Patent Publication Number: US-6709567-B1

Title: REDOX water treatment system

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/392,198, filed Sep. 8, 1999, entitled “Redox Water Treatment System,” now U.S. Pat. No. 6,342,150. This application also claims the benefit of U.S. Provisional Application Ser. No. 60/099,607, filed Sep. 9, 1998, entitled “Electrically Induced Redox Barriers for Water Treatment.” 
    
    
     TECHNICAL FIELD 
     The present invention is directed toward the treatment of contaminated water, and more particularly toward an electrolytic system promoting oxidation and reduction of ground water contaminants. 
     BACKGROUND ART 
     After decades of active research, cost effective mitigation of ground water contamination remains a major technical challenge. In general, most in situ treatment processes have proven both expensive and ineffective. Many of the more promising advances in the last few years have centered on natural or engineered in situ oxidation-reduction (REDOX) reactions. Through these reactions, toxic compounds are transformed in situ to non-toxic products. Examples include biological oxidation of benzene to carbon dioxide and water and abiotic reductive de-halogenation of trichloroethene to chloride and ethylene. Other examples of similar REDOX reactions are described by Weidemeier, Technical Protocol for Evaluating Natural Attenuation of Solvents in Ground Water, Air Force Center for Environmental Excellence, Brooks Air Force Base, San Antonio, Tex., November 1996. While the potential advantages of mitigation of ground water contamination through REDOX reactions has been recognized, effective engineering systems for driving such reactions have proven inadequate. Problems with existing technologies include excessive energy consumption and cost. 
     Several existing technologies use reduction reactions to degrade contaminants in water. Gillham, U.S. Pat. No. 5,266,213, describes the reductive de-chlorination of chlorinated contaminants in ground water using particulate metal. Gillham, U.S. Pat. No. 5,868,941, describes the use of an electrolytic system for the treatment of halogenated hydrocarbons that passes a plume of contaminated water through a bed of granular iron. An electric circuit is promoted for providing electrons for reducing the contaminant in the vicinity of the granular iron bed. By inducing a voltage in the current in the vicinity of the iron bed, Gillham contends his invention increases the rate at which halogenated hydrocarbons are broken down by reduction and deposition of iron and other precipitants is inhibited. However, known prior art systems fail to provide a suitable anode for promoting oxidation reactions which can be useful both in degrading intermediaries produced in the reductive de-halogenation of certain halogenated hydrocarbons and in degrading other contaminants by oxidation. 
     The present invention is directed to overcoming one or more of the problems discussed above. 
     SUMMARY OF THE INVENTION 
     An apparatus for treating a flow of water containing contaminants includes first and second permeable electrodes. In one embodiment, as illustrated in FIG. 1, a power supply is coupled to each of the first and second permeable electrodes to create an electrical potential therebetween. The first and second permeable electrodes are disposed within the flow of water containing contaminants with the first permeable electrode upstream from the second permeable electrode and the water containing contaminants flowing through and between the permeable electrodes. The permeable electrodes are spaced a select distance to promote an electric current in the water containing contaminants between the electrodes sufficient to sustain oxidation or reduction of the contaminants either at and/or near the electrodes. Laboratory data indicates treatment of target contaminants at a distance of ½ m up and downstream of the electrodes. Treatment of contaminants at larger distance is anticipated in field application. 
     The electrodes are preferably substantially planar plates disposed in parallel and substantially normal to the direction of water flow. The distance between the plates is between about 0.001 and 1 meter. A non-conductive spacer can be placed between the electrodes to maintain the select distance. In another embodiment, illustrated in FIGS. 8 and 9, the electrodes may also be closely spaced rods that approximate planar plates. The distance between the rods is approximately between 1 and 5 feet. Preferably the rods are disposed in rows which are laterally offset so that rods of adjacent rows lie laterally between each other forming a tortuous path for water traveling perpendicular to the rows. The electrodes and rods are made of a conductive material selected from the group including, but not limited to carbon black, vitreous carbon, graphite, stainless steel, platinized titanium, mixed metal oxides, aluminum, copper, gold and gold plated stainless steel. The electrodes are preferably in the form selected from the group including perforated plates, sintered metal, expanded metal, screens, wool (e.g., copper wool), felt and weave. The rods are preferably titanium or a titanium mixed metal oxide. When necessary or desired, the invention further contemplates more than one pair of first and second permeable electrodes disposed in series. Each of the first and second permeable electrodes is coupled to the power supply to create an electrical potential therebetween. The power supply may be a DC power supply having a positive terminal coupled to one of the first and second permeable electrodes and a negative terminal coupled to the other of the first and second permeable electrodes. Where multiple pairs of electrodes are provided in series, different voltages can be applied to the electrode pairs as may be necessary or desired to promote a given oxidation or reduction reaction. Additionally, the power supply may be applied continuously or may be pulsed on and off at a frequency sufficient to sustain the desired treatment, typically in the range of seconds to minutes. 
     Another aspect of the present invention is a method of treating water containing contaminants. In one embodiment, the method includes providing a pair of first and second permeable electrodes and flowing the water containing contaminants through and between the electrodes. A voltage is applied between each of the permeable electrodes of the pair sufficient to promote oxidation of the contaminants either at and/or near one electrode and reduction of the contaminants either at and/or near the other electrode. The contaminants treated must be subject to degradation through oxidation or reduction, such as halogenated hydrocarbons or the contaminants must be subject to a change in mobility due to a change in oxidation state of either the contaminant itself, such as a metal, or the change in oxidation state of some other compound that causes the contaminant to coprecipitate with that compound. The electrodes used in the method are preferably substantially planar plates disposed in parallel substantially normal to the direction of flow of water containing contaminants. The electrodes may also be offset rows of closely spaced rods that approximate planar plates. The method may further include periodically alternating the polarity of the electrodes to minimize formation of precipitants in the vicinity of the electrodes and to remove solid phase precipitates that may have formed on the electrode surface. More than one pair of first and second permeable electrodes may be provided. If so, a voltage applied to at least one pair of the permeable electrodes may be different from the voltage applied to another pair of permeable electrodes. The power supply may be applied continuously or may be pulsed on and off at a frequency sufficient to sustain the desired treatment. 
     The invention uses electrolytic technology to effect either the oxidation or reduction of ground water contaminant(s) to non-hazardous product(s). Treatable contaminants include, but are not limited to those subject to REDOX degradation, including, but not limited to compounds such as halogenated hydrocarbons, fuel hydrocarbons, nitrates, ammonium perchlorate, trinitrotoluene (TNT), Royal Demo Explosives (RDX) or methyl tertiary-butyl ether (MTBE). Illustrative is the reductive de-halogenation of perchloroethene (PCE). At the cathode, PCE is reduced to methane gas through the following reactions:                    
     At the anode the products will be oxidized through the following steps:                    
     The net reaction for the entire system is then:                    
     Thus, through sequential reduction and oxidation, PCE is degraded to non-hazardous products. Treatable contaminants also include metals including, but not limited to arsenic and metals associated with mining sites. A change in the oxidation state of the metals results in a change in mobility of the metal by causing the metal to either precipitate out of solution or be sorbed to some redox sensitive precipitate. 
     There is also the potential that the electrodes will generate electron donors or acceptors, such as hydrogen and oxygen, which are transported downstream of the electrodes and will degrade targeted contaminants downstream of the electrodes (an emitter). 
     Finally, there is also evidence that not only water quality, but also sediment mineralogy up and downstream of the electrodes is modified by the method described herein. The modification of the sediment around the electrodes also has the potential to effect treatment, even after the applied current has been turned off. For example, iron sulfide (FeS) may precipitate out of solution around the electrodes. The precipitated FeS may then react with other targeted contaminants present. 
     By varying the placement of the cathode and anode, the use of multiple cathodes and anodes, applying various voltages and varying electrode material the present invention can be used to degrade any REDOX sensitive constituent present in ground water through either sequential oxidation and reduction, sequential reduction and oxidation or multiple combinations of oxidation and reduction. The permeable electrodes or rods maximize surface area to fully promote the oxidative/reductive capacity of the system. The apparatus is modifiable and controllable through manipulation of applied voltage potential and electrode spacing to meet specific field conditions (e.g., flow rates and water quality objectives). Voltage applied across the electrode can be periodically reversed to avoid adverse precipitation of solid phase constituents, a common constraint of existing in situ treatment systems and to remove solid phase precipitates that may have formed. Selection of electrode/rod material can be specific to the contaminant to be treated as well as economic and logistical concerns. Representative electrode/rod materials can include carbon black, vitreous carbon, graphite (as a pure or fractional component) aluminum, copper, stainless steel, platinized titanium, gold, gold plated stainless steel, mixed metal oxides or other conductive or semiconductive materials. The electrodes are preferably in the form selected from the group including perforated plates, sintered metal, expanded metal, screens, wool (e.g., copper wool), felt and weave. The rods are preferably made of titanium or a titanium mixed metal oxide. The chemical thermodynamic conditions of the intraelectrode treatment zone can also be controlled through variation in voltage potential between electrode plates to optimize treatment of specific contaminants. 
     The system relies upon the natural flow of the ground water to move contaminants through the system and to encourage electron transfer. Most prior art systems encourage contaminant mitigation by electro-osmosis or electro kinetics and therefore require large electrode spacings and significant voltage drops to generate electromotive force that draw water, contaminants and/or flushing solutions through a targeted zone. However, the costs associated with large power requirements necessary to drive these systems have limited their application to narrow niches. As a result of the low energy consumption of the present invention, it presents a highly effective, simple and low cost treatment alternative. The voltages need only be sufficient to overcome reaction activation energies and provide the thermodynamic conditions necessary to make the desired oxidation or reduction favorable. Amperages need only be sufficient to address the stoichiometry of the oxidation and reduction reactions occurring at the electrodes. With the low energy requirements of this technology, power could be supplied by any number of low voltage sources, including passive solar panels. The simplicity of the apparatus, its low construction cost, its low operating costs and its versatility all support its widespread application. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of an apparatus for promoting oxidation and reduction of water contaminant in accordance with the present invention. 
     FIG. 2 is a schematic representation the electrodes used in the apparatus of FIG.  1 . 
     FIG. 3 is a schematic representation of a vertical cross-section of a permeable electrode which illustrates the current distribution bars connected to the electrode. 
     FIGS. 4A +  4 B are detailed schematic representations of embodiments of composite electrode panes of this invention. 
     FIG. 5 is a graph of ideal or theoretical amperage required to promote reduction of PCE. 
     FIG. 6 is a schematic representation of an in situ application of the apparatus for promoting oxidation and reduction of water contaminants. 
     FIG. 7 illustrates the electrically induced oxidation and reduction zones in the vicinity of the electrodes of FIG.  3 . 
     FIG. 8 is a schematic representation of rod electrodes driven or drilled into the ground in lines approximating the continuous electrode panels of FIGS. 1 and 2. 
     FIG. 9A is a schematic representation of an individual well in which a rod electrode has been inserted. 
     FIG. 9B is a schematic representation of rod electrodes in lines of wells approximating the continuous electrode panels of FIGS. 1 and 2. 
     FIG. 10 is a schematic representation of a vertical cross-section of a single electrode panel in a frame. 
     FIG. 11 is a schematic representation of a vertical cross-section of a sequence of interconnected electrode panels in frames. 
     FIG. 12 is a schematic representation of a horizontal cross-section of a frame and interlocks between electrode panels. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One embodiment of an apparatus for promoting oxidation and reduction of water REDOX susceptible contaminants  10  is illustrated schematically in FIG.  1 . This apparatus would be suitable for laboratory or industrial application of the present invention. The apparatus  10  consists of a containment vessel  12  having an inlet  14  in fluid communication with a source of water containing contaminants  16  and an outlet  18 . Within the containment vessel  12  a number of planar porous electrodes  20 ,  22 ,  24  and  26  are disposed in parallel normal to the direction of flow of water containing contaminants through the containment vessel  12 . Each of the planar porous electrodes is electrically coupled by connectors  28 ,  30 ,  32  and  34  to a power supply  36 . While the power supply  36  may be an alternating current supply, the power supply  36  is preferably a DC power supply having its negative terminal  38  coupled to electrodes  20  and  26  through connectors  28 ,  34  and its positive terminal  40  coupled to the electrodes  22 ,  24  via connectors  30 ,  32 . 
     The planar porous electrodes are preferably part of an assembly  42  shown schematically in FIG.  2 . The direction of water containing contaminant flow is illustrated by the arrow  44 . Each of the planar porous electrodes  20 ,  22 ,  24  and  26  are separated by non-conducting spacers  46 . The non-conducting spacers  46 , which could be made of, for example, a thermal plastic or glass, maintain the planar porous electrodes spaced a select distance. In the assembly  42 , electrodes consist of two electrode pairs each consisting of a negative electrode and a positive electrode. The first electrode pair consists of the negative electrode  20  and the positive electrode  22  and the second electrode pair consists of the positive electrode  24  and the negative electrode  26 . The planar porous electrodes can be varied to be arranged negative-positive-positive-negative as shown in FIG. 2, negative-positive-negative-positive or in any other combination. 
     In another embodiment of the invention, illustrated in FIG. 4, the spacers  46  are comprised of Geonet and the electrodes  20 ,  22  and  24  and Geonet spacers are separated by Geotextile filters  47 . Geonet is a high density polyethylene spacer or net having a high porosity and is available from GSE Lining Technology, Inc. of Houston, Tex. Geotextile filters are cloth filters that prevent any solids that form on the electrodes and are subsequently released from the electrodes by reversing polarity, from passing through the filters and thus cause them to precipitate out of solution. Geotextile filters also prevent hydrogen and oxygen gases that form on the negative and positive electrodes, respectively from mixing together. Geotextile filters are available from GSE Lining Technology, Inc. of Houston, Texas. A representative combination of the Geonet spacer and Geotextile filter can be purchased from GSE Lining Technology, Inc. of Houston, Tex. as product number GSE TP-275-66. This product is a triplanar Geonet core with a 6 oz/yd 2  non-woven Geotextile heat bonded to one or both sides of the Geonet core. 
     The electrodes may be made of any number of conductive or semiconductive materials. Representative materials include, but are not limited to vitreous carbon, carbon black, graphite felt, stainless steel, copper, gold plated stainless steel, gold, copper wool, platinized titanium, mixed metal oxides, graphite or aluminum. The selected electrode material is preferably stable in the applied environment. The electrodes also are preferably configured to have a large surface area so as to promote oxidation and reduction. Thus, the electrodes can be in the form of a wool (e.g., copper wool), a screen, a fabric, perforated plates, sintered metal, expanded metal, felts, weaves or the like. 
     In use, water containing contaminants is flowed into the inlet  14  and through and between the planar porous electrodes  20 ,  22 ,  24  and  26  and out the outlet  18  as a treated effluent. The negative electrodes  20  and  26  act as electron donors to promote reduction of contaminants either at and/or near the electrodes. Positive electrodes  22  and  24  act as electron receptors to promote oxidation either at and/or near the electrodes. Laboratory data indicates treatment of target contaminants at a distance of ½ m up and downstream of both the negative and positive electrodes. In one embodiment, sensors (e.g. reference electrodes  200 ) are installed immediately up and downstream of the electrodes in order to optimize the degradation of target contaminants (See FIG.  6 ). The reference electrodes measure electrical potential in solution relative to a known standard. The electrodes can be set to a predetermined voltage, relative to the reference electrode, which is needed to degrade targeted contaminants, e.g. 1.3 v. Based on the measurement obtained from the reference electrode voltage and/or amperage can be automatically adjusted to sustain targeted conditions. This not only optimizes the degradation of targeted contaminants, but also makes the system self-regulating. 
     The invention contemplates the use of a single pair of electrodes  20 ,  22  or multiple pairs of electrodes as illustrated in FIG.  2 . The exact configuration is a matter of design choice and based upon a number of factors including the contaminant to be treated, the concentration of the contaminant, the flow rate of the water containing contaminants, the temperature of the water and many other factors known in the art. Another important factor is the spacing of the planar porous electrodes. The spacing may vary between about 0.001-1 meters, depending upon the various factors set forth above. However, at least in laboratory applications, closer spacing has proven advantageous. Closer spacing decreases the voltage required to generate the necessary current and thereby increases the efficiency of the system. The potential between plate pairs can be the same or varied, depending upon treatment requirements. Thus, if desired, a separate power supply can be associated with each pair of planar porous electrodes. In one embodiment, electrical current distribution bars/straps  1  are used to uniformly distribute a constant voltage to the electrodes  20 ,  22 ,  24 , as illustrated in FIGS. 3 and 4A. In a preferred embodiment, the electrical distribution bars/straps are comprised of a piece of titanium which has been welded onto the length of the electrode. The titanium is then coupled to the power supply and operates to uniformly deliver the current to the electrode. This is particularly advantageous in the case of very large electrodes. 
     The system requires a relatively low potential of between 10 and 40 volts between electrodes of an electrode pair, depending upon a variety of factors such as the particular contaminant, flow rate, temperature, electrode material, electrode spacing and the like. In addition, extremely low currents are required to facilitate desired oxidation and reduction reactions. For example, based on the PCE mass flux through the apparatus and Faraday&#39;s Law, the ideal amperage or faradaic current for the treatment of PCE can be calculated as follows:          I   PCE     =           V   w                   φ                   AC   PCE         MW   PCE                     z                     96487                 C     mole                       
     where: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 I = 
                 amperage 
               
               
                   
                 V W  = 
                 seepage velocity 
               
               
                   
                 f = 
                 porosity 
               
               
                   
                 A = 
                 cross sectional area of electrodes 
               
               
                   
                 C PCE  = 
                 concentration of PCE in aqueous phase 
               
               
                   
                 MW PCE  = 
                 molecular weight of PCE 
               
               
                   
                 z = 
                 electron equivalents 
               
               
                   
                 C = 
                 coulombs 
               
               
                   
                   
               
            
           
         
       
     
     This equation describes the governing principles for a flow through electrolytic cell. FIG. 5 is an application of this equation that describes ideal amperage and cost associated with the apparatus for promoting oxidation and reduction of water contaminants. FIG. 5 illustrates that amperages can be adjusted for varying concentrations and that the cost of electricity for the system, assuming the cost of 10¢/kilowatt hour, one foot per day seepage velocity, and porosity of 0.3, is very low. 
     An in situ application of the apparatus for promoting oxidation and reduction is illustrated in FIG.  6 . In this embodiment a trench  50  is excavated normal to the path of ground water flow. An assembly of planar porous electrodes  52  is inserted in the trench, again, normal to the direction of ground water flow. As discussed above, the assembly may consist of a single pair of electrodes, or multiple pairs of electrodes, depending upon the treatment requirements. For ease of illustration, the assembly illustrated in FIG. 6 consists of a negative electrode  54  and a positive electrode  56 . Although not shown in FIG. 6, a suitable non-conductive spacer, as discussed above, is preferably disposed between the electrodes  54  and  56  to maintain them a select distance apart. Once installed in the trench, the trench is back filled with a suitable porous granular material  58 , such as gravel. A variable voltage DC power supply  60  is coupled to the electrodes  54 ,  56 . Because of the lower power requirements of the system, the power supply  60  could come from any number of passive sources including solar or wind electrical generators. As the ground water flows through and between the porous planar electrodes  54 ,  56 , reduction zone  61  is promoted either at and/or near the negative electrode  54  and an oxidation zone  62  is generated either at and/or near the positive electrode  56  as illustrated in FIG.  7 . As noted above, laboratory data indicates treatment of target contaminants at a distance of ½ m up and downstream of both the negative and positive electrodes. 
     It may be more practical in some instances to install rod electrodes that approximate planar plates. This embodiment of the invention is illustrated in FIGS. 8 and 9. With reference to FIG. 8, metal rods (e.g.  70 ,  72 ,  74 ) are driven into the ground in such a manner as to form when taken together substantially planar plates. Each of the rods is electrically coupled by connectors  92 ,  94  to a power supply  36 . As discussed above, while the power supply  36  may be an alternating current supply, it is preferably a DC power supply having its negative terminal  38  coupled to electrodes  70 ,  72 ,  74 , etc. through connector  94  and its positive terminal  40  coupled to the electrodes  76 ,  78 ,  80 , etc via connector  92 . Additionally, as discussed above, the DC current may be applied continuously or in some instances the current may pulsed on and off at a frequency sufficient to sustain the desired treatment, typically seconds to minutes. The direction of water containing contaminant flow is illustrated by the arrow  44 . The rods may be made of any number of conductive or semi-conductive materials. In a preferred embodiment, the rods are comprised of a metal, preferably titanium or a titanium mixed metal oxide. The spacing of the rods may vary from approximately 1-5 feet, depending upon various factors set forth above. The depth of the rods will depend upon the depth of the groundwater to be treated. The rods of adjacent rows may be laterally offset to define a tortuous path for water passing therebetween. 
     FIG. 9B illustrates the use of rod electrodes in lines of wells approximating continuous electrode panels. Each individual rod electrode  80 ,  82 ,  84 ,  86 ,  88 ,  90 , etc. depicted in FIG. 9B represents an individual well in which a rod electrode has been placed as illustrated in FIG.  9 A. 
     In cases in which very large areas of water must be treated, it may be more practical to use a series of single panel electrodes interconnected in a frame. This embodiment of the invention is illustrated in FIGS.  4  and  10 - 12 . FIG. 10 illustrates schematically a vertical cross-section of a single electrode panel  100 , as depicted in FIG. 4A in a frame  102  (FIG.  10 ). In a preferred embodiment, electrical current distribution bars  1  (FIGS. 3 and 4) are attached to the electrodes to uniformly conduct current to the individual electrodes. The electrodes are sandwiched into a composite panel  104  consisting of a geotextile filter  47 , which limits both sediment invasion into the panel and the mixing of evolved gasses, a geonet spacer  46  (FIG.  4 ), which provides a nonconductive spacer between the electrode and allows sediment/scale to drop through the panel to a sediment trap/washout  106  at the bottom of the electrode panel and the individual permeable electrodes  20 ,  22 ,  24  (FIG.  4 ). The frame  102  for the composite panel is comprised of a washout/sediment trap  106  at the base, a header  108  for collection/venting of gases through a vent pipe (not shown) connected to saddles  109 , vertical risers  110  conducting distribution bars  1  to the surface, and interlocks  112  (FIGS. 11 and 12) for connecting multiple electrode frames into a lateral extensive system  114  (FIG.  11 ). The frame can be constructed from any non-conducting material. In a preferred embodiment, the frame is made from a plastic material, preferably polyvinyl chloride (pvc). A flush pipe  210  is preferably in fluid communication with one side of the washout/sediment trap  106  and an outlet pipe  212  is in fluid communication with the other side of the washout/sediment trap  106  to enable flushing and removal of sediment from the washout/sediment trap  106 . Rods  214  which may be made of fiberglass, run perpendicular to the panel frame  102  to support them above the washout/sediment trap  106 . FIG. 12 illustrates in more detail two different embodiments of the interlocks  112  between the electrode panel frames  102 . The interlocks  112  are designed prevent gaps in the system. 
     EXAMPLE 1 
     Electrically induced degradation of PCE to non-hazardous byproducts through reduction and oxidation has been accomplished under laboratory conditions. A removal rate of approximately 85% was achieved under the following conditions using a single pair of electrodes: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Negative electrode: 
                 2 cm thick copper wool 
               
               
                   
                 Positive electrode: 
                 stainless steel screen with gold plate 
               
               
                   
                 Electrode spacing: 
                 approximately 5 cm 
               
               
                   
                 Applied voltage: 
                 40 volts 
               
               
                   
                 Flow rate: 
                 approximately 1 foot per day 
               
               
                   
                 Influent PCE concentration: 
                 110 mg/liter; 
               
               
                   
                 Resultant amperage: 
                 approximately 30 mA 
               
               
                   
                   
               
            
           
         
       
     
     EXAMPLE 2 
     Experiment using multiple, sequence electrodes have yeilded higher PCE removal rates. A PCE removal rate of 95% was obtained under the following operating conditions using two pairs of electrodes. 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 Upstream electrodes: 
                 identical perforated high-density poly- 
               
               
                   
                 ethylene impregnated with carbon black 
               
               
                   
                 (approx. 1 mm thick) 
               
               
                 Down stream 
               
               
                 electrode pair: 
               
               
                 Negative electrode: 
                 perforated high-density polyethylene 
               
               
                   
                 impregnated with carbon black (approx. 
               
               
                   
                 1 mm thick) 
               
               
                 Positive electrode: 
                 stainless steel screen with gold plate 
               
               
                   
                 configuration of electrodes 
               
               
                 Electrode configuration: 
                 the upstream electrode pair was configured 
               
               
                   
                 with the positive electrode leading the 
               
               
                   
                 negative electrode and the down stream 
               
               
                   
                 electrode pair was configured with the 
               
               
                   
                 negative electrode leading the positive 
               
               
                   
                 electrode 
               
               
                 Electrode spacing: 
                 approximately 2 cm for all electrodes 
               
               
                 Applied voltage: 
                 40 volts for each electrode pair 
               
               
                 Flow rate: 
                 approximately 1 foot per day 
               
               
                 Influent PCE concentration: 
                 110 mg/liter 
               
               
                 Resultant amperage: 
                 Upstream pair: 40 mA Downstream pair: 
               
               
                   
                 10 mA