be useful
for establishing the extent of competitive adsorption in this study.
Also the effects of dissolved organic carbon (DOC) on adsorption
capacity were not analyzed. Since the same water source was used for
all tests it is assumed that DOC was similar for all tests and its
competitive adsorption effects are constant for all tests. The DOC of
the tap water was not determined and the pH of the tap water was not
measured.
56
The results of the adsorption isotherm tests are plotted per the
Freundlich equation on Figures 49, 50, and 51 and the data values are
listed on Tables 3, 4, and 5. The results were generally as expected
with TCE being more strongly adsorbed than CT at all concentrations.
DCB, however, exhibited a very steep 1/n slope which resulted in a very
high adsorption capacity at high equilibrium concentrations and very low
capacity at low equilibrium concentrations.
o
•••, •r-
1-1
IN*
don
57
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= 4.) S...
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q-() C "0
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Ili t I I I I t tilIl t I
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58
50
40
100
59
Il 1 I II till I I 1 I I l I I t
-
_
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WO
-
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2 345 10 20 30 40 100
Equilibrium Concentration, Cf (ug/l)
Figure 51. Freundlich Isotherm of 1,4 DCB in Presence ofCT and ICE on Activated Carbon.
OPTIMIZATION AND ECONOMICS
The principal advantages of air stripping followed by GAC
adsorption are found in the ability of the system to treat waters
containing multiple contaminants having different volatilities and
adsorption characteristics. Using air stripping as a pretreatment step
will remove all or most of the highly volatile compounds as well as
various portions of the lesser volatile compounds thus reducing the
organic load on the activated carbon. Since the replacement or
regeneration of activated carbon is typically very expensive, it is
obviously advantageous to minimize the carbon usage rate. However, the
capital and operating costs for the air stripper must also be considered.
To determine the optimum design of an air stripper and adsorption
system there will be many variables to consider. It is likely that some
of the variables may be evaluated on a qualitative or intuitive basis
whereas others will require a more rigorous quantitative approach. For
example, the variables to consider for the air stripper include;
o packing media type, size
o media depth
o removal efficiency
o water loading rate
o air to water ratio
o temperature
60
61
o tower diameter
. pumping costs
o blower costs
and for the adsorption system;
. carbon type
. contactor type
. removal efficiency
. pumping costs
. carbon costs
A qualitative approach may be satisfactory for selecting the
packing media and the carbon type. There is generally sufficient
background information available from the manufacturer or prior
research or designs that a reasonably good choice can be made without
extensive lab or pilot tests for comparison. However, the price or
availability of certain products may result in compromises.
The next step is to determine the best combination of values for
the remaining variables based on the results of actual lab or pilot
tests using either the actual contaminated groundwater or synthesized
substitutes. It is also important to identify the range of variability
for certain parameters such as;
o influent contaminant concentrations
o influent contaminant types
o temperature
o changes in MCLs
o treated water flow rate requirements.
62
The treatment system should be designed to allow for fluctuations in
the above parameters during the effective design life. The means of
meeting the worst case treatment objectives may be accomplished by
either over-designing or by providing for easy expansion, augmentation
or operation modifications of the base design.
Many researchers have presented information on methods and
considerations for optimizing an air stripping design in general terms
[6, 9, 10, 14, 27, 49, 50]; however, Nirmalakhandan et al [9] has
recently developed a computer simulation which evaluates the effects of
changing any given design variable. The benefits include being able to
determine the optimum design values and also identifying which
variables have the most or least effect on overall treatment costs. It
is likely that his results and conclusions are valid for most air
stripping designs. Some of his conclusions are;
(1) The optimum water loading rate and air to water ratio can
be found by plotting overall cost contours for those parameters.
(2) The overall treatment cost is relatively insensitive to
modest variations of the operating variables in the vicinity of the
optimum region, but increases rapidly as conditions deviate further.
(3) Capital costs may influence overall treatment costs more
than power costs.
(4) Overall treatment cost is directly dependent on
temperature.
(5) Selection of packing media should be based on pressure loss
characteristics and KLa rather than material cost.
63
(6) Varying the air loading rate is generally the most
efficient way to adjust operating conditions to react to changes in
treatment objectives or temperature.
This study examined the following treatment system variables
with the objective of removing three organic contaminants (CT, TCE and
1,4 DCB).
o Two different packing media types and sizes were compared.
o Removal vs. media depth was examined.
o Removal vs. air to water ratio was examined.
o Both packings were analyzed for pressure drop vs. liquid
loading and air loading rates.
o The effect of influent concentration was examined.
o The effects of competitive adsorption was accounted for.
The three VOCs, carbon tetrachloride, trichloroethylene and
1,4 dichlorobenzene represent a relatively broad range in volatility
and adsorptivity. Carbon tetrachloride is the most volatile but the
least adsorbable whereas 1,4 dichlorobenzene is the least volatile and
most adsorbable. Trichloroethylene is in-between and represents a good
reference compound.
The results of the laboratory studies indicate that the
following choices should be made regarding the design variables.
(1) The tripacks media will allow for much higher liquid loading
rates and lower pressure losses than the pall rings. Although the KLa of
the tripacks is lower than the pall rings, the tripacks can still achieve
satisfactory removals with only slightly greater media depths.
64
(2) The air to water ratio should be fairly low, 39:1 or less
for tripacks media. The higher A:W ratios did not improve removals
significantly.
(3) Filtrasorb 400 is effective in adsorbing the combined
compounds to desired effluent concentrations.
(4) Since 1,4 DCB will achieve the least removal by air
stripping, it will probably be the controlling compound for the carbon
polishing step.
For this study, optimization of the design variables will be
accomplished as follows;
(1) Selection of packing media for air stripping: Use tripacks
because of lower pressure loss and greater liquid loading rate.
(2) Determine air to water ratio: The results of experiments
10 through 14 indicate that an A:W ratio of 39:1 will accomplish most
of the removal of CT and TCE and a significant portion of the 1,4 DCB.
Higher A:W ratios will not accomplish much greater removals.
(3) Determine a liquid loading rate: The tripacks media offer
a wide range of possible water loading rates. Figure 8 indicates that
at a low air loading rate, the liquid loading rate may vary
considerably with very little change in column pressure loss. For the
purposes of this study the liquid loading rate was set at 13.9 gpm/ft 2
and will be used for further design calculations so that the removal
data and KLa values will be valid.
(4) Optimize the final two design parameters which are packing
media depth and carbon usage. For the purposes of this study, the
65
treatment objective is to achieve an effluent concentration of 1 ugh 1
or less. The influent concentrations are assumed to be 40 ugh 1 for
carbon tetrachloride and trichloroethylene and 70 ugh1 for
1,4 dichlorobenzene (150 ugh1 total VOCs).
A simple mathematical approach will be applied to determine the
approximate overall cost minimum for the variables versus percent
removal. In order to present that methodology it is necessary to
establish several unit cost factors and make certain assumptions with
regard to system conditions and configurations.
Assumed unit costs are estimated as follows; carbon cost = $1.25
per pound. Air stripping tower and packing varies from $9,000 per
vertical foot at 3-feet height to $3,333 per vertical foot at 30-feet
height. Column shell is assumed to be carbon steel and 2-inch plastic
media is used. Pump costs include the pump, motor, valves, piping and
electrical equipment needed to lift the process water flow from ground
level to the top of the tower. The cost varies slightly according to
packing height, $20,000 for 3-feet height and $25,000 for 30-feet
height. Reference [2] is used as a basis of these costs. Pump
operating cost also varies as a function of the packing height. Using
an electrical power cost of $0.10 per Kwhr, the pump operating costs
will range from $2.00 per million gallons (MG) at 3-feet height to
$12.00 per MG at 30-feet height.
The capital costs associated with the tower, packing and pump
equipment have been amortized over 20 years at an annual interest rate
of 10% in order to assess the cost per MG value. Other equipment and
66
operating costs such as the air blower and carbon contactor have been
considered to be relatively constant with respect to changes in the
packing height.
A water flow rate of 700 gpm (1 MGD) is assumed for calculation
purposes. It will operate constantly and the influent contaminant
concentrations will not vary.
Figure 52 shows the packing media depths necessary to achieve up
to 99% removal by air stripping. The transfer unit model was used,
with NTU, R and KLa values as determined in laboratory experiments for
this study. Figure 53 shows estimated carbon usage that would occur
for various levels of air stripping pretreatment. The results of the
carbon adsorption isotherm experiments were used to estimate the carbon
usage requirements. The adsorption capacity, X/M value varied in
relation to the level of air stripping pretreatment. In other words,
after obtaining a given level of air stripping pretreatment
(% removal), that effluent VOC concentration then becomes the activated
carbon influent and C f concentration for which a corresponding
adsorption capacity, X/M value exists. The carbon usage, in terms of
lb/MG, were then calculated from the C f and X/M values. For CT and TCE
this relationship resulted in a gradual decline in carbon usage (lb/MG)
as the level of air stripping pretreatment increased (% removal).
However, for 1,4 DCB, the 1/n slope of the adsorption isotherm is so
steep that the carbon usage actually increases as the level of air
stripping pretreatment increases.
40—
35
30
Optimum Packing Height,11 Ft. 1,4 DCB
25
MIIMN•
1 0
CT
0 50 60
170 80 90
1
Amount Removed by Air Stripping (%)
Figure 52. Packing Depth vs. Percent Removal by Air
Stripping Pretreatment.
67
30
40
50
60
70
80
90
100
Amount Removed by Air Stripping (%)
Figure 53. Activated Carbon Use vs. Percent Removal by AirStripping Pretreatment.
68
69
The optimum design combination of air stripping and activated
carbon adsorption is determined by finding the cost minimum in terms of
dollars per million gallons treated for column height versus carbon
usage. Figure 54 shows the combined costs and the optimum design
point. For this particular combination of compounds and treatment
objectives it appears that 1,4 DCB is the controlling compound. The
cost minimum is reached at a packing height of 11 feet, a carbon usage
rate of 20 lb/MG and an overall treatment cost of $50/MG. The air
stripping pretreatment will remove greater than 99% of the CT and
greater than 98% of the ICE contaminants but only about 78% of the 1,4
DCB. Thus the activated carbon is necessary to accomplish additional
1,4 DCB removal to at least 98.6%
For comparison purposes, if all three compounds were to be
removed by air stripping alone, it would be necessary to have a packing
height of about 40 feet (1,4 DCB controls). The approximate treatment
cost would be $72/MG. However, the capital costs associated with the
carbon contacting system would have to be less the $70,000 in order for
air stripping alone to be economical. Conversely, if all three
compounds were to be removed by activated carbon adsorption alone (CT
controls), the carbon usage rates would be about 280 lb/MG or $350/MG.
The advantages of using air stripping as a pretreatment step to
activated carbon adsorption is quite apparent in this example design.
Although most of the contaminant removal will occur in the air
stripper, the carbon does allow for a more economical operating cost.
Furthermore the activated carbon step may prove to be indispensable if
1000
TCE Adsorption Cost
CT Controls
1,4 DCB Controls
Minimum Cost Point
70
CT Adsorption Cost
500
400
300
200
-c--3,e,--Overall Treatment Cost
1,4 DCB Adsorption Cost4-)
50
4-) 40
30Cd
F— 20
10
5
4
3
2
1 0 5 10 15 20 25 30 35
Packing Height (Ft.)
Figure 54. Treatment Costs vs. Packing Height, OptimumDesign Point.
100
be.n•n•
Air Stripper Cost
71
highly non-volatile compounds are present. Additionally, any
fluctuations in contaminant concentrations during the design life of
the facility will most likely be handled by adjusting the A:W ratio of
the air stripper or by increased carbon usage. These combined
processes offer a wide range of operational flexibility not available
with any single unit process.
CONCLUSIONS
It is apparent that treatment of groundwater containing multiple
contaminants may best be accomplished by a combination of air stripping
and activated carbon adsorption. In general, if the compounds are of
varying volatilities and different adsorption capacities, then the
combined treatment processes will be most economical.
The results of this study reveal that there are two controlling
compounds which define the optimization. At low packing heights (0 to
11 ft.), CT is controlling and at higher packing heights (11 to
15 ft.), DCB is controlling. Other types or combinations of VOCs may
result in only a single compound controlling the optimization.
Furthermore, in actual practice, other water constituents such as humic
substances or dissolved organic carbon would influence the adsorption
capacity of activated carbon for VOCs [50].
Although the method of optimization utilized in the study was a
simple economics problem a more detailed and comprehensive approach
could be developed by applying computer model simulation as was done by
Nirmalakhandan [9]. This would be a good direction for further
research.
72
APPENDIX
TABULATIONS AND GRAPHICAL PLOTSOF RESULTS OF THE AIR STRIPPING
EXPERIMENTS
(Includes Tables 7 through 16and Figures 9 through 48.)
73
PackingMedia
Exp#
L 9 A:W(gpm/ft') (vol:vol)
Table 7. Sample Concentrations
Carbon Tetrachloride
for VOC Experiments (ug/1)
Trichloroethylene 11_4 Dichlorobenzene0 ft. 1.92 ft. 3.92 ft. 5.67 ft. 0 ft. 1.92 ft. 3.92 ft. 5.67 ft. 0 ft. 1.92 ft. 3.92 ft. 5.67 ft.
Pall Rings 1 7.3 0 40.4 24.5 10.3 7.3 44.7 30.7 25.0 20.0 162 110 86.0
Pall Rings 2 7.3 75 41.8 7.40 1.40 0.70 60.2 15.0 2.50 1.10 170 69.0 43.1 22.7
Pall Rings 3 7.3 150 40.0 1.20 1.20 42.8 26.0 1.70 1.80 166 52.9 27.9
Pall Rings 4 7.3 224 42.7 5.40 1.20 1.00 64.1 10.0 1.40 1.30 162 57.3 45.9 36.9
Pall Rings 5 7.3 314 90.0 8.50 2.30 1.40 45.4 15.9 3.90 148 49.8 22.8
Pall Rings 6 7.3 0 14.6 9.80 6.60 5.30 38.3 32.4 25.3 22.5
Pall Rings 7 7.3 150 14.2 4.75 1.85 0.95 37.7 12.6 5.25 2.60
Pall Rings 8 7.3 224 15.0 4.75 2.15 0,75 39.0 12.4 5.80 2.30
Pall Rings 9 7.3 314 14.6 3.15 1.70 0.80 38.3 8.80 4.95 2.55
Tripacks 10 13.9 0 90.7 62.0 57.9 50.6 86.1 71.6
Tripacks 11 13.9 39 100 34.3 10.5 5.00 75.3 45.9 17.8 8.20 108 50.5 40.1 33.7
Tripacks 12 13.9 79 128 37.5 11.7 7.50 76.8 46.3 19.2 11.5 60.3 57.7 34.8 29.3
Tripacks 13 13.9 118 90.9 31.0 12.3 5.00 92.4 40.7 19.6 7.5 63.5 53.4 35.1 23.1
Tripacks 14 13.9 205 86.4 22.7 10.6 5.50 82.0 31.0 16.9 8.3 69.6 50.4 33.1 18.3
74
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0-1.92 ft. Trichloroethylene
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Pall Rings 1 7.3 0 39.3 58.0 29.1 31.3 18.6 20.0 32.1 __ 22.0(20.5) (29.0) (16.6) (16.3) (9.3) (11.4) (16.7) (--) (5.9)
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Pall Rings 4 7.3 224 87.4 77.8 16.7 84.4 86.0 7.1 64.6 20.0 19.6(45.5) (38.9) (9.5) (44.0) (43.0) (4.1) (33.6) (10.0) (11.2)
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Tripacks 10 13.9 0 31.6 6.6 12.6 -- -- -- 16.8 -- --(16.5) (3.3) (7.2) (--) (--) (--) (8.8) (--) (--)
Tripacks 11 13.9 39 65.8 69.4 52.4 39.0 61.2 53.9 53.4 20.6 16.0(34.3) (34.7) (29.9) (20.3) (30.6) (30.8) (27.8) (10.3) (9.1)
Tripacks 12 13.9 79 70.7 68.8 35.9 39.7 58.5 40.1 4.3 7.7 15.8(36.8) (34.4) (20.5) (20.7) (29.3) (22.9) (2.2) (3.9) (9.0)
Tripacks 13 13.9 118 65.9 60.3 59.3 55.9 51.8 61.7 15.9 34.3 34.2(34.3) (30.2) (33.9) (29.1) (25.9) (35.3) (8.3) (17.2) (19.5)
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