2.0 air stripping

23

2.0 Air Stripping

Andrew Stocking, P.E.

Hinrich Eylers, Ph.D.

Michael Wooden

Terri Herson

Michael Kavanaugh, Ph.D., P.E.

2.1 Background

2.1.1 Air Stripping Application for MTBE Removal from Drinking Water

Although MTBE has been commercially used in the United States since 1979, there arerelatively few cases where drinking water supplies have been contaminated with MTBE atconcentrations requiring treatment. Based on a limited review of such cases, only two majorpublic water supplies have been identified where MTBE treatment has been implementedand the effluent from the treatment system is used as drinking water: Rockaway Township,New Jersey and LaCrosse, Kansas.

In Rockaway Township, the groundwater was contaminated with several volatile organiccompounds (VOCs), including MTBE. Initially, GAC was used, but costs were excessive(McKinnon and Dyksen, 1984). Subsequent modifications to the treatment system includedusing a packed tower air stripper prior to GAC polishing. The combined treatment process ofair stripping followed by liquid phase GAC reduced initial MTBE concentrations atapproximately 96 µg/L to below detection limits (approximately 5 µg/L). The volumetricair/water ratio used in the air stripping system was 200.

In LaCrosse, Kansas, influent MTBE concentrations were as high as 900 µg/L. The treatmentsystem, which went into operation in 1997, consists of two packed tower air strippersoperated in series with an air/water ratio of 175 in each tower. The first air stripping towertypically reduces MTBE concentrations by approximately 90 percent and the second towerhas consistently removed any remaining MTBE to below the treatment goal of 10 µg/L andcommonly much further (detection limit is 0.2 µg/L). Appendix 2B provides a detaileddescription of the LaCrosse, Kansas facility as well as some operating data.

In these two cases, MTBE is being successfully removed from drinking water using a packedtower air stripper, although the air/water ratios (based on a volumetric basis) are relativelyhigh. For comparison, the removal of trichloroethylene (TCE) in drinking water applicationstypically requires a much lower air/water ratio of less than 30.

Air stripping technologies are widely used for removing halogenated VOCs from drinkingwater supplies prior to distribution and use of the water for public consumption. Packedtower aeration is the most common air stripping technology for drinking water treatment.Packed tower aeration is a well-understood and proven technology (Roberts et al., 1985;Kavanaugh and Trussel, 1980), and there are many equipment vendors and packing manu-facturers who provide the external and internal components for packed tower systems(Lamarre and Shearhouse, 1996). Other air stripping technologies have been used, butprimarily at low flow rates (<100 gpm) or in a remediation context. Other air strippingtechnologies include spray towers, bubble aerators, low profile aerators, surface aerators, andaspiration or centrifugal aeration devices.

25

2.1.2 Objectives of the Evaluation

Although air stripping using a packed tower is a widely used technology for VOCs, itsapplication to MTBE removal from drinking water has been limited. In addition, this tech-nology has several potential disadvantages that may limit its widespread use in drinking waterapplications. These limitations include: increased costs due to the potential need for off-gastreatment; delay due to permitting; aesthetic constraints due to tower heights; and someconcerns over mechanical reliability. Also, the use of packed tower air strippers brings thegroundwater in contact with the atmosphere, which may add other contaminants to the water.

Consequently, the objectives of this chapter are to review air stripping and off-gas treatmenttechnologies commonly used in drinking water applications and determine the most cost-effective and reliable air stripping system option for MTBE removal from drinking water.This chapter will review the technologies available for air stripping and off-gas treatment anddiscuss each technology in terms of cost-effectiveness, reliability, and ease ofimplementation. The chapter will then conclude with recommendations for air stripping andoff-gas treatment combinations for a variety of water treatment scenarios.

26

2.2 Description of Technology - Air Stripping

2.2.1 Background

As previously mentioned in Chapter 1, MTBE is a polar chemical and, as a result, is relativelysoluble in water (48,000 mg/L at 20°C). MTBE has a relatively low Henry’s constant and,thus, requires a higher air/water ratio in air stripping towers compared to the requiredair/water ratio for benzene or TCE. However, air stripping has been effectively used toremove MTBE from water, particularly from dilute solutions (i.e., less than 1 mg/L) that aretypically associated with groundwater contamination from leaking underground fuel tanks(LUFTs) (Creek and Davidson, 1998).

2.2.2 Process Principles

The effectiveness of air stripping technologies to remove organic contaminants from waterdepends upon the volatility of the compound from water and the physical design of the airstripping technology. Air stripping relies on an equilibrium phase transfer process where thecontaminant partitions between the aqueous phase and the air phase. The equilibriumpartitioning coefficient is called the Henry’s constant which, in dilute solutions, isdetermined by Raoult’s law using the vapor pressure of the pure compound and its watersolubility. In general, the higher the Henry’s constant for a contaminant, the more effectiveair stripping will be for that contaminant.

The Henry’s constant for MTBE was recently reported to range from 0.018 to 0.122(dimensionless) at 20°C (OSTP, 1997); however, the Henry’s constant is typically thought tobe closer to the lower end of this range. Figure 2-1 reports various Henry’s constants forMTBE as a function of temperature, as compiled by Paul Sun of Equilon Enterprises, L.L.C.(Sun, 1998). At 20°C, MTBE’s Henry’s constant is approximately 0.022. This value is severaltimes lower than Henry’s constants for common organic compounds found in groundwatersuch as TCE, PCE, or benzene. Thus, air stripping of MTBE is generally more difficult andmore costly than removal of these other compounds. The effectiveness of air strippingorganic compounds from water can be improved by raising the water temperature which, inturn, increases the Henry’s constant. However, unless the heat is free waste heat from anotherprocess (e.g., thermal off-gas treatment), heating the water is expected to be cost-prohibitive.

The low Henry’s constant of MTBE may require a larger air stripping system to achieve thedesired removal efficiency; however, air stripping may still be cost-effective relative to othertechnologies for MTBE treatment. One significant issue is the design of the contactingsystem between the contaminated water and the air used to strip out the organic compounds.In general, the goal is to maximize the extent of contact (maximum rate of mixing, highestspecific surface area) while minimizing energy costs associated with the equipment design.This provides the highest rate of mass transfer from water to air at the lowest operating cost.The most common mass transfer design for air stripping systems is the use of randomly

27

packed towers. Other options include spray towers, low profile units, bubble diffusers,aspirators, and surface aerators. Selection of the appropriate technology is often site-specific.

Figure 2-1. Estimated vs. experimental data for MTBE’s Henry’s constant as a function of temperature.

The following sections review the major established and emerging air stripping technologiesapplicable to MTBE treatment in drinking water applications. For a review of design issues,the reader is referred to standard textbooks on air stripping systems (Montgomery, 1985).

2.2.3 Aeration Technologies

Several types of air stripper technologies are currently available for removal of MTBE fromwater. This evaluation focuses on the capabilities and limitations of several major establishedand emerging air stripper technologies that are potentially applicable for MTBE removal.Applicability is evaluated based on the performance reported in engineering literature,vendor information, and professional experience with the equipment. Air strippertechnologies evaluated in this report include:

• Packed Tower Aeration

• Low Profile Aeration

• Bubble Diffusion Aeration

28

Shell EstimateCalgon DataWTC-BrutcherWTC-TangWTC-RoddenIT dataWTC-WilcoxMcKay Data

0.004

0.003

0.002

0.001

0.00010 15 20 25 30 35 40 45 50 55 60

Hen

ry's

Law

Con

stan

t for

MT

BE

, Atm

M3 M

ol

Temperature in °CKeq = Mol/L-gas / Mol/L-Liquid) = Hc / 0.0000802/(273 + °C)

H = 0.00052*(293T)*10 2500*(1/293-1T)

T = Temperature in °K

• Spray Towers

• Aspiration

A brief description of each technology, including its advantages and disadvantages, ispresented in Table 2-1. A detailed discussion of each technology follows.

Packed Tower Aeration

• System Description

In a packed tower, contaminated water flows downward by gravity through a circular orrectangular column that is filled with either randomly packed or structured packing material.Air is introduced into the tower below the packed bed and flows upward through the columncountercurrent to the flow of water. The air can either be dispersed into the bottom of thecolumn (forced draft system) or drawn out of the top of the column (induced draft system).

The packing material is designed to maximize available specific surface area for contactbetween the contaminated water and the process air, thereby providing the maximum specificsurface area possible for volatile contaminants to move from the liquid phase to the gasphase. Initial distribution of the influent water and process air over the entire cross section ofthe column is usually accomplished by vented orifice trays, influent troughs, or a spraynozzle header system. Of the three options, an orifice tray distributes air and water mosteffectively across the entire cross-sectional area of the column.

The mathematics of packed tower aeration have been studied extensively and can bedescribed relatively accurately by various correlations (e.g., Onda equations). While notpresented in this text, full mathematical explanations are available elsewhere (Perry et al.,1984).

An illustration of a packed tower system manufactured by Carbonair® EnvironmentalSystems, Inc. (New Hope, MN) is shown in Figure 2-2.

29

Table 2-1D

escription of Air S

tripping Treatment Technologies and T

heir Advantages and D

isadvantages

30

Air StrippingTechnology

Packed Tower

Low Profile

BubbleAeration

Spray Tower

Aspiration

BriefDescription

Water trickles downward over packingmedia, creating a thin film. These thinfilms of water are met by a counterflow of air blowing in from the bottomof the tower.

A series of stacked perforated trayswith countercurrent air streamsuspends water and volatilizescontaminants.

Rising bubbles and turbulenceprovide the air/water interface neededfor stripping without the need forpacking or media.

Water is sprayed downward throughnozzles into a collection sump. Air isblown upward counter currently to thewater and exits through a demister atthe top of the column.

A Venturi stripper uses highlyturbulent jets of water to shear andaccelerate fluid films within an openbore.

SystemComponents

• Supply Pumps• Air Blower• Influent and

Discharge Pipes


Discharge Pipes


Discharge Pipes


Discharge Pipes


Discharge Pipes

Advantages

• High flow capacity.• Removes difficult to strip compounds.• Low liquid pressure drop.• Proven technology.

• Compact (low profile).• Easily installed and maintained.• Proven technology.

• High liquid and air turndown ratio.• Simple device, low maintenance.• Short set-up time.• Low potential for fouling.

• Low pressure drop for gas(low blower cost).

• Simple operation (no mechanical parts).• Compact - low profile.

• Compact and low profile.• No significant problems with misting,

freeze up or slime growth.• Installation and operation can be easily

staged; units can be installed or removed.• Removal efficiency independent of air

temperature.• Low off-gas volume.

Disadvantages

• Fouling results in loss of efficiency, andincreased pressure drop.

• High gas pressure drop.• Transportation/set-up more complex

than low-profile systems.• Channeling of water through packing

may short-circuit treatment.• Highly visible (profile)

• Multiple units typically required forflows >100 gpm.

• Scale formation dramatically decreasestreatment efficiency.

• High gas-pressure drop requires highhorsepower blower.

• High removal efficiencies requiremultiple units.

• High pressure drop for liquids.• Packing may be required for MTBE

removal (subject to fouling).• Fouling of nozzle may reduce treatment

efficiency.• Low turndown ratio (unless nozzle is

changed).• May be high profile.

• Increased operating cost due to higherenergy demands.

• Large footprint for high (> 90%)removal efficiency and moderate flows(> 100 gpm).

Figure 2-2. An illustration of a packed tower system manufactured by Carbonair® (1995).

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OFF-GAS

INFLUENTDISTRIBUTOR

PACKING MEDIA

OPTIONALINFLUENT RISER PIPE

PACKING SUPPORTBLOWER

ANSI FLANGE(EFFLUENT)SIGHT GLASS

MANWAY

ANSI FLANGE(INFLUENT)

• Advantages/Disadvantages

Table 2-1 describes the primary advantages and disadvantages of various air strippingsystems. Additional advantages of packed tower systems are listed as follows (Lenzo, 1985;Lenzo, 1994):

• Packed towers are among the most widely implemented VOC removal systems availabletoday. They are commonly custom manufactured to meet the specific requirements of eachapplication, although it is also possible to buy an off-the-shelf system.

• Packed towers have been used successfully to remove MTBE from water in drinking waterapplications.

• Computer models are available to design and optimize packed tower air strippers.

• Manufacturers often provide proprietary design programs based on a database of empiricalperformance data that take into account non-ideal flow and the impacts of water quality oncost. Thus, packed tower air stripping can be designed to achieve a high degree of processreliability.

One disadvantage, however, is that operation and maintenance of packed towers can beimpacted by characteristics of the influent water unrelated to the contaminants of concern.Four common operational problems and their mitigation measures are as follows:

Corrosion. Contact between the water and the aluminum or steel tower can lead to corrosion,which weakens the tower frame and necessitates tower replacement. This can be avoided bycoating the aluminum or steel with epoxy or by using stainless steel or fiberglass reinforcedplastic (FRP) instead. However, FRP is less durable and becomes more prone to biologicalfouling as the ultraviolet-inhibiting materials within the FRP degrade.

Scaling. The contact between contaminated water and the air stream in a packed towertypically results in pH increases during treatment. In cases of high influent water hardness,scaling (i.e., precipitation of calcium carbonate or calcium sulfate onto the packing media,column internal structures, and effluent piping) can occur. Scaling can be reduced orprevented by lowering the influent water pH using acid feed or by injecting anti-scalingchemicals into the influent. Periodic cleaning (e.g. with acid solutions) of the packing media,internal structures, and piping and/or replacement of the packing material may be required ifscale prevention measures are not adequately employed in normal operation (Snoeyink andJenkins, 1980; Lenzo, 1994).

Iron Fouling. Groundwater is often low in dissolved oxygen and, therefore, contains ironmainly in the Fe2+ (ferrous iron) oxidation state. When ferrous iron comes in contact withoxygen during the aeration process, it is oxidized to ferric iron (Fe3+) which forms an

32

insoluble precipitate that will lead to fouling. Prevention of iron fouling in packed columnsis more difficult than prevention of scaling. In the most severe situations, periodic packingreplacement may be required.

Biological Fouling (Biofouling). If the packed tower internals are exposed to light and theinfluent water contains sufficient organic matter to sustain microbial growth, packedcolumns may be subject to biological fouling due to bio-growth or algae formation.Biofouling can be prevented by injection of a disinfectant (e.g., sodium hypochlorite) to thecolumn influent stream, but this is limited by the need to minimize disinfection by-productssuch as trihalomethanes (THMs).

It is important to note that the operational problems associated with corrosion, scaling, ironfouling, and biofouling also apply to the other air stripper technologies that are described below.However, the extent to which individual technologies are vulnerable to these factors may vary.

Other disadvantages of a packed tower include:

• Short-circuiting due to poor water or air distribution, which can limit the system’smaximum removal efficiency.

• Aesthetic concerns due to high visibility of packed towers.

• Key Variables/Design Parameters

The removal efficiency of organic contaminants by packed towers is a function of manyparameters (see Table 2-2). Manufacturers typically provide cylindrical towers with a limitedselection of diameters. Economic considerations determine the trade-off between tower volumeand air/water ratio as a function of standard air pressure drop and a given packing media.Because the tower volume directly affects capital costs, design optimization involves mini-mizing tower volume at a pressure drop that minimizes energy requirements. In any givenapplication, the optimal liquid loading rate, packing height, and air/water ratio will be functionsof site-specific characteristics of influent water quality, required VOC removal efficiencies,operational considerations, and economics as well as aesthetic concerns (see Section 2.6 forfurther discussion of system optimization). In addition, Table 2-2 shows the effects ofincreasing various parameters on the removal efficiency and cost (assuming a fixed towervolume, height, and packing) and the design of the packed tower (assuming a fixed removalefficiency). For example, in a groundwater treatment application, for a given tower design(fixed packing type, diameter, and height), increasing the water pumping rate to meet waterdemands will increase liquid loading. This causes a decrease in the air/water ratio, resulting ina decrease in removal efficiency and an increase in operating costs due to the greater volumeof air required to meet the target removal efficiency. Similarly, while raising the influent watertemperature will decrease the required tower volume for a given removal efficiency, it will also

33

increase operating costs substantially. In the case of tower design, the higher the design loadingrate, the greater the tower height needed to achieve design removal efficiencies.

• System Installations and Manufacturers

Packed towers, operated in parallel or in series, remove a wide range of volatile organiccontaminants at many water treatment facilities in the United States at flow rates rangingfrom less than 1 million gallons per day (694 gpm) up to 20 million gallons per day (approxi-mately 13,900 gpm). As mentioned in Section 2.1.1, the effluent water from packed tower airstrippers has been used as drinking water in many treatment cases; notably, the MTBEremoval sites in LaCrosse, Kansas and Rockaway Township, New Jersey (see Appendix 2Bfor a detailed description of the LaCrosse, Kansas packed tower air stripping operation).

Packed towers with diameters up to 15 feet are manufactured from a number of materials,including plastic, fiberglass, aluminum, and steel, and are being used to remove a variety ofcontaminants. The largest manufacturer of pre-engineered systems is Layne ChristensenCompany (Bridgewater, NJ) with over 400 installations nationwide (approximately 280installations in drinking water facilities). Layne Christensen Company has installedapproximately 10 packed towers specifically designed for MTBE removal; these are installedat remediation sites and not at drinking water facilities. Tonka Equipment Company (Plymouth,MN) is the second largest manufacturer with approximately 100 installations, mostly in theMidwest. Carbonair (New Hope, MN) also has approximately 150 packed tower drinking

34

Table 2-2Packed Tower Design Variables

Parameter

Liquid Loading Rate

Air/Water Ratio (AWR)

Water Temperature

Henry’s Constant

Packing Type and Size

Pressure Drop / Depth

Effect of Increasing (Õ) Parameteron Operations and Cost, Assuming

no Change in Tower Design

Ô Removal EfficiencyÕ Cost

Õ Removal EfficiencyÕ Cost

Õ Removal EfficiencyÕ Heating CostÕ Henry’s Constant

Õ Removal Efficiency

Õ the Size Ô Removal Efficiency

Õ Removal EfficiencyÕ Pump/Blower Cost

Effect of Increasing (Õ) Parameteron Tower Design, Assuming Removal

Efficiency is Maintained

Õ Tower Height (HTU)

Ô Packing Volume

Ô Packing Volume

Ô Packing Volume Ô AWR

Õ the Size Õ Packing VolumeÔ Pressure Drop

Õ AWR

water installations. Appendix 2C lists the addresses of Layne Christensen Company, TonkaEquipment Company, Carbonair, and other air stripper equipment vendors referenced in thischapter. A variety of plastic packing media is available. Both structured packing media andrandomly packed media are commercially available in various types of plastics and in a widevariety of sizes and geometric configurations. Most of these media, especially the newergeneration of high efficiency packing, are proprietary products.

• Technical Implementability

A packed tower air stripper requires supply pumps (sized only to provide the static headrequired to move the influent water to the top of the unit), air blowers, and a single influentand discharge pipe. There are no moving parts (blowers only) and, therefore, there areminimal daily maintenance requirements. If fouling is a concern, a chemical feed system isrequired, necessitating additional maintenance. Construction of a packed tower requires arelatively heavy foundation to allow the installation of seismic braces and wind supports.Tower heights can be reduced by placing two or more towers in series, which mitigates towersusceptibility to strong winds or light seismic activity.

Low Profile Aeration


In a low profile aeration system, contaminated water is pumped to the top of the stripperwhere it flows over an inlet weir onto a baffled aeration tray. Air is forced up throughperforations in the tray bottom by either a forced-draft or an induced-draft blower, creatinghighly turbulent conditions to maximize the contact of water and air. Multiple trays may bevertically stacked, with water flowing from upper trays to lower trays via overflow weirs. Thecontact time necessary to achieve the desired VOC removal efficiency is provided byselection of the tray size, liquid flow rate, and number of stacked trays. The air is dischargedat the top of the aeration unit. A minimum air flow is required to prevent water from enteringthe aeration holes.

An illustration of a low profile system manufactured by Carbonair® Environmental Systems,Inc. (New Hope, MN) is shown in Figure 2-3.

35

Figure 2-3. An illustration of a low profile system manufactured by Carbonair® (1995).

36

AIR FLOW

EXHAUST AIR

FLOW METER

WATER TEMPERATURE

SAMPLE TAP

PUMP DOWN

BLOWER MUFFLER

AIR TEMPERATURE

SIGHT GLASS

LEVEL CONTROL


See Table 2-1 for a summary of the advantages and disadvantages of low profile aerators. Amore detailed listing of the advantages of low profile air strippers includes:

• Skid-mounted configuration that allows the units to be placed on a concrete pad or levelfloor surface by forklift. Also, the unit can be placed in a heated building for cold-weatheroperation.

• Limited mechanical connections — blower inlet, stack gas discharge, and process waterinfluent and discharge lines.

• Pre-packaged power and control panel for remote monitoring, and operation of pumps andblowers as well as operation of strippers in batch or continuous mode.

• Easy maintenance and cleaning of fouled low profile systems by removal of interlockingtrays followed by pressure washing with or without chemical (e.g., acid) cleaning.

• Availability of vendor computer models based on empirical field-scale and bench-scaledata that model non-ideal flow and water quality scenarios for design optimization.

Disadvantages of the low-profile air stripper include those listed previously in this section forpacked towers (i.e., corrosion, scaling, iron fouling, and biological fouling) in addition to(Malcolm Pirnie, Inc., 1992):

• Performance drop off with scale formation, as described for packed towers. If scaling causesthe perforated trays to seal, performance can drop off much more rapidly than packedtowers.

• Minimal air turndown capacity due to the extreme performance drop off if process wateris no longer suspended on the tray surface and instead falls through the tray perforations.

• Not always weatherproof and, therefore, may require an enclosure for protection.


Similar to a packed tower aeration system, the removal efficiency of a low profile aerationsystem is a function of many parameters, including water temperature, air/water flow ratio orcontact time, number of trays, contact time, and the volatility of the contaminant. The effectsof these parameters on the removal efficiency of a low profile system is similar to the caseof a packed tower system (Table 2-2). The optimal system configuration will depend on site-specific characteristics of the influent water, required VOC removal efficiencies, operationalconsiderations, and economics.

37


Low profile aeration technology is a widely proven air stripping process for drinking waterand non-drinking water applications. The two primary vendors of low profile air strippers areNorth East Environmental Products, Inc., (West Lebanon, NH), manufacturer of theShallowTray™ stripper, and Carbonair® Environmental Systems, Inc. (New Hope, MN),manufacturer of Carbonair STAT.® North East Environmental Products is the largest distri-butor with more than 4,000 units in operation across the country for removal of a variety ofcontaminants. Three hundred of these units are used for small-community and municipalwater treatment applications, and approximately 1,000 units are used as household point-of-entry treatment units. Individual low profile units are capable of removing MTBE from waterfor flow rates up to 1,100 gpm, and parallel units are used for flow rates above this level insystems with capacities up to several million gallons per day. However, the flow rates areusually much lower (<100 gpm) (Shearhouse, 1998). There are over 200 low-profileinstallations in use for MTBE removal in groundwater remediation applications and at leastfive low-profile installations where the treated water is potable (there was no specificinformation identified for potable water installations, but these are likely to be smallsystems). See Appendix 2B for a description of the low profile system initially used atLaCrosse, Kansas.


As with packed towers, low profile air strippers require supply pumps, air blowers, and asingle influent and discharge pipe. Aside from the blowers and pumps, there are no othermoving parts; therefore, maintenance requirements are minimal. A discharge pump may berequired to move the effluent water to the next stage of the process train (Lenzo, 1994). Lowprofile units are generally easy to dismantle and clean. The units are typically skid-mounted(resulting in easy installation) and require a small footprint or foundation.

Bubble Diffusion Aeration


In a diffused bubble aeration system, air is released through fine bubble diffusers at thebottom of a water-filled tank, which is usually divided into baffled stages. Rising bubblescreate turbulent mixing, which provide the air/water contact area necessary for contaminantstripping. If longer residence time is need to increase contaminant stripping, bubble aerationsystems can be designed with baffles to create multiple chambers in a single unit or multipleunits to increase the total water depth. MTBE removal efficiencies greater than 90 percentare expected to require the use of multiple units in series. Design variables requiring optimi-zation include the basin depth, number of aeration stages, air flow rate, and water flow rate.

38


See Table 2-1 for a summary of the advantages and disadvantages of this technology. A moredetailed listing of the advantages of bubble aeration systems includes:

• Considerably lower profiles (less than 6-feet high) than packed towers.

• Skid-mounted configuration that allows the units to be placed on a concrete pad or levelfloor surface by fork-lift. Also, the unit can be placed in a heated building for cold-weatheroperation.

• High liquid and air turndown ratio (i.e., the ability to lower the liquid or air flow rateswithout radically decreasing performance).

• Limited mechanical connections — blower inlet, stack gas discharge, and process waterinfluent and discharge lines.

• Pre-packaged power and control panel for remote monitoring, and operation of pumps andblowers as well as operation of strippers in batch or continuous mode.

• Easy maintenance and cleaning of fouled baffles by removal of interlocking trays followedby pressure washing or pressure washing in combination with chemical (e.g., acid) cleaning.

Disadvantages, as listed in Table 2-1, of the bubble aeration system include those listedpreviously in this section for packed towers (i.e., corrosion, scaling, iron fouling, andbiological fouling) in addition to:

• High removal efficiencies (>90 percent) for high flow (>100 gpm) require several units inseries.

• No known drinking water applications for MTBE removal.

• Smaller air/water contact area than low profile systems; less efficient for chemicals with alow Henry’s constant such as MTBE.

• Significant performance drop off with scale formation on the diffuser, as described forpacked towers.

• High gas-pressure drop requirements across the diffuser requiring a high-horsepowerblower.

• Not weatherproof and, therefore, may require an enclosure for protection.

39


The key variables and design parameters required to design a diffused bubble aeration systeminclude the anticipated water flow rate, influent contaminant concentrations, air and watertemperature, required air/water ratio, water quality, and target effluent concentration. Removalof an organic compound by a diffused bubble aeration system is described by the followingequation (Montgomery, 1985):

where QG is the gas (air) flow rate, H is the compound’s Henry’s constant, CL and CLo arethe effluent and influent liquid concentrations of the organic compound, KLa is the overallmass transfer constant, VL is the total liquid volume, QL is the liquid flow rate, and QG/QLis the air/water ratio. According to this equation, as the air/water ratio increases, CL/CLodecreases and the removal efficiency increases, as expected. The relationship shows that forMTBE, bubble aeration (assuming H =̃ 0.03) requires either very high air/water ratios (>300vol/vol) to achieve high removal efficiencies (>90 percent) or multiple units operated inseries. Thus, this technology is not likely to be useful for MTBE removal if packed towers orlow profile air strippers are available.


Diffused bubble aeration systems are manufactured by Lowry Engineering, Inc. of Unity,ME (The Stripper™), Aeromix Systems, Inc. of Minneapolis, MN (BREEZE™), and CarbtrolCorporation of Westport, CT. Major vendors for bubble aeration stripping systems are listedin Appendix 2C. In general, a simple bubble aeration unit may be effective for 90 percentMTBE removal at low flow rates (<60 gpm) and high air/water ratios (>300). For higher flowrates, multiple units are required, thereby increasing capital costs. According to vendors,there are a few diffused bubble aeration systems installed in the United States for MTBEremoval; however, none are in drinking water applications.


On the basis of effectiveness and implementability, this technology is similar to low profileair stripping systems. The skid-mounted configuration allows the units to be placed on aconcrete pad or level floor surface by forklift, with mechanical connections limited to blowerinlet, stack gas discharge, and process water influent/discharge lines. As with low profileunits, pre-packaged power and control panels are available to operate pumps and blowers.Maintenance includes routine diffuser cleaning and typical pump and blower maintenance.Cleaning to remove scale build-up — a maintenance requirement common to nearly all airstrippers — involves disconnecting the diffusers followed by acid cleaning/ soaking orpressure washing.

40

CLCLO

=1

1+ H 1-expKLaVL

HQL

QGQL [ ])(

Spray Towers


In a spray tower aeration system, contaminated water is passed through one or more nozzlesand sprayed into a collection basin or tank. Spray aeration systems are typically used fordegassing applications, although they have also been used to remove VOCs from water. Ingeneral, there are three types of spray towers: cocurrent, countercurrent, and cyclonesystems.

Cocurrent. In a cocurrent spray tower, the water-feeding nozzle sprays water in the samedirection as the air flow. This configuration is typically less efficient than the other types ofspray tower and, consequently, is rarely used. Cocurrent towers will not be discussed further.

Countercurrent. In countercurrent spray towers, there are water-feeding nozzles at the top ofthe tower and a collection sump at the bottom. Air enters the bottom and is blown in anupward direction. The gas exits through a demister at the top.

Cyclone. Cyclone spray towers have a tangential air inlet on the side along the unit’s base. Airtravels in a spiraling motion up the column and exits at the top. Water, sprayed uniformlyacross the column from a manifold in the upper half of the column, collects at the bottomand exits out a drain.


See Table 2-1 for a summary of the advantages and disadvantages of spray towers. Majoradvantages of the spray tower system include:

• Low pressure drop required for air loading resulting in a smaller blower and less powerused compared to packed towers or low profile aerators. This could result in significantcost savings for off-gas treatment. However, if the spray tower is filled with packing, as isrequired for MTBE and other low volatility VOCs, the air flow rate and pressure drop isthe same as for a packed tower.

• Simple operation, since there are few moving parts and few parameters to monitor.

• Short setup time for smaller pre-fabricated units, although larger custom systems requireextensive transport and installation efforts.

• Easily adaptable tank and equipment designs for varying flow rates.

41

The disadvantages of spray tower systems include:

• High-pressure drop required for liquid loading due to the need to spray a fine mist of waterto achieve the necessary air/water ratio for contaminant stripping, compared to packedtower or low profile aerators.

• Dramatic removal efficiency drop at higher flow rates. Packing is required to achieve >90percent removal efficiency for MTBE and other low volatility compounds, which makesthis essentially a packed tower.

• Possibility of nozzle fouling, resulting in increased pressure drop, and internal packingfouling, as discussed for packed tower aerators.

• Low liquid turndown ratio (unless nozzles are changed).

• No known MTBE drinking water applications.


The mass transfer rate in a spray tower system is a function of the influent water droplet size,turbulence in the column, and distribution of influent water droplets in the tower.

Droplet Size. As the droplet size decreases, the surface area to volume ratio for a givenvolume of water increases, resulting in greater mass transfer efficiency. Mass transfer isproportional to surface area for a given volume of water. The droplet size is a function of thenozzle design and water pressure.

Turbulence. The column will operate most efficiently at maximum nozzle flow (i.e., highestturbulence). Because turndown of the liquid flow is not recommended, the best way to adjustthe system for different influent water flow rates is to change the size of the feed nozzles.This will allow the operator to maintain a maximum nozzle flow rate and, thus, maximizeturbulence inside the column.

Distribution. The spray must be distributed evenly throughout the tower, with a minimum ofspray striking the walls. The nozzle must also maintain proper distribution of droplet sizes atthe maximum flow rate. The maximum total liquid loading should be in the range of 1 to 3gpm/ft2. The maximum gas flow rate is approximately 800 lb/hr ft2 and is limited by liquiddroplet entrainment (i.e., flooding) (Fleming, 1989).


Spray towers have been used to remove low concentrations of MTBE from water in only afew remediation cases and with limited success. Generally, packing material must be added

42

to the column to achieve good (90 to 99 percent) removal efficiencies. If packing is added,spray towers are essentially packed towers and, therefore, have the same fouling challengesas packed towers.

Manufacturers of spray towers are listed in Appendix 2C. While there are currently no knowndrinking water applications of spray towers for MTBE removal, this technology has beenused to remove other VOCs in drinking water situations.


The countercurrent spray tower is the most common design selected for spray towers becauseit allows for more than one stage and, thus, greater removal efficiencies than the cyclonetower, which is limited to only one stage. Furthermore, a countercurrent spray tower requiresless maintenance and a smaller footprint than a cyclone spray tower for the same flow rate.Like the other air stripper technologies, spray towers require supply pumps, air blowers, anda single influent and discharge pipe. If there is no packing, the only piece of equipment thatis susceptible to scaling is the spray nozzle, which can be easily replaced.

Aspiration


Aspiration or centrifugal stripping involves injection of the contaminated water into a co-current, tangential-flow aspirator. Untreated and/or recirculated water is pumped into a collarand then through multiple orifices into the throat of the aspirator. As the water passes throughthe orifices, the orifices act like turbulent jets, which create a large water surface area andenhance the rate of mass transfer of the VOCs from the water to the air. The configuration ofthe collar and the type and number of orifices in the aspirator are designed to create a lowair/water ratio, which ranges from 5:1 to 30:1 for each water pass. For high removalefficiencies, the treated water must be recirculated many times, thus creating a treatmentsystem with an overall air/water ratio greater than 100:1 (Dempsey and Ackerman, 1989).Figure 2-4 shows the various components of an aspiration system manufactured by HazletonEnvironmental.


See Table 2-1 for a summary of the advantages and disadvantages of aspiration. Additionaladvantages of the aspirator stripper include:

• Scaling does not occur, or occurs only to a limited extent due to the high turbulence withinthe unit.

43

• The cocurrent air flow rate is induced by the turbulent water jets; this creates a low volumeof off-gas requiring treatment.

• Misting, freezing, and decreased atmospheric temperature do not generally impactaspirator strippers.

• Aspirator strippers are easily installed and constructed with self-contained modular quick-connect units. They are also relatively non-intrusive with low visual and noise impact,unless high removal efficiencies are required.

Disadvantages of the aspirator stripper include:

• Relatively high operating costs due to high energy demands from the high water pressuredrop in aspirators (similar to spray towers) compared to low profile aeration systems.

• Limited maximum removal efficiency with one system at any flow rate; higher flow ratesand higher removal efficiencies require multiple systems in parallel with significant waterrecirculation and high costs.

• Large footprint for high removal efficiency.

44

Figure 2-4. Components of an aspiration system (Maxi-Strip®) manufactured byHazleton Environmental (1998).

Primary Air Inlet

Deflector Plate

Water Inlet 1

Secondary Air Inlet

Water Inlet 2

Coalescing Chamber

Discharge

Containment Nozzles

Stripping Chamber

Diverging Nozzles

Deflector Feed Nozzles


The key variables and design parameters required to design an aspirator system includeinfluent liquid flow rate, influent contaminant concentrations, water temperature, waterquality, and the target final effluent concentration. As with spray towers, the rate of masstransfer is a function of the droplet size, turbulence, and liquid distribution. The jets minimizethe droplet size and maximize air/water turbulence while the collar distributes the waterevenly across the diameter of the aspirator. A single aspirator stripper can support up to aflow of 500 gpm; larger flows will require several aspirators operating in parallel (HazletonEnvironmental, 2000). The number of modules is determined by the flow through the systemand the removal efficiency required. A single aspirator can only achieve approximately 18percent MTBE removal efficiency for a single pass; therefore, water must be recirculated toachieve higher removal efficiencies or units must be designed in series. This will increase thenumber of aspirators and the costs required to meet treatment objectives.


There are over 100 aspirator systems in place to remove VOCs (Hazleton Environmental,1998). Although some of these systems are being used for drinking water applications in theUnited States, there are no known installations specifically designed for MTBE removal. Alimited number of manufacturers produce aspirator systems (see Appendix 2C). Of these, theMaxi-Strip® hydraulic venturi stripper — manufactured by Hazleton Environmental(Hazelton, PA) and distributed by Onion Enterprises (Walnut Creek, CA) — appears to bethe most well-established.


The Maxi-Strip® system can be trailer mounted for easy transport to and from multiplelocations. Minimal site preparation is required for its installation. A quick connect system isavailable, requiring only influent supply and discharge piping. Multiple units can be easilyinstalled or removed as influent concentrations and effluent criteria change. The only movingparts are centrifugal pumps that require no special equipment or training. The cocurrent airflow design reduces off-gas volume and associated air treatment equipment. For ease ofoperation, system controls and logic are also available.

45

2.3 Comparative Discussion of Air Strippers

2.3.1 Permitting

In general, all of the air stripper systems described in Section 2.2 require state and munici-pality-specific permits for construction, air emissions, and process water discharges (seeTable 2-3a). Construction permits may not be necessary for smaller low-profile installations(i.e., low profile, bubble diffusion, and aspirator units) if they can be installed in an existingenclosed treatment facility. Air emissions permits are generally required for any discharge oforganic compounds to the atmosphere (see Table 2-3b). The air emissions standards will varyfrom region to region, but for purposes of comparison, this chapter has used the South CoastAir Quality Management District (South Coast AQMD) standard of 1 lb. VOC/day as themaximum allowable air emission rate. Due to this stringent requirement, ease of permittingwill be defined by the volume of the off-gas stream and the concentration of VOCs in thatgas stream for the various air stripping technologies. Process water discharges from any ofthe air stripping systems described above will be subject to nearly identical discharge permit-ting issues.

2.3.2 Flow Rate

Flow rate is a governing factor in determining the removal efficiency from a given air stripperdesign. A summary of typical maximum hydraulic capacities for each of the technologiesdescribed in Section 2.2 is presented in Table 2-4. Design of larger systems may be feasibleif a custom-designed unit is constructed. Low profile, bubble diffusers, and aspirators aretypically bought as pre-designed units, whereas spray towers and packed towers are usuallycustom designed for the needed application. As indicated, packed towers are generally ableto accept the highest flows before multiple, parallel units are required.

47

Table 2-3aA

ir Stripping P

ermitting R

equirements

48

Technology

Air Stripping Technology

Packed Tower

Spray Tower

Bubble Aeration

Low Profile

Aspiration

Construction Requirements

• Free standing or guy wired.

• Requires concrete pad.

• May be installed outdoors.

• Free standing or guy wired.

• May require concrete pad.

• May be installed outdoors

• Typically requires process enclosure.

• Needs self-supporting stack.

• Typically requires process enclosure.

• Requires level surface.

• Larger units need self-supporting stack.

• May be installed in an enclosure or outdoors

• May require concrete pad.

• Larger units need self-supporting stack.

• Height of stack above ground level

• Interference from other buildings

Operational and Maintenance Requirements

• May require pretreatment to reduce fouling potential.

• Cleaning of packing likely necessary for fouling.

• Routine blower and pump maintenance required.

• May require pretreatment to reduce fouling.

• Cleaning of packing (for MTBE) and nozzles may be required if

fouling occurs.



• Cleaning of bubble diffusers and tank may be required if fouling

occurs.


• Needs enclosed heating if ambient temperature below freezing.


• Cleaning of trays is required if fouling occurs.


• Needs enclosed heating if ambient temperature below freezing.

• Routine pump maintenance required.

• 3 phase, 230/460 volt, 60 hertz, power typically needed.

• Emissions limit of organic compounds to the atmosphere (range:

1 to 10 lb/day).

• MTBE treated as an organic.

• Monitoring requirements: for new operation, daily monitoring; for

operation up to two years, monthly monitoring.

Table 2-3bO

ff-gas Treatment P

ermitting R

equirements

49

Technology

Vapor Phase GAC

Adsorption

Thermal Oxidation

Catalytic Oxidation

Biofilter

Construction Requirements

• Minimum distance to outer boundary of a school: 1000 feet.

• Minimum system design: two beds to provide standby

device in case of carbon breakthrough.


• Safety device required for system shutdown during periods

of low temperature.


• Safety device required for system shutdown during periods

of low temperature.

Operational and Maintenance Requirements

• Monitoring requirements: for new operation, daily

monitoring; for operation up to two years, monthly

monitoring.

• Temperature requirement:

1500°F at exit point of combustion chamber.

• Monitor VOCs at outlet.

• Temperature requirement: 550°F at inlet.

• Degradation: required to demonstrate.

• By-products monitored

Off Gas Treatment Technologies

2.3.3 Removal Efficiency and Flow Rate

All of the air stripping technologies are capable of meeting stringent removal efficiencyrequirements (>99 percent). High removal efficiencies are dependent on the process designof the system and the number of air stripping units included in the system. For a single unit,however, each of the technologies is limited to a maximum removal efficiency that can bepractically achieved (see Table 2-4).

For a single packed tower, the maximum design efficiency is usually less than or equal to 99percent. Removals greater than this are difficult to maintain because of possible non-idealflow through the tower, resulting in short-circuiting. The advantage of packed towers is thata single tower can achieve high removal efficiencies at very high flow rates whereas the othersystems require several treatment units in series or are not practical for high flow rates. Lowprofile aeration systems are also capable of achieving removal efficiencies as high as 99percent, but typically only for lower flow rates (<100 gpm).

Simple unit bubble diffusion aerators and spray towers without packing are capable ofachieving only 80 to 90 percent removal because of mass transfer limitations inherent to thetechnology. Both of these systems will likely not be selected for removal of MTBE becausehigh removal efficiencies would require several units or towers in series, which make thesesystems not economically feasible. A spray tower can achieve higher removal efficienciesonly if packing is used, which essentially makes the spray tower into a packed tower.

A single aspirator unit can only achieve 60 to 70 percent removal efficiency; however, theseunits are designed to allow for significant water recirculation to achieve higher removal

50

Table 2-4Typical Maximum Hydraulic Capacities for Commercially Available Systems*

*All information supplied by vendors; information is for a single unit system with maximum air to water ratio (AWR)for the given system.

Technology

Packed Tower

Low Profile

Bubble Diffusion

Spray Tower

Aspiration

Max. Hydraulic Capacity

for MTBE Removal

1,000 gpm

1,100 gpm

50 gpm

All custom designs

500 gpm

Typical “One-Pass” MTBE

Removal Efficiency (%)

(w/max. AWR)

90 - 99

90 - 99

80 - 90

80 - 90

18

Max. Hydraulic Capacity

for Other VOCs

Custom designs

550 gpm

200 gpm

All custom designs

500 gpm

efficiencies. To treat higher flow rates, aspirators must be placed in parallel. Recirculatingthe effluent water and placing multiple aspirators in parallel can result in a large footprint andhigh capital, operation, and maintenance costs. For this reason, aspirators are only practicalfor low flow rates (<60 gpm).

2.3.4 Impact of Water Quality

Water quality has a significant impact on all air stripping systems due to biofouling or scalingfrom iron precipitation and magnesium and calcium carbonate precipitation. Biofouling can bemitigated by pretreatment with a disinfectant. The overall impact of scaling on treatment can bemitigated by cleaning or scale removal, or by adding chelating agents to the air stripper influent.Packed tower and low profile aeration systems are generally the most sensitive to scaleformation, followed by bubble aeration and spray towers, which exhibit lower scale formationdue to a higher degree of turbulence. Aspirator systems are the least sensitive to fouling due tothe high level of turbulence, which does not allow scaling to form. Table 2-5 below summarizesthe range of iron and hardness levels that could lead to scaling. The low profile aeration systemmay be more susceptible to fouling than a packed tower because the air ports used to distributeair are small. However, the low profile stripper is more accessible for cleaning. Other waterquality parameters that impact the operations of an air stripping system include manganese andchloride concentrations, pH, alkalinity, temperature, and oil and grease.

Table 2-5Problematic Iron and Hardness Concentrations for Air Stripping Technologies

2.3.5 Other Factors

A comparative discussion of each of the air stripper technologies relative to effectiveness andimplementability issues is presented below. Specifically, each of the technologies isdiscussed relative to its reliability, flexibility, adaptability, and potential for modification.Table 2-6 presents a comparative summary of the technologies with respect to each of thesecriteria. Construction, operation, and maintenance issues are summarized in Table 2-3a.

51

*All information obtained from vendors.

Technology

Packed Tower

Low Profile

Bubble Aeration

Spray Tower

Aspiration

Iron Concentration (mg/L)

≥3-5

≥5-10

≥5-10

Dependent on Configuration

≥1,200

Hardness (mg/L)

≥300-500

≥50-100

≥1,000

Dependent on Configuration

≥2,000

Reliability

Reliability includes both process and mechanical reliability of the technology to meet treatedwater requirements consistently. In general, all of the technologies discussed above aremechanically reliable. There are few moving parts or mechanical equipment in any of thesystems, limiting the need for change-outs or replacements. Based on this criterion alone, allof the systems would receive a “MEDIUM” or better rating for mechanical reliability.Process reliability for MTBE removal from water in a single unit varies for each technology(see Table 2-4).

Combining mechanical and process reliability, bubble diffusion aeration and spray towerswere given a “LOW” rating because a single unit cannot achieve removal efficiencies greaterthan 90 percent (i.e., these technologies are not reliable for MTBE removal). For theremaining technologies, their ability to treat MTBE is generally a function of flow rate.Packed tower air strippers can achieve greater than 95 percent MTBE removal at higher flows(i.e., 600 to 6,000 gpm), whereas low profile and aspiration air strippers are capable of highremoval efficiencies for MTBE only at low flows (<100 gpm). To achieve the same degreeof reliability for each air stripping system, multiple units may be required.

Flexibility

Flexibility is defined as the ability of the technology to handle a wide range of flow rates.The ability to handle a wide range of flows is not necessarily tied to overall hydrauliccapacity but, rather, the ability of the unit to function as desired if flows drop significantlybelow, or increase significantly above, process design values. With the exception of theaspirator stripper and spray tower, the other air stripping technologies are able to handlevarying liquid flow rates without requiring significant design changes (i.e., these techno-logies have a high liquid turndown ratio). Due to the reliance of the aspirator and spray towerstrippers on high pressure liquid streams to effect mass transfer, these units are not well-suited to handle changing flow rates.

Adaptability

Adaptability is defined as the ability of a technology to handle fluctuating contaminantinfluent concentrations and other water quality parameters of interest, such as scaleformation and fouling. All of the technologies evaluated have specific removal efficienciesunder given influent flows that are relatively independent of the influent concentration. Ifinfluent concentrations increase and effluent concentration goals remain unchanged, it maybe necessary to increase the air/water ratio or recirculate the water to meet treated watergoals.

52

53

Table 2-6C

omparison of A

ir Stripping Technologies

Definition

Packed Tower

Low Profile

Reliability

• Proven technology for MTBE.• Low mechanical failure

potential.

HIGH• Oldest design (widely used).

95-99% removal of MTBE atwide flow range.

• Low mechanical failurepotential.

• Towers are weatherproof.• Orific plate distribution system

ensures even distribution ofboth water and air over entirecross-section; this results inmaximum AWR interface andyields consistent results.

• Minimum instrumentation.

HIGH/MEDIUM• Proven technology for MTBE at

low-medium flows.• Few moving parts means

minimal chance for mechanicalfailure.

• Minimal instrumentation.• Not weatherproof, thus

requiring construction ofprotective shelter.

Flexibility

• Able to handle a wide rangeof flows.

• Able to operate in batch orcontinuous mode.

HIGH• Orifice plate distribution system

allows a single unit to handlewide range of flows withoutaffecting performance.

• Not well-suited to batch flow.• Towers can be operated as

standalone units or in parallelor series.

• Use of variable frequencydrives on blower motormaintains constant AWR over awide range of water flowrates,resulting in significant energysavings at lower water flows.

HIGH• Able to handle wide range of

flows.• Can operate in continuous or

batch mode.

Adaptability

• Able to handle changinginfluent concentrations.

• Able to handle changing waterquality.

• Easily cleaned.

HIGH/MEDIUM• Can handle a wide range of

daily or seasonal fluctuations incontaminant loading.

• Subject to scaling if hardness/metals levels increase. Accessto packing for cleaning isdifficult and limited.

MEDIUM• Able to handle daily or seasonal

fluctuations in contaminantloading at low-medium flows.

• Subject to scaling if hardness/metals levels increase.

• Readily cleaned.

Potential for Modifications

• Readily supplemented withadditional or larger components(blower, tank, etc.) if influentconditions change.

• Readily combined with pre orpost-treatment equipment.

• Can be turned down.

HIGH/MEDIUM• May be modified with larger

blower.• 30 to 40% turndown rate for air

(can only be throttled back 60-70% of capacity beforeefficiency decreased.

• Height and weightconsiderations will limit amountof additional packing.

• Difficult to relocate tower.• Diameter cannot be increased

to handle more than maximumdesign flowrate.

MEDIUM• Flow/loadings above design

conditions typically requiresnew unit.

• Blower cannot be turned down.• Easily relocated due to size.• Size of unit cannot be modified

to handle water flow rates inexcess of maximum designflow rate.

Air Stripping Technology

54

Table 2-6 (Continued)

Com

parison of Air S

tripping Technologies

Bubble Aeration

Spray Tower

Aspiration

Reliability

LOW• Technology is proven to be

effective in certain applications.Minimal effectiveness forMTBE.


• Minimal instrumentation.• Not weatherproof, thus

requiring construction ofprotective shelter.

LOW• Technology is proven to be

effective in certain applications.Low removal efficiency forMTBE unless packing added.


MEDIUM• Successful Superfund

applications. No specific MTBEapplications identified;however, computer modelinghas been developed fromextensive testing to size anapplication and predict theperformance of the operatingsystem for MTBE removal.Good removals predicted atlow to high flows.

• Only moving parts arecentrifugal pumps (no blower)which are simple to maintain.

Flexibility


flows.• Can operate in continuous or

batch mode.

LOW• Water flow rate should be

maintained constant foreffective treatment.

• Not well-suited to batch flow ifpacking used.

MEDIUM• Not able to handle wide range

of flows.• Capable of batch and

continuous operation withoutmodification.

Adaptability

MEDIUM• Able to handle wide range of

contaminant loadings at lowflows.

• Resistant to scaling due tohardness/dissolved metals.


MEDIUM• Able to handle wide range of

contaminant loadings at lowflows.

• Subject to scaling if hardness/metals levels increase.

• Easily maintained unlesspacking is required.


contaminant loadings at low-medium flows.

• Resistant to scaling due to highdegree of turbulence.



MEDIUM• Flow loadings above design

conditions require new oradditional unit.

• Blower turn down is feasible.• Easily relocated due to size.• Size of unit cannot be modified

to handle water flow rates inexcess of maximum designflow rate.

MEDIUM• Readily tied into other larger

components.• Low turn down ratio (nozzle

must be changed to adjust flowrates).

HIGH• Additional modules can be

added for difficult applications.• Modules can be turned off if

influent concentrationsdecrease.

The efficiency of air strippers is not affected by other VOCs (including BTEX) in the wastestream due to the relatively high air/water ratio required for removal of MTBE. However,increased pH, iron, calcium, or magnesium in the influent stream could lead to increasedscale formation. Ease of cleaning in the event that scaling occurs is, therefore, a relatedadaptability criterion. Packed columns and spray aeration systems fitted with packing (as istypically necessary for MTBE removal) are most difficult to clean due to the generalinaccessibility of the packing material. In addition, the packing material provides a mediumfor scale and iron deposits. Alternatively, low profile systems have significant contactbetween the aerated process water and the tray surface, resulting in more rapid scaleformation; however, access to trays and cleaning is easier for low profile systems. Aspiratorand bubble aeration technologies do not generally rely on media surfaces to create masstransfer in the water column. Therefore, fouling is limited to the nozzles in these systems,which can be easily replaced.

Potential for Modification

The potential to implement equipment modifications is defined as the capability to changeequipment due to a change in design conditions (e.g., metals precipitation pre-treatmentequipment, activated carbon post-treatment). “Turndown” of system air flows to minimizeenergy costs if influent concentrations decrease is another example of system modifications.All the technologies considered have equal capability of being combined with pre- or post-treatment equipment, as the systems can be built or modified with influent feed tanks ordischarge tanks. Similarly, each technology has difficulty handling flows significantly abovemaximum hydraulic design capacity, although the modular layout of the low profile andaspirator system may facilitate supplementing the process with additional units if higherflows are required.

If off-gas treatment is required, it is advantageous to have an air flow rate as low as possible.Assuming a variable speed blower, the ability to turn down air flow rate is feasible for packedtowers, bubble aeration systems, and spray towers. However, unpacked spray towers do notrely heavily on counter-current air flow to effect removal efficiency, thus resulting in lowenergy savings potential if influent concentrations decrease. Low profile units have almostno air turndown capability because the water must be kept from falling through trayperforations. Aspirator units do not rely on any forced air source to effect volatilization and,therefore, are not subject to this criterion.

2.3.6 By-products

Aside from scale formation, by-products are not generally a concern with air strippingtechnologies. However, there is a potential for biological fouling that may produce biologicalsolids in the treated water. Disinfection of the air stripper influent or effluent water could beimplemented to prevent biological fouling. In addition to biological fouling, out-gassing ofcarbon dioxide will cause a rise in the pH, which may necessitate implementation of miti-

55

gation measures. In summary, none of the air stripping technologies demonstrates anyparticular advantage or disadvantage for minimizing by-product formation, although some ofthe air stripping technologies are more amenable to by-product removal (e.g., low profileaerators for scaling and fouling removal).

2.3.7 Cost Effectiveness

To compare capital costs for each of the air stripping systems evaluated, suppliers wereprovided with a number of treatment scenarios and asked to provide capital costs and motorhorsepower requirements (as part of O&M costs) to meet the treatment requirements. At leastone major manufacturer of each air stripping technology evaluated in Section 2.2 wascontacted and asked to provide model selection, number of units required in parallel or series,and capital costs for the following potential MTBE treatment scenarios:

• Influent flows of 60, 600, and 6,000 gpm.

• Influent MTBE concentrations of 20, 200, and 2,000 µg/L.

• Effluent MTBE discharge requirements of 20, 5, or 0.5 µg/L.

Table 2-7 provides a sample calculation of total capital costs, total annual costs, and unittreatment costs.

As indicated on Table 2-8, capital costs for spray towers, packed towers, and low-profilesystems are similar at low flow rates (60 gpm), with bubble diffusion and aspiration systemsrequiring significantly higher capital costs at higher influent concentrations due to the needfor multiple units to achieve equivalent removal efficiency. At higher flows and removalrates, packed columns and spray towers become significantly more cost-effective than thelow profile technologies, as predicted, while bubble diffusion and aspiration systems remainsignificantly higher in capital costs.

To estimate operating costs for each of the systems, manufacturer-recommended horsepowersizing for the stripping system blowers and pumps were requested for each of the threeinfluent flow rates and a required MTBE removal efficiency of 97.5 percent that corre-sponded to both the 20 µg/L to 0.5 µg/L and 200 µg/L to 5 µg/L influent/effluent scenarios.For the purpose of comparison, centrifugal feed and discharge pumps were not consideredbecause this equipment is common to all of the systems. Supplemental feed pumps capableof delivering the required flows at high pressure drops were, however, considered for thespray tower and aspiration systems as this equipment is integral to their stripping operation.

Table 2-9 presents a summary of annual O&M costs for each of the air stripping units underthe scenario described above. These cost estimates include electrical costs based on anaverage electric rate of $0.08/kWhr, maintenance and sampling labor costs at $80/hr, andanalytical costs at $200 per sample. The estimated sampling and maintenance labor hours are

56

presented in Table 2-10. The sampling requirements used to estimate analytical costs arepresented in Table 2-11. It is important to note that the frequency of sampling will likely bemuch higher for the first few years of operation while the system receives regulatoryapproval. As indicated in Table 2-9, spray towers, packed towers, and low profile systemshave competitive annual O&M costs at the low flow rate of 60 gpm while bubble diffusionand aspiration systems require considerably higher O&M costs (approximately three timeshigher) due to high electrical, labor, and analytical costs. At medium flow rates (600 gpm)and high flow rates (6,000 gpm), spray towers and low profile systems diverge considerablyin costs from packed towers due to higher electrical costs for the spray towers and higherelectrical, labor, and analytical costs for the low profile systems. In general, O&M costsincrease with the number of units required for a given flow rate since more units require morepower, maintenance, and sampling.

57

Line Item Cost

Treatment Unit $125,000

Piping, Valves, Electrical (30%) $37,500

Site Work (10%) $12,500SUBTOTAL $175,000

Contractor O&P (15%) $26,250SUBTOTAL $201,250

Engineering (15%) $30,188SUBTOTAL $231,438

Contingency (20%) $46,288

TOTAL CAPITAL $277,725

Amortized Annual Capital $22,381

Annual O&M $91,684

TOTAL ANNUAL COST $114,065

Annual Flow Treated (kgal) 315,000

UNIT TREATMENT COST ($/kgal)* $0.36

Table 2-7Sample Calculation of Capital, Annual, and Unit Treatment Costs for a 600 gpm Packed Tower,

2,000 to 20 µg/L MTBE

*To convert unit treatment cost to $/acre-ft, multiply by 326.Amortization based on a 30-year period at a 7% discount rate.

Capital expenses include: equipment, piping, valves, and electrical components (30%), site work (10%),contractor O&P (15%), engineering (15%), and contingency (20%).

O&M Costs include: 1. Power costs at $0.08/kWhr2. Labor costs:

Due to the lack of field data, annual labor costs for maintenance and sampling have been estimated as follows:60 gpm: ~6 hrs/week at $80/hr = $25,000/yr600 gpm: ~12 hrs/week at $80/hr = $50,000/yr6000 gpm: ~31 hrs/week at $80/hr = $130,000/yr

3. Analytical costs estimated at $200 per sample; see Table 2-11 for the number of samples required for apacked tower packed tower.

58

Flow(gpm)

Influent(µg/L)

Effluent(µg/L)

Removal Spray Tower Packed Tower Low Profile BubbleAeration

Aspiration

60 20 5 75.00% $55,545 $66,654 ND $69,827 $51,10160 20 0.5 97.50% $66,654 $88,872 $95,537 $174,567 $127,75460 200 20 90.00% $55,545 $66,654 $45,880 $104,740 $77,76360 200 5 97.50% $66,654 $88,872 $71,149 $174,567 $127,75460 200 0.5 99.75% $88,872 $111,090 $88,845 $244,394 $204,40660 2000 20 99.00% $77,763 $99,981 $52,443 $209,480 $215,51560 2000 5 99.75% $88,872 $111,090 $58,955 $244,394 $204,40660 2000 0.5 99.98% ND ND ND ND $333,270

600 20 5 75.00% $177,744 $222,180 $259,871 $628,441 $355,488600 20 0.5 97.50% $211,071 $288,834 $519,741 ND $944,265600 200 20 90.00% $177,744 $233,289 $337,543 $1,152,141 $555,450600 200 5 97.50% $211,071 $288,834 $675,085 ND $944,265600 200 0.5 99.75% $333,270 $299,943 $776,172 ND ND600 2000 20 99.00% $266,616 $277,725 $675,085 ND ND600 2000 5 99.75% $333,270 $299,943 $776,172 ND ND600 2000 0.5 99.98% ND ND ND ND ND

6000 20 5 75.00% $1,555,260 $1,999,620 $2,598,706 ND ND6000 20 0.5 97.50% $1,799,658 $2,221,800 $5,197,412 ND ND6000 200 20 90.00% $1,666,350 $2,021,838 $3,375,425 ND ND6000 200 5 97.50% $1,799,658 $2,221,800 $5,197,412 ND ND6000 200 0.5 99.75% $2,288,454 $2,788,359 $7,761,725 ND ND6000 2000 20 99.00% $1,999,620 $2,532,852 ND ND ND6000 2000 5 99.75% $2,288,454 $2,788,359 ND ND ND

Flow (gpm) Influent(µg/L)

Effluent(µg/L)

Removal Spray Tower Packed Tower Low Profile Bubble Aeration Aspiration

60 20 5 75.00% $48,933 $46,844 ND $151,557 $176,29460 20 0.5 97.50% $48,933 $47,888 $51,021 $151,557 $176,29460 200 20 90.00% $48,933 $46,844 $49,977 $151,557 $176,29460 200 5 97.50% $48,933 $47,888 $51,021 $151,557 $176,29460 200 0.5 99.75% $48,933 $48,410 $52,587 $151,557 $176,29460 2000 20 99.00% $48,933 $48,410 $55,720 $151,557 $176,29460 2000 5 99.75% $48,933 $48,410 $58,852 $151,557 $176,29460 2000 0.5 99.98% ND $51,021 ND ND $176,294

600 20 5 75.00% $102,126 $77,587 $226,294 $1,121,972 $370,946600 20 0.5 97.50% $102,126 $83,852 $249,789 ND $370,946600 200 20 90.00% $102,126 $81,242 $241,957 $1,121,972 $370,946600 200 5 97.50% $102,126 $83,852 $249,789 ND $370,946600 200 0.5 99.75% $102,126 $91,684 $281,114 ND ND600 2000 20 99.00% $102,126 $91,684 $249,789 ND ND600 2000 5 99.75% $102,126 $91,684 $281,114 ND ND600 2000 0.5 99.98% ND ND ND ND ND

6000 20 5 75.00% $489,953 $257,620 $857,343 ND ND6000 20 0.5 97.50% $489,953 $312,440 $1,092,286 ND ND6000 200 20 90.00% $489,953 $296,777 $1,013,972 ND ND6000 200 5 97.50% $489,953 $312,440 $1,092,286 ND ND6000 200 0.5 99.75% $489,953 $312,440 $1,405,544 ND ND6000 2000 20 99.00% $489,953 $328,103 ND ND ND6000 2000 5 99.75% $489,953 $328,103 ND ND ND

Table 2-8Initial Capital Expenses for Air Stripping Systems

Table 2-9Annual O&M Costs for Air Stripping Systems

ND = no data available, system may require custom design

O&M Costs include: 1. Power costs at $0.08/kWhr.2. Labor costs estimated at $80/hr; see Tables 2-13 to 2-17 for a breakdown of estimated annual labor

hours required for maintenance and sampling.3. Analytical costs estimated at $200 per sample; see Table 2-11 for the number of samples required for

each system.

ND = no data available, system may require custom design

Capital Expenses include:Equipment: Piping, valves, electrical (30%); site work (10%); contractor O&P (15%); engineering (15%);contingency (20%).

59

Table 2-10Estimated Labor Hours for Maintenance and Sampling of Air Stripping Systems* in Hours per Week

Table 2-11Sampling Requirements for Air Stripping Systems and Off-gas Treatment:

No. of Samples Collected Weekly

Air Stripper 60 gpm 600 gpm 6,000 gpm

Packed Tower 6 12 31Low Profile 6 31 72

Bubble Diffusion 12 48 N.E.Spray Tower 6 12 31

Aspiration 19 38 N.E.

N.E. = not evaluated

*Maintenance & sampling labor hours associated with selected off-gastreatment system are included in these estimates.

Treatment System 60 gpm 600 gpm 6,000 gpm

Air StripperPacked Tower 2 2 7

Low Profile 2 7 31Bubble Diffusion 6 51 N.E.

Spray Tower 2 2 7Aspiration 7 12 N.E.

Off-Gas TreatmentGAC 1 1 7

Recuperative Thermal Oxidation 1 1 1Recuperative Flameless Thermal Oxidation 1 1 1

Recuperative Catalytic Oxidation 1 1 1Non-Recuperative Catalytic Oxidation N.E. 1 N.E.

Notes:

2. N.E. = not evaluated.

water samples

gas samples

1. For the cost estimates, both water and gas sample analyses were priced atapproximately $200 per sample.

*Maintenance & sampling labor hours associated with selected off-gas treatment systemare included in these estimates.

Notes:

1. N.E. = not evaluated.2. Labor hours are dependent upon the number of units required.

Notes:

1. For the cost estimates, both water and gas sample analyses were priced at approximately $200 persample.

2. N.E. = not evaluated.3. Sampling requirements are dependent upon the number of units required.

Amortized annual capital costs and annual O&M costs were combined to determine the totalamortized operating cost for each system per 1,000 gallons of treated water (see Tables 2-12through 2-17). The equipment was amortized at a discount rate of seven percent over a 30-year period. At the low flow rate (60 gpm), packed towers, spray towers, and low profilesystems are the least expensive options. As the flow rate increases, unit costs drop at differentrates with spray towers and packed towers emerging as the least expensive options. Althoughspray towers are cost competitive with packed towers and low-profile systems for the 60 gpmscenario, they have not been considered because, when they are filled with packing (as isrequired for MTBE removal), they become nearly identical to packed towers. Therefore,based on this cost comparison, packed towers represent the least expensive option for alldrinking water applications. However, low profile strippers are competitive with packedtowers at low flow rates (60 gpm) and are expected to be used preferentially to packed towersdue to their ease of use and implementability. The results of this cost comparison indicate thatbubble diffusers and aspiration strippers are the most expensive air stripping options and are,thus, not likely to be optimal for drinking water applications where MTBE removal isrequired.

A sensitivity analysis was performed for the costs of the two most promising technologies,packed tower systems (Table 2-18) and low profile systems (Table 2-19), at a design flow rateof 600 gpm, an influent concentration of 200 µg/L MTBE, and an effluent concentration of5 µg/L MTBE. For this analysis, we assumed that influent water with high potential forfouling (i.e., high natural organic material [NOM] concentrations) should be pre-treated withsodium hypochlorite or a similar disinfectant (approximately 5 mg/L free chlorine) prior tobeing fed into the air strippers. The additional costs associated with disinfection are thecapital cost for a chemical feed system and annual chemical consumption. High potential forfouling results in a cost increase of approximately $0.13/1,000 gallons of treated water. As aresult of the high air/water ratio required for MTBE removal, BTEX loads of up to 200 µg/Lfor each compound are expected not to affect the performance of either the packed tower orlow profile systems. BTEX compounds have higher Henry’s constants than MTBE and,therefore, are more easily removed by air stripping. Thus, capital, annual O&M, and unitcosts are unaffected by BTEX loadings that are similar or lower than MTBE loadings on amass basis. As can be expected, shortening the design life of these systems is expected toresult in higher unit costs. Reducing the design life from 30 years to 2 years, whilemaintaining a seven percent discount rate, results in an approximate doubling of the unitcosts for the packed tower system ($0.34 to $0.77/1,000 gallons) and the low profile system($0.96 to $1.98/1,000 gallons).

60

61

Flow(gpm)

Influent(µg/L)

Effluent(µg/L)

Removal(%)

Spray Tower Packed Tower Low Profile BubbleAeration

Aspiration

60 20 5 75.00% $1.69 $1.66 ND $4.98 $5.7260 20 0.5 97.50% $1.72 $1.75 $1.86 $5.25 $5.9260 200 20 90.00% $1.69 $1.66 $1.70 $5.07 $5.7960 200 5 97.50% $1.72 $1.75 $1.80 $5.25 $5.9260 200 0.5 99.75% $1.78 $1.82 $1.89 $5.43 $6.1160 2000 20 99.00% $1.75 $1.79 $1.90 $5.34 $6.1460 2000 5 99.75% $1.78 $1.82 $2.02 $5.43 $6.1160 2000 0.5 99.98% ND ND ND ND $6.44

600 20 5 75.00% $0.37 $0.30 $0.78 $3.72 $1.27600 20 0.5 97.50% $0.38 $0.34 $0.92 ND $1.42600 200 20 90.00% $0.37 $0.32 $0.85 $3.85 $1.32600 200 5 97.50% $0.38 $0.34 $0.96 ND $1.42600 200 0.5 99.75% $0.41 $0.37 $1.09 ND ND600 2000 20 99.00% $0.39 $0.36 $0.96 ND ND600 2000 5 99.75% $0.41 $0.37 $1.09 ND ND600 2000 0.5 99.98% ND ND ND ND ND

6000 20 5 75.00% $0.20 $0.13 $0.34 ND ND6000 20 0.5 97.50% $0.20 $0.16 $0.48 ND ND6000 200 20 90.00% $0.20 $0.15 $0.41 ND ND6000 200 5 97.50% $0.20 $0.16 $0.48 ND ND6000 200 0.5 99.75% $0.21 $0.17 $0.64 ND ND6000 2000 20 99.00% $0.21 $0.17 ND ND ND6000 2000 5 99.75% $0.21 $0.18 ND ND ND

Table 2-12Total A

mortized O

perating Costs ($/1,000 G

allons Treated)*for A

ir Stripping S

ystems

ND = no data available, system may require custom design.*To convert costs to $/acre-ft, multiply by 326.Amortization based on a 30-year period at a 7% discount rate.

Capital expenses include: equipment, piping, valves, and electrical components (30%), site work (10%), contractor O&P(15%), engineering (15%), and contingency (20%).

O&M Costs include: 1. Power costs at $0.08/kWhr.2. Labor costs estimated at $80/hr; see Tables 2-13 to 2-17 for a breakdown of estimated annual labor hours required for

maintenance and sampling.3. Analytical costs estimated at $200 per sample; see Table 2-11 for the number of samples required for each system.

62

Flo

w(g

pm)

Sys

tem

Con

figur

atio

nIn

fluen

t(µ

g/L)

Effl

uent

(µg

/L)

Rem

oval

(%

)C

apita

l Cos

t ($

)A

nnua

l O&

M (

$)U

nit

Cos

t($

/100

0 ga

l)*

60

sing

le t

ower

20

57

5.0

0%

$5

5,5

45

$4

8,9

33

$1

.69

60

sing

le t

ower

20

0.5

97

.50

%$

66

,65

4$

48

,93

3$

1.7

2

60

sing

le t

ower

20

02

09

0.0

0%

$5

5,5

45

$4

8,9

33

$1

.69

60

sing

le t

ower

20

05

97

.50

%$

66

,65

4$

48

,93

3$

1.7

2

60

sing

le t

ower

20

00

.59

9.7

5%

$8

8,8

72

$4

8,9

33

$1

.78

60

sing

le t

ower

20

00

20

99

.00

%$

77

,76

3$

48

,93

3$

1.7

5

60

sing

le t

ower

20

00

59

9.7

5%

$8

8,8

72

$4

8,9

33

$1

.78

60

ND

20

00

0.5

99

.98

%N

DN

DN

D

60

0si

ngle

tow

er2

05

75

.00

%$

17

7,7

44

$1

02

,12

6$

0.3

7

60

0si

ngle

tow

er2

00

.59

7.5

0%

$2

11

,07

1$

10

2,1

26

$0

.38

60

0si

ngle

tow

er2

00

20

90

.00

%$

17

7,7

44

$1

02

,12

6$

0.3

7

60

0si

ngle

tow

er2

00

59

7.5

0%

$2

11

,07

1$

10

2,1

26

$0

.38

60

0si

ngle

tow

er2

00

0.5

99

.75

%$

33

3,2

70

$1

02

,12

6$

0.4

1

60

0si

ngle

tow

er2

00

02

09

9.0

0%

$2

66

,61

6$

10

2,1

26

$0

.39

60

0si

ngle

tow

er2

00

05

99

.75

%$

33

3,2

70

$1

02

,12

6$

0.4

1

60

0N

D2

00

00

.59

9.9

8%

ND

ND

ND

60

00

para

llel t

ower

s2

05

75

.00

%$

1,5

55

,26

0$

48

9,9

53

$0

.20

60

00

para

llel t

ower

s2

00

.59

7.5

0%

$1

,79

9,6

58

$4

89

,95

3$

0.2

0

60

00

para

llel t

ower

s2

00

20

90

.00

%$

1,6

66

,35

0$

48

9,9

53

$0

.20

60

00

para

llel t

ower

s2

00

59

7.5

0%

$1

,79

9,6

58

$4

89

,95

3$

0.2

0

60

00

para

llel t

ower

s2

00

0.5

99

.75

%$

2,2

88

,45

4$

48

9,9

53

$0

.21

60

00

para

llel t

ower

s2

00

02

09

9.0

0%

$1

,99

9,6

20

$4

89

,95

3$

0.2

1

60

00

para

llel t

ower

s2

00

05

99

.75

%$

2,2

88

,45

4$

48

9,9

53

$0

.21

Table 2-13Expense Summary for Air Stripping Systems – Spray Tower

ND = no data available, system may require custom design.*To convert unit treatment costs to $/acre-ft, multiply by 326.Amortization based on a 30-year period at a 7% discount rate.



Due to the lack of field data, annual labor costs for maintenance and sampling have been estimated as follows: 60 gpm: ~6 hrs/week at $80/hr = $25,000/yr.600 gpm: ~12 hrs/week at $80/hr = $50,000/yr.6000 gpm: ~31 hrs/week at $80/hr = $130,000/yr.

3. Analytical costs estimated at $200 per sample; see Table 2-11 for the number of samples required foreach system.

63

Flo

w(g

pm)

Sys

tem

Con

figur

atio

nIn

fluen

t(µ

g/L)

Effl

uent

(µg

/L)

Rem

oval

(%

)C

apita

l Cos

t ($

)A

nnua

l O&

M (

$)U

nit

Cos

t($

/100

0 ga

l)*

60

2.6’

dia

. to

wer

20

57

5.0

0%

$6

6,6

54

$4

6,8

44

$1

.66

60

2.6’

dia

. to

wer

20

0.5

97

.50

%$

88

,87

2$

47

,88

8$

1.7

5

60

2.6’

dia

. to

wer

20

02

09

0.0

0%

$6

6,6

54

$4

6,8

44

$1

.66

60

2.6’

dia

. to

wer

20

05

97

.50

%$

88

,87

2$

47

,88

8$

1.7

5

60

2.6’

dia

. to

wer

20

00

.59

9.7

5%

$1

11

,09

0$

48

,41

0$

1.8

2

60

2.6’

dia

. to

wer

20

00

20

99

.00

%$

99

,98

1$

48

,41

0$

1.7

9

60

2.6’

dia

. to

wer

20

00

59

9.7

5%

$1

11

,09

0$

48

,41

0$

1.8

2

60

ND

20

00

0.5

99

.98

%N

DN

DN

D

60

08.

3’ d

ia.

tow

er2

05

75

.00

%$

22

2,1

80

$7

7,5

87

$0

.30

60

08.

3’ d

ia.

tow

er2

00

.59

7.5

0%

$2

88

,83

4$

83

,85

2$

0.3

4

60

08.

3’ d

ia.

tow

er2

00

20

90

.00

%$

23

3,2

89

$8

1,2

42

$0

.32

60

08.

3’ d

ia.

tow

er2

00

59

7.5

0%

$2

88

,83

4$

83

,85

2$

0.3

4

60

08.

3’ d

ia.

tow

er2

00

0.5

99

.75

%$

29

9,9

43

$9

1,6

84

$0

.37

60

08.

3’ d

ia.

tow

er2

00

02

09

9.0

0%

$2

77

,72

5$

91

,68

4$

0.3

6

60

08.

3’ d

ia.

tow

er2

00

05

99

.75

%$

29

9,9

43

$9

1,6

84

$0

.37

60

0N

D2

00

00

.59

9.9

8%

ND

ND

ND

60

00

6 x

11.5

’ dia

. pa

ralle

l tow

ers

20

57

5.0

0%

$1

,99

9,6

20

$2

57

,62

0$

0.1

3

60

00

6 x

11.5

’ dia

. pa

ralle

l tow

ers

20

0.5

97

.50

%$

2,2

21

,80

0$

31

2,4

40

$0

.16

60

00

6 x

11.5

’ dia

. pa

ralle

l tow

ers

20

02

09

0.0

0%

$2

,02

1,8

38

$2

96

,77

7$

0.1

5

60

00

6 x

11.5

’ dia

. pa

ralle

l tow

ers

20

05

97

.50

%$

2,2

21

,80

0$

31

2,4

40

$0

.16

60

00

6 x

11.5

’ dia

. pa

ralle

l tow

ers

20

00

.59

9.7

5%

$2

,78

8,3

59

$3

12

,44

0$

0.1

7

60

00

6 x

11.5

’ dia

. pa

ralle

l tow

ers

20

00

20

99

.00

%$

2,5

32

,85

2$

32

8,1

03

$0

.17

60

00

6 x

11.5

’ dia

. pa

ralle

l tow

ers

20

00

59

9.7

5%

$2

,78

8,3

59

$3

28

,10

3$

0.1

8

Table 2-14Expense Summary for Air Stripping Systems – Packed Tower

ND = no data available, system may require custom design*To convert unit treatment costs to $/acre-ft, multiply by 326.Amortization based on a 30-year period at a 7% discount rate.



Due to the lack of field data, annual labor costs for maintenance and sampling have been estimated as follows: 60 gpm: ~6 hrs/week at $80/hr = $25,000/yr600 gpm: ~12 hrs/week at $80/hr = $50,000/yr6000 gpm: ~31 hrs/week at $80/hr = $130,000/yr


64

Flo

w(g

pm)

Sys

tem

Con

figur

atio

nIn

fluen

t(µ

g/L)

Effl

uent

(µg

/L)

Rem

oval

(%

)C

apita

l Cos

t ($

)A

nnua

l O&

M (

$)U

nit

Cos

t($

/100

0 ga

l)*

60

Sin

gle

unit

20

57

5.0

0%

ND

ND

ND

60

Sin

gle

unit

20

0.5

97

.50

%$

95

,53

7$

51

,02

1$

1.8

66

0S

ingl

e un

it2

00

20

90

.00

%$

45

,88

0$

49

,97

7$

1.7

06

0S

ingl

e un

it2

00

59

7.5

0%

$7

1,1

49

$5

1,0

21

$1

.80

60

Sin

gle

unit

20

00

.59

9.7

5%

$8

8,8

45

$5

2,5

87

$1

.89

60

Sin

gle

unit

20

00

20

99

.00

%$

52

,44

3$

55

,72

0$

1.9

06

0S

ingl

e un

it2

00

05

99

.75

%$

58

,95

5$

58

,85

2$

2.0

26

0N

D2

00

00

.59

9.9

8%

ND

ND

ND

60

03

in p

aral

lel

20

57

5.0

0%

$2

59

,87

1$

22

6,2

94

$0

.78

60

06

in p

aral

lel

20

0.5

97

.50

%$

51

9,7

41

$2

49

,78

9$

0.9

26

00

3 in

par

alle

l2

00

20

90

.00

%$

33

7,5

43

$2

41

,95

7$

0.8

56

00

6 in

par

alle

l2

00

59

7.5

0%

$6

75

,08

5$

24

9,7

89

$0

.96

60

06

in p

aral

lel

20

00

.59

9.7

5%

$7

76

,17

2$

28

1,1

14

$1

.09

60

06

in p

aral

lel

20

00

20

99

.00

%$

67

5,0

85

$2

49

,78

9$

0.9

66

00

6 in

par

alle

l2

00

05

99

.75

%$

77

6,1

72

$2

81

,11

4$

1.0

96

00

ND

20

00

0.5

99

.98

%N

DN

DN

D

60

00

30 in

par

alle

l2

05

75

.00

%$

2,5

98

,70

6$

85

7,3

43

$0

.34

60

00

60 in

par

alle

l2

00

.59

7.5

0%

$5

,19

7,4

12

$1

,09

2,2

86

$0

.48

60

00

30 in

par

alle

l2

00

20

90

.00

%$

3,3

75

,42

5$

1,0

13

,97

2$

0.4

16

00

060

in p

aral

lel

20

05

97

.50

%$

5,1

97

,41

2$

1,0

92

,28

6$

0.4

86

00

060

in p

aral

lel

20

00

.59

9.7

5%

$7

,76

1,7

25

$1

,40

5,5

44

$0

.64

60

00

ND

20

00

20

99

.00

%N

DN

DN

D6

00

0N

D2

00

05

99

.75

%N

DN

DN

D

Table 2-15Expense Summary for Air Stripping Systems – Low Profile



O&M Costs include: 1. Power costs at $0.08/kWhr.2. Labor costs:



65

Flo

w(g

pm)

Sys

tem

Con

figur

atio

nIn

fluen

t(µ

g/L)

Effl

uent

(µg

/L)

Rem

oval

(%

)C

apita

l Cos

t ($

)A

nnua

l O&

M (

$)U

nit

Cos

t($

/100

0 ga

l)*

60

5 in

par

alle

l2

05

75

.00

%$

69

,82

7$

15

1,5

57

$4

.98

60

5 in

par

alle

l2

00

.59

7.5

0%

$1

74

,56

7$

15

1,5

57

$5

.25

60

5 in

par

alle

l2

00

20

90

.00

%$

10

4,7

40

$1

51

,55

7$

5.0

7

60

5 in

par

alle

l2

00

59

7.5

0%

$1

74

,56

7$

15

1,5

57

$5

.25

60

5 in

par

alle

l2

00

0.5

99

.75

%$

24

4,3

94

$1

51

,55

7$

5.4

3

60

5 in

par

alle

l2

00

02

09

9.0

0%

$2

09

,48

0$

15

1,5

57

$5

.34

60

5 in

par

alle

l2

00

05

99

.75

%$

24

4,3

94

$1

51

,55

7$

5.4

3

60

5 in

par

alle

l2

00

00

.59

9.9

8%

ND

ND

ND

60

050

in p

aral

lel

20

57

5.0

0%

$6

28

,44

1$

1,1

21

,97

2$

3.7

2

60

0N

D2

00

.59

7.5

0%

ND

ND

ND

60

050

in p

aral

lel

20

02

09

0.0

0%

$1

,15

2,1

41

$1

,12

1,9

72

$3

.85

60

0N

D2

00

59

7.5

0%

ND

ND

ND

60

0N

D2

00

0.5

99

.75

%N

DN

DN

D

60

0N

D2

00

02

09

9.0

0%

ND

ND

ND

60

0N

D2

00

05

99

.75

%N

DN

DN

D

60

0N

D2

00

00

.59

9.9

8%

ND

ND

ND

60

00

ND

20

57

5.0

0%

ND

ND

ND

60

00

ND

20

0.5

97

.50

%N

DN

DN

D

60

00

ND

20

02

09

0.0

0%

ND

ND

ND

60

00

ND

20

05

97

.50

%N

DN

DN

D

60

00

ND

20

00

.59

9.7

5%

ND

ND

ND

60

00

ND

20

00

20

99

.00

%N

DN

DN

D

60

00

ND

20

00

59

9.7

5%

ND

ND

ND

Table 2-16Expense Summary for Air Stripping Systems – Bubble Aeration






66

Flo

w(g

pm)

Sys

tem

Con

figur

atio

nIn

fluen

t(µ

g/L)

Effl

uent

(µg

/L)

Rem

oval

(%

)C

apita

l Cos

t ($

)A

nnua

l O&

M (

$)U

nit

Cos

t($

/100

0 ga

l)

60

6 un

its in

ser

ies

20

57

5.0

0%

$5

1,1

01

$1

76

,29

4$

5.7

2

60

6 un

its in

ser

ies

20

0.5

97

.50

%$

12

7,7

54

$1

76

,29

4$

5.9

2

60

6 un

its in

ser

ies

20

02

09

0.0

0%

$7

7,7

63

$1

76

,29

4$

5.7

9

60

6 un

its in

ser

ies

20

05

97

.50

%$

12

7,7

54

$1

76

,29

4$

5.9

2

60

6 un

its in

ser

ies

20

00

.59

9.7

5%

$2

04

,40

6$

17

6,2

94

$6

.11

60

6 un

its in

ser

ies

20

00

20

99

.00

%$

21

5,5

15

$1

76

,29

4$

6.1

4

60

6 un

its in

ser

ies

20

00

59

9.7

5%

$2

04

,40

6$

17

6,2

94

$6

.11

60

6 un

its in

ser

ies

20

00

0.5

99

.98

%$

33

3,2

70

$1

76

,29

4$

6.4

4

60

011

uni

ts in

ser

ies

20

57

5.0

0%

$3

55

,48

8$

37

0,9

46

$1

.27

60

011

uni

ts in

ser

ies

20

0.5

97

.50

%$

94

4,2

65

$3

70

,94

6$

1.4

2

60

011

uni

ts in

ser

ies

20

02

09

0.0

0%

$5

55

,45

0$

37

0,9

46

$1

.32

60

011

uni

ts in

ser

ies

20

05

97

.50

%$

94

4,2

65

$3

70

,94

6$

1.4

2

60

0N

D2

00

0.5

99

.75

%N

DN

DN

D

60

0N

D2

00

02

09

9.0

0%

ND

ND

ND

60

0N

D2

00

05

99

.75

%N

DN

DN

D

60

0N

D2

00

00

.59

9.9

8%

ND

ND

ND

60

00

ND

20

57

5.0

0%

ND

ND

ND

60

00

ND

20

0.5

97

.50

%N

DN

DN

D

60

00

ND

20

02

09

0.0

0%

ND

ND

ND

60

00

ND

20

05

97

.50

%N

DN

DN

D

60

00

ND

20

00

.59

9.7

5%

ND

ND

ND

60

00

ND

20

00

20

99

.00

%N

DN

DN

D

60

00

ND

20

00

59

9.7

5%

ND

ND

ND

Table 2-17Expense Summary for Air Stripping Systems – Aspiration






67

Sensitivity Parameter Capital Cost($)

Annual O&M($)

Total AnnualCost ($)

Unit Cost($/1000 gal)

NOM Fouling

BTEX Load

Design Life3

2 years $288,834 $83,852 $243,604 $0.77

10 years $288,834 $83,852 $124,976 $0.40

30 years $288,834 $83,852 $107,129 $0.34

Literature review indicates that packed tower performance should beunaffected by BTEX. 2

Low Fouling

Moderate Fouling

High Fouling 1

$288,834 $83,852 $107,129 $0.34

$288,834

$288,834

$344,379

$83,852

$83,852

$107,129

$107,129

$0.34

$0.34

BTEX at 20 µg/L each


$118,865 $146,618 $0.46

No BTEX present


Table 2-18Sensitivity Analysis for Air Stripping Systems – Packed Tower600 gpm system, influent = 200 µg/L, effluent = 5 µg/L MTBE

1 Increased costs include a NaOCl feed system (chemical storage tank, feed pump, controls, piping, etc.)for a dosage of 5 mg/L free chlorine. 12.5% NaOCl solution estimated at $80/30 gallon totes.

2 BTEX compounds are more easily removed through air stripping due to their higher Henry’s constants. 3 At a 7% discount rate.

68

Sensitivity Parameter Capital Cost($)

Annual O&M($)

Total AnnualCost ($)

Unit Cost($/1000 gal)

NOM Fouling

BTEX Load

Design Life3

2 years $675,085 $249,789 $623,173 $1.98

10 years $675,085 $249,789 $345,906 $1.10

30 years $675,085 $249,789 $304,191 $0.96

Moderate Fouling

High Fouling 1

Literature review indicates that low profile performance should beunaffected by BTEX. 2


$675,085 $249,789 $304,191 $0.96

$0.96

$675,085 $249,789 $304,191 $0.96Low Fouling

No BTEX present



$284,802 $343,681 $1.09

$675,085 $249,789 $304,191

$730,630

Table 2-19Sensitivity Analysis for Air Stripping Systems – Low Profile

600 gpm system, influent = 200 µg/L, effluent = 5 µg/L MTBE

1 Increased costs include a NaOCl feed system (chemical storage tank, feed pump, controls, piping, etc.)for a dosage of 5 mg/L free chlorine. 12.5% NaOCl solution estimated at $80/30 gallon totes.

2 BTEX compounds are more easily removed through air stripping due to their higher Henry’s constants. 3 At a 7% discount rate.

2.4 Off-gas Treatment

Off-gas treatment of the MTBE-contaminated air stream from an air stripping system is oftenrequired prior to discharge to the atmosphere. Treatment alternatives for removing MTBE inthe air discharge include: carbon adsorption, thermal and catalytic oxidation, biologicaltreatment, and gas-phase chemical oxidation. This section will evaluate alternative techno-logies for off-gas treatment for air streams containing low concentrations of MTBE (less than10 ppmv or parts per million on a volume basis). Selection of off-gas treatment technologiesfor MTBE is a key factor in cost estimates for the overall air stripping technology evaluation(Figure 2-5). The cost of off-gas treatment is generally proportional to the volume of gas (air)being treated. Thus, the higher the air/water ratio in an air stripping system, the higher thecosts. Typical volumetric air/water ratios for MTBE removal range from 100 to 200. Thisresults in large gas volumes with very low concentrations of MTBE. For example, given amedium sized groundwater treatment system (600 gpm) with an influent water concentrationof 200 µg/L MTBE, an air stripper operating at 90 percent efficiency emits air containing0.3 ppmv MTBE.

The regulations that define the maximum emissions limit of MTBE into the atmosphere varyfrom state to state and district to district. Under the Clean Air Act, MTBE is categorized asa hazardous air pollutant (HAP). In most states, agencies require off-gas treatment inconjunction with an air stripper. In the South Coast AQMD, for example, the emission level

69

Figure 2-5. Cost of off-gas treatment technologies as a function of air flow rate.

Total Costs ($/yr) for Off-Gas Treatment of5 ppmv MTBE

0

1,000,000

2,000,000

3,000,000

4,000,000

5,000,000

6,000,000

0 20,000 40,000 60,000 80,000 100,000 120,000 140,000

air flow rate (cfm)

$/yr

GACRecuperative Thermal OxidationRecuperative Flameless Thermal OxidationRecuperative Catalytic Oxidation

air flow rate (cfm)

$/yr

Total Costs ($/yr) for Off-gas Treatmentof 5 ppmv MTBE

at which control is required is 1 lb/day of VOC release. Other districts may require differentemission standards based on criteria including location of the site and regional ambient airquality (attainment or nonattainment status).

In this chapter, the 1 lb/day emissions limit is used for illustrative purposes and it has beenassumed that MTBE is the only VOC in the off-gas stream. In Table 2-20 below, massemissions of MTBE produced by an air stripper are shown as a function of the flow rate andinfluent concentration. In the case of the highest flow rate scenario (6,000 gpm), off-gastreatment must be employed for groundwater containing 200 µg/L and 2,000 µg/L MTBEwith respective required removal efficiencies of 93 and 99.3 percent. In the case of the lowestflow rate scenario (60 gpm), only groundwater systems containing a concentration of 2,000µg/L MTBE or higher will require off-gas control.

A brief description of each off-gas treatment technology, including its advantages anddisadvantages, is presented in Table 2-21.

Table 2-20MTBE Air Stripper System Off-gas Removal Rates Required to Meet 1 lb/day Discharge Limit

70

*All information obtained from vendors.

CWATER

µg/L

2,000

200

20

CAIR

ppmv(MTBE)

3.5

0.3

0.02

lb/dayEmission

1.44

0.14

0.01

RemovalRequired for

Off-GasControl

30%

N/R

N/R

lb/dayEmission

14.4

1.41

0.11

RemovalRequired for

Off-GasControl

93%

29%

N/R

lb/dayEmission

144

14.1

1.08

Removal Required

for Off-GasControl

99.3%

93%

7.4%

Groundwater Flow Rates

60 GPM 600 GPM 6,000 GPM

1. Based on an AWR ratio of 160 and a groundwater effluent target concentration of 5 µg/L; assumes a 1 lb/daycontrol limit.

2. N/R = off-gas control not required under the scenarios stated above.3. A higher AWR ratio will result in lower CAIR but the same lb/day emission assuming the same removal

efficiencies.4. The presence of other volatile organic compounds will result in higher lb/day emissions.

71

Table 2-21D

escription of Off-gas Treatm

ent TechnologiesOff-Gas

TreatmentTechnology

GAC

ThermalOxidation

CatalyticOxidation

Biofiltration

AdvancedOxidationProcess

BriefDescription

Removal of pollutants bymeans of physical adsorptiononto activated carbon grains.

Destruction of pollutants bythermal oxidation.

Alternative oxidation processincorporating a reducedtemperature burner andcatalyst bed.

Adsorption onto natural orinert media wheremicroorganisms degrade andoxidize the pollutant.

Oxidation of organic compoundsusing reagents such asozone, hydrogen peroxide,titanium oxide and UV.

SystemComponents

• Fan• Pretreat heater• Adsorber vessels• GAC• Monitoring

Instrumentation

• Fan• Heater• Combustion chamber• Fuel• Stack

• Fan• Heater• Catalyst• Fuel• Stack

• Blower• Humidifier• Biofilter bed

• UV Photolytic reactor• Ozone generator• Reaction Vessel

Advantages

• 100% removal ratesattained.

• Regenerative carbon bedsallow material recovery.

• Efficient at removing amany contaminants fromgas streams in whichconcentrations and flow areapt to vary.

• Catalyzed reactionsproceed at lowertemperatures and allowlower energy requirements.

• Environmentally safe andcreate no secondarypollution.

• Isothermal techniquerequiring no fuel.

Disadvantages

• Spent carbon transport anddisposal may require hazardouswaste handling permits.

• Humid air streams require heatingfor MTBE control in the presenceof competing water vapor.

• Necessity for high temperatureoperation and supplementaryfuel

• Nitrogen oxides generated bycombustion process.

• Potential catalyst poisoningcaused by dust and heavymetals

• Increase in VOC content maycause temperature rise anddestroy the catalyst.

• Limited examples of MTBEoxidation.

• Limited by the compound’sinherent biodegradability.

• Can require large areas due toslow biodegradation rates.

• Microbial activity is verysensitive to biofilter conditions.

• Oxidation process without theproduction of nitrogen oxides.

• Gas phase application of AOP islimited compared to the treat-ment of aqueous systems.

2.4.1 Vapor Phase GAC Adsorption

System Description

Vapor phase GAC adsorption is a well-known technology used to remove a wide range oforganic compounds from air streams. Removal of organic compounds from air occurs bymeans of physical adsorption on activated carbon. The air passes through a fixed bed ofactivated carbon until the capacity of the carbon is nearly exhausted. Activated carbons arederived from a variety of carbonaceous materials, including hard wood, coal, petroleum coke,fruit pits, and coconut shells. Gas phase carbon adsorbers are designed as either single-passor regenerative beds, depending on the mass of chemicals in the feed stream. For detailedinformation on GAC treatment systems in liquid-phase applications, see Chapter 4.

Advantages/Disadvantages

See Table 2-21 for a summary list of advantages and disadvantages for this technology. Themost important advantages of GAC include the following:

• GAC operates effectively in cases of low MTBE concentrations (less than 100 ppmv);whereas the effectiveness of alternative off-gas treatment technologies, including thermaland catalytic oxidation, is not well documented for very dilute air streams.

• GAC is readily available from local suppliers.

• GAC adsorbers are simple to install and easy to operate compared to technologiesinvolving thermal or catalytic processes.

• GAC is not operated at high temperatures.

The key disadvantages of GAC include:

• MTBE adsorbs poorly compared to other BTEX compounds on GAC.

• The capacity of GAC for MTBE removal is reduced by competing adsorbates (e.g., otherorganics, water vapor).

• Off-gas streams produced by air stripping are near 100 percent humidity, which willdecrease the effectiveness of GAC and, thus, require heating or dehumidifying of the airstream.

• Raising the temperature of the off-gas to 85°F reduces the relative humidity to 50 percent.However, high air temperatures (above 80°F) will in turn reduce the adsorptive capacity ofGAC.

• If regenerative beds are not used on-site, the carbon needs to be regenerated off-site. Incomparison, thermal processes or biofiltration do not generate secondary waste streams.

72

Key Variables/Design Parameters

The two key design factors for GAC systems include: (1) identification of the most effectivecarbon adsorbent for the removal of MTBE, and (2) selection of the bed length and cross-sectional area. The key design parameters and their effect on regeneration frequency arelisted in Table 2-22. For example, increasing relative humidity (greater than 50 percent)causes premature breakthrough resulting in increased regeneration frequencies. Aspreviously noted, the air streams from air strippers will contain 100 percent relative humidity.Heating of the air to reduce its relative humidity or dehumidifying of the air stream is oftenrequired to increase GAC capacity for the organic compound of concern.

System Installation and Manufacturers

There are no known full-scale installations of GAC beds designed to remove MTBE from thevapor phase. In general, carbon adsorption has been used for at least four decades to providesolvent recovery and odor control, and treatment of gases containing other VOCs. Manycommercial grades of activated carbon are produced throughout the United States by morethan 20 vendors. Representative manufacturers include Calgon Carbon Corporation(Pittsburgh, PA), U.S. Filter/Westates (Los Angeles, CA), Carbochem, Inc. (Ardmore, PA),Tigg Corporation (Bridgeville, PA), and Nucon International Inc. (Columbus, OH). Adsorberbeds are produced by many manufacturers, including Indusco Environmental (Atlanta, GA),Norit Americas Inc. (Atlanta, GA), and Tigg Corporation (Bridgeville, PA). A variety ofservices for supplying and periodically replacing GAC are available (see Chapter 4 for moreinformation).

73

DesignParameter

Air Temperature

Humidity >50%

InfluentConcentration

Superficial Velocity

Bed Length

Isotherm Constant(K)

Effect of Increasing DesignParameter on Regeneration

Frequency

Õ Regeneration Frequency




Ô Regeneration Frequency

Ô Regeneration Frequency

Effect ofIncreasing Design Parameter

on Costs

Õ Annual Costs

Õ Annual Costs Õ Capital Costs

Õ Annual Costs

Õ Annual Costs

Õ Annual Costs Õ Capital Costs

Ô Annual Costs

Table 2-22GAC Design Variables

Based on MTBE adsorption equilibrium data from manufacturers, the air phase adsorptioncapacity of GAC ranges from 6 to 10 percent (g MTBE/100 g carbon) at 1 atm and 60°F forconcentrations up to 100 ppmv (Calgon Carbon Corporation, 1998). However, an air stripperdesigned for MTBE removal employs a relatively high air/water ratio, diluting the MTBE airstream concentrations to much less than 10 ppmv (see Table 2-20). The more dilute theMTBE concentration, the lower the carbon adsorption capacity. In addition, humid off-gasesfrom an air stripper reduce the capacity of GAC for MTBE adsorption. Therefore, GAC forMTBE likely ranges from two to five percent by weight.

Technical Implementability

GAC adsorbers are either packaged or custom built according to site conditions. Componentscan be separately installed, connected, and then filled with carbon. In addition to the vessel,the system includes auxiliary valves, gauges, and bed sampling ports. The exhausted carbonmay be disposed of or regenerated at an off-site facility.

2.4.2 Thermal and Catalytic Oxidation

System Description

Other applicable technologies to treat MTBE off-gases generated from air stripping arethermal and catalytic oxidation. Thermal and catalytic oxidation processes can reliably andsafely destroy up to 99 percent of the MTBE emissions. Based on manufacturer data, MTBEwill readily burn in an oxidizer and will be converted to carbon dioxide and water as easilyas other low molecular weight hydrocarbons (Thermatrix, Inc., 1998). However, a full-scaleapplication of thermal processes to treat an air stream containing MTBE has not been reported.

Thermal oxidizers are devices in which the air stream containing organic compounds ispassed over or through a burner flame or other pre-heat device into a chamber where theorganic compounds are oxidized. The units are typically single-chamber, refractory-linedoxidizers equipped with a propane or natural gas burner and a stack (Thermatrix, Inc., 1998).

Catalytic oxidizers incorporate a bed of catalytic surfaces to initiate and promote oxidationat lower temperatures (600 to 800°F) than would be used in thermal oxidation (1,350 to1,800°F). Typical catalysts are composed of a ceramic or metal substrate with a high surfacearea to volume ratio. Covering the ceramic or metal substrate is a thin layer of catalyticmaterial. The most common catalytic materials used in the environmental industry are nobleand base metals, such as platinum, palladium, and vanadium, dispersed on substratesincluding aluminum oxide, silicon oxide, titanium oxide, or crystalline alumina.

In most cases, thermal or catalytic oxidation process costs can be lowered using heat recoveryequipment. Oxidizers incorporate heat exchangers as recuperative or regenerative systems.In recuperative oxidizers, an air-to-air heat exchanger transfers the energy of the exhaust to

74

the incoming process gas stream to reduce auxiliary fuel costs; up to 70 percent of the heatof the exhaust gases can be recovered. In regenerative oxidizers, beds of ceramic materialsare used as a medium to transfer the heat to the incoming process stream; a maximum of 95percent heat recovery can be obtained. Typically, the capital costs for regenerative oxidizersare higher than recuperative systems (Advanced Environmental Systems, 1998).


See Table 2-21 for a summary list of the typical advantages and disadvantages of thermal andcatalytic oxidation systems. The most important advantages of thermal oxidation include thefollowing:

• Upon startup, destruction efficiencies consistently reach 99.9 percent for a variety oforganic compounds.

• Heat recovery allows for a recuperation of heating costs. This is an advantage over carbonadsorption, where heating of humid air is required and the costs are not recovered.

• Thermal oxidation is highly flexible and performs reliably under changing conditions(when concentrations and flow vary). Thermal oxidization is also an advantage when theair stream contains catalyst inhibitors (e.g., sulfur).

The key disadvantages of thermal oxidizers include:

• Compared to catalyst beds, thermal oxidation requires high temperature operation andrelatively large amounts of supplementary fuel unless heat recovery is efficiently utilized.This translates into higher annual costs.

• Thermal oxidizers employ specialty materials required to withstand extreme temperatures(1,800°F). The cost of construction materials can be greater than the associated capital costfor a catalytic oxidizer.

• There is limited data on the efficiency and costs of treating high volume air streams withlow MTBE concentrations.

• Generation of combustion by-products are possible (e.g., carbon monoxide [CO], oxidesof nitrogen [NOx], oxides of sulfur [SOx]).

The key advantages of catalytic oxidizers include:

• Catalytic oxidizers operate at lower temperatures compared to thermal oxidation, allowinglower energy requirements and directly translating into improved economics for fuel use.

• Catalytic oxidizers do not require temperature-resistant construction materials.

75

The most significant disadvantages of catalytic oxidizers include:

• If the composition of the gas stream is known to vary, a sudden increase in VOC contentcan cause temperatures to reach levels that deactivate or destroy catalysts. For MTBEtreatment applications, periods of high MTBE loadings in the air stripping off-gas are notexpected and, therefore, high temperature surges are not likely to occur within the catalystbed for MTBE treatment.

• Catalyst beds are more easily clogged and destroyed by dust compared to thermaloxidizers.

• There is limited data on the efficiency and costs of treating high volume air streams withlow MTBE concentrations.

• Generation of combustion by-products is possible (e.g., CO, NOx, SOx).

Key Variable/Design Parameters

Thermal Oxidizers. The key design parameters for thermal oxidizers are gas flow rate,temperature and pressure of the inlet gas, concentration of the contaminant, fuelrequirements and residence time in the reaction chamber. The following design conditionsare typically specified: residence time in the combustion chamber ranges from 0.5 to 1.0seconds, and operating temperatures are within the limits of 1,350 to 1,800°F (730 to 980°C).Gas flow rates can range from 300 to 30,000 standard cubic feet per minute. The efficiencyof thermal oxidation for some VOCs is 90 percent at lower temperatures (1,350°F), butreaches up to 99.9 percent above 1,500°F for most organic compounds.

Catalytic Oxidizers. Similar to thermal oxidization, design variables for catalytic oxidizersare gas flow rate, temperature and pressure of the inlet gas, concentration of the contaminant,fuel requirements, residence time in the reaction chamber, and space velocity in the catalystbed. The space velocity is defined as the volumetric gas rate at operating conditions dividedby the volume of the catalyst chamber. Catalytic oxidizers meet stringent air emissionsrequirements and demonstrate removal efficiencies up to 99 percent.

System Installations and Manufacturers

There are no reported pilot-scale or full-scale applications of thermal processes to treat gasstreams containing MTBE; however, effective treatment is expected. Thermal oxidation iseffective for treating most mixed hydrocarbon-laden fumes and deodorizing foul-smellinggases. Catalytic oxidation has also been widely used and is now a mature technology used tocontrol air streams from wastewater, groundwater, and soil remediation projects. Catalyticoxidation technology continues to evolve with new heat recovery systems and catalystmaterials to improve efficiency. There are over 40 vendors of thermal oxidizers nationwide(see Appendix 2D) and several proprietary variations are available. Catalytic oxidationsystems specifically designed for remediation are manufactured by more than 20 companies.

76

ABB Preheater Inc. (Wellsville, Inc.) patented Combu-Changer® regenerative thermaloxidizer system, Cor-Pak® thermal oxidizer, and Cor-Pak® catalytic oxidizer. FlamelessThermal Oxidizer is offered by Thermatrix, Inc. (San Jose, CA) and AdvancedEnvironmental Systems Inc. (Elkton, MD). Other manufacturers include MegtecCorporation (De Pere, WI), North American Manufacturing Co. (Cleveland, OH), CatalyticProducts International (Lake Zurich, IL), and CVM Corporation (Wilmington, DE).


Oxidizers are constructed either as package or field-erected units. The only moving part isthe blower that will likely be connected to the influent air stream of the air stripper.Monitoring of the oxidizer is augmented by instrumentation that allows the operator tomonitor in real time the working temperature, flow rate, and oxidation efficiency; mainten-ance requirements are minimized using these controls. Installation includes site preparationand construction of ductwork and foundations. One disadvantage of full-scale applicationswhere air volumes are high is that the oxidizer footprint can be large — greater than 30 feetin length and 20 feet in height (Advanced Environmental Systems, 1998). Smaller units canbe installed without any restrictions.

2.4.3 Biological Treatment

System Description

Another option for off-gas control of organic compounds is the use of gas-phase biologicalaerobic oxidation. Studies have shown that MTBE can be degraded to carbon dioxide andwater in biofilters, provided that sufficient residence time is available for growth of theMTBE biodegraders. Lag times ranging from 3 weeks to 12 months have been reported(Eweis et al., 1998). Biotreatment technologies employ either a biofilm on inert media (e.g.,GAC) or a compost matrix.


See Table 2-21 for a summary list of the advantages and disadvantages of biologicaloxidation systems. The most important advantages of biofiltration include:

• Oxidation of organic chemicals is mediated by microbes in a natural media, withoutthermal processes.

• Biological filters contain environmentally safe components that create no secondarypollution.

• Operating costs are likely to be low.

77

The key disadvantages of biofiltration include the following:

• The biofiltration process is limited by the contaminant’s inherent rate of biodegradability.Process may be highly sensitive to changing influent conditions.

• Performance may be low during the initial period of microorganism acclimation.

• There may be a slow transient response of biological systems to variations in the inletconcentration and flow.

• MTBE degradation appears to be slow and may not be sustainable due to low cell yields.However, research is on-going and biodegradation of MTBE may prove to be more feasiblein the future.

Key Variable/Design Parameters

The key design parameters for biofilters are gas velocity across the bed face, length andvolume of the bed, contaminant loading, and inlet gas flow rate. Volumetric productioncapacity, typically expressed as biomass per unit volume per unit time, tends to increase withcontaminant concentration. Controlling variables are temperature, moisture content, pH,porosity, and nutrient concentrations. The optimum operating ranges for these parametersdepends on the compound. Humidity must be carefully monitored in biofilters to avoid waterclogging of the compost matrix pores.

System Installations and Manufacturers

Small biofilters are used extensively in the United States in compost piles and wastewaterleach fields, removing odorous gases. This technology has also been implemented in Europeand Japan where at least 500 permanent biofilters are in operation (Bohn BiofilterCorporation, 1998). The technology has recently received more attention in the UnitedStates. Ametek® Rotron Biofiltration Products (Saugerties, NY) has installed full-scaleBiocube™ Aerobic Filters to treat gasoline hydrocarbon gases from soil vapor extractionsystems. Bohn Biofilter Corporation (Tucson, AZ) has been involved in more than 20permanent biofilter beds in the United States. Monsanto Enviro-Chem (St. Louis, MO) hasalso recently become a major supplier of biofilters.

Specific strains of bacteria may be introduced into the filter to preferentially degrade MTBE.Bench-scale reactors used in laboratory studies have maintained stable activity for MTBEdegradation. However, without enrichment and recycle, cell growth rates and cell yields arelow (0.05/d and 0.1 to 0.3 g/g-MTBE, respectively). MTBE biofiltration research is currentlyon-going at several universities and industrial laboratories, including the University ofCalifornia, Davis and Equilon Enterprises, LLC. (Eweis et al., 1998; Salanitro et al., 1999).While isolated strains of microorganisms are capable of degrading MTBE, field tests arerequired to determine persistence, stability, and metabolic activity in engineered processesunder actual processing conditions.

78


In compost biofilters, the medium is sometimes turned at intervals and must be replacedevery 2 to 4 years. Systems using inert material are operated without any replacementschedule. The system components include a humidifier to saturate the vapor, blower, biofilterbed, distribution ducts beneath the bed, and a drainage pipe for recycling water to thehumidifier. However, in order to treat the off-gas from an air stripping system, a humidifierand blower are not likely needed. Most biofilters also include a direct irrigation systemconsisting of sprinklers above the medium or soaker hoses within the medium. These can beused to add water when the humidified gas stream is not sufficient to meet the needs of themicrobes and nutrients to maintain peak biological activity. Some systems incorporateinterlocking trays to design for expandable modules.

79

2.5 Evaluation and Screening ofOff-gas Treatment Technologies

2.5.1 Permitting

Among the off-gas treatment technologies, thermal oxidation, catalytic oxidation, or GACadsorption systems would likely be accepted most easily by regulatory agencies as aneffective means of treating vapor emissions contaminated with MTBE. When used inconjunction with the selected air stripping technology, both GAC adsorption and thermaloxidation are listed as best available control technologies (BACT) and, therefore, areexpected to meet stringent requirements for removal efficiency. With respect to biofiltration,the compliance issues focus primarily on the need to demonstrate reliable and consistentbiodegradation of MTBE. Periods of failure and low microorganism performance have beenreported in laboratory studies (Salanitro et al., 1994). Regulators will likely inspect the siteand review biofilter operations on the basis of stable performance, complete destruction ofMTBE, and by-product formation. Permitting is highly site-specific depending on localdemographics, site location, and the regional ambient air quality.

2.5.2 Flow Rate

All of the chosen off-gas treatment technologies can handle a wide range of air stream flowrates. Carbon adsorption, biofiltration, and thermal and catalytic oxidation can all operatebetween several hundred cubic feet per minute (cfm) to over 100,000 cfm. However, capitalinvestment and operating costs increase proportionally to the flow rate. Capital expenses,annual O&M costs, and total amortized operating costs are listed in Tables 2-23, 2-24, and2-25, respectively. Biofiltration is not addressed in these tables because no vendor estimateswere provided, as discussed at the end of this section.

2.5.3 Removal Efficiency

Some California air districts (e.g., South Coast AQMD) enforce an emission control standardof 1 pound VOC/day from an air stripper. For this analysis, off-gas treatment has beenimplemented when MTBE concentrations at the air stripper exit result in emissions in excessof the 1 lb/day mass limit as shown in Table 2-26. An off-gas treatment unit would berequired to destroy enough MTBE to fall below the 1 lb/day discharge limit.

At the low MTBE air concentrations expected in the off-gas from air strippers, GAC, andthermal and catalytic oxidizers are expected to achieve greater than 99 percent removal ofMTBE. However, due to the low affinity for MTBE adsorption onto vapor phase GAC, fieldverification of a steady, continuous removal efficiency is required.

Typical destruction efficiencies of thermal and catalytic oxidation are 99 percent and 97percent, respectively, for concentrations of most organic compounds greater than 5,000 ppmv

81

(Schen et al., 1993). However, potential concentrations of MTBE generated from airstripping are three orders of magnitude lower than 5,000 ppmv. There is little or no inherentheating value in the concentrations of MTBE emitted from an air stripper. Costs for MTBEdestruction will, therefore, be governed by the amount of energy required to operate the off-gas treatment system, and depend very little on the influent MTBE gas concentration.

Biofiltration has demonstrated between 90 to 98 percent removal efficiencies of otherorganic compounds at concentrations between 20 ppmv and 1,000 ppmv. Consistent removalefficiencies have not been demonstrated for concentrations of VOCs below 20 ppmv or forany concentrations of MTBE. Thus, it is expected that a biofiltration off-gas treatment unitfor MTBE removal will require significant field verification prior to regulatory approval.

82

83

Flow(gpm)

InfluentMTBE(ppmv)

GACRecuperative

ThermalOxidation

RecuperativeFlamelessThermal

Oxidation

RecuperativeCatalyticOxidation

Non-RecuperativeCatalytic Oxidation

60 0.5 $11,100 $199,800 $732,600 $166,500 ND

600 0.5 $111,000 $666,000 $1,443,000 $510,600 $333,000

6000 0.5 $1,010,100 $3,889,440 $5,550,000 $3,152,400 ND

60 5 $11,100 $199,800 $732,600 $166,500 ND

600 5 $111,000 $666,000 $1,443,000 $510,600 $333,0006000 5 $1,010,100 $3,889,440 $5,550,000 $3,152,400 ND Table 2-23

Initial Capital E

xpenses for Off-gas Treatm

ent System

s

ND = no data available.

Capital Expenses include:EquipmentPiping, valves, electrical (30%)Site work (10%)Contractor O&P (15%)Engineering (15%)Contingency (20%)

Data is based on vendor information provided for the following scenarios:MTBE Removal:

Influent 5 ppmv, Effluent 0.03 ppmv.Air Temperature: 65°F.

Flow Rate: 60 gpm system: 1,200 cfm (AWR 150:1).600 gpm system: 12,000 cfm (AWR 150:1).6000 gpm system: 120,000 cfm (AWR 150:1).

84

Flow(gpm)

InfluentMTBE(ppmv)

GACRecuperative

ThermalOxidation

RecuperativeFlameless Thermal

Oxidation*

RecuperativeCatalytic Oxidation


60 0.5 $16,076 $21,083 $50,006 $24,332 ND

600 0.5 $67,165 $117,229 $61,920 $150,063 $382,403

6000 0.5 $640,448 $1,078,688 $354,786 $1,407,032 ND

60 5 $57,704 $21,083 $50,006 $24,332 ND

600 5 $483,440 $117,229 $61,920 $150,063 $382,4036000 5 $4,803,200 $1,078,688 $354,786 $1,407,032 ND

Table 2-24A

nnual O&

MC

osts for Off-gas Treatm

ent System

s

ND = no data available

O&M Costs include: 1. Power costs (at $0.08/kWhr).2. Fuel costs (at $5.00/ MBtu).3. Analytical costs estimated at $200 per sample; see Table 2-11 for the number of samples required for each system.4. GAC includes GAC changeout at $1.50/ lb, use rate of 0.05 lb/1000 ft3 air at 5 ppmv MTBE and 0.006 lb/1000 ft3 at 0.5 ppmv MTBE.5. Recuperative Flameless Thermal Oxidizer (at 600 gpm) incorporates a rotor concentrator.

Data is based on vendor information provided for the following scenarios:1. MTBE Removal:

Influent 5 ppmv, Effluent 0.03 ppmv: 99.4% removal.Air Temperature: 65°F (except for GAC - air is heated to 85°F).

2. Flow Rate: 60 gpm system: 1,200 cfm (AWR 150:1).600 gpm system: 12,000 cfm (AWR 150:1).6000 gpm system: 120,000 cfm (AWR 150:1).

Cost estimate data provided by:Carbon Adsorption: Calgon Carbon (Pittsburgh, PA).Recuperative Thermal Oxidation: Advanced Environmental Systems (Elkton, MD).Recuperative Flameless Thermal Oxidation: Thermatrix (San Jose, CA).Recuperative Catalytic Thermal Oxidation: Advanced Environmental Systems (Elkton, MD).Non-Recuperative Catalytic Thermal Oxidation: Advanced Environmental Systems (Elkton, MD).

85

Flow (gpm)InfluentMTBE(ppmv)

GAC RecuperativeThermal Oxidation

RecuperativeFlamelessThermal

Oxidation

RecuperativeCatalyticOxidation


60 0.5 $0.54 $1.18 $3.46 $1.20 ND

600 0.5 $0.24 $0.54 $0.57 $0.61 $1.30

6000 0.5 $0.23 $0.44 $0.25 $0.53 ND

60 5 $1.86 $1.18 $3.46 $1.20 ND

600 5 $1.56 $0.54 $0.57 $0.61 $1.30

6000 5 $1.55 $0.44 $0.25 $0.53 ND

Table 2-25Total A

mortized O

perating Costs ($/1,000 G

allons Treated)* for Off-gas Treatm

ent

N/D = No data available.Amortization based on a 30-year period at a 7% discount rate.*To convert costs to $/acre-ft, multiply by 326.

O&M Costs include: 1. Power costs (at $0.08/kWhr)2. Fuel costs (at $5.00/ MBtu)3. Analytical costs estimated at $200 per sample; see Table 2-11 for the number of samples required for each system.4. GAC includes GAC changeout at $1.50/ lb, use rate of 0.05 lb/1000 ft3 air at 5 ppmv MTBE and 0.006 lb/1000 ft3 at 0.5 ppmv MTBE.5. Recuperative Flameless Thermal Oxidizer (at 600 gpm) incorporates a rotor concentrator.

Data is based on vendor information provided for the following scenarios:1. MTBE Removal:

Influent 5 ppmv, Effluent 0.03 ppmv: 99.4% removal.Air Temperature: 65°F (except for GAC - air is heated to 85°F).

2. Flow Rate: 60 gpm system: 1,200 cfm (AWR 150:1).600 gpm system: 12,000 cfm (AWR 150:1).6000 gpm system: 120,000 cfm (AWR 150:1).


2.5.4 Other Characteristics

Table 2-27 contains a summary of the characteristics of off-gas technologies with respect toreliability, flexibility, adaptability, and potential for modification.

Reliability

Off-gas treatment is not well documented for the removal of MTBE, although experience withother VOCs indicates that GAC and thermal and catalytic oxidation technologies meetreliability criteria. A fixed bed carbon adsorber has demonstrated a high degree of systemreliability due to process simplicity; however, reliability for MTBE removal will likely requirefield demonstration. Thermal oxidizers have few moving parts and are not known tomechanically fail and are, thus, expected to be highly reliable. The problems associated withrecuperative or regenerative systems are likely to be related to heat exchanger fouling.Catalytic oxidation systems demonstrate only moderate reliability since feed streamscontaining dust and particulates can plug the catalyst section. Biofilters demonstrate lowreliability because the activity of the microorganisms for MTBE destruction is unknown andcould periodically drop to zero if influent concentrations are low. Also, there is less experiencerelative to the other off-gas treatment technologies with biofilter maintenance and operation.

86

Notes:1. Based on an air temperature of 65°F.2. The presence of other volatile organic compounds in the air stream will result in a higher total lb/day emission.

100

150

180

3.7 ppmv

2.5 ppmv

2.0 ppmv

0.4 ppmv

0.3 ppmv

0.2 ppmv

0.04 ppmv

0.03 ppmv

0.02 ppmv

Air Stripper Flow Chart

60 GPM 600 GPM 6,000 GPMAir/Water Ratio

Effluent Air Stripper Concentration of MTBE initiating the need for off-gas treatment

(emissions limit of 1 lb/day MTBE)

Table 2-26Air Stripper Exit Concentrations Initiating Off-gas Treatment

87

Table 2-27C

omparison of O

ff-gas Treatment Technologies

Off-Gas TreatmentTechnology

GAC

Thermal Oxidation

Catalytic Oxidation

Biofiltration

Reliability

HIGH• Carbon adsorption cycle can be

relied upon provided thatoperating procedures arefollowed.

HIGH• Oxidizers can be relied upon

provided that operatingprocedures are followed.

MEDIUM• Failure of catalysts caused by

certain heavy metals, dust,sulfur and chlorine can occur.

LOW• Process is not subject to

significant mechanical failure,but biological activity can beunreliable.

Flexibility

MEDIUM• Air flow rate design can range

from 300 to 100,000 cfm.• Molecular weight of target

compounds varies from 45 and130.

• Any compound will beadsorbed in proportion to itsgas phase concentrations, butretention on the bed dependson volatility and nature of thecompound.

HIGH• Air flow rate design can range

from 100 to 100,000 cfm.• Most organic compounds and

hydrocarbons will becompletely destroyed.

HIGH• Air flow rate design can range

from 100 to 100,000 cfm.• Most organic compounds and

hydrocarbons will becompletely destroyed.

LOW• Changing air flow rates may

reduce removal efficiency.• Vapor phase MTBE degradation

not well demonstrated.• Lag time delays onset of MTBE

oxidation.

Adaptability

MEDIUM• Variations in air quality can

reduce the number ofadsorption sites available to thecontaminant of concern.

• Particles clog the adsorbentbed.

HIGH• Thermal oxidizers handle

variations in influent conditionsand air quality, continuallyremoving VOCs to desirablelevels.

LOW• Variations in air quality and

influent conditions affect theVOC-air mixture and may causea temperature rise resulting indecreased performance.

LOW• Variation in air quality and

influent conditions candecrease or eliminate biologicalactivity.


MEDIUM• New units can be easily added

in modular fashion.

LOW• Thermal oxidizers can be

modified by raising theoperating temperature, but itleads to higher fuelrequirements.

LOW• Raising temperatures in excess

of the maximum designdeactivates catalysts.

• Replacement of the catalystsection can be cost prohibitive.

LOW• Biofilters can be enhanced by

seeding differently, adding co-substrates, etc. but responsetime to modified conditions isslow.

Flexibility

GAC is moderately flexible, referring to its effectiveness over a range of air flow rates, targetcompound properties, and target compound concentrations. All organic compounds will beadsorbed to the GAC in proportion to their gas phase concentration, but retention on thecarbon bed depends on the physical-chemical nature of the compound (see Chapter 4 formore information). Thermal treatment exhibits high flexibility and can be designed for bothlow and high flow rates (100 to 170,000 cfm). Biofiltration is currently considered to beminimally flexible (subject to change as a result of on-going research) due to the long lagtime for initial MTBE oxidation.

Adaptability

Thermal oxidization is the most adaptable off-gas treatment technology. The performance ofa thermal oxidizer is not affected by changing influent gas concentrations. Catalytic beds andbiofilters are much less adaptable. Catalytic beds are temperature sensitive; a rise intemperature from changing VOC-air mixtures may result in decreased performance orcatalyst failure. In biofiltration, fluctuating loadings of MTBE can be toxic to microbes, orcause microbes to preferentially metabolize other carbon substrates, leading to MTBEbreakthrough.

Potential for Modification

GAC systems can be modified easily by addition of parallel units. The other off-gastreatment technologies are not readily suited for modifications. The potential to enhanceremoval in a thermal oxidizer primarily rests on increasing the temperature but this requireshigher fuel use. Catalyst replacement is cost prohibitive and system modifications to thecatalyst section are technically limited. Modifications to operating biofilters are also limited.

2.5.5 By-products

It is not likely that large amounts of NOx, SOx, or CO will be generated by thermal oxidizersbecause advances in the design and operation have significantly decreased or eliminated theproduction of these oxides. Catalytic systems operate at relatively low temperatures,requiring less fuel than thermal oxidizers and normally have lower oxide emissions. Byproduct formation during biofiltration is not well documented. GAC treatment of off-gassesis expect to produce no by-products.

88

2.5.6 Cost Effectiveness

GAC

The capital costs associated with the installation of an on-site GAC adsorption system aresignificantly less than thermal and catalytic oxidation systems, but higher than biofiltration.Based on a 5 ppmv influent MTBE concentration, O&M costs are expected to be high dueto a relatively high frequency of regeneration or replacement. O&M cost estimates for GACincluded carbon changeouts and analytical costs (see Table 2-24) while those of the oxidationsystems included fuel costs, electric costs, and analytical costs. Estimates of carbon usagerates range from 50 to 150 lb GAC/day at 1,200 cfm (air flow rate from a 60 gpm watersystem using an air/water ratio of 150). For the highest assumed flow rate of 6,000 gpm, thecarbon usage rate increases to approximately 8,600 lb GAC/day based on a usage rate of 0.05lb GAC per 1000 cubic feet of air treated. Results of this cost analysis based on vendor dataare shown in Tables 2-23, 2-24, 2-25, 2-28 and 2-29. The unit costs ($/1,000 gallons treatedwater) of GAC gas-phase adsorption at 60, 600, and 6,000 gpm range from $1.86 to $1.55for a 5 ppmv influent MTBE concentration and $0.54 to $0.23 at 0.5 ppmv influent MTBEconcentration (see Table 2-25). At higher MTBE influent gas concentrations (i.e., 5 ppmv),these costs are generally not competitive with thermal and catalytic oxidation systems, aswould be predicted from the high carbon usage rate (8,600 pounds GAC/day).

At lower influent MTBE gas phase concentrations of 0.5 ppmv, carbon usage rates andannual O&M costs decrease (see Table 2-24) while thermal and catalytic oxidation costs areunaffected. This trend causes GAC to become the most cost-effective off-gas treatmenttechnology at the lower influent concentrations. The other factor that could cause GAC tobecome more cost-effective than thermal or catalytic oxidation is the projected number ofyears of operation. Because carbon capital costs are substantially lower than oxidation capitalcosts, a shorter amortization period could cause GAC to become more cost-effective thanoxidation. Consequently, if a drinking water treatment technology is only needed for a fewyears (10 years for a 60 gpm system, 2.5 years for a 600 gpm system, and 1 year for a 6,000gpm system), rather than the 30 years assumed for this evaluation, it is more cost effective toinstall a GAC system.

Thermal Treatment

Compared to other off-gas treatment technologies, the capital costs associated with thermaltreatment are the most expensive. The system has high capital and installation costs andvariable O&M costs. Estimated capital and O&M costs for oxidation systems are presentedin Tables 2-23, 2-24, 2-25, 2-28 and 2-29.

89

90

Flow(gpm)

System Name and Configuration Influent(ppmv)

Effluent(ppmv)

Removal (%) Capital Cost ($) Annual O&M ($) Unit Cost($/1000 gal)*

60 RTO: Burner, CombustionChamber, Stack

5 0.03 99.40% $199,800 $21,083 $1.18

60 Flameless RTO: Burner,Combustion Chamber, Stack

5 0.03 99.40% $732,600 $50,006 $3.46

60 RCO: Burner, Catalyst Bed, Stack 5 0.03 99.40% $166,500 $24,332 $1.20

60 GAC (1,000-lb bed) 5 0.03 99.40% $11,100 $57,704 $1.86


5 0.03 99.40% $666,000 $117,229 $0.54


5 0.03 99.40% $1,443,000 $61,920 $0.57

600 RCO: Burner, Catalyst Bed, Stack 5 0.03 99.40% $510,600 $150,063 $0.61

600 CO (No heat recovery): Burner,Catalyst Bed, Stack

5 0.03 99.40% $333,000 $1,407,032 $1.30

600 GAC (dual bed unit with 16,000 lbtotal GAC)

5 0.03 99.40% $111,000 $483,440 $1.56


5 0.03 99.40% $3,889,440 $1,078,688 $0.44


5 0.03 99.40% $5,550,000 $354,786 $0.25

6000 RCO: Burner, Catalyst Bed, Stack 5 0.03 99.40% $3,152,400 $1,407,032 $0.53

6000 GAC (7 dual-bed units, each with22,000 lb total GAC)

5 0.03 99.40% $1,010,100 $4,803,200 $1.55

Table 2-28C

ost Sum

mary for O

ff-gas Treatment Technologies at 5 ppm

v MT

BE

*To convert unit treatment costs to $/acre-ft, multiply by 326.


91

Flow(gpm) System Name and Configuration

Influent(ppmv)

Effluent(ppmv) Removal (%) Capital Cost ($) Annual O&M ($)

Unit Cost($/1000 gal)*


0.5 0.03 94.00% $199,800 $21,083 $1.18


0.5 0.03 94.00% $732,600 $50,006 $3.46

60 RCO: Burner, Catalyst Bed, Stack 0.5 0.03 94.00% $166,500 $24,332 $1.2060 GAC (1,000-lb bed) 0.5 0.03 94.00% $11,100 $16,076 $0.54


0.5 0.03 94.00% $666,000 $117,229 $0.54


0.5 0.03 94.00% $1,443,000 $61,920 $0.57

600 RCO: Burner, Catalyst Bed, Stack 0.5 0.03 94.00% $510,600 $150,063 $0.61600 CO (No heat recovery): Burner,

Catalyst Bed, Stack0.5 0.03 94.00% $333,000 $1,407,032 $1.30

600 GAC (dual bed unit with 16,000 lbtotal GAC)

0.5 0.03 94.00% $111,000 $67,165 $0.24


0.5 0.03 94.00% $3,889,440 $1,078,688 $0.44


0.5 0.03 94.00% $5,550,000 $354,786 $0.25

6000 RCO: Burner, Catalyst Bed, Stack 0.5 0.03 94.00% $3,152,400 $1,407,032 $0.536000 GAC (7 dual-bed units, each with

22,000 lb total GAC)0.5 0.03 94.00% $1,010,100 $640,448 $0.23

Table 2-29C

ost Sum

mary for O

ff-gas Treatment Technologies at 0.5 ppm

v MT

BE

*To convert unit treatment costs to $/acre-ft, multiply by 326.


The primary factors that affect the cost of oxidation systems include air flow rates, desiredlevel of destruction, and desired energy efficiency (Van der Vaart et al., 1991). The latterparameter — desired energy efficiency — becomes more important with increasing air flowrates and longer hours of operation. Based on vendor data, recuperative (i.e., with heatrecovery) thermal oxidizers are more cost-effective than non-recuperative systems. Thehigher capital costs of recuperative oxidizers are offset by lower fuel usage costs and, thus,the increased initial capital costs pay for themselves within a short period of time. Forexample, in the case of a catalytic oxidation system designed for a water flow rate of 600gpm, the total amortized operating cost of a non-recuperative system can be over twice asmuch as a recuperative system ($1.30 vs. $0.61/1,000 gallons treated; see Table 2-25).

Biofiltration

No engineering cost estimates were provided by vendors for biofiltration. In general, capitalcosts associated with biofiltration are similar to low compared to other technologies;however, the major cost savings typically result from operating costs (Dharmavaram, 1991).Operating costs are estimated to be less than $0.09 per 1,000 gallons of water treated. ForMTBE, no full-scale vapor phase biofilters have been reported at drinking water facilities.Therefore, performance and costs of biofilters for MTBE treatment under actual processconditions are unknown at this time. However, a substantial amount of research is currentlyongoing to investigate the feasibility of using biofilters for removal of MTBE from drinkingwater (e.g., University of California at Davis, University of California, Riverside, andRutgers). Consequently, biofilters may prove to be an effective treatment technique in thefuture, but they are not yet proven to be reliable for off-gas treatment field operation.

92

2.6 Optimization of Air Stripping Technologies

2.6.1 Introduction and Design Equations

One of the objectives of this document is to determine an appropriate range of design andoperating parameters to demonstrate the cost-effectiveness of MTBE removal in air strippingdrinking water applications. As explained previously, air stripping theory for packed towersis well developed and can be applied to process design, using few empirical correlations(Roberts et al., 1985; Kavanaugh et al., 1980; Ball et al., 1984; Perry et al., 1984; Hand etal., 1986). Using the defining equations, the purpose of this section is to explain howchanging the initial operating assumptions will change the packed tower design, and how theoperating parameters — with a given packing diameter, height, and media — will affectperformance of a packed tower. This section is intended to illustrate the relationship betweenair stripping variables and design parameters and should not be used as an exact basis for airstripping design. For this reason, the safety factor, which would typically be divided by theheight of the transfer unit (and set at approximately 0.8), has been set at unity. In addition,this section has relied upon a Henry’s constant for MTBE of 0.0311 at 20°C. The actualHenry’s constant for MTBE may be lower (see Figure 2-1), which will affect the size of theair stripping unit. Furthermore, the results presented in this section are not explicitly tied tothe cost estimates previously provided, as all costs presented in this paper are based onassumptions and vendor estimates presented in Appendix 2A. For a packed tower, the designequations are as follows (Montgomery, 1985):

Where Z is the packing height (ft); HTU is the height of a transfer unit (ft); NTU is thenumber of transfer units; L is the liquid loading rate (ft3/min/ft2); KLa is the overall masstransfer constant (1/min); Cin is the influent concentration (µg/L); Cout is the effluentconcentration (µg/L); S is the stripping factor (dimensionless); H is the Henry’s constant(dimensionless); G/L is the volumetric air/water ratio (dimensionless); and G is the airloading rate (ft3/min/ft2).

Using the above equations, in conjunction with empirical data on air pressure drop as afunction of gas flow rate and packing, it is possible to evaluate the effect of the various designparameters on overall tower design. Table 2-30 below summarizes the design drivers that are

93

Z = (HTU)(NTU)

S = H *

HTU =

NTU =

LKLa

GL

SS-1

(S –1)+1

Sln

CinCout )(

inherent to the air stripping design for treating contaminated water, the design parametersthat must be determined prior to tower construction, and the design and operating para-meters, some of which can be altered during tower operation. Figures 2-6 through 2-12 weredeveloped to visualize the effect graphically of these design and operating parameters onsystem performance and cost.

2.6.2 Design Parameters

In order to design a packed tower air stripper, an air/water ratio, liquid loading, pressure drop,and air loading rate must be selected (design and operating parameters). This will define thetower height, diameter, and packing type (the design parameters) for given contaminants andremoval efficiencies (design drivers). The design of an optimized packed tower is a complexfunction of the design and operating parameters. Figures 2-6 through 2-12 were developedby varying one parameter and calculating the effect on a second parameter, while keepingother parameters constant. A remediation technology design software package developed atMichigan Technical University was used for this purpose (Crittenden et al., 1998).

Packing Volume

Packing volume is a function of tower diameter and height. Each unit of height achieves thesame removal efficiency, as shown in the equation for HTU. The diameter of the tower is afunction of the stripping factor and liquid loading. If the air/water ratio is too large for a givendiameter, the tower will flood. Figure 2-6 demonstrates that an increase in the removalefficiency requires an increase in the packing volume. As shown in Figure 2-7 there is anoptimal stripping factor for a given removal efficiency to minimize the packing volume.Although Figure 2-7 shows that for stripping factors greater than two, an increase in thestripping factor slightly lowers removal efficiency, this is an artifact of setting the pressuredrop and liquid flow rate constant. If these parameters are held constant, an increase in theair/water ratio increases the diameter of the tower, which increases the packing volume.

94

Design Drivers

Influent Concentration

Removal Efficiency

Chemical Properties

Water Quality

Design Parameters

Packing Size and Type

Tower Height

Tower Diameter

Design & Operating Parameters

Influent Flow Temperature

Air / Water Ratio

Liquid Loading Rate

Air Loading Rate

Pressure Drop

Table 2-30Design and Operating Parameters

95

0

20

40

60

80

100

120

140

Removal Efficiency

Req

uir

ed P

acki

ng

Vo

lum

e (m

3)

20o

5oC

10o

90 99.999

Assumptions:S=5 (AWR= 161)600 gpmTripacks No. 2Pressure Drop = 50 Pa/m

Figure 2-6. Effects of increasing the removal efficiency on the packing volume as a function of inlet

water temperature.

0

20

40

60

80

100

120

140

1 3 5 7 9

Stripping Factor, S = H*(AWR)

Pac

kin

g V

olu

me

(m3)

99% Removal

90% Removal

95% Removal

Increasing

Removal

Efficiency

Assumptions:Pressure Drop = 50 Pa/mT=10 oCInfluent Concentration = 200 µg/L600 gpm flow rate

Figure 2-7. Effects of increasing the AWR on the packing volume as a function of removal efficiency.

Req

uire

d P

acki

ng V

olum

e (m

3 )

Pac

king

Vol

ume

(m3 )

Removal Efficiency


Assumptions:Pressure Drop = 50 Pa/mT = 10°CInfluent Concentration = 200 µg/L600 gpm flow rate

96

0

20

40

60

80

100

120

140

160

1 3 5 7 9 11

Stripping Factor, S=H*(AWR)

Pac

kin

g V

olu

me

(m3)

Tripacks No. 1/2 (1 Inch)

Tripacks No. 2(3.5 Inch)

Tripacks No. 1(2 Inch)

Assumptions95% Removal Efficiency600 gpm flow ratePressure Drop = 50 Pa/m200 µg/L influent

Figure 2-8. Effects of changing AWR and packing media on packing volume for a given removalefficiency.

0

100

200

300

400

500

600

700

1 3 5 7 9 11


Pre

ssu

re D

rop

(P

a/m

) Tripacks No. 1/2(1 Inch)

Tripacks No. 2(3.5 Inch)

Tripacks No. 1(2 Inch)

Assumptions:Tower Dimensions =8’ diameter; 30’ tall600 gpmTemp = 10oC

Figure 2-9. Effects of changing AWR and packing media on pressure drop for fixed tower dimensions.

Pac

king

Vol

ume

(m3 )

Pre

ssur

e D

rop

(Pa/

m)



Assumptions:Tower Dimensions =8' diameter; 30' tall600 gpmTemp = 10°C

Assumptions:95% Removal Efficiency600 gpm flow ratePressure Drop = 50 Pa/m200µg/L influent

97

0

5

10

15

20

25

30

35

40

0 50 100 150 200 250 300 350

Pressure Drop (Pa/m)

Pac

kin

g V

olu

me

(m3)

0

10

20

30

40

50

60

70

To

tal Brake P

ow

er (kW)

Packing Volume: TriPacks No. 1/2 (I inch) Packing Volume: TriPacks No. 1 (2 inches)

Packing Volume: TriPacks No. 2 (3.5 inches) Power: TriPacks No. 1/2 (I inch)

Power: TriPacks No. 1 (2 inches) Power: TriPacks No. 2 (3.5 inches)

Figure 2-10. Effects of changing pressure drop on total brake power and the packing volume as afunction of packing.

Assumptions:Temperature = 10°CS = 5 (AWR = 161)600 gpm95% Removal Efficiency

Pressure Drop (Pa/m)

Pac

king

Vol

ume

(m3 )

Total Brake P

ower (kW

)

Assumptions:Temperature = 10°CS = 5 (AWR = 161)600 gpm95% Removal Efficiency

Packing Volume: TriPacks No. 1/2 (1 inch)

Packing Volume: TriPacks No. 2 (3.5 inches)

Power: TriPacks No. 1 (2 inches)

Packing Volume: TriPacks No. 1 (2 inches)

Packing Volume: TriPacks No. 1/2 (1 inch)

Power: TriPacks No. 2 (3.5 inches)

98

Figure 2-11. Effects of changing the AWR on pressure drop for a given tower cross-sectional area.

70%

75%

80%

85%

90%

95%

100%

105%

1 3 5 7 9 11Stripping Factor, S=H*(AWR)

Rem

ova

l Eff

icie

ncy

1000 gpm

60 gpm

600 gpm

Assumptions:200 µg/L influentAir Stripper design for 95%removal at600 gpmdesign flow(S=5)Tripacks No. 2Temperature = 10oC

Figure 2-12. Demonstration of air stripping flexibility (ability to handle a variety of flow rates) for a givendesign.

Assumptions:200 µg/L influentAir Stripper design for95% removal at600 gpm design flow(S = 5)Tripacks No. 2Temperature = 10°C


Rem

oval

Effi

cien

cy

00 2 4 6 8 10 12

50

100

150

200

250

6' Diameter

12' Diameter


Pre

ssu

re D

rop

[P

a/m

]

21' Diameter

Assumption:600 gpm

Packing Media

There are many different types and sizes of packing materials available, with rings, saddles,and spheres being the most prevalent packing shapes. Figures 2-8, 2-9, and 2-10 illustrate theeffect of various sized Tripack media on required packing volume, pressure drop, and totalpower, respectively. As is seen in Figure 2-8, the largest packing size requires the mostpacking volume for a given removal efficiency. However, packing volume appears to beindependent of size for Tripack media less than 2 inches. Figure 2-9 shows that as thestripping factor increases, packing size plays a more important role in determining thepressure drop. As expected, a smaller packing size causes a larger pressure drop for a givenair/water ratio. Figure 2-9 also demonstrates that if a larger packing is chosen (2 inch or 3.5inch), the stripping factor can be changed dramatically without significant changes in thepressure drop, relative to the 1-inch packing.

The third variable to consider when choosing a packing media is the total amount of brakehorsepower required to achieve the desired removal efficiency. Brake horsepower (the idealpower plus the frictional power requirements) represents the largest O&M cost for a packedtower air stripper and packing volume represents the capital costs. Figure 2-10 shows thetrade-off between capital costs and O&M costs for a packed tower. As the pressure dropincreases, the brake power will increase and the required packing volume will decrease for agiven packing media. For a given removal efficiency, stripping factor, and water flow rate,the height of the packed tower increases as the size of the media increases. While this largermedia increases the total brake power (see Figure 2-10), it does not significantly increase thepacking volume. This is due to the trade-offs mentioned above. Finally, Figure 2-10 showsthat, for a constant packing volume, increases in packing size require an increase in pressuredrop to maintain a constant removal efficiency. As the packing size increases, turbulentmixing (i.e., contaminant removal) decreases, requiring greater packing depths to achieve thesame removal efficiency.

2.6.3 Operating Parameters

Temperature

A simple, yet expensive, way to increase air stripping removal efficiency (see Figure 2-6) isto increase water temperature, either by adding heat to the influent water, or by using steamas the stripping fluid in place of air. As the water temperature increases, the Henry’s constantand overall mass transfer coefficient will increase, thus reducing air/water ratios and thevolume of packing required. In typical air stripping operations, the influent water tempera-ture is not altered from the natural temperature of the groundwater. Figure 2-6 shows that ifa higher inlet water temperature is used in the design, packing volume and capital costs canbe reduced substantially. However, these capital cost savings are offset by an increase inO&M costs. For large-scale drinking water applications, use of steam stripping or heating ofthe water is not cost-effective unless there is a low cost source of heat available from anotherprocess (i.e., waste heat from a thermal oxidizer or other unrelated process).

99

Stripping Factor and Pressure Drop

Increases in the air/water ratio for a fixed packing and flow rate will increase the air pressuredrop. Figure 2-11 shows that, for a given tower diameter, an increase in the stripping factorwill cause an increase in the pressure drop for a constant liquid flow rate. The magnitude ofthe pressure drop decreases as the tower diameter increases.

Flow Rate and Removal Efficiency

Figure 2-12 shows the effect of changing the water flow rate on the removal efficiency for agiven tower design. As the water flow rate increases, the stripping factor must increaseaccordingly to achieve the same removal efficiency. As the water flow rate decreases, thestripping factor can be decreased without decreasing the removal efficiency. In practice,Figure 2-12 illustrates that a packed tower air stripper should be designed for the maximumflow rate that may need treatment. This will guarantee that the air stripper will be able toachieve the desired removal efficiency for all flow rates.

2.6.4 Summary

The purpose of this analysis, besides demonstrating the relationship between the designdriver variables, design parameters, and operating parameters, is to determine the range ofoptimum design parameters for an air stripper. Based on the above analysis, the followingrange of design and operating parameters for a packed tower stripper is recommended:

Packing Size. Medium-sized packing (2 inch) offers the optimal trade-off between pressuredrop, packing volume, and total brake power. As noted, 2-inch packing requires a largerpressure drop than 3-inch packing, but requires less total brake power (lower O&M costs) toachieve the same removal efficiency. Alternatively, 2-inch packing requires a lower pressuredrop than 1-inch packing, but not a larger packing volume for a given removal efficiency.

Stripping Factor. As explained previously, as the stripping factor increases, the O&M costsalso increase. However, the higher the stripping factor, the higher the removal efficiency fora given packed tower volume. For this reason, stripping factors between four and sevenrepresent an appropriate testing range. A field study should be designed to allow testing ofthis range of stripping factors. Performance data should be obtained over this range becausethe lower the acceptable stripping factor, the lower the off-gas capital and O&M costs.

Other Factors. Liquid loading rate and possibly influent temperature should also be tested.As the liquid loading rates increase for a given tower volume, the stripping factor decreases.Changing the flow rate can significantly increase the O&M costs while augmenting theremoval efficiency of MTBE from water.

100

2.7 Conclusions and Recommendations for Future Research

2.7.1 Recommended Technologies

Air stripping is a mature technology that has been widely used to produce potable water fromsources contaminated with volatile organic compounds. As noted in this evaluation, however,there are only a few examples where air stripping has been used to remove MTBE or otherfuel oxygenates from potable water sources. Thus, a pilot- or full-scale testing program iswarranted to demonstrate the capabilities and possible limitations of this technology forproducing potable water from sources contaminated with MTBE. In addition to testing theair stripping system, a pilot test should be performed to evaluate a compatible off-gastreatment technology.

MTBE water treatment scenarios are expected to vary from site to site. Two primaryscenarios are envisioned: 1) a remediation scenario with high initial MTBE concentrations(>1,000 µg/L) and low (<100 gpm) flow requirements; and 2) a drinking water scenario withlow initial concentrations (<1,000 µg/L), but generally higher capacity requirements (>100gpm). The recommendations discussed below will focus primarily on the application of airstripping systems in the second scenario: drinking water applications with lower MTBEinfluent concentrations.

Recommended Stripping Technologies

Packed tower aeration is superior to other air stripping technologies from a cost perspective,regardless of hydraulic capacity, removal efficiency requirements, or initial MTBE concen-trations (see Table 2-12). At higher flow rates (>600 gpm) and removal efficiencies(>95 percent), packed towers are not only less expensive but, often, the only technologycapable of achieving the treatment goal (see Figure 2-13). However, for lower flow rates(<100 gpm), low profile air strippers become cost competitive with packed towers ($1.80-$1.86 vs. $1.75 per 1,000 gallons treated, respectively, for 97.5 percent MTBE removal). Lowprofile air strippers are generally easier to install, maintain, and adapt to changing flow andwater quality conditions than packed towers. Thus, for drinking water scenarios requiringhydraulic capacities less than 100 gpm, which may include either a remediation scenario ora drinking water scenario, a low profile air stripper is recommended. For hydraulic capacitiesgreater than 100 gpm, the packed tower aeration technology is recommended.

101

102

Figure 2-13. Domains for cost-effectiveness of air strippers at varying removal efficiencies.

Off-gas Treatment Technology Recommendation

For this evaluation, it is assumed that off-gas treatment is required when MTBE gas phaseloadings exceed 1 lb/day (see Table 2-12). If off-gas treatment is required and MTBE influentconcentrations are low (<200 µg/L), GAC is the most cost-effective off-gas technologybecause carbon usage rates are low (as a result of the very dilute MTBE stream) and, thus,O&M costs remain low. If MTBE influent concentrations are much higher (e.g., the 2,000µg/L scenario), oxidation is the recommended technology for an air stream from a packedtower system. The results of the cost analysis show that there is a small difference in costsbetween catalytic and thermal oxidation. Thus, thermal oxidation is the technologyrecommended for evaluation in conjunction with the selected aeration technology. Thermaloxidation using heat recovery and GAC are proven technologies. Because both technologiesdemonstrate an equally high level of reliability, flexibility, and removal efficiencies, cost-effectiveness becomes the determining factor in the choice of an off-gas technology.

Recommended Combined System

Based on the above evaluation, the recommended combined technologies are a packed towerair stripper for high flows (>100 gpm) and the low profile system for low flows (<100 gpm).If necessary, both systems can be combined with the off-gas technology identified above forthe respective flow rates and MTBE influent concentrations. The primary bases for thisrecommendation are reliability and economics. Both recommended aeration technologieshave a long track record of successful operation in drinking water situations for removal ofVOCs other than MTBE. As noted, unit costs for this combination of technologies are lowerthan costs for competing technologies in most cases. The recommended technologycombinations are listed in Table 2-31. The associated costs are listed in Table 2-32.

2.7.2 Recommendations for Future Research

As stated previously, air stripping is a well understood technology with many installationsacross the country. However, besides the packed towers at LaCrosse, Kansas and RockawayTownship, New Jersey, there appears to be a lack of published data for air stripperapplications used to remove MTBE in a drinking water context. Consequently, collection ofcost and operational data from a variety of air stripping sites would better demonstrate theapplicability and cost-effectiveness of air strippers in MTBE treatment scenarios. Cost datashould include both real capital and operational and maintenance costs. Operational datashould include influent concentrations, removal efficiencies, air and water flow rates, and airconcentrations.

103

104

Flow RatesInfluent MTBE

Concentration = 2000 µg/L

Influent MTBEConcentration =

200 µg/L


20 µg/L

Effluent Concentration (µg/L)

6000 gpm

600 gpm

60 gpm

20

–

PT + TO

LP + TO

5

–

PT + TO

LP + TO

0.5

–

–

–

20

PT + GAC

PT + GAC

LP

5

PT + GAC

PT + GAC

LP

0.5

PT + GAC

PT + GAC

LP

20

–

–

–

5

PT + GAC

PT

LP

0.5

PT + GAC

PT

LP

Flow RatesInfluent MTBE

Concentration = 2000 µg/L


200 µg/L


20 µg/L

Effluent Concentration (µg/L)

6000 gpm

600 gpm

60 gpm

20

–

$0.90

$3.08

5

–

$0.91

$3.20

0.5

–

–

–

20

$0.38

$0.57

$1.07

5

$0.39

$0.59

$1.80

0.5

$0.40

$0.62

$1.89

20

–

–

–

5

$0.13($0.36)

$0.30

$1.66

0.5

$0.16($0.39)

$0.34

$1.86

PT = Packed Tower; LP = Low profile; GAC = Activated Carbon Off-gas; TO = Thermal Oxidation;CO = Catalytic Oxidation; — = not evaluated; ( ) = close to discharge limit, may or may not be required

Table 2-31Air Stripping and Off-gas Technology Combination Recommendations

Table 2-32Cost for Technology (Air Stripping and Off-gas Treatment)

And the corresponding costs are:

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2.0 air stripping

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