Application of a packed column air stripper in the removal of volatile organic compounds from...

Rev Chem Eng 2014; aop

Mohammed Evuti Abdullahi , Mohd Ariffin Abu Hassan * , Zainura Zainon Noor

and Raja Kamarulzaman Raja Ibrahim

Application of a packed column air stripper in the removal of volatile organic compounds from wastewater

Abstract: Addressing environmental degradation and

ensuring environmental sustainability are inextricably

linked to all methods of reducing volatile organic com-

pounds (VOCs) from the environment. A packed column air

stripper is a typical example of such technologies for the

removal of VOCs from polluted water. The present review

is devoted to the applications of a packed column air strip-

per and, in comparison with previous reviews, presents

further elaborations and new information on topics such

as modeling and simulation of the dynamic behavior of

the air stripping process in a packed column air stripper.

The paper observed that a knowledge gap still exists in the

synthesis of this knowledge to formulate practically appli-

cable mathematical relationships to describe the process

generally. Therefore, further researches are still required

in the area of air stripper performance optimization, par-

ticularly in the development of a mathematical model

and the optimization of an air stripper using a statisti-

cal experimental design method. Such a determination

is critical to the understanding of the interactive effect of

process variables such as temperature, air-to-water (A/W)

ratio, and height of packing on air stripper performance.

Keywords: modeling; packed column air stripper; volatile

organic compounds; wastewater.

DOI 10.1515/revce-2014-0003

Received January 14 , 2014 ; accepted June 5 , 2014

1 Introduction

There is an increasing public concern on the need to

remove volatile organic compounds (VOCs) from ground-

water and wastewater because certain VOCs are known to

be hazardous to human health and the environment ( Gold-

stein and Galbally 2007 ). Some VOCs have been identified

as odorous, ozone precursors, carcinogenic, and potential

air toxicants ( Inoue et al. 2011 ). About 189 hazardous air

pollutants (HAPs) have been identified by U.S. Environ-

mental Protection Agency (USEPA), out of which 97 are

VOCs ( USEPA 1990 ). There also exists mounting evidence

that long-time exposure to low concentrations of certain

organic chemicals can be an important factor in the devel-

opment and manifestation of some chronic diseases. It

is further believed that between 80% and 90% of cancer

cases are of environmental origin; therefore, the contami-

nants present in potable water supplies come under suspi-

cion ( Bedding et al. 1982 ). More also, certain VOCs are also

known as greenhouse gases ( Derwent 1995 , Wuebbles and

Hayhoe 2002 , IPCC 2006 , Lelieveld et al. 2009 ). Because

these gases effectively absorb radiated energy from the

Earth, the rise in their concentrations within the Earth ’ s

atmosphere has been associated with the global warming

phenomena ( AEA Group 2007 , Mohammed et al. 2012 ). In

addition, the presence of VOCs in wastewater reduces the

possibility of water reuse, such as in irrigation, thereby

placing a higher demand on the limited existing primary

water resources ( Bedding et al. 1982 ).

This increasing concern is also due to the increase

in the numbers and quantities of VOCs in use, which

have made the water resources used for portable supply

to have become increasingly susceptible to contamina-

tion by VOCs from various sources. Abdullahi and Chian

(2011) , in their study of VOCs in drinking water in pen-

insular Malaysia, detected 54 different VOCs species in

samples analyzed from 11 states, which were attributed

to improper disposal practice. The result showed that

the number of significant compounds detected increased

with the extent of infrastructure growth in the state, with

*Corresponding author: Mohd Ariffin Abu Hassan, Institute of

Environmental and Water Resources Management, Universiti

Teknologi Malaysia-Johor Bahru, Johor Bahru, Johor 81310,

Malaysia, e-mail: [email protected]

Mohammed Evuti Abdullahi: Faculty of Chemical Engineering,

Department of Chemical Engineering, Universiti Teknologi Malaysia-

Johor Bahru, Johor Bahru, Johor 81310, Malaysia

Zainura Zainon Noor: Institute of Environmental and Water

Resources Management, Universiti Teknologi Malaysia-Johor Bahru,

Johor Bahru, Johor 81310, Malaysia

Raja Kamarulzaman Raja Ibrahim: Advanced Photonic Science

Institute, Faculty of Science, Physics Department, Universiti

Teknologi Malaysia-Johor Bahru, Johor Bahru, Johor 81310, Malaysia

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2 M.E. Abdullahi et al.: A packed column air stripper for VOC removal

Negeri Sembilan and Johor having the highest of 12 VOCs

each. The contributory sources were identified to include

pharmaceutical industries, pesticides, insecticides, paint

industries, disinfection byproducts, solvents, adhesives,

cleaning agents, plastics manufacturing, resin and chlo-

rinated rubber, dyes, and petroleum products. A study by

the US Geological Department from 1999 to 2000 shows

that pharmaceutical byproducts form 80% of the contami-

nants in 139 streams across 30 states ( Kolpin et al. 2002 ,

Gupta et al. 2006 ).

The pharmaceutical industry is one of the largest

users of organic solvents per amount of the final product

( Slater et al. 2006 , Grodowska and Parczewski 2010 ).

These include toluene, xylene, ethanol, or isopropyl

alcohol. Significant emissions of VOCs from pharmaceu-

tical industry come from chemical synthesis and extrac-

tion phases. In natural product extraction, toluene is

used as a solvent to remove fats and oils that will con-

taminate the products and some VOCs are used extract

the product (such as plant alkaloids). Xylene is also

used in the laboratory to make baths with dry ice to cool

reaction vessels and as a solvent to remove synthetic

immersion oil from the microscope objective in light

microscopy. It is also used to sterilize many substances

( Grodowska and Parczewski 2010 , Kartheek et al. 2011 ).

Also, according to Kuroki et al. (2010) , xylene is used as

solvent and fixative. Chemical fixatives are used to pre-

serve tissue from degradation and to maintain the struc-

ture of the cell and the subcellular components such as

cell organelles (e.g., nucleus, endoplasmic reticulum,

and mitochondria).

Treatment methods of VOCs are generally classified

into two categories. One is by trapping these compounds

and removing them from the system with the possible uti-

lization of valuable compound referred to as nondestruc-

tive methods, while the others involve converting them

chemically into harmless compounds called destructive

methods ( Berenjian et al. 2012 , Preis et al. 2013 ). However,

the type and concentration of the VOCs determine the

choice of treatment method. The destructive methods

include advanced oxidation processes (AOPs) such as

photocatalysis and photo-Fenton ozone-based processes

( Schultz 2005 , Grote 2012 ). AOPs are based on the genera-

tion of very reactive species such as · OH and SO 4 ·

- radicals

that quickly oxidize a broad range of organic pollut-

ants. Sulfate radicals act as a relatively selective oxidant

that reacts with certain organic compounds, especially

benzene derivatives with ring activating groups. AOPs

can be separated into heterogeneous and homogeneous

depending on the number of phases involved. Examples

of homogeneous are UV/H 2 O

2 and UV/O

3 systems, while

examples are heterogeneous are UV/TiO 2 and ZnO/UV

systems ( Saien et al. 2011 , Loures et al. 2013 ).

Other destructive methods include thermal oxidation

( Surinder et al. 1992 ), bioreaction ( Gomez et al. 2009 ), cat-

alytic oxidations ( Peng et al. 2003 ), sonochemical ( Goel

et al. 2004 , Yaqub and Ajab 2013 ), electrochemical ( Nav-

aladian et al. 2007 ), and nonthermal plasma ( Hammer

1999 , Vandenbroucke et al. 2011 ), while the nondestruc-

tive method include air stripping ( Harisson et al. 1993 ),

absorption ( Zhu et al. 2008 ), adsorption ( Cho et al. 2007 ),

membrane-based separation ( Garba 2008 ), and conden-

sation ( Dwivedi et al. 2004 ).

Studies have reported the limitations of these con-

ventional methods. Thermal and catalytic oxidations

are not suitable for the treatment of dilute VOCs ( < 1000

ppm) because of high energy consumption. This is con-

trary to the case of high VOC concentrations, where the

exothermic reaction will result in a self-sustained process

( Subrahmanyam et al. 2007 ). In addition, the process may

generate appreciable air pollutants, particularly toxic

degradation products and particulate matter. Biological

treatment is a common method due to its cost-effective-

ness and versatility. An important aspect of this process is

the minimal impact on the environment and the complete

destruction of pollutants. However, the reaction process is

too slow and cannot meet the demand of emergency rapid

response to water pollution. It is also only effective for

dilute effluents and it is not effective for nonbiodegrada-

ble pollutants such as chlorinated carbon compounds

( Russsell et al. 1992 , Melcer 1994 , Schultz 2005 , Berenjian

et al. 2012 , Lin et al. 2012 ).

A major weakness of adsorption is that adsorbents

usually become progressively saturated and inactive.

Moreover, the saturated adsorbent itself becomes a haz-

ardous waste that must be treated or disposed properly.

Also, high dissolved organic carbon (DOC) and other con-

taminants can compete with some VOCs such as trichlo-

roethane (TCE) for binding sites available on the sorbent.

A concentration of 10 ppm natural organic matter in river

water has been shown to reduce TCE adsorption by 70%

( Melcer 1994 ). However, the adsorbent could be reused

after an appropriate regeneration step. Thermal regen-

eration and solvent regeneration are two methods of

exhausted adsorbent regeneration. Thermal regeneration

is carried out at high temperature usually above 1073 K

and therefore requires high energy consumption and con-

siderable carbon loss, while solvent extraction has low

regeneration efficiency because of adsorbent pores block-

age ( Lu et al. 2011 ). Worrall and Zuber (1996) reported that

$ 250,000 capital cost and annual operating costs result-

ing from freight to/from kiln, kiln fuel, carbon makeup,



M.E. Abdullahi et al.: A packed column air stripper for VOC removal 3

Table 1 Performance of an packed column air stripper for different VOCs.

Types of pollutant Pollutant concentration

A/W ratio

Removal efficiency (%)

References

Chlorobenzene 1 – 10 mg/l NA 99 Lin et al. 2012

Chloroform 50 ° 300 μ g/l 20:1 87.4 Samadi et al. 2004

1,2-DCE NA 40:1 90.6 Harisson et al. 1993

1,2-Dibromo-3-chloropropane NA 762.1 89.2 Nirmalakhandan et al. 1993

TCE 440 μ g/l NA 99 Byer and Morton 1985

1,1,2,2-Tetrachloroethane 350 μ g/l NA 94 – 98 Byer and Morton 1985

NA, not available.

etc., exceeding $ 1,200,000 is required to treat 500 gal/min

(1892.7 l/min) of wastewater.

Liquid-phase electrical discharge reactors have also

been investigated and are being developed for several

applications in drinking water and wastewater treatment

( Locke et al. 2006 , Gerrity et al. 2010 ). Generally, strong

electric fields when applied to water (electrohydraulic

discharge) initiate both chemical and physical process

such as shockwaves, cavitation, and light emissions

( Locke et al. 2006 ). However, the high density of liquid

prevents electrons from accelerating and to undergo dis-

sociative collision unless. The electric field is several

orders of magnitude higher compared to the plasma ’ s at

atmospheric pressure; therefore, electron avalanches are

almost impossible inside a liquid because of low mobility

and high recombination rate ( Locke et al. 2006 , Vanden-

broucke et al. 2011 ). Also, there are reports of some cases

of harmful byproducts formation due to the incomplete

destruction of the VOCs ( Anders et al. 1997 , Creyghton

1997 ).

Air stripping is a technology that uses an air strip-

per for VOC removal from wastewater by increasing the

surface area of the contaminated water that is exposed

to air, and it is widely used for the removal of VOCs from

wastewater in the process industries ( Huang and Shang

2006 ). The types of air stripper include packed column,

sieve tray, and diffused aeration ( Kutzer et al. 1995 , Linek

et al. 1998 , Alam and Hossain 2009 , El-Behlil and Adma

2012 , El-Behlil et al. 2012 ). The applicability of each tech-

nology is based on its performance as reported in engi-

neering literatures, vendor information, and professional

experience with the equipment ( Wang et al. 2006 , Mourad

et al. 2012 ).

It has also been observed that the focus of the

researches on VOC treatment nowadays is more on new

areas, such as membrane-based separation and nonther-

mal plasma applications ( Garba 2008 , Gerrity et al. 2010 ,

Lin et al. 2012 ); however, air stripping as a technique still

remains most useful where there is an economic interest

in higher concentrations of valuable VOCs through recov-

ery. Over 99% VOC recovery using air stripping has

been reported in the literature ( Chuang et al. 1992 , Nir-

malakhandan et al. 1993 , Negrea et al. 2008, Zareei and

Ghoreyshi 2011 ). Air stripping has also been used along

with other treatment methods such as adsorption, cata-

lytic oxidation, pervaporation, and nonthermal plasma as

an integrated system for the treatment of VOCs ( Chuang

et al. 1992 , Worrall and Zuber 1996 , Zareei and Ghoreyshi

2011 , Abdullahi et al. 2013a ). Although some reviews on

air stripping are available ( Kutzer et al. 1995 , Brown et al.

1997 , Berenjian et al. 2012 ), this paper presents further

elaborations and new information on some topics in the

application of a packed column air stripper in VOC treat-

ment from wastewater. These include a review of the

physical and chemical properties of the VOCs and how

they affect the air stripping process. Second, the summary

tables of the various applications containing information

on process variables provided by this paper ( Tables 1 and

2 ) will serve as an easy and ready access to the literature.

The review will also cover available works on the mode-

ling and simulation of a packed column air stripper.

2 VOC contamination of water Many VOCs are manmade (anthropogenic) chemicals

that are used or produced in the manufacture of paints,

adhesives, petroleum products, pharmaceuticals, and

refrigerants. Many are also compounds of fuels, solvents,

hydraulic fluids, paint thinners, and dry cleaning agents

commonly used in urban settings such as bleach ( Zor-

goski et al. 2006 ). Some are of natural origin, produced

by plants, animals, microbes, and fungi (biogenic). When

VOCs are spilled or disposed of on or below the land

surface, a portion evaporates, contributing to air pollution

problems, but some can be carried deep into soil by rain-

water or melting snow ( Minnich 1993 ). Once they enter




groundwater, VOCs can remain there for years, decompos-

ing slowly because of the cool, dark environment. These

chemicals move with the groundwater and pose a threat

to the nearby drinking water. The USEPA estimates that

VOCs are present in one fifth of the nation ’ s water supplies

( Moran et al. 2006 ). Several factors increase the likelihood

that a water supply will be contaminated. These factors

are illustrated in Figure 1 .

One factor is the distance between a well and a source

of contamination. Many wells contaminated with VOCs

are located near industrial or commercial areas, gas sta-

tions, landfills, or railroad tracks. A second factor is the

amount of VOCs dumped or spilled. Some spills are small

and localized. Others occur over a long period of time or

involve large quantities of contaminants. When a large

quantity of chemicals has leaked or spilled, as may occur

with leaking underground tanks or industrial spills, a

large geographical area may be affected. Third, the depth

of a well can be a factor. Shallow wells are often affected

sooner and more severely than deep wells when contami-

nants have been spilled on surface soils. A fourth factor is

local geology. Groundwater covered by thin, porous soil

or sand layers is most vulnerable. Dense, thickly layered

soils may slow down the movement of contaminants and

may help to absorb them. The fifth factor affecting con-

tamination of water is time. Groundwater typically moves

very slowly. A spill may take years to reach nearby wells,

so wells may not be contaminated until months or years

after the spill is discovered. Although many VOCs found

in drinking water are due to contamination, others may be

formed when drinking water is treated with chlorine. The

chlorine reacts with organic materials found in water and

forms certain VOCs known as chlorination byproducts

( Minnich 1993 , Simpson et al. 2006 ).

3 Packed column air stripping Air stripping is the process of removing VOCs from liquid

(water) by providing contact between the liquid and gas

(air) in drinking water treatment, industrial wastewater

purification, and treatment of heavily polluted under-

ground water ( Brown et al. 1997 , Mead and Leibbert 1998 ,

Beranek 2001 , Wang et al. 2006 , El-Behlil and Adma 2012 ,

Abdullahi et al. 2013a ). A typical packed column air strip-

ping system is shown in Figure 2 .

3.1 Theory of air stripping

Air stripping is a mass transfer operation involving the

transfer of dissolved VOCs in water from liquid phase to

gas phase ( Mead and Leibbert 1998 , Beranek 2001 , Nava-

ladian et al. 2007 , Abdullahi et al. 2013b ). The air and the

water interact in a column specially designed to maximize

Table 2 Treatment of VOCs from wastewater using a combination of technologies.

Researcher Technologies Pollutants Findings

Chuang et al. 1992 Air stripping and oxidation using

hydrophobic catalyst

Benzene, toluene, and

xylene (BTX)

95% conversion of BTX

Shortcoming: Each compound needs an

appropriate catalyst

Russsell et al. 1992 Air stripping and GAC adsorption TCE 95 – 99% recovery of TCE

Shortcoming: High cost and need to

regenerate adsorbent

Worrall and Zuber 1996 Air stripping and GAC adsorption

using nitrogen as stripping gas

Benzene, toluene, ethyl

benzene, and xylene

(BTEX) from refinery

wastewater

99% BTEX captured by the carbon bed.

Increased carbon working life

Shortcoming: Nitrogen is expense

Francke et al. 2000 Air stripping and plasma catalytic

treatment (dielectric barrier

discharge)

Vinyl chloride, cis-DCE,

TCE, toluene, xylene, and

ethylene benzene

85% removal of each compound

Shortcoming: Effect of field conditions such

as temperature and mechanism of the plasma

decomposition process not investigated

Zareei and Ghoreyshi 2011 Air stripping and vapor

permeation system

Chloroform, 1,1,2-

TCE, DCE, and

dichloromethane

High VOCs recovery efficiency

Shortcoming: Membrane instability




Homesteadwith

septic field

Dieselstorage

Water table

Ground water

Shallow aquifer

Clay(aquitard)

Bedrock aquifer

BarnDrivingshed

Figure 1 Underground water contamination sources ( Simpson et al. 2006 ).

Reproduced with permission from the Ministry of Agriculture, Food and Rural Affairs, Ontario, Canada.

Exhaust (Gas outlet)

Flow meter

Air stripper

Air compressor

Flow meter

Collection tank

Heater

Pump

Feed tank

Figure 2 A typical packed column air stripping system.




the contact surface area between the water and the air.

Equilibrium is reached when the two phases are brought

into contact. This means that the water in contact with

air evaporates until the air is saturated with water vapor,

and the air is absorbed by the water until it becomes satu-

rated with the individual gases. The equilibrium relation-

ship is linear and it is defined by Henry ’ s law. Henry ’ s law

states that for a low concentration of volatile compound

a, at equilibrium, the partial pressure of a gas ( P a ) above

a liquid is directly proportional to the mole fraction of the

gas ( x a ) dissolved in the liquid. This can be mathemati-

cally stated in Equation (1):

a a aP H x=

(1)

The proportionality constant H a is known as Henry ’ s con-

stant. It is a dimensionless partition coefficient expressed

as the ratio of the mass (or molar) concentration in the gas

phase to that in the liquid phase ( Robert et al. 1985 ). This

constant is a primary indicator of a compound ’ s poten-

tial for removal by air stripping and it increases with the

increase in temperature. The application of air stripping is

limited to compounds with Henry ’ s constant values > 100

atmospheres ( Richardson et al. 2002 , Mourad et al. 2012 ).

Applying Henry ’ s law, a mass balance of VOCs in and

out of Figure 3 will be as follows:

out out out out in in in inL x G y L x G y⋅ + ⋅ = ⋅ + ⋅

(2)

where G is the gas flux, L is the water flow, x is the VOCs con-

centration in water, and y is the VOCs concentration in air.

Assuming an uncontaminated air supply,

in in out in out0; andY L L L G G G= = = = =

Liquid inLinXin

Gas inGinYin

Liquid outLoutXout

Gas outGoutYout

Strippingcolumn

Figure 3 A countercurrent flow air stripping column ( Mourad et al.

2012 ).

Reproduced with permission from the International Association for

Sharing Knowledge and Sustainability (IASKS), Canada.

in out outL x L x G y⋅ = ⋅ + ⋅

(3)

Rearranging Equation (3) yields

out in out( - )G y L x x⋅ =

(4)

in out

out

-x xG

L y

⎛ ⎞=⎜ ⎟⎝ ⎠

(5)

Equation (5) is the gas-liquid ratio. The percentage

removal of the VOCs ( E ) is used to evaluate the efficiency

of the air stripper and it is expressed as

in out

in

-100(%)

x xE

x= ×

(6)

3.1.1 Design equation

In a countercurrent air stripping operation as shown in

Figure 3, air stripping is effectively achieved with the water

flowing downwards over the packing as a film, while the

air flows upward as the continuous phase. The design

equations are derived elsewhere ( Montgomery 1985 , Hand

et al. 2011 ) for the general cases of absorption and desorp-

tion (stripping). A simple case of isothermal desorption

of a trace, volatile solute is presented in this paper. The

packed height Z required to achieve a desired separation is

Z HTU NTU= × (7)

HTU , the height of transfer unit (m), is the ratio of

superficial velocity, u o (m/s), to the transfer rate constant,

K L a (s -1 ), and NTU is the number of transfer units (dimen-

sionless). HTU is defined by Equation (8).

0 m

( )L L L

u LHTU

K a K aρ= =

(8)

where L m

is liquid mass flux (kg/m 2 /s), ρ L is liquid density

(kg/m 3 ), and K L a is the overall mass transfer coefficient

per second based on liquid-phase driving force, which is

also the product of the overall mass transfer coefficient

and the specific interfacial area ( Robert et al. 1985 ). NTU

can be calculated using Equation (9):

, 2 , 1

, 1 , 1

-( -1) 1

ln-1 -

L G

c

L G

c

C CS

HSNTU

S C C

H S

⎡ ⎤⎢ ⎥+⎢ ⎥= ⎢ ⎥⎛ ⎞⎢ ⎥⎜ ⎟⎢ ⎥⎝ ⎠⎣ ⎦

(9)




where C L , 2

and C L , 1

are the solute concentrations in the

influent and effluent liquid, respectively, and C G , 1

is the

concentration in the influent gas. H c is Henry ’ s constant

and S is the stripping factor that represents the capac-

ity for transfer relative to equilibrium condition in an air

stripper.

G c

L

Q HS

Q=

(10)

Q G and Q

L are the volumetric flow rates of gas and liquid,

respectively. If S > 1, it means that there is enough gas to

convey away all of the solute in the entering liquid and

complete removal by stripping is then possible, given a

sufficiently tall column. If S < 1, however, the system per-

formance is limited by equilibrium, and the fractional

removal is asymptotic to the value of the stripping factor

( Robert et al. 1985 ).

If the stripping gas does not contain any contaminant

of interest, substituting C G , 1

= 0 into Equation (9), it can be

simplified to

, 2

, 1

( -1) 1

ln( ) -1

L

Lm

L L

CS

CL SZ

K a S Sρ

⎡ ⎤⎛ ⎞⎢ ⎥+⎜ ⎟⎜ ⎟⎢ ⎥⎛ ⎞ ⎛ ⎞ ⎝ ⎠⎢ ⎥= ⎜ ⎟⎜ ⎟ ⎝ ⎠ ⎣ ⎦⎝ ⎠

(11)

Equation (11) can be used in design calculations to

estimate the packing height necessary to achieve a given

treatment objective.

3.2 The process and the equipment

A packed column air stripper as shown in Figure 4 con-

sists of a cylindrical column that contains packing materi-

als (usually Raschig rings) and a spray nozzle for water

distribution at the top of the tower. The column also has

an air distribution system at the bottom. The water trick-

les down through the spaces between the packing materi-

als to the bottom of the column while air moves upwards

in a countercurrent operation. The packing increases the

surface area of the contaminated water that is exposed to

air, thereby maximizing the amount of the VOCs moving

from the water to the air. The purified water is collected

at the bottom of the column, while the VOC-rich air leaves

the column at the top ( Mead and Leibbert 1998 , Beranek

2001 , Richardson et al. 2002 , Mourad et al. 2012 ). A typical

packed column air stripper is shown in Figure 4.

Air stripping is therefore only a mere phase separation

and the VOC-rich off-gas from the air stripper may have

to undergo further treatment to meet the emission limits.

This is usually done using integrated systems such as air

stripping and nonthermal plasma reactor ( Abdullahi et al.

2013a ), air stripping and catalytic oxidation ( Chuang et al.

1992 ), air stripping and adsorption ( Russsell et al. 1992 ,

Worrall and Zuber 1996 ), air stripping and vapor permea-

tion ( Zareei and Ghoreyshi 2011 ), etc.

This technology has been used mainly for the removal

of VOCs from dilute aqueous waste streams. Several

studies have reported high VOC removal efficiencies using

air stripper as shown in Table 1. Samadi et al. (2004) com-

pared the performance of an air stripper to granulated

activated carbon (GAC) in the removal of chloroform from

Tehran drinking water. The removal efficiencies were

89.9% and 71.2% for the air stripper and GAC columns with

deionized water samples, respectively, while, for chlorin-

ated Tehran tapwater, 91.2% and 76.4% removal efficien-

cies were obtained, respectively. This result shows that an

air stripper is more effective in chloroform removal.

Studies show that an economical solution to VOC

removal from wastewater can be best achieved using a

combination of technologies ( Hammer 1999 ). A summary

of related works on the application of integrated system

for the treatment of VOCs is shown in Table 2. The goals

of integrated (hybrid) processes are to decrease pollu-

tion from wastewater, to decrease air pollution by return-

ing clean air to the atmosphere, and to either recover the

VOCs or decompose them into smaller and less harmful

byproducts ( Zareei and Ghoreyshi 2011 ).

The USEPA Engineering Bulletin published a review

on the effectiveness of air stripping on general contami-

nant groups present in aqueous solution. The review

showed that successful treatability tests at some scale

have been recorded for halogenated and nonhalogenated

volatile organics. Experts are also of the opinion that the

technology can be applied for halogenated semivolatile

organics but will not be applicable for some organic com-

pounds such as nonhalogenated, semivolatile polychlo-

rinated biphenyls (PCBs), pesticides, dioxins and furans,

organic cyanides, and organic corrosives ( USEPA 1991 ,

Bansode et al. 2003 ).

The application of air strippers in groundwater treat-

ment have been age long. According to the USEPA, in the

late 1960s, an air stripper was used to remove TCE from

groundwater polluted by a refrigerator manufacturing

firm in the United States with an efficiency of 78%. High

removal efficiency was also reported by a joint research by

United States Air Force and Coast Guard, which assessed

the efficiency of a countercurrent packed column air strip-

per that uses centrifugal force to drive the liquid ( Surinder

et al. 1992 ). The efficiency of an air stripper depends on




Air out

Cover

Water in

Mist eliminator

Distributiontray

Packingmaterial

Packingmaterial

Packingmaterial

Air Pip

eP

ipe

Column

Redistributor(Typical)

Supportplate

Manometerdevice

Water

Water outlet

Influentwater

Air in

Air duct

Centrifugal blower

Damper

Centrifugal pump

Concrete pad

Water

Flowmeter

Flowmeter

Valve

Air outletscreen

Figure 4 A packed column air stripper ( Suthersan 1999 ).

Reproduced with permission from CRC Press, USA.

many factors such as the characteristics of the volatile

material (partial pressure, Henry ’ s constant, gas trans-

fer resistance, etc.), water and ambient air temperature,

turbulence in gas and liquid phases, area-to-water ratio,

column height, and exposure time. Packed column strip-

pers operate most efficiently over a narrow range of water

flows of 0.8 – 1.8 (m 3 /min)/m 2 of the tower cross-sectional

area. This means that a packed column air stripper uses

less air for the same water flow rate than a sieve tray air

stripper. Packed column air strippers are operated over a

wide range of flow rates typically from 1.5 to 76 (m 3 /min)/m 2

of the column cross-sectional area – this means that, if the

water flow rate to the column decreases, the air flow rate

will also decrease. This will reduce the cost of treating air

emissions.

Henry ’ s constant, a primary indicator of a com-

pound ’ s potential for removal in an air stripper, increases

with the increase in temperature. Also, compounds with

low volatility at ambient temperature may require preheat-

ing to achieve high removal efficiency. In addition, differ-

ent VOCs also have different values of Henry ’ s constant

( Chuang et al. 1992 , Nehra et al. 2008 ). Packed column air

strippers exhibit lower pressure drop characteristics than

the pressure drop through a sieve tray air stripper. This




allows for a smaller blower and motor, with reduced elec-

trical operating costs. In general, packed column air strip-

pers are generally less expensive and easier to construct.

They are also preferred for liquids that have a tendency to

foam, since they have significant interference on the effi-

ciency of a tray air stripper. However, tray strippers have

shorter heights and smaller diameters than an equivalent

packed column stripper.

4 Factors affecting air stripping in a packed column air stripper

The performance of an air stripper is affected by physi-

cal and chemical properties of the contaminants and the

operating parameters ( Mourad et al. 2012 , Abdullahi et al.

2013b ). The physical and chemical properties of VOCs give

an indication of the physical state of the compound and

its mobility. These properties include molecular weight,

melting point, boiling point, vapor pressure, water solu-

bility, specific gravity, liquid surface tension, liquid-water

interfacial tension, and Henry ’ s constant ( Reidy et al.

1990 ). These factors determine the operating parameters.

Several studies investigated the influence of operating

parameters on the removal efficiency. In general, VOC

removal efficiency was found to depend on temperature,

air-to-water (A/W) ratio, hydraulic loading rates, packing

materials, size, depth and diameter, gas pressure drop,

and Henry ’ s constant of the contaminant ( Chuang et al.

1992 , Nirmalakhandan et al. 1993 , Alam and Hossain

2009 , Mourad et al. 2012 , Abdullahi et al. 2013b ).

4.1 Effect of temperature

The mass transfer of VOCs occurs through volatilization,

which may be induced by mechanical surface aeration.

The process of removal of dissolved gas from liquid pro-

ceeds through the following consecutive steps ( Montgom-

ery 1985 ):

1. Transfer from the bulk fluid to the interface,

2. Transfer across the interface, and

3. Transfer away from the interface into the bulk of new

phase.

According to the two-film model, laminar films exist at

the gas-liquid interface. The resistance to the rate of mass

transfer is therefore estimated by summing the resistances

offered by the liquid- and gas-phase boundary layers. The

rate of mass transfer of a VOC from wastewater to the

4.0Air flowrate 1.0 l/minAir flowrate 1.5 l/minAir flowrate 2.0 l/minAir flowrate 2.5 l/minAir flowrate 3.0 l/min

3.5

3.0

2.5

2.0

KL

(m

in-1

)

1.5

0.5

0.00 5 10 15 20

Temperature (°C)

3025

1.0

Figure 5 Effect of temperature on A/W mass transfer coefficient ( Lin

et al. 2012 ).

Reproduced with permission from Taylor & Francis Group, UK.

atmosphere across an air-wastewater interface can be

described by ( Montgomery 1985 , Hand et al. 2011 , Abdul-

lahi et al. 2014 ).

--

g

L

C CdMK A

dt H

⎛ ⎞= ⎜ ⎟⎝ ⎠

(12)

From a thermodynamic analysis, the temperature depend-

ence of Henry ’ s constant can be modeled by a Van ’ t Hoff-

type relation, given in the integrated form by ( Hand et al.

2011 )

0-log( )

HH C

RT

Δ⎛ ⎞= +⎜ ⎟⎝ ⎠

(13)

Equations (12) and (13) show that temperature plays

an important role on the mass transfer phenomena, as

K L increases as temperature is increased. This is also

reported by Lin et al. (2012) in the study of air stripping of

chlorobenzene as shown in Figure 5 .

It was explained that the change of temperatures

influences the physical properties of both the liquid and

gas and has a significant impact on the mass transfer

process. For example, an increase in temperature would

lead to a pronounced reduction of liquid viscosity and

surface tension of air bubbles, resulting in the formation

of small and stable bubbles, and increases the probability

of coalescence. It is this duality that alters the interfacial

area and influences significantly on the mass transfer

process ( Chuang et al. 1992 , Zareei and Ghoreyshi 2011 ,

Lin et al. 2012 , Abdullahi et al. 2014 ). In addition, the

change in physical properties of the liquid and gas change

with increasing temperature has also a great influence




on diffusion coefficient. Viscosity decrease leads to a

decrease in the thickness of the stagnant film at the gas-

liquid interface, resulting in a lower mass transfer resist-

ance, and hence increases the diffusion coefficient. Both

effects lead to an increase in K L . Furthermore, it is interest-

ing that a jump of K L values was found at around 30 ° C for

a given air flow rate as illustrated by Figure 6 , which indi-

cates that stripping at higher temperature could achieve a

higher chlorobenzene removal efficiency even at lower air

flow rate ( Lin et al. 2012 ).

Similarly, the VOC removal efficiency increases with

temperature because increase in temperature causes a

decrease in the solubility of organic compounds in water

and increases Henry ’ s coefficient and hence improves

removal efficiency ( Chuang et al. 1992 , Kutzer et al. 1995 ,

Zareei and Ghoreyshi 2011 ). The decrease in the solubil-

ity of organic compounds in water as the temperature

increases can be explained using the second law of

Rashingring

A B C D

E F

G H I

Lessingring

Intalox saddle

Pall ring Nor pacring

Tri-pac

Tellerette

Partitionring Beri saddle

Figure 6 Examples of packing material for air stripping towers ( Hand et al. 2011 ).

Reproduced with permission from McGraw-Hill Education Ltd., UK.

thermodynamics. Heating a solution of a gas enables the

particles of gas to move more freely between the solu-

tion and the gas phase. The second law predicts that

they will shift to the more disordered, more highly dis-

persed, and therefore more probably gas state. This leads

to an increase in Henry ’ s coefficient and hence improves

removal efficiency ( Zareei and Ghoreyshi 2011 ).

Reidy et al. (1990) attributed the increase in VOC

removal efficiency, as the temperature rises to increase in

vapor pressure with temperature. Thus, as the vapor pres-

sure increases, the ease of contaminant removal increases.

Table 3 shows the vapor pressure of water measured at

four different temperatures.

Table 3 shows a nonlinear increase in pressure as

the temperature increases. Thus, a plot of vapor pres-

sure against temperature does not give a straight line but

rather gives a curve that rises faster and faster as the tem-

perature increases. An increase temperature will result in




rise in vapor pressure and hence Henry ’ s constant, which

therefore increases the ease of contaminant removal

( Reidy et al. 1990 , Lin et al. 2012 ).

If it is assumed that the effect of temperature on

Henry ’ s constant is due almost entirely to changes in

vapor pressure, the relationship between Henry ’ s con-

stant and temperature can be approximated by the Clau-

sius-Clapeyron equation given as Equation (14)

1 1

2 2 2 1

1 1ln ln -V

H P H

H P R T T

Δ⎛ ⎞ ⎛ ⎞ ⎛ ⎞≈ =⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠ ⎝ ⎠

(14)

Bass and Sylvia (1992) reported that the Henry ’ s con-

stant for methyl tert -butyl ether (MTBE) is doubled by a

17 ° C temperature increase, from 12 ° C to 29 ° C. Therefore,

heating the wastewater by this amount before treatment

would reduce the stripping air requirement by half.

4.2 Effect of A/W ratio

The research results show that, at any given tempera-

ture, the increase in A/W ratio results in higher VOC

removal efficiency. This is because increased air flow rate

increases the interfacial area, decreases gas-phase resist-

ance, and hence increases the efficiency of mass trans-

fer. Another effect of increased A/W ratio is it causes a

decrease in partial pressure of the solute in the gas phase,

decreases its solubility, and improves its removal effi-

ciency ( Chuang et al. 1992 , Alam and Hossain 2009 , Zareei

and Ghoreyshi 2011 , Abdullahi et al. 2014 ). In another

study, Lin et al. (2012) reported that the effect of air flow

rate on chlorobenzene removal was more significant

when the system was operated at a lower flow rates (1 – 2

l/min) than higher flow rates especially for higher tem-

perature. It was explained that the increase in interfacial

area while the air flow rate increases is nonlinear. Tradi-

tionally, the decreasing of bubble size (or the increasing

in bubble concentration) would lead to an increase in air-

aqueous interfacial area at lower air flow rate. However,

frequent bubble collision at higher air flow rate condition

would increase the diameter of air bubbles during the air

Table 3 Equilibrium vapor pressure of water at four different tem-

peratures ( Lowe 1990 ).

T ( ° C) P(mm Hg) Change in P (mm Hg)

0 4.85 –

10 9.21 4.63

20 17.54 8.33

30 31.82 14.28

stripping; as a result, the interfacial area does not linearly

increase with an increase in air flow rate. Moreover, since

the total A/W interface surface area is proportional to the

number and size of the air bubbles, an increase in air flow

rate will result in an increase in the K L values and hence

the removal efficiency. Alam and Hossain (2009) also

reported an increase in ammonia removal from industrial

wastewater with increase in A/W flow ratio.

According to Nirmalakhandan et al. (1993) , the design

of most countercurrent packed towers for air stripping

in environmental application is for operation under low

liquid and gas loading rates. Mourad et al. (2012) reported

that the typical gas-liquid ratio for packed bed air strip-

per will range from 10:1 to 30:1. However, the use of higher

A/W flow ratios during air stripping has reported in some

literatures. Kutzer et al. (1995) studied the influence of

the operating conditions on the removal efficiencies of

perchloroethene, TCE, 1,2-dichloroethane (DCE), and

1,1,2-TCE at A/W ratios of 10 – 100. Also, Alam and Hossain

(2009) investigated the effect of A/W ratio on the air strip-

ping of ammonia at A/W flow ratio of 1250 – 2000.

4.3 Effect of initial VOC concentration

Initial VOC concentration is another very important factor

that affects the performance of the air stripper. This is

because the rate of volatilization of VOCs is dependent

on its concentration. Research results show that increas-

ing the initial concentration enhances the VOC removal

( Alam and Hossain 2009 , Lin et al. 2012 ). According to

Lin et al. (2012) , the positive effect of increasing initial

chlorobenzene concentration is related to a large concen-

tration gradient of chlorobenzene concentration on the

air-liquid interface, which is feasible for the diffusion of

chlorobenzene during air stripping. In addition, a linear

relationship was found between the obtained K L and the

initial concentration of chlorobenzene, which is essential

for choosing K L under different pollution concentration in

practical operation. This is illustrated by Figure 7 .

4.4 Effect of shape and height of packing materials

The packing material may consist of individual pieces

randomly dumped into the column or structured. The

structured column packing is formed from vertical sheets

of corrugated thin gauge ceramic/metal/plastic with the

angle of the corrugations reversed in adjacent sheets to

form a very open honeycomb structure with inclined flow




10

8

6

4

CB

con

cent

ratio

n (m

g/l)

2

0

0 2 4 6 8 10Time (min)

12

C0=10 mg/l

C0=7 mg/l

C0=5 mg/l

C0=3 mg/l

C0=1 mg/l

curve fittedtemperature 15°Cair flow rate 2.5 l/min

14 16

Figure 7 Effect of initial concentration on chlorobenzene (CB)

removal ( Lin et al. 2012 ).

Reproduced with permission from Taylor & Francis Group, UK.

channels and a relatively high surface area. Packing mate-

rial provides a large A/W interfacial area resulting in an

efficient transfer of the volatile contaminant from the

water to the air ( Wang et al. 2006 , Alam and Hossain 2009 ,

El-Behlil et al. 2012 ). Packing materials normally employ

at least one of the following mechanisms ( Ali 2012 ):

1. Dividing the gas into small bubbles in a continuous

liquid phase.

2. Spreading the liquid into thin film that flow through a

continuous gas phase.

3. Forming the liquid into small drops in a continuous

gas phase.

There are several different varieties of commercially

available packing material with their physical properties

usually provided by the manufacturers ( Hand et al. 2011 ).

Some examples are displayed in Figure 6. The types and

shape of packing materials determine the pressure drop

in the air stripper. Packing offers resistance to flow under

given air and water loading rates. The packing media

“ hold up ” a certain amount of liquid depending on the

relative amount of space (gap) left between them, which is

referred to as voidage ( Nirmalakhandan et al. 1993 ).

The choice of packing materials should therefore be

based on the ratio of their absolute pressure drop to avoid

the phenomenon of flooding. Modern plastic lattice pack-

ings are superior to classic packings such as stainless-steel

pall rings or ceramic saddles due to its high voidage, which

reduces the tendency of plugging. To minimize the pressure

drop of packings, many manufacturers offer large random

packings up to a nominal size of 90 mm ( Kutzer et al. 1995 ,

El-Behlil et al. 2012 ). However, ceramic random column

packing is highly suitable for the conditions of higher and

lower temperatures and can be much more resistant to all

kinds of organic acid, inorganic acid, and solutions, except

for the hydrofluoric acid than metal packing. Random

ceramic packing or column ceramic packing is widely used

in drying towers, absorbing towers, and cooling towers

in the fields of chemical and petrochemical industries.

Acid-resistant ceramic packings rings are widely used in

refineries, chemical engineering, acid plants, gas plants,

oxygen plants, steel plants, and pharmaceutical plants

( Cameron and Chang 2010 ). Ceramic packings are mainly

used as linings of reaction vessel in washing towers,

cooling towers, reclaiming towers, desulfurization towers,

drying towers, and absorbing towers. They can also be

used as lining bricks in anticorrosion pools and channels

( Cameron and Chang 2010 , El-Behlil et al. 2012 ).

The effect of packing materials on the removal effi-

ciency of VOCs from wastewater was studied by Alam

and Hossain (2009) using four different packing materi-

als (coal chips, plastic ring, stone chips, and wood chips).

They observed that, among the four types of packing

materials used, a plastic ring gave the best performance

due its high surface area. Plastic is also lighter and will

therefore not add much to the weight of column. Alam

and Hossain (2009) reported that removal efficiency

increased with an increase in packing height, which was

due to the increment of contact time between air and

water. They observed an increase in percentage removal

from 63.5% to 91.60% when the height of the packing was

being gradually increased from 1 to 5 ft during air strip-

ping of ammonia using plastic rings. However, the choice

of optimum height is a tradeoff between pressure drop,

packing volume, and total brake power. The total brake

horsepower is the ideal power plus the frictional power

requirements for stripping in a packed tower, and as the

pressure increases, the brake horsepower will increase.

Lower packing height requires a larger pressure drop for

a given A/W ratio but requires less total brake horsepower

to achieve the same removal efficiency and vice versa

( Wang et al. 2006 ).

4.5 Effect of physical and chemical proper-ties of the VOCs

Air stripping of VOCs from wastewater depends on their

physical and chemical properties. These properties

include: molecular weight, melting point, boiling point,

vapor pressure, water solubility, density (specific gravity),

liquid surface tension, liquid-water interfacial tension

and Henry ’ s constant ( Reidy et al. 1990 , Russsell et al.

1992 , Huang and Shang 2006 , El-Behlil et al. 2012 ). Thus,




the first step in determining which treatment method is

the evaluation of known properties of the contaminants

( Spencer and Witco 2008 ). Knowledge of a compound ’ s

physicochemical tendencies can be used to alter the

behavior and fate of that compound in the environment

( Russsell et al. 1992 ).

Density (specific gravity) and molecular weight:

Density can be defined as the concentration of matter and

is measured by the mass per unit volume. In relation to

liquids, these units are grams per milliliter. The density

of a substance is usually referenced to pure water, which

is taken to be 1 g/ml. A spill of sufficient magnitude of

high-density compounds often results in the formation

of a plume or pool(s) of dense nonaqueous phase liquid

( Russsell et al. 1992 ). The molecular weight of a com-

pound provides indirect information about the size and

chemical complexity of a compound. For most organic

compounds having an intermediate molecular weight

(e.g., benzene), vapor pressure increases three to four

times for each 10 ° C rise in temperature. Density is also a

function of temperature. It decreases as the temperature

rises. Therefore, heavy organic compounds in wastewater

are not strippable with air and may require steam strip-

ping ( Bravo 1994 ).

Boiling point: VOCs are organic chemicals with high

vapor pressure at ordinary, room temperature conditions.

This is due to their low boiling points, which causes large

numbers of molecules to evaporate from the liquid or

solid form of the compound into the surrounding air. For

instance, formaldehyde with a low boiling point of -19 ° C

(-2 ° F) slowly leaves the paint into the air. Some common

examples include acetone, benzene, ethylene glycol,

formaldehyde, methylene chloride, perchloroethylene,

toluene, xylene, and 1,3-butadiene ( Zorgoski et al. 2006 ,

Goldstein and Galbally 2007 ).

Solubility: The solubility of the compound as defined

by solubility constants indicates the extent to which the

compound will dissolve in water. In another words, it is

the maximum concentration of a solute that can be carried

in water under equilibrium conditions and is generally

given as parts per million or milligrams per liter ( Russsell

et al. 1992 , Spencer and Witco 2008 ). For the purposes of

water quality and treatment, solubility is the amount of a

contaminant that can dissolve in water at near ambient

temperatures (25 ° C). The water solubility limit of TCE is

1000 mg/l, the maximum concentration of TCE that can

be in aqueous solution at 20 ° C. Water solubility of a com-

pound has a direct relation on its distribution coefficients.

A compound that is relatively insoluble in water will

prefer to partition into another phase (volatilize) ( Russsell

et al. 1992 ).

The volatility of the compound as defined by Henry ’ s

constant indicates how well a contaminant will be removed

by air stripping. Partition coefficients and adsorption

isotherm data are available for many compounds ( Russ-

sell et al. 1992 ). Most organic substances become more

soluble as the water temperature increases, unless the

organic compound is volatile ( Huang and Shang 2006 ,

El-Behlil et al. 2012 , Abdullahi et al. 2013). Solubility of

organic compounds in water is controlled primarily by the

polarity of the compound. Nonpolar organic compounds

do not dissolve well in the polar solvent water. Solubility

values < 1 mg/l (1000 μ g/l) are considered insoluble, thus

allowing the compound to be easily removed from water.

Highly soluble compounds, such as ethylene glycol or ter-

tiary butyl alcohol, are very difficult to remove from water.

Very soluble compounds do not transfer to other solvents

or adsorbents, nor will they bind to soils or activated

carbon ( Spencer and Witco 2008 ).

The equilibrium concentration of a solute or contami-

nant in air is directly proportional to the concentration

of the solute in water at a given temperature. This is sup-

ported by Henry ’ s constant, which states that the amount

of gas that dissolves in a given quantity of liquid, at con-

stant temperature and total pressure, is directly propor-

tional to the partial pressure of the gas above the solution.

Therefore, Henry ’ s constant describes the tendency of a

given compound to separate between a gas and a liquid

and is a special case of solubility, as the compound is

soluble in both water and in air ( Spencer and Witco 2008 ).

4.6 Hydraulic loading rates

Liquid mass loading rate is defined as the mass of con-

taminant entering the stripper per unit area. An air strip-

per may be operated under high or low liquid loading

rates, but low hydraulic loading may cause low mass

transfer efficiency (Berenek 2001). The advantage of high

loading rate is that it results in shorter packing depths

and faster remediation. However, the operating cost may

escalate because of the need to use high-performance,

costly packing materials. Most countercurrent packed

towers used for air stripping in environmental applica-

tions are generally designed to operate under low liquid

loading and gas rates ( Nirmalakhandan et al. 1993 ). For a

given packing material, the transfer efficiency of a given

chemical increases with the air and water loading rates.

The packing while providing the surface area for mass

transfer also offers resistance to flow, resulting in a pres-

sure built up in the gas phase, and this increases with the

increasing air and water loading rates. The loading rates




should therefore be chosen to maximize mass transfer

while keeping the pressure gradient under nominal levels.

Excessive loading prevents water from flowing down,

resulting in the accumulation of the water at the top of

the packing called “ flooding ” . An exponential increase in

pressure gradient is usually observed when loading rates

approach the flooding limit and the air loading rate at this

point is called “ flooding velocity ” . The opposite situation

occurs when the air loading rate is far above the liquid

loading rate, resulting in the entrainment of the liquid.

The pressure drop in the tower should be between 200 and

400 N/m 2 per meter of the tower to height to avoid flood-

ing ( Srinivasan et al. 2011 ).

5 Mathematical models for a packed column air stripping process

The complexity and high cost of direct plant operation

prompted the development of models to study system

behavior. According to Djebbar and Narbaitz (2002) , pilot-

scale studies are a relatively expensive method of deter-

mining the overall mass transfer coefficient ( K L a ) in an

air stripping system. The simulation of models enhances

improved estimation of system variables and examines the

effects of process changes on the efficiency of the air strip-

ping system ( Mourad et al. 2012 ). The use of a mathemati-

cal model can therefore be said to be a complementary

approach to laboratory and pilot plant experimentation,

since it can simulate the dynamic behavior of the air strip-

ping process and can select the optimum process design

( Melcer 1994 ). Mathematical models that effectively

describe the air stripping of VOCs in packed columns are

useful tools for the design and scaleup of an air stripping

process, with predictive models being of special interest to

assess process feasibility preliminarily and/or to validate

experimental data for process scaleup.

Modeling water treatment method has been taken

much consideration by companies (suppliers) in the last

decades, while many researchers have worked on prepared

models. For example, the Swedish Water and Wastewa-

ter Association developed the quantitative microbial risk

assessment (QMRA) modeling tool to investigate a prob-

ably waterborne outbreak of calicivirus that occurred in

the municipality of Lilla Edet, Sweden, after a period with

heavy rain ( Heinicke et al. 2009 ). Elias et al. (1996) also

developed a helpful tool for the study of anaerobic biolog-

ical reactors in order to optimize production of methane

and the quality of the treated wastewater. Zhang and

Cheng (2000) , on the contrary, developed a mathematical

model to describe the removal of cyanogen chloride from

a gas stream passing through a bed packed with activated

carbon impregnated with copper, chromium, and silver.

Lee et al. (2000) also developed a model to predict the per-

formance of fixed-bed absorbers.

Different types of models have been developed over

the years in the area of packed column air strippers. They

include model equations that describe the VOC removal

mechanisms as presented in the “ Theory of air stripping ”

section with differing degrees of refinement relating to

mass transfer assumptions and methods of calculating

rate coefficients ( Melcer 1994 ); models for the prediction

of the system parameters such as mass transfer coeffi-

cient ( Robert et al. 1985 , Djebbar and Narbaitz 2002 ); and

models for design, cost, and performance assessment

( Dzombak et al. 1991 , Lo and Alok 2000 , Mourad et al.

2012 ).

5.1 Models for the prediction of mass transfer coefficient

A packed column air stripper differs from other types of

stripper due to the presence of the packing materials,

which increase the interfacial surface area between the

liquid (water) and gas (air) phase and enhance the inter-

phase mass transfer of contaminant from water to air

( McKinney and Lin 1996 ). There are a number of design

equations that can be used to predict the performance

of packed column air strippers. A very important design

variable, utilized in the majority of these equations, is the

mass transfer coefficient ( Djebbar and Narbaitz 2002 ). This

coefficient is a function of the compound to be stripped,

A/W ratio, air and water temperature, hydraulic loading

rates, types and size of packing material, size depth and

diameter, gas pressure drop, and Henry ’ s constant of the

contaminant ( McKinney and Lin 1996 , Djebbar and Nar-

baitz 2002 ). Moreover, the A/W ratio of VOCs depends on

Henry ’ s constant and the hydraulic stability of the column

( Mourad et al. 2012 ).

The overall resistance to interphase mass transfer can

be considered as the sum of the two resistances, a gas- and

liquid-phase resistance referred to as the two-resistance

theory. This can be represented as

1 1 1

L L GK a K a Hk a

= +

(15)

where K L a is the overall (liquid-phase-based) mass trans-

fer coefficient and H is the Henry ’ s law constant expressed

as the ratio of the VOC mass concentration in the gas




phase to that in the liquid phase ( Djebbar and Narbaitz

2002 ). If the gas-phase resistance can be considered suf-

ficiently small to be neglected, then the transfer is liquid

phase controlled ( Robert et al. 1985 ).

Three most popular models for predicting mass

transfer coefficient are those of Sherwood and Hollo-

way, Shulman et al., and Onda et al. ( Robert et al. 1985 ,

Djebbar and Narbaitz 2002 , Kim and Deshusses 2008 ).

The Sherwood-Holloway model presumes that the liq-

uid-phase resistance controls, thus neglecting the gas-

phase resistance; it is the model most widely used to

date in the studies of VOC stripping for water treatment.

The models of Shulman et al. and Onda et al. evaluate

the gas-phase resistance as well as the liquid-phase

resistance and hence qualify as two-resistance models

( Robert et al. 1985 , Djebbar and Narbaitz 2002 ). Onda ’ s

correlations are known for their good fit with experimen-

tal data and have been recommended by many chemi-

cal engineering handbooks ( Kim and Deshusses 2008 ).

However, Onda ’ s correlations were developed from only

a few plastic packing materials with limited sizes, which

restricted their applicability to a few packings and limited

their accuracy to about 20% ( Djebbar and Narbaitz 2002 ,

Kim and Deshusses 2008 ). To overcome this, Djebbar and

Narbaitz (2002) developed a neural network (NN) model

that was able to simulate the sudden increase in K L a at

high gas loading rates. Also, it simulated more realisti-

cally the effect of the packing depth and liquid flow with

an absolute error of < 19%. In another research, Kim and

Deshusses (2008) developed correlations that allow the

determination of gas film mass transfer coefficients and

liquid film mass transfer coefficients for packing materi-

als used in biofilters and biotrickling filters for air pol-

lution control. In their research, lava rock, polyurethane

foam (PUF) cubes, pall rings, porous ceramic beads,

porous ceramic Raschig rings, and various compost-

woodchips mixtures were used as packing material, and it

was observed that most of the fitted data fell within ± 20%

of the experimental values.

Robert et al. (1985) evaluated two-resistance models

for air stripping of volatile organic contaminants in a coun-

tercurrent packed column air stripper. In their research,

mass transfer of six volatile organic solutes was studied

to assess the validity of previously proposed, generalized

correlations for predicting the stripping behavior of trace

organic contaminants. The assumption of liquid-phase

control of the transfer rate was confirmed for most volatile

solutes, oxygen, and dichlorodifluoromethane as against

moderately volatile solutes such as carbon tetrachloride,

tetrachloroethylene, trichloroethylene, and chloroform;

the gas-phase resistance was found to be affected by both

gas- and liquid-phase resistance. The Onda model pre-

dicted the transfer rate constants within an average stand-

ard deviation of 21%. Also, a comparison of empirically

generated mass transfer coefficient using Onda correla-

tion to actual mass transfer coefficients in an air stripping

tower for the removal of dissolved gasoline constituents

from groundwater by Wolf et al. (1989) gave 31% standard

deviation.

5.2 Models for the design, cost, and performance assessment

Mourad et al. (2012) developed a trace organic treatment

model (TOTM) by reviewing and adopting the available

analysis and design procedure for a packed column coun-

tercurrent air stripper such as the Onda correlation. The

model considered some regulated trace organic contami-

nants, including the maximum allowable concentration

levels (MCL) and some of their physical and chemical

properties. The model built using visual basic was devel-

oped to be user-friendly considering all possible con-

taminant characteristics, initial concentrations, final

concentrations, and method of treatment. This software

gives the user properties of contaminants and available

packing materials. It was successfully used to investi-

gate the effect of temperature and packing material on

air stripper volume ( Mourad et al. 2012 ). To predict water

treatment cost, the Best Available Technology Evaluator

(BATE) was developed by Lo and Alok (2000) , which can

be used to evaluate the cost and performance of the air

strippers.

Moreover, Air Stripper Design and Costing (ASDC) is

another microcomputer-based program written in C lan-

guage for air stripper design and cost optimization. This

model has features that enable the modification of some

cost factors such as packing material unit cost and elec-

tricity rate. The adjustments for inflation are also included

in the program ( Dzombak et al. 1991 ).

David W. Hand of the National Center for Clean Indus-

trial and Treatment Technologies (CenCITT) of Michigan

Technological University also developed a computer-

based model named Aeration System Analysis Program

(ASAP ™ ) (ASAP ™ 2013). The mathematical models con-

tained in ASAP ™ can be used to

– Assess the preliminary design and feasibility of using

air stripping processes,

– Plan pilot plant studies and interpret their results,

and

– Provide process design when site-specific model

parameters are available.




Also, ASAP ™ has the ability to provide a value of the

overall mass transfer coefficient, K L a , through the use of

the Onda correlation with a factor of safety.

The ASAP ™ Packed Tower Aeration Model is designed

to predict the performance of countercurrent packed

tower air strippers. This model calculates the removal effi-

ciency using several simplifying assumptions, including

steady-state, plug-flow reactor conditions for both air and

water streams, clean influent air stream, and equilibrium

of contaminant concentrations in air and water phases, as

described by Henry ’ s law. The model can be used to assess

the preliminary design and feasibility of air stripping pro-

cesses, plan pilot-scale studies, or interpret pilot-scale

results ( NWRI 2006 ).

The Iowa State University also developed a computer-

based program named AirStrip v.1.2, which uses the mass

transfer correlations of Onda et al. to design and rate air

strippers (Raschig Jaeger Technologies 2006). This model

was tested for the removal of MTBE by Butillo et al. (1994) .

This was done to evaluate the effectiveness of raising

Table 4 Summary of packed column air stripper models.

Name Developers/Source Description References

TOTM Khaldoon Mourad and Ronny Berndtsson

(Lund University, Lund, Sweden 22100)

and Wail Abu-El-Sha ’ r and Abdalla M.

Qudah (Jordan University of Science and

Technology, Irbid 22110, Jordan)

TOTM is packed column countercurrent air

stripper model based on Onda correlation

built using Visual Basic. This software gives

the user properties of contaminants and

available packing materials and was used

to investigate the effect of temperature and

packing material on air stripper volume.

Mourad et al. 2012

BATE Irenem C. Lo (Department of Civil

Engineering, The Hong Kong University of

Science and Technology, Clear Water Bay,

Hong Kong, P.R. China) and Pota A. Alok

(Department of Civil Engineering, Kansas

State University, Manhattan, USA)

BATE has been developed for evaluating the

cost and performance of the best available

technologies for removing VOCs from

drinking

Lo and Alok 2000

ASAP ™ David W. Hand, Ph.D., CenCITT, Michigan

Technological University, 1400 Townsend

Drive, Houghton, MI 49931, USA

ASAP ™ Features: Hand 1991

– Extensive vendor tower packing database

– Design and rating mode options

– Henry ’ s law database and parameter

estimation methods

– Physical and chemical properties database

including compounds from USEPA ’ s Title III

Consolidated Chemical List

NN prediction of

K L a (NN model)

Y. Djebbar (Senior Project Engineer,

Greater Vancouver Regional District, 4330

Kingsway, Burnaby, BC, Canada V5H 4G8;

e-mail: [email protected]) and R.M.

Narbaitz (Professor, University of Ottawa,

Civil Engineering Department, Ottawa, ON,

Canada KIN 6N5)

The NN model can simulate the sudden

increase in K L a at high gas loading rates and

to simulate the effect of the packing depth

and liquid flow on K L a

Djebbar and Narbaitz

2002

ASDC D.A. Dzombak, H. Fang, and S.B. Roy,

Department of Civil and Environmental

Engineering, Carnegie Mellon University,

Pittsburgh, PA 15213, USA

Interactive, microcomputer-based program

written in C language for air stripper design

and cost optimization

Dzombak et al. 1991

AirStrip v.1.2 Iowa State University AirStrip v.1.2 is a computer program that

uses the mass transfer correlations of Onda

et al. to design and rate air strippers

Raschig Jaeger

Technologies 2006

Development mass

transfer coefficient

for different

packing materials

S. Kim and M.A. Deshusses Correlations that allow the determination of

gas film mass transfer coefficients and liquid

film mass transfer coefficients for different

packing materials used in biofilters and

biotrickling filters for air pollution control

Kim and Deshusses

2008




the influent water temperature to increase stripping

efficiency. For the elevated temperatures, the computer

model predicted the following MTBE removal efficiencies:

99.41%, 99.78%, and 100% at temperatures of 65 ° F, 80 ° F,

and 100 ° F, respectively. However, the laboratory ana-

lytical result of effluent water sample indicates an MTBE

removal efficiency of 99.98%. This shows a good predic-

tion, although the actual system operated with efficiency

greater than the engineered model.

In another research to assess the performance of air

stripping to remove MTBE from contaminated groundwa-

ter, the California MTBE Research Partnership undertook

this project to assess the accuracy of several available

models used to predict the cost and performance of packed

tower and low profile air strippers. Data from nine case

study sites operating during the late 1990s were obtained

and analyzed. Two models were chosen for evaluation:

the ASAP ™ and Packed Tower Model and the North East

Environmental Products (NEEP) ShallowTray ® Modeler

software. Commercially available models were found to

predict actual removal efficiencies within 15%, demon-

strating that modeling can be a valuable tool for assess-

ing air stripper cost and performance during conceptual

design or remedy selection ( NWRI 2006 ).

A summary of the various packed column air stripper

models is shown in Table 4 .

6 Conclusion The packed column air stripper is a proven, effective means

of removing VOCs from groundwater and wastewater. It

has been used in many sites, either alone or in conjunc-

tion with other methods with effective results. As shown

in the present review, extensive experimental and theoret-

ical researches have been carried out by a lot of research-

ers, and a large volume of knowledge on the application

of a packed column air stripper in the removal of VOCs

from wastewater has been collected. Various models have

been developed to determine the overall mass transfer

coefficient. Empirically generated and actual mass trans-

fer coefficients in air stripping towers for the removal of

various compounds are available. In addition, different air

stripper design software have been developed. However, a

knowledge gap still exists in the synthesis of this knowl-

edge to formulate practically applicable mathematical

relationships for the general description of the process.

Mathematical models capable of effectively describing

the air stripping of VOCs in packed columns are useful

tools for the design and scaleup of air stripping process.

Predictive models make it possible to assess process fea-

sibility preliminarily and/or to validate experimental data

for process scaleup. Further attention is therefore required

in the area of air stripper performance optimization. There

has been no research on the development of a mathemati-

cal model and optimization of an air stripper using statis-

tical experimental design method. Such a determination

is critical to the understanding of the interactive effect

of process variables such as temperature, A/W ratio, and

height of packing on an air stripper performance.

Acknowledgments: The authors would like to express

their sincere appreciation to Universiti Teknologi Malaysia

(UTM) for the financial support under the exploratory

research grant scheme (ERGS) and international doctor-

ate fellowship.

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Mohammed Evuti Abdullahi obtained his first degree in Chemical

Engineering from the Ahmadu Bello University, Zaria-Nigeria, in

1994 and his Master ’ s degree in Chemical Engineering from the

Federal University of Technology, Minna-Nigeria, in 2005. After

9 years of experience as a lecturer in the polytechnic sector, he

moved to the University of Abuja-Nigeria in 2008. He is presently

a PhD research fellow in the Faculty of Chemical Engineering,

Universiti Teknologi Malaysia (UTM). His research interest is in the

area of wastewater treatment and environmental pollution control.

He is a registered engineer and has many academic publications to

his credit.

Mohd Ariffin Abu Hassan is an Associate Professor in the Faculty of

Chemical Engineering in UTM. He obtained his first degree in Chemi-

cal Engineering from UTM and was awarded the ESSO medal. He

performed his Master ’ s degree in Engineering Management at UTM

in 1998 and obtained his PhD from UMIST, Environmental Technol-

ogy Centre, Chemical Engineering Department in 2004. His research

interest is in the area of wastewater treatment.




Zainura Zainon Noor started her career at UTM in 1999 as a research

officer in the Chemical Engineering Pilot Plant. A well-trained chemi-

cal engineer specializing in environmental engineering, Dr. Zainura

has established and strengthened her expertise in Green Technol-

ogy, which includes cleaner production, life cycle assessment (LCA),

water and carbon footprints, and greenhouse gas inventory and pro-

jection as well as sustainable development. She is an accomplished

project manager and is currently leading the Green Technology

Research Group (Green Tech RG) at one of UTM ’ s prominent centers

of excellence, the Institute of Water and Environmental Management

(IPASA). She is now an Associate Professor in the Faculty of Chemi-

cal Engineering, UTM.

Raja Kamarulzaman Raja Ibrahim is currently a senior lecturer in

the Faculty of Science and an associate researcher at the Advanced

Photonic Science Institute in UTM. He obtained his first degree in

Physics from UTM in 2002 and his Master ’ s degree in Optoelectron-

ics from the University of Southampton, UK, in 2005. He obtained

his PhD in Chemical Engineering and Analytical Science from the

University of Manchester, UK, in 2012 for his work on mid-infrared

diagnostics of the gas phase in nonthermal plasma applications.

His research interests include the development of optical sensor

systems for various applications and the development of nonther-

mal plasma reactors for environmental pollution applications and

gas-phase analysis using optical techniques.



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