vvEPA United States Environmental Protection Agency EPA/540/SR-98/504 March 1999 SUPERFUND INNOVATIVE TECHNOLOGY EVALUATION Emerging Technology Summary Integration of Photocatalytic Oxidation with Air Stripping of Contaminated Aquifers Rajnish Changrani, Gregory B. Raupp, and Craig Turchi In a recently completed test program, bench-scale laboratory studies at Ari- zona State University (ASU) in Tempe, AZ, and pilot-scale studies in a simu- lated field-test situation at Zentox Cor- poration in Ocala, FL, were performed to evaluate the integration of gas-solid ultraviolet (UV) photocatalytic oxidation (PCO) with air stripping of an aquifer contaminated with chlorinated volatile organic compounds (VOCs). Chlori- nated ethylenes such as trichloroethyl- ene (TCE) can be destroyed in a wide process window, although chlorinated ethanes such as trichlorethane (TCA) are nonreactive. Water vapor signifi- cantly inhibits the chlorinated ethylene destruction rate. For this reason, PCO units should be placed downstream of a dehumidification unit located between air strippers and the PCO unit, with targeted reduction of the relative hu- midity in the contaminated air stream to less than 50%. Principal carbon-con- taining products of PCO identified ex- perimentally at the bench scale include carbon dioxide, carbon monoxide, and phosgene (COCI2). Failure to close car- bon mass balances under some pro- cess conditions suggests that not all byproducts were identified. Further studies are needed in this area. A panel bed was identified as the preferred pho- toreactor configuration. This unit is characterized by simplicity of construc- tion, ease of maintenance, and high UV photon utilization efficiency. This Emerging Technology Summary was developed by EPA's National Risk Management Research Laboratory, Cin- cinnati, OH, to announce key findings of the research project that is fully docu- mented in a separate report of the same title (see Project Report ordering infor- mation at back). Introduction Contamination of drinking water aqui- fers with volatile chlorinated organic sol- vents is a widespread problem across industrialized areas of the U.S. The. global objective of this work was to evaluate the integration of gas-solid ultraviolet (UV) pho- tocatalytic oxidation (PCO) downstream of an air stripper unit as a technology for cost-effectively treating water pumped from an aquifer contaminated with chlorinated volatile organic compounds (VOCs). The photocatalytic oxidation process integrated with air stripping is shown schematically in Figure 1. In this configuration, the air stripper off-gases are fed directly to the PCO reactor without pretreatment. In the Printed on Recycled Paper ------- continuous flow PCO reactor, the con- taminated air stream contacts the surface of a near-ultraviolet irradiated titania (TiO2) catalyst, causing photochemical destruc- tion of the contaminants at or near room temperature. Exhaust gas from the PCO reactor is fed to a dry scrubber for re- moval of HCI and CI2 generated during tho oxidation of chlorinated solvents. The primary objectives of this research project were as follows: • Define the optimum gas residence time, catalyst characteristics, UV light intensity, and photoreactor configuration that achieves greater than 95% destruction of the primary VOCs (trichloroethylene and trichtorethane) in air stripper off-gases. • Characterize the overall performance of the integrated air stripper/PCO process by quantifying contaminant destruction during short-term transients generated by air stripper startup, shutdown, and upsets, and by quantifying destruction perfor- mance over an extended test period. The secondary objectives of this project included the following: • Quantify catalyst lifetime independent of factors related to integration of the PCO unit with an air stripper. • Identify reaction byproducts and conditions that Inhibit or enhance their formation. Most of these objectives were success- fully achieved through bench-scale and pilot-scale controlled testing described in the body of this report. The first objective was mot through the following strategy. Statistically designed experiments enabled efficient process performance character- ization as a function of key process vari- ables. The derived empirical response surface models allowed prediction of con- ditions for which high VOC destruction efficiency could be achieved. These pre- dictions were subsequently confirmed in a long-term bench-scale test and in several short-term pilot-scale tests. Because we wore unable to obtain actual field test data, the second primary objective listed above could not be achieved. Experimental Methods The bench-scale PCO system at ASU Is shown schematically in Figure 2. A CO2- free synthetic air mixture contaminated to a predetermined level is prepared by con- tinuously mixing gases from pressurized gas cylinders using mass flow controllers (mfc). Water vapor is added to the feed flow by a separate nitrogen flow through a saturator. The humid, contaminated air is fed to an annular photoreactor incorporat- VOC-laden humid air Air f - T Stripping T T Towers *^ *^ VOC-contaminated groundwater Surge Tank *—fi Utility air Stripped water Figure 1. Integrated air stripping with photocatalytic oxidation process. ing a 1.5" OD 20 W UV black lamp in a 2.25" ID glass tube. The annular space is filled with titania-coated packing; bed length is 90 cm. Ultraviolet light fluxes at the lamp and at the reactor outer wall were measured with a Minolta integrating photometer. The system incorporates three in-line sensors (Sensl, Sens2 and SensS) and three discrete sampling locations (Sam1, Sam2 and SamS). Sam1 and Sam2 are automatic multiple port GC sampling valves which send a well-defined volume of gas to one of two columns in the Varian 3700 GC for VOC and carbon dioxide analysis, respectively. The third sampling location (SamS) allows gas samples to be with- drawn for off-line analysis by gas detec- tion tubes. Sensl is an in-line relative humidity sensor (Vaisala). Sens2 and SensS are in-line electrochemical sensors for HCI (Detcon) and CO (Sierra) mea- surement, respectively. Flow, temperature and pressure measuring devices are also included as indicated in the figure. Mea- surements are recorded using automatic data acquisition driven by a Hewlett- Packard 486 processor based PC. Testing Procedures For a given performance evaluation run, gas flows are set and stabilized and the feed gas is analyzed with the sensors and GCs until a steady reading is achieved. At this point, the UV lamps are illuminated, and the reactor outlet is regularly ana- lyzed. Once steady operating conditions are achieved (typically several hours with fresh catalyst beds, several minutes with conditioned beds), measurements are re- corded for several more hours. It is these average steady state values (usually six to eight points taken over two hours) tnat are reported. i Three unique sets of experiments were run. Set 1 was designed to character12® the performance of the bench-scale ij"™ under process conditions that are repi'jf" sentative of those found in the field at jne site to be tested. Groundwater at the tp~ get industrial field site is currently bejn9 pumped and treated by a 660 gallons |pr minute remediation plant employing \atr stripping and carbon bed adsorption the stripper off-gases. In the planned fi test, the PCO unit would have been e ployed to treat a slipstream of the stripper off-gases. The expected total concentration ranges from 10 to 18 P with typically two-thirds of the total attrr" uted to TCE. An in-line dehumidifier rr" duces the relative humidity (RH) in tlPe off-gases to about 50% upstream of tle carbon beds. Process characterization was efficiency achieved by employing statistical desigjn of experiments (DOE) surface respon6 methodology (SRM). Table 1 summariz the independent process variables and le els. The chosen 1/2 fraction of a 25 fact rial design augmented with axial runs center point replicates, or "face center cubic" design, provides a complete r sponse surface and has a reasonably stable variance over a large portion of th design region. Set 2 is designed to efficiently chara terize the performance of the bench-scal unit for conversion of complex contain nant mixtures. Set 3 experiments wen designed to test the long-term stability the photocatalyst in a reactive enviro ment. f ------- GC Carrier Gas#1 Sam1 Sam2 Sam3 PCO Reactor Effluent VOC in Air Feed Preparation Section Sam1 - 3 Sampling locations Sens'! - 3 Sensing locations mfc mass flow controller P Pressure gauge T Temperature sensor Figure 2. Bench-scale photocatalytic oxidation apparatus. ablel. Process Parameters and Levels actor Dtal inlet VOC concentration (ppmv) elative Humidity (%) V Intensity),^, (mW/cm2) smperature (°C) i ?sidence tiine (s) Code A B C D E Low(-1) 10 20 0.5 40 0.5 Center (0) 20 50 2.0 60 1.0 High (+1) 30 80 3.5 80 1.5 " 'The principal measured responses are conversion Xvoc of the VOC and selectiv- ity SC02 of the reaction towards producing CO2. Conversion is a direct measure of destruction efficiency, while the selectivity is a measure of the percentage of the VOCs destroyed that are converted to the desired complete combustion product car- bon dioxide. Results and Discussion Set 1 Results Under the conditions of these experiments, TCA conversion is es- sentially negligible, with measured values ranging between about 2 and 6%. For purposes of brevity, subsequent DOE mod- els for VOC conversion described below and the accompanying discussion are for TCE conversion only. Trichloroethylene conversion data are described by the fol- lowing quadratic model: Xvoc =111.02- 0.22 • VOC -2.81 • RH + 9.02 • UV - 0.23 • T + 42.0 • T - 15.11 • i? - 0.07 • RH • UV + 0.008 'RH'T where VOC = VOC concentration (ppm ), RH = Relative Humidity (%), UV = UV intensity (mW/cm2), 7= Temperature (K), and -r = residence time (s). The model reveals that all five independent factors are significant. The three-dimensional re- sponse surfaces are shown in Figures 3 and 4. Increasing residence time and UV intensity increase VOC conversion, whereas increasing RH decreases VOC conversion. Inhibition of the TCE photo- catalytic oxidation rate by water vapor is a well-established phenomenon first reported by our laboratory. The following linear model adequately describes the observed selectivity behav- ior: SC02 = -23.90 - 1.61 • VOC -0.19' RH + 3.93 • UV + 0.43 • T- 12.51 • r + 0.0002' VOC'UV'T where variables are as defined previously. Increasing VOC concentration decreases selectivity, while increasing light intensity (UV) and increasing temperature (T) in- crease selectivity. In addition, there is an interaction among VOC concentration, UV and T. The magnitude of the interaction ------- VOC conversion 60 VOC concentration (ppm) 1.5 Residence time (s) Figure 3. Response surface for VOC conversion versus residence time and inlet VOC concentration. Twenty percent RH, 3.5 mW/cm2UV intensity, 40°C. 60 VOC conversion 20 Ritativ* Humidity (%) 30 1.5 Residence time (s) Figure 4. Response surface for VOC conversion versus residence time and RH. Ten ppmv VOC concentration, 3.5 mW/cm2UV intensity, 40°C. term, while statistically significant, is small relative to the main factor effects. The main selectivity responses can be ex- plained within the framework of a surface- mediated, free-radical sequential oxidation mechanism. In this model, hydroxyl radi- cals created through UV excitation of the titania photocatalyst initiate oxidative at- tack on adsorbed VOCs to create VOC radicals. The radical sites are then at- tacked by oxygen to create peroxy radi- cals. These reactive peroxy radicals readily decompose to produce partial oxidation products on the catalyst surface. This se- ries hydroxyl radical attack, oxygen addi- tion sequence continues until complete combustion products are produced. We hypothesize that TCE is first oxidized to dichloroacetaldehyde, then to dichloroace- tic acid, and ultimately to chlorinated for- mic acid intermediates. Under dry conditions <10% RH), hydroxyl radicals may not be the primary initiator of oxida- tion, and an alternative reaction path through phosgene becomes dominant. With increasing relative humidity, the pro- duction of phosgene and other undesir- able intermediates decreases. For sufficiently high humidity (>40-50%) and residence time, essentially complete con- version to carbon dioxide is observed. Assuming typical Langmuir-Hinshelwood- Hougen-Watson (LHHW) reaction kinetics determine the TCE destruction rate, in- creasing TCE concentration should in- crease the average surface coverage by TCE. For otherwise fixed conditions, the production rate of reactive hydroxy) radi- cals will be essentially the same. At steady state, there will therefore be relatively fewer hydroxyls to attack adsorbed partial oxi- dation intermediates, leading to a reduced integrated selectivity to CO2. A similar ar- gument can be posed to explain the UV dependence. With increasing UV inten- sity, the production rate of hydroxyl radi- cals increases. A relatively greater number of hydroxyls will be available to attack partial oxidation intermediates, leading to an increased integrated selectivity to CO2. No VOCs other than TCE and TCA were observed in the GC chromatograms. Carbon monoxide levels were between 0.5 to 1.5 ppmv for all experimental runs. Molecular chlorine (Cl) and phosgene (COCI2) levels were observed at levels <0.2 ppmv; no significant hydrogen chlo- ride was detected. The calculated carbon mass balances for the Set 1 experimental runs range from ca. 60 to 100%. In geh- eral, highest percent closures were ob- tained for low inlet VOC concentration. Low inlet TCE concentrations also favpr higher selectivity. Lack of closure could not be attributed to inaccuracy in C02 measurements but is instead most likely due to non-detect of volatile partial oxida- tion products including, but not necessar- ily limited to, dichloroacetyl chloride. A Cl atom balance is not reported since, ex- cept for unconverted TCE, only very small amounts of chlorine-containing compounds were detected in the reactor outlet for all runs! It is likely that chlorine buildup on the catalyst surface is responsible for the low levels of chlorine-containing com- pounds detected in the reactor effluent. ------- Sef 2 Results At the target Superfund site, co-contaminant or secondary VOCs present along with the primary VOCs are, 1,1-dichloroethylene, cis 1,2- dichloroethylene, vinyl chloride, tetra- chloroethylene (also known as perchloroethylene), and 1,2-dichloroben- zene. Each of these co-contaminants is present in varying amounts depending on air stripper operation, but under no circumstance have levels higher than 1 ppmv been detected. A mixture experi- mental design in which the chlorinated ethylenes were lumped together to create a "secondary chlorinated ethylenes" pseudocomponent was employed. This strategy yields the following three-compo- nent mixture: trichloroethylene (mole frac- tion x,), secondary chlorinated ethylenes (mole fraction x2), and 1,2-dichlorobenzene | (mole fraction x3). The initial concentra- tions for each of the chlorinated VOCs in the lumped secondary chlorinated ethyl- enes category were held in the same ratio for all experiments, at levels designed to , approximate the average ratios found at the Superfund site. A 32 factorial mixture experiment was designed to capture the maximum pseudo- component ratios R, = x, / x2 and R2 = x, / x3 expected at the Superfund site. Table 2 summarizes these ratios and the corre- spibnding pseudo mole fractions of the thr,ee pseudocomponents for the nine ex- peHmental runs performed in this set. Otner process conditions were fixed at representative values as follows: TCE in- let, concentration = 10 ppmv, RH = 20%, UV Intensity = 3.5 mW/cm2, Temperature = 40°C., and residence time = 0.5 s. These process values were chosen to yield a TCE conversion that was reasonably high . (order of 70%) but not so high (i.e., >95%) thp conversion sensitivity to the presence of| co-contaminants would be masked by pqor measurement statistics. :A statistical analysis of the experimen- tal data revealed that nearly the entire Table 2. Mixture Experimental Design Run ID 1 2 3 4 5 6 Ri 2 4 6 2 4 6 2 4 R2 6 6 6 11 11 11 16 16 x, 0.60 0.70 0.75 0.63 0.74 0.80 0.64 0.76 X2 0.30 0.18 0.13 0.31 0.19 0.13 0.32 0.19 X3 0.10 0.12 0.12 0.06 0.07 0.07 0.04 0.05 sum of squares is explained by the sum of squares for the mean, I.e., that the effect of the co-contaminants on TCE con- version is not statistically significant. Se- lectivity depends on the pseudocomponent ratios according to the following quadratic model: Sco2 = ~39-19 +10.69 • R, + 9.23 • R2 + 0.71-Rf -0.26• R22 -0.525•R1-R2 Figure 5 shows this response visually. The response shows a complex depen- dence on the pseudocomponent ratios. As the ratio (R,) of TCE to secondary chlorinated ethylenes increases, selectiv- ity increases regardless of the chloroben- zene level. The increase is most dramatic at the lowest ratio of TCE to chloroben- zene. The Selectivity tends to go through a broad maximum as the ratio (R2) of TCE to chlorobenzene increases, with the lo- cation of the maximum shifting to lower R2 ratios as the ratio of TCE to secondary chlorinated ethylenes increases. Carbon mass balance closures range from ca. 60 to near 100% (basically the same range observed for the factorial ex- periments in Set 1). As for Set 1, a chlo- rine atom balance is not reported since essentially no chlorine-containing com- pounds other than those found in uncon- verted VOCs were detected in the reactor outlet for all runs. Set 3 Results These experiments were designed to test the long-term stability of the photocatalyst in a reactive environ- ment. Process conditions were chosen so that high TCE conversion (>95%) and se- lectivity (100%) were obtained during the initial stages of the run. To simplify gas analysis, TCE was the only VOC in the contaminated air stream. A freshly-pre- pared catalyst bed was employed. Over the course of the first six days of operation, TCE conversion held steady at greater than 95%, while Selectivity was constant at approximately 100%. The C mass balance was essentially closed in all measurements (=100%). The Cl atom mass balance was only about 3-4%. After six days on stream, a dramatic change in performance was observed. The Cl atom balance closure jumped to the 70-80%' closure range. This improved closure is associated with the appearance of sub- stantial concentrations of HCI and CI2, as well as lesser amounts of COCI2. Selectiv- ity fell below 100% at this point as a portion of the inlet C appeared in the COCI2 byproduct. Over the remainder of the run, the TCE conversion gradually fell, although it remained above 90% over the entire duration. The selectivity also gradu- ally fell, although it remained above 95%. The findings in this experiment are con- sistent with a model in which the fresh 14 12 10 R2 (TCE: Aromatic) Figure 5. Influence of pseudocomponent ratios R, and R2 on selectivity. 5 R1 (TCE: Cl- ethylenes) ------- surface becomes chlorinated as TCE is dochlorinated and oxidized in the PCO process. Initially this deposited Cl is non- volatile and an inventory of adsorbed Cl builds up on the surface as TCE-contain- ing air Is treated. At the point of Cl atom saturation on the surface, TCE conver- sion continues but chlorine produced sub- sequently Is volatile, appearing in the gas phase as HCI, CI2, and COCI2. Pilot-Scale Experiments A commercial scale reactor should be designed to achieve high integrated pho- ton utilization efficiencies, while providing low pressure drop, high throughput opera- tion in a reasonably compact physical con- figuration. The preferred reactor configuration is a key technical issue ad- dressed in this research. Figure 6 com- pares the two principal reactor configurations tested in this research project. Configuration 1 is the packed bed design, in which a cylindrical photoreactor containing a number of equally-spaced par- allel UV lamps is packed with a catalyst- coated support material. The air flows parallel to the lamps that irradiate the sup- ported catalyst. Configuration 2 is panel bed design, in which a rectangular duct- shaped photoreactor contains a number of alternating UV lamp banks and a re- movable panel-type catalyst support ma- terial. In this design, the air flows perpendicular to the lamps that irradiate the panels. Set 1: Reactor Configuration 1 Table 3 summarizes TCE conversion perfor- mance for a set of controlled pilot-scale runs at Zentox's production facility with Reactor Configuration 1. Note that the tem- perature reported refers to the ambient air temperature, not the temperature of the reactor, which should be substantially higher (20 to 40°C.) due to thermal en- ergy dissipation of the UV lamps. Inhibi- tion of the VOC conversion rate by the presence of water vapor is substantial at the pilot-scale as evidenced by a com- parison of Run 1 with Run 2, and by a comparison of Run 7 with Run 8. The inhibition effect appears to be more se- vere at the lower TCE concentrations (Runs 1 and 2). Second, higher destruc- tion efficiencies are achieved for lower air flow rates (compare Runs 3 and 4). This behavior is essentially a residence time effect expected for rate-based processes. Although this reactor configuration yielded acceptable performance charac- teristics with respect to throughput and light utilization, the packed bed design used a support material that was difficult to coat with catalyst, and the loosely packed bed was cumbersome to exchange if a catalyst replacement was necessary or desired. Moreover, because the sup- port was organic polymer based, it tended to degrade under UV irradiation. An im- proved reactor configuration was there- fore designed, constructed and tested based on experience gained in the labo- ratory and in these pilot-scale tests. Set 2: Reactor Configuration 2 Table 4 summarizes TCE conversion perfor- mance for a set of short-term, controlled pilot-scale runs at Zentox's production fa- cility with a layered, perpendicular flow PCO reactor (Reactor Configuration 2). This design employed consecutive, alter- nating banks of lamps and catalyst-coated panels. In a general qualitative sense, this re- actor configuration exhibits the same per- Airin Catalyst-coated packing UV lamps UV lamp bank Catalyst panels Air in Air out Figure 6. Photocatalytte oxidation reactor configurations. Configuration 1 (top) is a packed bed design, and Configuration 2 (bottom) is a panel bed desig 6 ------- Table 3. TCE Conversion Results for Reactor Configuration 1 Run UV Air Flow Temp. TCE RH No. Lamps (scfm) (ppmv) Conversion Photon (%) Efficiency (%) 1 2 3 4 5 6 7 8 22 ,22 22 22 22 0 22 22 28 28 28 20 38 39 39 39 21 33 24 24 24 22 22 32 22 19 21 31 15 67 69 64 76 23 45 45 44 68 68 25 55 90 90 96 90 5 77 90 1.2 1.7 1.9 2.1 1.9 — 7.4 8.0 Table 4. TCE Conversion Results for Reactor Configuration 2 Run No. 10 11 12 13 14 15 16 17 18 )9 20 UV Lamps 18 18 18 18 18 18 18 46 46 50 50 Air Flow (scfm) 28 43 27 43 22 41 25 50 50 75 75 Inlet Temp. (°c.) 28 29 23 24 24 26 26 29 32 27 41 TCE Inlet (Ppmv) 85 55 81 29 29 53 76 45 46 52 49 " RH (%) 24 24 47 45 43 60 45 40 20 70 21 TCE Conversion (%) 90 81 85 77 89 62 87 91 99 36 92 Photon Efficiency (%) 9.4 8.4 8.0 4.3 2.5 6.0 7.1 3.5 3.9 2.2 5.3 i 10 _ 8 0 Configuration 2 © Configuration 1 6.. 4. . o £ 2 0 10 20 30 40 50 60 70 80 90 Inlet VOC Concentration (ppm) .Figure7. Integrated photon efficiency versus inlet TCE concentration for the two pilot reactor configurations. formance behavior as the first reactor con- figuration. A direct performance compari- son of the two reactor configurations is not possible since the reactors were not tested under identical conditions of flow rate, VOC concentration, relative humid- ity, and temperature. Apparent photon ef- ficiency is a performance metric adopted by PCO equipment suppliers to allow com- parison of the inherent energy efficiencies of different photoreactor designs. Figure 7 is a visual comparison of the effective photon utilization in each configuration ver- sus inlet VOC concentration. Although there are significant run-to-run variations in process parameters, the figure shows that the two designs behave roughly the same at equivalent inlet VOC concentra- tions in terms of effective photon utiliza- tion. The lower fabrication cost and simpler maintenance make the second configura- tion a superior design. Conclusions Controlled testing of two pilot-scale re- actor systems revealed that they exhibit qualitatively similar performance charac- teristics; Integrated photon utilization efficiencies depend on inlet TCE con- centration, with higher efficiencies ob- served for higher inlet concentration. The primary advantages of the second configuration lie in ease of fabrication and maintenance. In particular, the second or panel configuration utilizes a catalyst sup- port that is simple to produce, and that allows simple, rapid installation or ex- change of the active catalyst loaded pan- els. Based on the results of our bench-scale studies, we can make the following con- clusions on PCO process performance for the target application: • Although a large process conditions window exists for which TCE can be destroyed, frichlorethane (TCA) is nonreactive. Thus, PCO as presently employed is not a viable technology for treating TCA-contaminated air streams. • Increasing incident UV light intensity and mean gas residence time increase TCE conversion. The quantitative operating window (combination of process conditions) required to yield 95% TCE destruction can be predicted through an empirical response surface model. In general, UV intensities greater than 3.5 mW/ cm2 and residence times greater than 2.5 s will yield greater than 95% TCE destruction. • Water vapor significantly inhibits the . VOC destruction rate for chlorinated ------- ethylenes such as TCE. For this reason, PCO units should be placed downstream of a dehumidification unit located between air strippers and the PCO unit. For high VOC conversion operation in the PCO unit, these dehurnldiflers should reduce the RH to less than 50%. Principal carbon-containing products of POO identified experimentally at the bench scale include CO2, CO, and phosgene (COCL). Failure to close carbon mass balances under some conditions suggests that not all byproducts were identified. The presence of secondary chlorinated ethylenes and chloro- benzene did not significantly affect the conversion of the primary VOC TCE. However, selectivity to the desired complete combustion product carbon dioxide decreased with increasing co-contaminant concen- tration, suggesting that the presence of the co-contaminants enhanced the production of undesirable partial oxidation products. TCE destruction activity could be maintained for up to thirty days on stream. Results suggested that chlorine atoms build up on a fresh catalyst surface during an initial period of operation until the surface is saturated. During this time, only small amounts of chlorine-containing products are detected in the reactor effluent. After this initial period of operation, Cl-containing compounds, including HCI, CI2 and COCI2 are evolved from the surface. Recommendations Based on the research described in this report, the following recommendations are made for further investigation: • The hypothesis that the lack of carbon mass balance closure under some conditions was due to undetected/ unidentified volatile partial oxidation intermediates should be tested. • The preferred PCO reactor Configuration 2 should be tested at an acceptable field site so that the objectives related to practical Rajnlsh Changrani and Gregory B. Raupp are with Arizona State University, Tempo, AZ85287-6006. Craig Turchiis with NEPCCO Environmental Systems, Oca/a, FL 34470. Norms M. Lewis Is the EPA Project Officer (see below). The complete report, entitled "Integration of Photocatalytic Oxidation with Air Stripping of Contaminated Aquifers," (Order No. PB99-127920; Cost: $25.50, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-605-6000 Tha EPA Project Officer can be contacted at: National Risk Management Research Laboratory U,S. Environmental Protection Agency Cincinnati, OH 45268 integration of PCO with air stripping, and perhaps vacuum soil vapor extraction can be achieved. • First principles modeling of the preferred PCO reactor configuration should be undertaken to allow reactor optimization to be performed in a rapid, efficient and scientifically sound manner. The modeling effort should include submodels for the UV radiation field and intrinsic VOC destruction rate expressions, as well as the reactor- scale advection-diffusion-reaction model. • Testing efforts should be focused on sites that are contaminated with chlorinated ethyienes. Avoid sites contaminated with such recalcitrant compounds as aromatics 'and chlorinated ethanes. • Equipment development efforts should continue to focus on advanced reactor designs. Based on previous experience, it is likely that a large margin for process performance improvement exists through this avenue. The full report was submitted in fulfill- ment of contract number CR 821100-01-0 by Arizona State University under the sponsorship of the United States Environ- mental Protection Agency. The report cov- ers a period from October 1, 1993, to June 30, 1997, and work was completed as of June 30, 1997. ! ------- ------- ------- ------- United States Environmental Protection Agency Center for Environmental Research Information Cincinnati, OH 45268 Please make all necessary changes on the below label, detach or copy, and return to the address in the upper left-hand corner. If you do not wish to receive these reports CHECK HERE D; detach, or copy this cover, and return to the address in the upper left-hand corner. PRESORTED STANDARD POSTAGE & FEES PAID EPA PERMIT No. G-35 Official Business Penalty for Private Use $300 EPA/540/SR-98/504 ------- |