5 Benefits & Process

Transcription

1 5 Benefits & Process Improvements of Pretreatment with Air Strippers Christopher Hessler - QED Environmental Systems Inc. Dexter, Michigan - San Leandro, California Copyright QED Environmental Systems, Inc. 2013; all rights reserved. The information contained within this document may not be published, broadcast, rewritten or otherwise distributed without the prior written authorization from QED.

2 Topics of Discussion

3 Topics of Discussion Section 1: Air Stripping & Pre-treatment Air stripping 101 The E-Z Tray air stripper Process & operating considerations Section 2: Process Improvements Pre-treatment for activated carbon Improving oxidation processes Optimizing biological treatment Improving performance / maintenance for membranes Degassing & aeration applications Section 3: Final Remarks

4 Process Optimization & Pre-treatment Environmental treatment can be complicated Broad range of contaminants & unpredictable No one size fits all answer to achieving goals Even effective technologies for a task do not run optimally using a heavy lifting approach Pre-treatment and careful technology placement can vastly reduce operating & maintenance costs for the system life-cycle Air stripping provides an exceptionally suited platform for VOC reduction to improve operations as part of the larger treatment goals

5 Process Optimization & Pre-treatment Mass transfer defined by Henry s law (partitioning) Equilibrium process where contaminants driven into air through high air-to-water ratio Contaminant are not destroyed during process Counter-current flow design creates optimal driving force for removal cleanest air is contacting cleanest water

6 Air Stripping Technology Overview Why air stripping? Fundamental process modeling Any text book EPA evaluated / sanctioned High VOC removal efficiency 4-log reduction (99.9%) Low A/W Limited process impediments Volatility is insensitive Below solubility limits Clean air Contaminated air

7 Air Stripping Technology Overview Contaminants defined by Henry s law constant Higher Henry s law constant = more volatile contaminant Henry s law constant is temperature dependent (increase with increasing temp) Increasing A/W can improve 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

8 Air Stripping with the E-Z Tray E-Z Tray Model 6.4 (65 gpm max) E-Z Tray Model 16.4 (150 gpm max) E-Z Tray Model 24.4 (250 gpm max) E-Z Tray Model 96.6 (1000 gpm max)

9 How the E-Z Tray Works A mixture of air and water creates a turbulent froth of bubbles on each sieve tray Small bubbles have high surface area for mass transfer Contaminants partition into the bubbles on a massive scale and exit in the off gas Multiple equilibrium contacts improve removal efficiency

10 Comparison of Air Strippers Sliding Tray Tower Stacking Tray Air Strippers Air Strippers Air Strippers Reduced footprint for installation and maintenance Modular Resistant to fouling Small footprint but very tall structure often required Construction demands Difficult process modeling More space needed for disassembly, lifting from all sides, pipe disconnection and tray stage stacking

11 The E-Z Tray Design & Operating Advantage Front hatch provides easier access Visual observation of process No forklifts, gantries, multi-person disassembly crew Minimal space constraints Light weight removable sieve trays Single person removal Quicker re-assembly Better serviceability All plumbing connections are fixed No re-sanitizing No hose / flex re-assembly

12 The E-Z Tray Design & Operating Advantage Split tray Components Replace 6 ft. trays Components Weigh less than 35 lbs.!!!! Small, easy to manage

13 E-Z Online Modeling & Support Access the Web based Modeler 24 / 7 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.

14 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

15 Why the E-Z Tray? Highly efficient for VOC removal 99.9% reduction of solvents / BTEX Low A/W Turn-down flexibility Results in low cost / 1000 gal. Implementing the E-Z Tray Replacing existing aerators or towers (less efficient) Easy process integration Small foot print reducing space / construction demand Ease of maintenance E-Z Tray provides distinct cost advantage over drilling a new well. Easy to implement in existing space. Easy to service & operate (trays).

16 The E-Z Tray Design & Operating Advantage 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 Consider 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.

17 Temperature Higher water temp increases volatility Better stripping efficiency Thermal equilibrium between A/W Free product Creates second equilibrium (oil / water) Reduces apparent Henry s constant Lower process efficiency Surfactants Increase solubility of contaminants Reduce air stripper efficiency Process Considerations Cause foaming through air / water contact Upstream pre-treatment can optimize the efficiency of air stripping!

18 Process Considerations Contaminant in equilibrium with oil, water, & air phases!!! Stripping removes dissolved portion of contaminants in water. Some organics partition into the free-phase component and re-equilibrate after stripping.

19 #1 Reducing the Load on GAC systems Activated carbon is a well understood staple for contaminant removal. Much like air stripping, contaminants are moved to another phase, but not destroyed in the process. Adsorption is based upon contaminant partitioning between solid & liquid phase. Carbon is effective & efficient in most cases but also non-selective. When adsorption sites fill, contaminant breakthrough occurs and the GAC must be removed & replenished Carbon exchange costs have greatly increased and drive the life cycle cost for this technology!!!

20 #1 Reducing the Load on GAC systems (cont.) Air stripping can prolongs GAC life by removing highly volatile fractions 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 x-fer driving force Oversized contactor / load Example (assumes 25% loading rate) 200 ppb 1,000 gpm = 400 lbs. / day reduction in GAC loading 20 ppb 1,000 gpm = 961 lbs. / day reduction in GAC loading

21 #1 Case Study: The classic pump & treat Common practice for groundwater treatment & recovery Oil / Water separator used to recover floating / sinking product from the waste stream. More volatile VOCs removed efficiently using an air stripper. Activated carbon polishes the water and removes less volatile or nonvolatile components. For a 50 gpm P/T system (BTEX / 25% solubility) with an air stripper upstream of polishing GAC: A/S removes 99% of BTEX % 29% DRO Carbon vessels optimized for DRO removal Reduced carbon loading of 252 lbs VOCs / day!!!

22 #2 Increasing the Efficiency of Oxidation Processes Oxidation & advanced oxidation reactors emerging for the destruction of potentially dangerous & persistent contaminants. Reaction of organics with radical species End product CO 2 Reduction of taste & odor compounds Oxidation of dissolved inorganic species Many commercialized variants. Oxidation processes are effective for contaminant destruction. Process is non-selective and must be designed for total oxidant demand not just targeted contaminant.

23 #2 Increasing the Efficiency of Oxidation Processes (cont.) Destruction efficiency function of: Reaction stoichiometry Oxidant dose / contact time Scavenger demand By-product formation Reduction of oxidant with pre-treatment based on reaction 1000 gpm and NO constraining demand: 400 ppb TCE = 33 lbs. / hr ozone reduction 400 ppb VC = 66 lbs. / hr ozone reduction 400 ppb benzene = 335 lbs. / hr ozone reduction Air stripping can reduce oxidant demand by removing: Chlorinated solvents BTEX Alkanes Oxidation processes is optimized to target complex or non-volatiles: 1,4-dioxane MTBE TBA

24 #2 Case Study: Groundwater Treatment with a Complex Contaminant Matrix Price s Pit Superfund Site Illegal chemical dump site impacting groundwater & drinking water wells. Complex mixture of solids, heavy metals, VOCs, and other toxic organics. Unit operations include: clarification, bio- degradation, air stripping & hydrogen peroxide polishing. Treatment processes selected & staged to optimize efficiency at each stage. Recalcitrant organics were targeted in the final stage with a minimal dose of peroxide.

25 #3 Improving the Performance of Biological Treatment Systems Aerobic & anaerobic biological reactors degrade contaminants through meta- bolic processes (biochemical & enzymatic) Mass transfer into bio-matrix & process sensitivity to operating conditions. Different mechanisms for different contaminants: Chlorinated solvents not efficiently removed in aerated systems Hydrocarbons aerobically degraded Aerobic processes typically quicker degradation than anaerobic

26 #3 Improving the Performance of Biological Treatment Systems (cont.) Reactor size & performance impacted by multiple biochemical & process variables: DO Alkalinity Temperature Consortium Fluctuation in chemistry Many VOCs experience mass x-fer limitation. Aerated system can partition volatile contaminant without degradation into large air volume! Air stripping removes low MW compounds with variable biodegradability for more stabile biological treatment. Permits for more easily controlled emissions.

27 #3 Case Study: Reducing off-gas treatment from an 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 Before After Thermal Oxidizer VOC VOC Less VOC Treatment Plant Treatment Plant

28 #4 Maintenance Reduction for Membrane Systems Membrane systems provide high quality effluent through: Size exclusion Permeability Chemical properties Significant maintenance requirements which requires monitoring & back flushing: Biofouling Irreversible fouling Chemical attack of membranes Excessive loading In potable water systems, membrane systems implemented downstream for effluent polishing. In wastewater applications, membranes achieve high rejection rate at estimated operating costs of $ / gallon.

29 #4 Maintenance Reduction for Membrane Systems (cont.) As an upstream pretreatment, air stripping can removed or chemically alter contaminants which cause membrane damage or premature irreversible fouling: Hydrogen sulfide Chlorinated VOCs Hydrocarbons Soluble metals (through oxidation) Low A/W ratios result in 99+% H 2 S removal with ph adjustment. Dissolved iron precipitation typically 80 90% and removed via filtration

30 #4 Case Study: Stripping H 2 S and precipitating iron prior to RO High purity water producer Ground water wells unusable: High sulfides (~250 ppm) High iron (~2 ppm) Damage & increased maintenance cost for RO systems led to increased use of municipal water source Air stripper used to remove H 2 S & precipitate iron. Post air stripping: Non-detect H 2 S 90% iron precipitation into media filters prior to RO

31 #5 Degassing and Aeration Many phenomena generate dissolved gasses which can saturate aqueous streams or extraction liquids. Presence of gasses can confound or damage down stream processes Dissolved gasses can also contribute to fugitive emissions & permitting requirements Contaminant Henry s (atm) 25 o C CO 2 1,216 1,480 CH 4 35, H 2 S 545 3,740 Radon 4,680 1,724 Solubility (ppm) 25 o C Typically, gasses have a high VP & high Henry s constant, so exceptional removal efficiency expected

32 #5 Degassing and Aeration (cont.) Advantages of degassing with the E-Z Tray High removal efficiency at low A/W ratio Lower total airflow & improved emission control Compact / modular installation Reduced operating footprint & construction demands compared to build-in-place basin Methane emissions at saturation are BELOW the LEL If emissions control is necessary consider cost of covering a basin in addition to off-gas treatment Assume: 1.5 MGD basin Basin: $ / 1000 gal treated E-Z Tray: $ / 1000 gal treated

33 #5 Case Study: Reducing Corrosion through ph Adjustment Copper corrosion in tanks & water lines AWWA study investigating reduction of copper & lead leaching from water storage tanks & lines 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

34 Is Air Stripping Viable for Pre-treatment? Pilot Testing! The clear & obvious solution to messy applications Small (~50 gpm max) skidded unit placed in field Ability to gather performance data Control & measure operating conditions Acquired data used to back calculate effective Henry s law constant for scale-up Additional Benefits Low monthly rental charge Opportunity to work with equipment Test results provide data for scale-up

35 In Conclusion. Pre-treatment processes can be invaluable to improve overall treatment efficiency & reduce operating / maintenance costs. Technologies can be selected & implemented in key locations of the process for targeting contaminants which may impact downstream technologies. Air stripping using the E-Z Tray is ideal for efficient removal of VOCs, whose presence contribute to operating & maintenance demands on down stream technologies. Removal of VOCs & dissolved gases are shown to improve efficiency & operating costs of activated carbon, oxidation, biological, & membrane treatment systems. The E-Z Tray design provides a robust & highly efficient platform for VOC & dissolved gas removal, while vastly reducing maintenance demands & installation requirements of other aeration technologies.

36 Questions? Christopher Hessler QED Environmental Systems, Inc. Ann Arbor, MI and San Leandro, CA Toll-free: Direct: x 263