Drinking Water Treatment Using Air Strippers

Transcription

1 Drinking Water Treatment Using Air Strippers Dave Fischer QED Environmental Systems Inc. Dexter, MI / San Leandro, CA Copyright QED Environmental Systems, Inc ; all rights reserved.

2 Topics Overview 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

3 Processes for VOC removal Activated Carbon Inefficient loading for low concentration contaminants, significant operating costs Air Stripping Capable of high removal efficiency, low maintenance costs, operating cost is blower electrical Oxidation process High capital equipment costs, significant operating cost RO / Ultrafiltration Not always capable of >99% removal, high energy use, some organics can damage membranes

4 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.

5 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

6 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.

7 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

8 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

9 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

10 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)

11 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

12 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

13 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

14 E-Z Tray Advantages Modeling E-Z Tray Tower Stacking Tray Air Strippers Air Strippers Air Strippers Easily modeled online by customer to help process evaluation More complex design process due to structural aspects, assistance normally required Online modeler not offered

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 Successful Process Requirements 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 Clean air Contaminated air Impact of dirty air less driving force for mass transfer

18 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.

19 Sliding Tray Advantages NSF Certification Live Safer. 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

20 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

21 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 Haloacetic Acids (HAAs) are not removed by air stripping

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

23 Chloroform Removal THM Removal Chloroform (ppb) after 24 hours CHCl3 in CHCl3 out In 24 hour Out 24 hour 10 Immediately after stripping Time (days) THM redevelopment due to residual chlorine and available precursor organics.

24 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 chlorine residual reduction can occur if precursor organics continue to be converted to THMs.)

25 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.

26 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 = atm/mol-frac)

27 Reducing Corrosion through ph Adjustment Copper corrosion in tanks & water lines AWWA study investigating reduction of copper & lead leaching from water storage tanks & lines using ph adjustment methods Air stripper installed to re-circulate water tanks Air stripping shown effective for removing CO 2 and increasing 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

28 Air stripping can extend GAC life by removing VOC prior to downstream adsorption of recalcitrant organics. Reducing the Load on GAC 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

29 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

30 Case Study 1 Cheyenne, WY 4/2009 6/2009 8/ / /2009 2/2010 4/2010 6/2010 8/ / /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 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)

31 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

32 Case Study 2 Cedarburg, WI 8/ / /2010 2/2011 4/2011 6/2011 Preliminary, through pre-po submittals QED PO Ship 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

33 Case Study 2 Cedarburg, WI Tower located in residential area Worries about access safety, especially in winter months Experience with media spillage from access ports $20K media replacement cost

34 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 utility garage

35 Case Study 3 Edina, MN 10/ /2009 2/2010 4/2010 6/2010 8/ / /2010 2/2011 4/2011 6/2011 8/ / /2011 Preliminary, through pre-po submittals Pilot study + design QED PO Ship 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

36 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

37 Case Study 4 Madison, WI Modeling, preliminary - submittals PO Ship 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 -- voc-mitigation

38 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

39 Case Study 5 Santa Cruz, CA Modeling, preliminary - submittals Rev B Quote PO Ship 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 Full Case Study available at --

40 E-Z Tray Air Stripper Installations

41 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 - Well maintained air stripping equipment will provide many years of effective service

42 Questions? David Fischer QED Environmental Systems, Inc. Tel: s: WEB: