Drinking Water Treatment Using Air Strippers

Transcription

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

2 Presentation Overview US Drinking Water Sources VOCs in Public Water Supplies VOC Treatment Methods Air Stripping and Types of Air Strippers QED E-Z Tray The QED Air Stripper Modeler Process Requirements THM Removal Ammonia, H2S, Radon and Methane Case Studies

3 Drinking Water Sources 52,000 of the 156,000 public drinking water systems in the United States are defined as community water systems (CWS) 92% of the US population receive their water from a CWS 40,600 or 80% obtain part or all of their water from groundwater supply wells

4 VOCs in Public Water Supplies Chloroform Methyl tert-butyl ether (MTBE) Perchloroethene (PCE) Bromoform Dibromochloromethane Trichloroethene (TCE) Bromodichloromethane 1,1,1-Trichloroethane (TCA) 1,1-Dichloroethane (1,1-DCA) Dichlorodifluoromethane (CFC-12) cis-1,2-dichloroethene (cis-1,2-dce) 1,1,-Dichloroethene (1,1-DCE) Trichlorofluoromethane (CFC-11) Trans-1,2-Dichloroethene (trans-1,2-dce) Toluene DETECTION FREQUENCY IN PERCENT

5 Traditional Treatment Methods Filtration with granular activated carbon (GAC) Chemical treatment systems Biological treatment methods Reduction of THMs Reducing organic carbon prior to disinfection Alternate disinfectant (e.g., ozone) Removal of THMs downstream

6 Air Stripping A process that removes or strips volatile organic compounds from contaminated water by contacting clean air with contaminated t water across a high surface area, causing the volatile compounds to move from the water into the air. Process is governed by Henry s Law. Driving dissolved volatile organic contaminants from water into air.

7 Air Stripping Air Flow Contaminated Counter-current flow means the cleanest air contacts t the cleanest water. This ensures efficient mass transfer throughout the entire e flow path. water Clean water

8 Air Stripping Air water contact happens in the active tray areas. High air to water surface for transfer is created by the turbulent froth mixture.

9 Air Stripping The froth in action.

10 Air Stripping Some physical elements Temperature affects the process higher h 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

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

12 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 p to 60-80% TTHM reduction with 1 pass through an E-Z Tray stripper) 1. Violation Prompts Treatment Change, July 2011 Opflow Spray Nozzles

13 Tower Stripper Method Thin 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

14 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.

15 Tower Stripper vs. Tray Stripper Biggest issue is cleaning - towers are hard to access and often require flooding with acid solution; high O&M costs Very tall structure - wind loading, thermal issues, unsightly in some locations Operating conditions difficult to observe Complex design process due to structural issues No web based performance model, models harder to use

16 E-Z Tray vs. Tower O&M Example Site in Sturgis, MI treating 250 GPM water containing: 111ti 1,1,1-trichloroethane th c-1,2-dichloroethylene hexachlorobutadiene methylene chloride naphthalene tetrachloroethylene (PERC,PCE) trichloroethylene (TCE) Oversized tower replaced with a 500 GPM E-Z Tray Historical tower cleaning with acid cost about $54,000/year Pressure washing the E-Z Tray every days estimated at $8,000/year

17 Stacking Tray Stripper Stacking tray strippers are a series of stacked rectangular boxes with bottom perforations Trays layers are sealed with gaskets and fastened together th 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 Stacking Tray Design More information at --

18 Sliding Tray Stripper (E-Z Tray) Method 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)

19 E-Z Tray Advantages Cleaning E-Z Tray Tower Stacking Tray Air Strippers Air Strippers Air Strippers Single person cleaning (Tray weight is 24 3lb ) Packing access and removal is lbs) difficult needed Major disassembly and multi person crew

20 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

21 E-Z Tray Advantages Monitoring E-Z Tray Tower Stacking Tray Air Strippers Air Strippers Air Strippers Easy process monitoring and inspection, even Condition of packing and air flow distribution are very Difficult or impossible to observe air and liquid flow distribution while in operation difficult to observe during operation

22 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

23 E-Z Tray Design One piece shell with single gasketed door cuts down on possible air leaks Clear front door gives easy visual check of air stripper performance Front access cleaning means piping stays put no need to break connections

24 Ancillary Equipment Skid Blower Infeed Pump Discharge Pump Gravity Drain Control Panel Pressure Switch Liquid Flow Meter Air Flow Meter Bag Filters Solenoid Valve Tank Level Switches

25 QED Air Strippers E-Z Tray y Model 6.4 E-Z Tray y Model 16.4 E-Z Tray y Model 24.4 E-Z Tray y Model 96.6

26 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 The performance modeler is based on the design S = stripping factor Kh = Henry s Law constant L = liquid flow rate G = gas flow rate 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, AIChEJournal,Vol.47,No.6,June2001.Also--Perry,R. H., and D. W. Green, Perry s Chemical Engineer s Handbook, 7th ed., McGraw-Hill, New York 1997.

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

28 Process Variables Temperatures (air and water) Altitude (air density) Flow rates (air and water) - Process impacts - Hydraulic impacts 2 Other Information

29 3 Pick a Stripper First pass pick the stripper model that matches project flow Metric units available on Model Site

30 4 Contaminant Concentrations Concentration in ppb (ug/l) 1000ppb = 1ppm Each contaminant operates independently

31 (URL listed to allow easy remodeling)

32 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

33 Stripper Performance Impacts Air flow restrictions Liquid flow issues Major water/air temperature changes Free phase product or other sorptive compounds that decrease stripping Surfactants or other polar organic chemicals that can lower H for target organics

34 Free Phase Product Coats walls Residual source in sump Partitioning issues Interferes with performance in an unpredictable way

35 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

36 Surfactants Example Normal froth Surfactant impacted froth

37 Types of Tray Air Stripper Fouling Metal oxides 1. 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

38 Tray Fouling Knowing When to Clean Normal stripper sump pressure = 4-6 inch H2O / tray stage

39 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

40 Stripper Cleaning Cleaning frequency and effort is highly site- specific Example ppm TDS, 260ppm total hardness, 0.03ppm iron - stripper requires cleaning every 3 weeks Time to clean an E-Z ZTray 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

41 THM Removal Trihalomethanes (THMs) can form in drinking water when disinfectant i t (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

42 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

43 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. Perc cent Percent THM reduction vs. Water Temperature Water Tem mp. (F) % THM Reduction Water temp

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

45 Dibromochloromethane Removal THM Removal DBCM (ppb) 15 DBCM in DBCM out In 24 hour Out 24 hour Time (days)

46 Impact on Chlorine Residual Chlorine Residual Change (before / after stripping) Cl2 (ppm) 0.8 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)

47 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.

48 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 Rental skids available from QED and some equipment contractors contact us for more information.

49 Additional Site Information for Design Site history of DNAPL and/or LNAPL Parameters that t 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)

50 Other Stripper Applications Ammonia hard to strip Hydrogen Sulfide easy to strip Radon removal extremely easy to strip Methane removal extremely easy to strip

51 Case Study 1 - NY Gasoline spill in high value residential area Space for process equipment limited Need for low profile - sound and appearance Strippers housed in metal shipping i containers

52 Case Study 2 - Australia Large industrial chlorinated cleanup Strippers one component in a multi-unit / multiphase process string EZ-Tray design allowed space saving stripper placement Strippers used in series for higher removal Air In Air Out Liquid id In Liquid Out Max. plant flow = 4MGD (3000gpm; 15MLD) Normal operating range = MGD ( gpm)

53 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

54 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 on-line computer tools - Air stripping equipment needs to be maintained to ensure continued design removals

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