drinking water treatment using air strippers · 2017-07-06 · air stripping – technology...

Dave Fischer

QED Environmental Systems Inc.

Dexter, MI / San Leandro, CA

Copyright © QED Environmental Systems, Inc. 2007-2016; all rights reserved.

Drinking Water Treatment Using Air Strippers

• VOC Removal Overview / The Air Stripping Process

• Methods of Air Stripping

• QED E-Z Tray®

Strippers

• Modeling the process

• Process requirements

• THM removal

• H2S, CO2, radon and methane removal

• Corrosion control

• Case Studies

Topics Overview

• Activated CarbonInefficient loading for low concentration contaminants, significant operating costs

• Air StrippingCapable of high removal efficiency, low maintenance costs, operating cost is blower electrical

• Oxidation process

High capital equipment costs, significant operating cost

• RO / UltrafiltrationNot always capable of >99% removal, high energy use, some organics can damage membranes

Processes for VOC removal

• 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

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

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

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

E-Z Tray® Advantages … Modeling


• Easily modeled online by customer to help process evaluation

• More complex design process due to structural aspects, assistance normally required

• Online modeler not offered

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)

• Dissolved volatile organics

in a water matrix

• No free-phase organics

• Clean air (concentration gradient

driven)

• High surface area of contact

between air and water

• High air to water ratio

• Sufficient contact time

• No surfactants or other H lowering

factors (dissolved polar organics)

• Stripper is level

Impact of dirty air – less “driving force” for mass transfer

Successful Process Requirements

Clean air Contaminated air

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.

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

Sliding Tray Advantages

NSF CertificationLive Safer.®

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 redevelop after removal through stripping if organic precursors are still available in the presence of residual disinfectant

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

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 worked with a

partner company to

conduct THM

removal studies at

several small

drinking water

treatment facilities

Results show

consistent THM

removal of > 85%

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

)

after 24 hours

Immediately after stripping

THM redevelopment due to residual chlorine and available precursor organics.

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 chlorine residual reduction can occur if precursor organics

continue to be converted to THMs.)

THM Removal – Process Design

A successful THM removal

process design needs to 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 solutions.

Dissolved Gas Stripper Applications

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

• (H = 545 atm/mol-frac)

• CO2 – very easy to strip (the cause of normal metal-oxide scaling in strippers)

• (H = 1216 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)

Reducing Corrosion through pH Adjustment

Copper corrosion in tanks & water lines

AWWA study investigating reduction of copper &

lead leaching from water storage tanks & linesusing pH adjustment methods

Air stripper installed to re-circulate water tanks

Air stripping shown effective for removing CO2 andincreasing ground 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

Air stripping can extend GAC life by

removing VOC prior to downstream adsorption of recalcitrant

organics.

Air stripping is effective for removing

organics not readily adsorbed:

• Vinyl chloride

• Methylene chloride

Air stripping is cost effective upstream

treatment at low concentration

• Low mass transfer driving force for GAC alone

• Oversized contactor/load for reasonable run-time

Example (assumes 25% loading rate, 1000gpm)

200 ppb TCE = 400 lbs. / day reduction in GAC loading

20 ppb benzene = 961 lbs. / day reduction in GAC loading

Reducing the Load on GAC

Case Study 1 – 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

Case Study 1 – Cheyenne, WY

4/2009 6/2009 8/2009 10/2009 12/2009 2/2010 4/2010 6/2010 8/2010 10/2010 12/2010 2/2011 4/2011

Preliminary, through 1st RFQ Re-bid, contractor work QED PO + Approvals Equipment ships

Project time-line

• Early consideration of tower approach

• Four E-Z Tray 96.6 strippers, special shaft mount blowers (75HP), vibration and bearing heat sensors on blowers, custom electric junction panels

• Detailed and extended submittal process

• Units in service since 6/2011 – working flawlessly

• Electrical usage initially about 49 kWh (per unit) @ 6400cfm reduced to 5200cfm and this dropped to 40 kWh (49-40 kWh x 0.08 x 24 x 30 = $2800 to $2300/month to run one blower)

Case Study 2 – Cedarburg, WI

• Landfill near a 700gpm supply well causing vinyl chloride hits (below MCL)

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

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

• Sequestering agent used for tower (and planned for E-Z Tray) – tower never cleaned

• E-Z Tray footprint critical

Project time-line

• Did not want another of tower (see next slide)

• One E-Z Tray 72.6 stripper, standard blower (40HP), hinged doors and split trays

• Used the existing pump house with a small addition to keep project costs low

• Units started up in 9/2011 – working very well


6/20118/2010 10/2010 12/2010 2/2011

Preliminary, through pre-POsubmittals

QEDPO

Ship

4/2011


• Tower located in residential area

• Worries about access safety, especially in winter months

• Experience with media spillage from access ports

• $20K media replacement cost

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

possible tower location

E-Z Tray units located in existing space within utilitygarage

Project time-line

• Ease of vinyl chloride stripping (H = 1245) allowed 2-stage stripper and lower air flow (4400cfm vs. standard 5200cfm – 15% under normal)

• Four special E-Z Tray 96.2 strippers, special low pressure blowers (25HP and split trays

• New design gravity drains

• Units started operation in March of 2012

Case Study 3 – Edina, MN8/2011 10/2011 12/20116/201110/2009 12/2009 2/2010 4/2010 6/2010 8/2010 10/2010 12/2010 2/2011

Preliminary, through pre-PO

submittalsPilot study + design

QED

POShip

4/2011

Case Study 4 – Madison, WI

• 2200 GPM Supply well impacted with low level PCE and TCE

• Two E-Z Tray 96.6 strippers -each handling 1100 GPM

• E-Z Tray footprint allowed the city to add on to an existing building situated over the clear well

• H2S is added after the stripper to slightly reduce pH

Project time-line

• Gravity drain discharge enclosures

• Tray racks and spare trays for fast change-out and off-line cleaning capability

• Units started operation in July of 2013

• Excellent experience, city evaluating this technology for another supply well

City Project Document --

https://www.cityofmadison.com/water/projects/unit-well-15-voc-mitigation

Case Study 4 – Madison, WI

PO Modeling, preliminary - submittals

2010 2011 2012 2013

Ship

Case Study 5 – Santa Cruz, CA

• Treating 350 GPM for THM/DBPs

• One E-Z Tray 36.6 stripper

• City piloted a tower and worried about observed fouling issues

• Evaluated the E-Z Tray units against some alternative sliding tray designs and liked the ease of access and stainless construction

Project time-line

• Meeting removal requirements

• Minimal fouling

• Added sun screens to prevent algae build-up

• Working very well for them –exceeding bromide removal predictions

• Also using an on-line THM analysis system

Case Study 5 – Santa Cruz, CA

PO ShipRev BQuote

Modeling, preliminary -submittals

2013 2014

Full Case Study available at --http://www.qedenv.com/Products/Santa_Cruz_Meets_THM_DBP_Challenge_with_Unique_Air_Stripping_Process

E-Z Tray Air Stripper Installations

• 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

• Well maintained air stripping equipment will

provide many years of effective service

Summary

Questions?

David FischerQED Environmental Systems, Inc.

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

WEB:www.qedenv.com

drinking water treatment using air strippers · 2017-07-06 · air stripping – technology...

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