Dave Fischer

QED Environmental Systems Inc.Ann Arbor, MI / San Leandro, CA

Copyright © QED Environmental Systems, Inc. 2007- 2012; all rights reserved.

Air Stripping for VOC Removal

- Advanced Topics

• Review - the Air Stripping process• The Impact of Fouling Conditions• Tower vs. Tray• Review - the QED Air Stripper Modeler• Extending the Model• Complicated Removal Design (VOC, THM,

ammonia)• Case Studies

Topic Overview

Air StrippingMass transfer Process governed by Henry‟s Law

Counter-current flow ensures efficient mass transfer throughout the entire flow path

High air to water surface for transfer is created by the turbulent froth mixture

Air Stripping

The froth in action.

Sliding Tray Type StripperMethod

Air bubbles - froth and turbulent mixing creates mass transfer surface area

Advantages

• Easy access• Less prone to fouling• Less intrusive at site• Wide flow turn-down

Disadvantage

• Requires higher pressure blower (HP)

Selected QED Air Strippers

E-Z Tray

Model 6.4

(65 gpm max)

E-Z Tray

Model 16.4

(150 gpm max)

E-Z Tray

Model 24.4

(250 gpm max)

E-Z Tray

Model 96.6

(1000 gpm max)

Stacking Tray Stripper

Stacking Tray Design

• Stacking tray strippers are a series of stacked rectangular boxes with bottom perforations

• Trays layers are sealed with gaskets and fastened together with clamps around outer edges

• Cleaning requires lifting trays and breaking pipe connections, often requires two or more people or an overhead crane

• Requires access to all sides for installation and maintenance

More information at -- http://www.qedenv.com/davislf/

• High air to water ratio (A/W)#1. Process parameter

• High surface area of contact between air and water

• Clean air (concentration gradient driven process)

• Dissolved volatile organics in a water matrix (modeling valid for levels < 25% of water solubility)

• No free-phase organics• No surfactants or other H lowering

factors (dissolved polar organics)• Stripper is level

Impact of dirty air

Successful Process Requirements

Clean air Contaminated air

Stripper Performance Impacts

• Air or liquid flow restrictions• Significant water or air temperature changes• Free phase product or other sorptive

compounds that decrease stripping, such as organic solids

• Surfactants or other polar organic chemicals that can lower H for target organics

• Contaminated air

Air Stripping

• Temperature impacts the process – higher temperature = better stripping• Process temperature is roughly = water temperature• Freezing is not a concern for continuous operation• Discharged air is saturated at the process temperature (consider condensation and thermal impacts on air treatment units)

Some physical elements

Temperature EffectsThermal mass of water >> that of air – example (200gpm flow; 1300cfm air flow):

Water Temp (F)

55

55

55

Air Temp (F)

55

ProcessTemp (F)

50

70

80

100

54.9

55.2

55.3

55.6

Hotter air is less dense (also for higher elevation) – so stripping will decrease – example (Tw = 55F; MTBE in = 10,000 ppb; 4-tray stripper)

Air Temp (F)

40

50

80

MTBE (ppb)

4248

4343

4545

Discharge Air

• Entrained water droplets and air at 100% RH at the process temperature• High efficiency mist eliminator for droplet removal• Improper demister sizing or fouling can cause water blow by• Water knock out, downstream process insulation, etc. for condensation issues

Air Flow

Types of Tray Air Stripper Fouling

• Metal oxides1.

• Hardness (scale)• Suspended solids• Bio solids, slimes• Oils & Greases• Free phase non-

aqueous phase liquids (NAPL)

1. CO2 stripping can cause a slight pH increase, leading to insoluble metal oxide formation

Bio Fouling

Example – pH adjustment to minimize inorganic tray fouling causeda fungus to rapidly develop a protective slime.

Tray Fouling – Knowing When to Clean

Normal stripper sump pressure = 4-6 inch H2O / tray stage

Tray Fouling – What Does it Look Like?

Expected performance impact is gradual as air flow decreases, due to tray fouling.

E-Z Tray Tower Stacking Tray

Air Strippers Air Strippers Air Strippers

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

E-Z Tray® Advantages … Cleaning

• Single person cleaning

• Packing access and removal is difficult

• Major disassembly and multi person crew needed

Stripper Cleaning• Cleaning frequency and effort is highly site-

specific• Example -

– 1000ppm TDS, 260ppm total hardness, 0.03ppm iron - stripper requires cleaning every 3 weeks

• Time to clean an E-Z Tray stripper– Two 1000gpm, E-Z Tray 96.6 units (8 doors, 48

trays) takes 8-10 minutes/tray to fully remove, pressure wash and reinstall all the trays in this system

• Clean trays– Backup tray set

• Sequestering agents (decrease cleaning frequency)

– inorganic polyphosphates

• Bio-fouling– Ozone, etc.

• pH adjustment– In/out

• Pre-stripper oxidation and filtration

Fouling - Preventative Measures

Tower StripperMethodThin film of water flows over a high surface area packing

Advantages

• Lower energy use in the air mover, due to lower overall pressure drop

Disadvantages

• Flow turn-down difficult• Difficult to clean• Tall structure• Short circuiting

Tower Stripper

If fouling conditions develop, the tower can quickly loose mass transfer area. Small local areas of deposition can produce flow short circuiting that further limits available contact area.

Tray vs.Tower Stripper

• Hard to access for cleaning (high O&M costs)• Very tall structure (wind loading, thermal issues)• Operating conditions difficult to observe• Complex design process due to structural issues• No web based performance model, models harder to use

E-Z Tray® vs. Tower O&M ExampleSite in Sturgis, MI treating 250gpm water containing:

1,1,1-trichloroethanec-1,2-dichloroethylenehexachlorobutadienemethylene chloridenaphthalenetetrachloroethylene (PERC,PCE)trichloroethylene (TCE)

Oversized tower replaced with a 500gpm E-Z Tray 48.6 model

Historical tower cleaning with acid cost about $54,000/year

Pressure washing the E-Z Tray every 40-50 days estimated at $8,000/year

Modeling the Process

Xin = aqueous concentration entering the air stripperXout = aqueous concentration exiting the air stripperYin = gas concentration entering the air stripperNth = number of theoretical trays in the air stripperS = stripping factorKh = Henry’s Law constantL = liquid flow rateG = gas flow rate

Web based Model

http://www.qedenv.com/modeler

The performance modeler is based on the designprocedure discussed in -- Kibbey, T. C. G., K. F. Hayes andPennell, K.D., „„Application of Sieve-Tray Air Strippers tothe Treatment of Surfactant-Containing Wastewaters‟‟,

AIChE Journal, Vol. 47, No. 6, June 2001. Also -- Perry, R.H., and D. W. Green, Perry’s Chemical Engineer’s Hand-book, 7th ed., McGraw-Hill, New York 1997.

Henry‟s Constant (H)Larger H = more easily stripped (atm/mol-frac)

• vinyl chloride - 1245• TCE – 648• benzene - 309

• MTBE - 32• acetone - 2.4

1 – Pick Contaminants

• Temperatures (air and water)• Altitude (air density)• Flow rates (air and water)

- Process impacts- Hydraulic impacts

2 – Other Information

Process Variables

• First pass – pick the stripper model that matches project flow

3 – Pick a Stripper

Metric units available on Model Site

• Concentration in ppb (ug/L) – 1000ppb = 1ppm• Each contaminant behaves independently

4 – Contaminant Concentrations

5 – Review Model Results

(URL listed to allow easy remodeling)

How certain field analytical results are modeled

TPH, DRO, GRO, Total-BTEX, TVPH, F1 – F4, C6-C10, etc. All represent groups of organic compounds, with ranges of Henry‟s constant (H). A representative compound is used to stand for the group. Typical practice:

• BTEX – modeled as benzene (lowest H out of the BTEX)• TPH – modeled as either benzene (TPH-GRO) or naphthalene

(TPH-DRO)• F1 (C6-C10) – model as GRO = benzene• F2 (C10-C18) – model as DRO = naphthalene• F3 and above (>C16) = difficult to strip

This method carries RISK if the group actually has more lower H components than that of the representative. Model individual components if you need to meet specific targets.

We Can Help Model Special Cases• Flow very close to rated maximum for a given stripper model• Air flow conditions different than the standard (+/- 15-20%)• Strippers with a non-standard number of trays • Blended flow calculations• Strippers in series (use effluent from first model run as influent

for second)• Contaminants that are not listed in the model contaminant table• Calculation of “effective H” from field pilot data• Results less than 1ppb• Pilot cases where concentrations are >> 25% water solubility

Special Cases

Standard 4&6 Tray Custom 7 Tray Standard Series

Series – same air(like an 8 Tray) Parallel – different flow rates

Blended discharge

Complicated Removal Situations

• Free phase NAPL• Surfactants / H altering non-strippable

components• THM Removal• Ammonia Removal

Free Phase Organics

• Dramatically lower removal efficiency

• Can coat walls and accumulate in the sump to act as an ongoing VOC source

• Can cause partitioning effects where a percentage of certain VOCs are “sequestered” from the stripping process

Free Phase Organics - Partitioning

Contaminated Water

Stripping removes dissolved portion of contaminants – some NAPL moves through the system. Some organics may also partition into the free-phase component.

Organics re-equilibrate in the sample vial -increasing the dissolved concentrations in the treated water.

Free Phase Organics - Example

Example – site handling tanker ballast water with a combination of free phase hydrocarbons and VOCs dissolved in water

20.00% 40.00% 60.00% 80.00% 100.00%0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

Water + Hydrocarbon

pilotmodel

(% Maximum Flow)

(% V

OC

Rem

oval

)

Surfactants / Polar Organics

• Lower effective H for all contaminants due to solubilization of organic compounds by surfactant micelles.

• Can cause foaming– Sometimes subtle (and not required when polar organics

are present) – Demister fouling and blower back-pressure increase– Control = Anti-foam additives (does not recover stripping

effectiveness )– Control = Knock-out tank prior to demister

Closed DOD site with low level dissolved TCE. Visual indication of excessive foam in upper stripper trays. TOC 2-3X greater than sum of target organics.

Surfactants / Polar Organics – Example

Normal froth Surfactant impacted froth

Surfactants / Polar Organics – Example (cont.)

Field results show a consistent 60-85% reduction in stripping capability vs. theoretical performance prediction for TCE.

THM Removal

• Trihalomethanes (THMs) can form in drinking water when disinfectant (chlorine) breaks down precursor organic compounds, normally organic solids

• Air stripping is an effective way to reduce THMs• THMs can re-form after stripping if organic precursors

are still available

Chloroform Removal

0 5 10 15 20 25 30 350

5

10

15

20

25

30

35

40

45

THM Removal

CHCl3 in CHCl3 out In – 24 hour Out – 24 hour

Time (days)

Chl

orof

orm

(ppb

)

THM Removal – Possible Process

Successful THM removal process design will need to account for remaining THM precursors, while providing sufficient residual disinfection. Clear well loop design or remote reservoir loop may provide the best solution.

Ammonia Removal

• Dissolved ammonia gas can be stripped from water• H is very low (very hard to strip) – requiring high A/W to

achieve significant removals• pH adjustment is required• Pilot testing required to understand the balance

between ionic and dissolved gas forms

Ammonia Removal

pH adjusted > 11 prior to air stripper, then adjusted back to required level

Model assumes the dissolved gas NH3

Ammonia RemovalFrom EPA article (ref. below) -- A/W of 300-500 cfm/gallon are typical for ammonia removal. Using QED‟s normal dimensionless A/W – (ft3/min air / ft3/min water) this equates to A/W of 2200 - 3700.

Normal A/W for VOCs are 50-200.

The only way to achieve high A/W with the E-Z Tray systems is to lower the liquid flow rate.

Ammonia removal is viable for low liquid flow situations.

EPA Waste Water Technology Fact Sheet – Ammonia Stripping – EPA 832-F-00-019, Sept. 2000

Additional Site Information for Design

• Site history of DNAPL and/or LNAPL• Parameters that are > 25% of water solubility + are

hard to strip (DRO, C12-C28 hydrocarbons, etc.)• Is O&G above detection limit (is limit low enough)• Is there air contamination near the blower inlet• Does stable foam form if target water is shaken in a jar• Is there an offset between TOC and the target organics• Site history of surfactant use• Are high shear pumps used to capture the water (stable

emulsions of NAPL)

Pilot Testing

• Prepackaged, just add electricity

• Rental• Used for scale-up

design• Allows H correction

from results when NAPLs, surfactants, etc. are known to be present

Case Study – Use of Pilot Data• Target contaminant = TCE at 140ppb• Model predicts <1ppb result (100% removal)• Field results show 1.4ppb result (98.7-99% removal)• TOC checked – modeled contaminants add to 280ppb, TOC in is

1300ppb (TOC after stripper is 1100ppb)• There is about 1000ppb of unknown• Original tower stripper also never met modeling prediction –

supplier blamed fouled media• Slight abnormal foaming observed• Normal H for TCE is 648 (atm/mol-frac); pilot results show an

effective H between 97 and 236 (atm/mol-frac)• Used effective H values from pilot data to increase air to water ratio

(A/W) two stripper model steps to achieve target

Other Stripper Applications

• Hydrogen Sulfide – easy to strip (pH needs to be dropped < pH = 6)

• (H = 545 atm/mol-frac)

• Radon removal – extremely easy to strip• (H = 4680 atm/mol-frac)

• Methane removal – extremely easy to strip• (H = 35390 atm/mol-frac)

Case Study 1

• VOC treatment of tanker ballast water

• Strippers replaced an aging activated sludge treatment process that was unable to handle changes in flow and concentration

• Process string includes free-phase removal and air treatment

• Pilot testing used prior to design

Case Study 2• VOC reduction prior to SBR

treatment of pharmaceutical wastewater

• Stripper air flow rate much lower than flow from SBR

• Allowed smaller CATOX air treatment unit

VOC

Treatment Plant

Before

Treatment Plant

After

Thermal Oxidizer

Less VOCVOC

Case Study 3 – Cheyenne, WY• Abandoned Atlas Missile

sites contaminated city wells with chlorinated solvent

• US Army Corps is QED‟s customer

• Strippers will treat city water during high demand, summer months (4000gpm capacity)

• Excellent equipment reliability required to ensure continuous water treatment

• System started June 2011

Questions?

David FischerQED Environmental Systems, Inc.

Tel: 800-624-2026E-mail: dfischer@qedenv.com

WEB:www.qedenv.com