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

Transcription

1 Air Strippers for VOC and Dissolved Gas Removal with a focus on Water and Wastewater Applications Dave Fischer QED Environmental Systems Inc. Ann Arbor, MI / San Leandro, CA Copyright QED Environmental Systems, Inc ; all rights reserved.

2 Topics Overview 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

3 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.

4 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. Air Flow Contaminated water Clean water

5 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 Dissolved gases (methane, carbon dioxide) Chlorinated solvents Light hydrocarbons (BTEX) Heavy hydrocarbons (DRO, naphthalene) MTBE Ammonia Hardest to strip

6 Air Stripping Methods Simple Storage Tank Aeration Tower Strippers Stacking Tray Strippers Sliding Tray Stripper (QED E-Z Tray)

7 Simple Storage Tank Aeration 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% removal 1. (compare to 60-80% TTHM reduction with 1 pass through an E-Z Tray stripper) 1. Violation Prompts Treatment Change, July 2011 Opflow Spray Nozzles

8 Stripping Methods Tower Thin film of water flows over a high surface area packing Stacked Tray Air bubbles - froth and turbulent mixing creates mass transfer surface area Sliding Tray Air bubbles - froth and turbulent mixing creates mass transfer surface area

9 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)

10 E-Z Tray Advantages Cleaning E-Z Tray Tower Stacking Tray Air Strippers Air Strippers Air Strippers Single person cleaning Packing access and removal is difficult Major disassembly and multi person crew needed

11 E-Z Tray Advantages Footprint E-Z Tray Tower Stacking Tray Air Strippers Air Strippers Air Strippers 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

12 E-Z Tray Advantages Monitoring E-Z Tray Tower Stacking Tray Air Strippers Air Strippers Air Strippers 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

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

14 Modeling the Process Web based Model 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 The performance modeler is based on the design procedure discussed in -- Kibbey, T. C. G., K. F. Hayes and Pennell, K.D., Application of Sieve-Tray Air Strippers to the Treatment of Surfactant-Containing Wastewaters, AIChE Journal, Vol. 47, No. 6, June Also -- Perry, R. H., and D. W. Green, Perry s Chemical Engineer s Handbook, 7th ed., McGraw-Hill, New York 1997.

15 Henry s Constant (H) Larger H = more easily stripped (atm/mol-frac) vinyl chloride TCE 648 benzene MTBE - 32 acetone - 2.4

16 (URL listed to allow easy remodeling)

17 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

18 THM Removal Henry s Law predicts that the THM compounds will strip in the following order: Chloroform - easiest to strip Bromodichloromethane Dibromochloromethane Bromoform - hardest to strip Parameter H (atm/mol-frac) benzene chloroform bromodichloromethane 63.1 dibromochloromethane 44.4 MTBE 32 bromoform 29.5 Haloacetic Acids (HAAs) are not removed by air stripping

19 THM Removal Some Pilot Data 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. Percent Percent THM reduction vs. Water Temperature % THM Reduction Water temp Water Temp. (F)

20 Chloroform Removal THM Removal Chloroform (ppb) CHCl3 in CHCl3 out In 24 hour Out 24 hour Time (days)

21 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.

22 Impact on Chlorine Residual Chlorine Residual Change (before / after stripping) Cl2 (ppm) Influent Effluent /13/10 10/15/10 10/17/10 10/19/10 10/21/10 10/23/10 10/25/10 10/14/10 10/16/10 10/18/10 10/20/10 10/22/10 10/24/10 Sample Date (Further residual reduction possible if precursors continue to be converted to THMs)

23 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

24 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)

25 Special Situations 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)

26 Special Situations Standard 4&6 Tray Custom 7 Tray Standard Series Series same air (like an 8 Tray) Parallel different flow rates Blended discharge

27 Special Situations (cont.) 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

28 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.

29 Process Economics for high efficiency 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.

30 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

31 Additional Operating Factors 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

32 Tray Fouling What Does it Look Like? Low Air Flow (Weeping) Weeping is when water drops through tray sieve (air) holes Expected performance impact is gradual as air flow decreases, due to tray fouling.

33 Impact on Process Performance (A/W decrease) Aggressive iron scale formation can be removed with mild acid or a brush DO 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

34 Air Flow Control 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.

35 Stripper Fouling Inorganic Fouling precipitation of inorganics which attach on air stripper surface Aeration increases system ph Iron Fe 2+ into Fe(OH) 2 Hardness Ca 2+ into CaCO 3 For air strippers, aggressive fouling impacts maintenance frequency and potentially process efficiency. Fouling water benchmarks for air strippers o Hardness > 400 ppm; Iron > 5 ppm accelerated cleaning frequency o Hardness = ppm; Iron = 2 4 ppm regular maintenance o Hardness < 100 ppm; Iron < 1 ppm infrequent cleaning required

36 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)

37 Tray cleaning 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

38 Tray Fouling - Preventative Measures 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 Oil and grease + bio-slime impacted tray from a fuel terminal site

39 Stripper Cleaning Cleaning frequency and effort is site-specific Example 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)

40 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 days estimated at $8,000/year

41 Example O&M Cost Estimate 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

42 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

43 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

44 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

45 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 utility garage

46 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

47 Case Study 5 Pre-Treatment for SBR 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 Thermal Ozidizer Before After Thermal Oxidizer VOC VOC Less VOC Treatment Plant Treatment Plant

48 Case Study 6 Reducing Corrosion through ph Adjustment 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 CO 2 & 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

49 Summary 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 online computer tool - Air stripping equipment needs to be maintained to ensure continued design removals

50 Survey + Questions? David Fischer QED Environmental Systems, Inc. Tel: s: dfischer@qedenv.com WEB: