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 Results

3 Processes for VOC removal Activated Carbon Low capital cost, inefficient loading for low concentration contaminants, significant operating costs ($0.95-$1.57/Kgal*) Air Stripping Medium capital cost, capable of high removal efficiency, low maintenance costs, operating cost is blower energy (tray type: $0.10-$0.35/Kgal, tower type: $0.48/Kgal) Oxidation process High capital equipment costs, high operating costs - energy and chemicals ($0.88 $2.42 /Kgal) RO / Ultrafiltration Not always capable of >99% removal, high energy use, some organics can damage membranes ($0.40-$0.70/Kgal) (*web sources; flat 10-year applied capital equipment cost assumed)

4 Air Stripping - Basics 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 transfer from the water to the air. Driving dissolved volatile organic contaminants from water into air.

5 Air Stripping Primary Factors 1. Air to water ratio (A/W) 2. Increasing air to water ratio (A/W) increases removal efficiency 3. Henry s law constant 4. Henry s law constant is temperature dependent (increases with increasing temperature) 5. Some contaminants can not be stripped (1,4 dioxane, methanol, tertbutyl 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 - Process 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 they are transferred. Air Flow Contaminated water Clean water

7 Common Air 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

8 QED E-Z Tray Advantages Stripper Type Modeling Footprint Monitoring Cleaning E-Z Tray On-line and easy to use Reduced footprint Clear door for observation Single person cleaning Tower Engineering design required Small footprint but tall Packing difficult To inspect Packing access and removal is difficult Stacking Tray Some are now on-line Extra space on sides plus piping disconnects Limited porthole observation Extra disassembly multi person crew often needed

9 Stripper Fouling Inorganic Fouling precipitation of inorganics which attach on air stripper surfaces Aeration increases system ph (CO2 is stripped) 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

10 Stripper Fouling Evaluation Tool Iron <0.1ppm 0.1-1ppm 1-5ppm 5-20ppm >20ppm low fouling potential modest fouling potential (not bio) significant fouling potential (optimum for bio) serious fouling potential, heavy O&M (manageable) 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

11 Example Tray Stripper O&M Cost Estimate Assume an E-Z Tray 1000gpm capacity unit treating 800gpm and 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 Operating Cost = $0.125 / Kgal treated

12 Chemical Metering Information Antifoam Stripper (gpm) Typical Feed Rate (GPD) 5 ppm 10 ppm 50 ppm Redux 390 Typical Feed Rate (GPD) Stripper (gpm) 75ppm 85ppm 100ppm

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

14 (URL listed to allow easy remodeling)

15 Successful Stripping 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

16 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.

17 Pilot Testing Pilot test and demo trailer currently located in California

18 Some System Design Notes Standard E-Z Tray material is 304SS 316SS, SL, 2205SS available (chlorides > 80ppm) Induced draft piping recommended for systems with VGAC Gravity drain designs need to have a water trap Site elevation (>1000 ft-measl) is important for blower sizing Check down-stream air piping delta-p this can also impact blower sizing Confirm site electrical specifications Will operation be intermittent? In the strippers the water flow has near 100% turndown; but air flow must stay within +/- 15% of recommended Air will be 100% saturated at process temperature Process temperature is normally equal to the water temperature Induced draft

19 E-Z Tray Drinking Water Applications THM Removal Dissolved gas removal Lead and copper applications Improving GAC efficiency Bottled water and drink bottling plants

20 E-Z Tray NSF Certification Live Safer. QED s sliding tray air stripper (E-Z Tray) was 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

21 THM Removal Trihalomethanes (THMs; DBPs) 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 In Texas over 50% are DBP violations Source -FluksAqua

22 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

23 TTHM Experience / Results Santa Cruz Plant Data $0.095/k-gal treated ($ /kwh power basis; capital cost not included) Plant scale TTHM reductions between 78% - 98% So. Cal April 2016 pilot study Pilot scale TTHM reductions between 61% - 85% -- note the role of A/W in lowest removal Data from AMS on-line THM analysis system.

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

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. 1 TTHM Regrowth 2

26 Why High Efficiency Removal is Important A THM regrowth study at the CA Pilot Study showed that different distribution locations can have different regrowth rates and shows the importance of maximum initial THM removal. Aeration is not the same as Air Stripping.

27 Tank Aeration Methods Spray aeration with venting 39-28% Surface aeration with venting 41-12% -- with most < 30% aeration technologies within the OMT WTP clear well will not be sufficient to lower distribution system TTHM concentrations below MCL levels. Reducing Volatile Disinfection By-Products in Treated Drinking Water Using Aeration Technologies Water Environment & Reuse Foundation (WERF) Web Report #4441 (2015)

28 Cl2 (ppm) Does Stripping Impact Chlorine Residual? Chlorine Residual Change (before / after stripping) 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.)

29 TTHM Removal Economics Model / Tool Tool provides water treatment cost estimates at different target TTHM reduction levels. Evaluates 2 and 3 tray level design options to help lower blower energy costs. Automatically calculates number of units required for high flow rate cases. Need an assumption changed for your project? Contact us and we can customize the results report with your inputs. Assumptions: water and air temperatures of 55F, site elevation of 0-ft MEASL, labor rate of $50/hour, two cleanings per year, 10 year simple capital amorization, 365 days/year operation, 0.1 $/kw-hour energy cost. THM distribution taken from pilot and plant data at five different municipal drinking water plants. Contact QED for a customized report with assumptions for your plant and THM distribution conditions. Note: flows below 250gpm will use the E-Z Tray Model 24.X size air stripper. This is currently the smallest model available with NSF-61 Certification. Lower flow models (fabricated of identical materials) are available - contact QED for more information.

30 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)

31 Stripping and Inorganics CO2 and ph Results from several pilot studies ph and copper

32 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

33 Increasing GAC Utilization 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

34 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 periods (4000gpm capacity) Excellent equipment reliability required to ensure continuous water treatment

35 Case Study 1 Cheyenne, WY 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 working flawlessly since 6/2011 Electrical usage saving of more than $2500/month per blower from high efficiency removal (air flow operated below nominal level).

36 Case Study 2 Cedarburg, WI Landfill near a 700gpm supply well with vinyl chloride hits (below MCL) System modeling based on a long list of possible future contaminants, based on LF monitoring data E-Z Tray footprint critical Started up in Sept City operates a tower stripper on another well treating an unrelated TCE issue in operation 18 years (next slide) Sequestering agent used for tower (and planned for E-Z Tray) tower never cleaned

37 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

38 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 also a key design factor possible tower location E-Z Tray units located in existing space within utility garage

39 Case Study 3 Edina, MN 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

40 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 Low dose NSF approved sulfuric acid is added after the stripper to slightly reduce ph

41 Case Study 4 Madison, WI Used QED gravity drain discharge units 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

42 Case Study 5 Santa Cruz, CA Treating 350 GPM for THM/DBPs One E-Z Tray 36.6 stripper (20HP blower) City piloted a tower and worried about observed fouling issues Evaluated the E-Z Tray units against some alternative stacked tray designs and liked the ease of access and stainless construction

43 Case Study 5 Santa Cruz, CA 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 (Aqua Metrology Systems AMS) Full Case Study available at --

44 E-Z Tray Air Stripper Installations

45 Summary Air strippers are effective at removing dissolved volatile organic compounds and gases from water The primary process factors are air to water ratio (A/W) and Henry s constant (H) of target VOC or gas The process can be modeled using QED s on-line computer tool - Well maintained air stripping equipment will provide many years of effective service

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