Air Strippers for VOC and Dissolved Gas Removal with a ... · Air Stripping Counter-current flow causes the cleanest air to contact the cleanest water. This ensures efficient mass

Dave Fischer

QED Environmental Systems Inc.

Ann Arbor, MI / San Leandro, CA

Copyright © QED Environmental Systems, Inc. 2007-2015; all rights reserved.

Air Strippers for VOC and Dissolved Gas Removal with a focus on Water and

Wastewater Applications

• Description of the Air Stripping process

• Methods of Air Stripping

• E-Z Tray product features/benefits

• Modeling the process and stripper sizing

• Disinfection by-products (DBP) stripping

• Additional design information

• System maintenance

• Operating costs

• Case Studies

Topics Overview

Air Stripping

A process (governed by Henry’s Law)

that removes or “strips” volatile

organic compounds from

contaminated water by contacting

clean air with contaminated water

across a high surface area, causing

the volatile compounds to move from

the water into the air.

Driving dissolved volatile organic contaminants from water into air.

Air Stripping

Counter-current flow causes the cleanest air to contact the cleanest water. This ensures efficient mass transfer throughout the entire flow path. Simple aeration is not air stripping.

Contaminants are not destroyed during process.

Clean water

Air Flow Contaminated water

Air Stripping – Technology Overview

Higher Henry’s law constant = more volatile contaminant

Henry’s law constant is temperature dependent (increases with increasing temp).

Increasing air to water ratio (A/W) improves removal efficiency for marginally volatile contaminants.

Some contaminants will not respond to air stripping (1,4 dioxane, methanol, tert-butyl alcohol).

Easiest to strip

Hardest to strip

Dissolved gases (methane, carbon dioxide)

Chlorinated solvents

Light hydrocarbons (BTEX)

Heavy hydrocarbons (DRO, naphthalene)

MTBE

Ammonia

Air Stripping Methods

• Simple Storage Tank Aeration

• Tower Strippers

• Stacking Tray Strippers

• Sliding Tray Stripper (QED E-Z Tray)

Simple Storage Tank Aeration

Spray Nozzles

• Water is circulated within a holding tank and discharged from spray nozzles

• Nozzles develop significant back pressure (30-40psi)

• Mass transfer occurs as droplets fall through free air

• Multiple passes are needed to provide THM removal

• 8 passes required to reach 50-60% removal1. (compare to 60-80% TTHM reduction with 1 pass through an E-Z Tray stripper)

1. “Violation Prompts Treatment Change”, July 2011 Opflow

Stripping Methods

TowerThin film of water flows over a high surface area packing

Stacked TrayAir bubbles - froth and turbulent mixing creates mass transfer surface area

Sliding TrayAir bubbles - froth and turbulent mixing creates mass transfer surface area

Stripping Methods

Tower

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

Sliding Tray

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

Disadvantage

• Requires higher pressure

blower (HP)

E-Z Tray Tower Stacking TrayAir 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

E-Z Tray® Advantages … Footprint


• Reduced footprint for installation and maintenance

• Small footprint but very tall structure often required

• Lots of space needed for disassembly, lifting from all sides, pipe disconnection and tray stage stacking

E-Z Tray® Advantages … Monitoring


• Easy process monitoring and inspection, even while in operation

• Condition of packing and air flow distribution are very difficult to observe

• Difficult or impossible to observe air and liquid flow distribution during operation

Site Data

Often the highest historical analytical result for each parameter is used for design modeling

Modeling the Process

Xin = aqueous concentration entering the air stripper

Xout = aqueous concentration exiting the air stripper

Yin = gas concentration entering the air stripper

Nth = number of theoretical trays in the air stripper

S = stripping factor

Kh = Henry’s Law constant

L = liquid flow rate

G = gas flow rate

Web based Modelhttp://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

(URL listed to allow easy remodeling)

THM (DBP) 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

Henry’s Law predicts that the THM compounds will strip in the following order:

Chloroform - easiest to stripBromodichloromethaneDibromochloromethaneBromoform - hardest to strip

Haloacetic Acids (HAAs) are not removed by air stripping

THM Removal

Parameter H (atm/mol-frac)benzene 309.2chloroform 225.2bromodichloromethane 63.1dibromochloromethane 44.4MTBE 32bromoform 29.5

THM Removal – Some Pilot Data

11-2 11-3 11-4 11-5 11-911-11

11-17

11-23

12-1

% THM Reduction 90 91 90 90 88 86 88 85 84

Water temp. 55.4 55.2 54.5 52.8 52.0 52.6 49.6 50.5 42.8

40.0

42.0

44.0

46.0

48.0

50.0

52.0

54.0

56.0

58.0

82

83

84

85

86

87

88

89

90

91

92

Wa

ter

Te

mp

. (F

)

Pe

rce

nt

Percent THM reduction vs. Water Temperature

QED working with a

partner company to

conduct THM

removal studies at

several small

drinking water

treatment facilities.

Results show

consistent THM

removal of 85% or

more.

Chloroform Removal

0 5 10 15 20 25 30 35

0

5

10

15

20

25

30

35

40

45

THM Removal

CHCl3 in

CHCl3 out

In – 24 hour

Out – 24 hour

Time (days)

Ch

loro

form

(p

pb

)

THM Removal – Process Design

Successful THM removal

process design will needs

account for remaining THM

precursors, while providing

sufficient residual disinfection.

Clear well loop design, remote

reservoir loop or stripping prior

to immediate use may provide

the best solution.

Impact on Chlorine Residual

10/13/10

10/14/10

10/15/10

10/16/10

10/17/10

10/18/10

10/19/10

10/20/10

10/21/10

10/22/10

10/23/10

10/24/10

10/25/10

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Chlorine Residual Change

(before / after stripping)

Influent

Effluent

Sample Date

Cl2

(p

pm

)

(Further residual reduction possible if precursors continue to be

converted to THMs)

Other Stripper Applications

• Ammonia – very hard to strip

• CO2 – somewhat easy to strip

• Hydrogen Sulfide – easy to strip

• Radon removal – extremely easy to strip

• Methane removal – extremely easy to strip

Additional Site Information for Design

• Site history of DNAPL and/or LNAPL

• Parameters that are hard to strip (DRO, etc.)

• Is O&G above detection limit (is O&G MDL 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 sum of the target organics

• Is there a site history of surfactant use

• Are high shear pumps used to capture the water (stable emulsions of NAPL)

• Adjustments to unit specifications (higher air flow, slightly

higher water flow, etc.)

• Power considerations

• Operation modes (batch, extended shut-down)

• Environmental

• Strippers with a non-standard number of trays

• Blended flow calculations

• Strippers in series (use effluent from first model run as

influent for second)

Special Situations

Standard 4&6 Tray Custom 7 Tray Standard Series

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

Blended discharge

Special Situations

• 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 Situations (cont.)

Pilot Testing

• Prepackaged,

just add electricity

• Rental

• Used for scale-up

design and fouling

studies

• Allows H correction

from results when

NAPLs, surfactants,

etc. are known to be

present

Rental skids available from QED and some equipment contractors –

contact us for more information.

• Sliding Tray Stripper – $0.10-$0.35/Kgal

• Tower Stripper – $0.48/Kgal

• Activated Carbon (GAC) – $0.95-$1.57/Kgal

• Oxidation process – $0.88 – $2.42 /Kgal

Considering 10 year project life & equivalent removal efficiency – equipment

cost, install cost, operating / maintenance cost (energy, GAC replenishment),

and annual flow treated (x / 1000 gallons). Legacy & lifecycle costs are

becoming a major design requirement.

Process Economics for high efficiency VOC removal

Key Operating Considerations

• Air-to-water ratio (A/W) = controls process efficiency and performance

• Air pressure = sufficient pressure needed to hold water on tray (back pressure)

• Differential pressure = pressure drop across trays expected / excessive pressure drop restricts air flow & impacts performance

• Liquid flow rate = unit designed for maximum hydraulic capacity / high turn-down

• Liquid level in integral sump = minimum water level prevents air bypass / maximum level prevents flooding

• Seals = equal torque on front hatch to prevent leaks / trays properly latched to prevent air / water bypass / down comers in place to prevent bypass

• Temperature affects the process – higher

temperature = better stripping

• Process temperature is roughly equal to water temperature; air temperature not a big factor

• Freezing is not a concern for continuous operation

• Discharged air is saturated (high humidity) at the process temperature, so consider condensation and thermal impacts if air treatment is planned

Additional Operating Factors

Tray Fouling – What Does it Look Like?

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

Low Air Flow (Weeping)

Weeping is when water drops through tray sieve (air) holes

Impact on Process Performance (A/W decrease)

Aggressive iron scale formation – can be removed with mild acid or a brushDO NOT use detergents to clean trays

Scale on surface will not impact performance unless tray perforations are reduced in size –back pressure increases, lowering A/W

OPEN

CLOSED

Blower throttle controls air flow on system –expressed as differential pressure across trays(check Magnahelic)

Throttle “chokes” air output as function of the blower curve. Used for air “tuning”.

Air Flow Control

Inorganic Fouling – precipitation of inorganics which attach on air stripper surface

• Aeration increases system pH• Iron – Fe2+ into Fe(OH)2

• Hardness – Ca2+ into CaCO3

Fouling water benchmarks for air stripperso Hardness > 400 ppm; Iron > 5 ppm – accelerated cleaning frequencyo Hardness = 200 – 300 ppm; Iron = 2 – 4 ppm – regular maintenanceo Hardness < 100 ppm; Iron < 1 ppm – infrequent cleaning required

For air strippers, aggressive fouling impacts maintenance frequency and potentially process efficiency.

Stripper Fouling

Stripper Fouling Rules of Thumb

Iron <0.1ppm low fouling potential

0.1-1ppm modest fouling potential (not bio)

1-5ppm significant fouling potential (optimum for bio)

5-20ppm serious fouling potential, heavy O&M (manageable)

>20ppm extreme, control options required

Langelier Saturation Index (LSI*)

<1 no scale forming, corrosive potential

0 neutral

>1 scale forming potential

Microbial – Biological Activity Test (BART – Hach kit)

> 1000 cfu/ml – concern for bio-fouling

>10,000 cfu/ml – expect serious bio-fouling

(*calculate LSI with calcium hardness, total alkalinity, pH and water temperature)

• DO NOT use detergents for cleaning!

• Dilute acid for cleaning• We recommend dissolved citric acid

• Apply acid solution to soak and dissolve scales

• Light mechanical scrubbing and/or pressure wash• Release heavy deposits

• Open up sieve holes

• Rinse with fresh water

• Rinse A/S sump and flush

Tray cleaning

• 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 for severe cases

Tray Fouling - Preventative Measures

Oil and grease + bio-slime impacted tray from a fuel terminal site

Stripper Cleaning

• Cleaning frequency and effort is site-specific

• Example -

– 0.03 ppm iron, high hardness scales - stripper requires cleaning every 3 weeks

• Cleaning time for the largest 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 (about 1/2 day per stripper)

E-Z Tray vs. Tower O&M Example

• Site in Sturgis, MI treating 250 GPM water containing a range of chlorinated solvents (PCE, TCE, etc.)

• Oversized tower replaced with a 500 GPM E-Z Tray

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

• Required special protective measures for personnel, flooding tower with acid and collection and disposal of cleaning solutions

• Costs for pressure washing the E-Z Tray every 40-50 days estimated at $8,000/year

Assume an E-Z Tray 96.X stripper (1000gpm capacity, our largest

unit), treating 800gpm, cleaned 4 times/year – this unit processes

420,500 kgal/year. Example does not include capital costs.

Cleaning – 2 person crew ($80/hr + supplies), 4-hours/event =

$3600/year

Power – 75HP = 0.75 kW running 24/7/365 at $0.10/kWh =

$49K/year

Cost =

$0.125 / 1000-gallons treated

Example O&M Cost Estimate

E-Z Tray Advantage - Safer by design

Live Safer.®

QED’s sliding tray air stripper (E-Z Tray) is the first self-container air stripper to

achieve certification from NSF International to NSF/ANSI Standard 61: Drinking

Water System Components – Health Effects

Nationally recognized health effects standard for all products that come in contact

with drinking water

All water contacting materials in the E-Z Tray units are safe for drinking water

systems use

Case Study 1 – Cheyenne, WY

• Abandoned Atlas Missile sites contaminated city wells with chlorinated solvent

• US Army Corps is QED’s customer

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

• Excellent equipment reliability required to ensure continuous water treatment

• System started June 2011

Case Study 2 – Cedarburg, WI

• Landfill near a 700gpm supply well causing low level vinyl chloride hits

• System modeling based on a long list of possible future contaminants, based on LF monitoring data

• City operates an older tower stripper on another well treating an unrelated TCE issue – in operation 18 years

• Sequestering agent used for tower and E-Z Tray

• E-Z Tray footprint helped to keep project costs low

Case Study 3 – Edina, MN

• Supply well impacted with low level vinyl chloride hits

• Engineer was considering a tower

• Site location issues weighed against the tower

• Creative use of the E-Z Tray footprint allowed the city to use available space below the parking garage

• Energy use was also a key design factor

• Units began operation in March of 2012

possible tower location

E-Z Tray units located in existing space within utilitygarage

Case Study 4 – CA Water Treatment Plant

• Municipal water treatment facility

• Treatment of DBPs in reclaimed water feed portion

• Pilot testing on a packed tower stripper showed gumming and fouling of packing

• Visited a nearby Army facility using an E-Z Tray for remediation – liked the ease of maintenance

• E-Z Tray footprint, NSF61 approval and on-line model are other positives of the QED system

VOC reduction prior to SBR

treatment of pharmaceutical

wastewater

Stripper air flow rate much lower

than flow from SBR – high

contaminant concentration

Allowed smaller CATOX

air treatment unit

VOC

Treatment Plant

Before

Treatment Plant

After

Thermal Oxidizer

Less VOCVOC

Thermal Ozidizer

Case Study 5 – Pre-Treatment for SBR

Copper corrosion in tanks & water lines

• AWWA study investigating reduction of copper & lead leaching from water storage tanks & lines with pH adjustment methods

• Air stripper installed to re-circulate water within tanks

• Air stripping shown effective for removing CO2 & and increasing water pH w/o chemical addition

• Air stripping provides a cost competitive platform to increase pH w/in acceptable levels (~ 1pt.) and maintain EPA compliance

Case Study 6 – Reducing Corrosion through pH Adjustment

• Air strippers are effective at removing dissolved

volatile organic compounds from water

• The primary process factor is air to water ratio

• The process can be modeled using QED’s on-

line computer tool -

http://www.qedenv.com/modeler

• Air stripping equipment needs to be maintained

to ensure continued design removals

Summary

Survey + Questions?

David FischerQED Environmental Systems, Inc.

Tel: 800-624-2026E-mails: [email protected]

WEB:www.qedenv.com

Air Strippers for VOC and Dissolved Gas Removal with a ... · Air Stripping Counter-current flow causes the cleanest air to contact the cleanest water. This ensures efficient mass

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