Patent Publication Number: US-2010108588-A1

Title: Method of and apparatus for converting biological materials into energy resources

Description:
This application is a continuation of U.S. application Ser. No. 12/409,457, filed on Mar. 23, 2009, now U.S. Pat. No. 7,645,382, which is a continuation of U.S. application Ser. No. 11/198,703, filed on Aug. 5, 2005, now U.S. Pat. No. 7,507,341, which (i) claims the benefit of U.S. Provisional Patent Application No. 60/599,355, filed on Aug. 6, 2004, which application is incorporated herein by reference in its entirety, and (ii) is also a continuation-in-part of U.S. application Ser. No. 10/795,944, filed Mar. 8, 2004, now U.S. Pat. No. 7,001,520, which is continuation of U.S. application Ser. No. 10/270,420, filed Oct. 15, 2002, now U.S. Pat. No. 6,709,594, which is continuation-in-part of U.S. application Ser. No. 10/107,614, filed Mar. 26, 2002, now U.S. Pat. No. 6,540,919, which is continuation of U.S. application Ser. No. 09/612,776, filed Jul. 10, 2000, now U.S. Pat. No. 6,395,176, which is continuation-in-part of U.S. application Ser. No. 09/468,427, filed on Dec. 21, 1999, which is continuation of U.S. application Ser. No. 09/229,279, filed Jan. 13, 1999, now U.S. Pat. No. 6,030,538, which is continuation-in-part of U.S. application Ser. No. 08/934,548, filed Sep. 22, 1997, now U.S. Pat. No. 5,893,979, which is continuation-in-part of U.S. application Ser. No. 08/552,226, filed Nov. 2, 1995, now U.S. Pat. No. 5,695,650, which applications are hereby incorporated by reference in their entirety in the present application. 
    
    
     BACKGROUND 
     This patent is directed to a method and apparatus for converting biological materials into energy resources, and, in particular, to a method and apparatus using pulsed electric fields to release intracellular materials from biological materials in a method and apparatus for converting the biological materials into energy resources. 
     Significant energy potential exists in biological materials, including biological wastes such as municipal and industrial wastes. It has been estimated that the animal waste produced on an annual basis in the United States has an energy value equivalent to 21 billion gallons of gasoline. Elsewhere, researchers have stated that the organic content of human wastewaters produced in the United States has an annual energy value equivalent to 0.11 quadrillion BTUs, with an estimated annual monetary value of $2 billion. See Logan, Extracting Hydrogen and Electricity from Renewable Resources, Envtl. Sci. and Tech., vol. 41, pp. 161-167 (2004), hereby incorporated by reference in its entirety. Researchers have also stated that animal wastewaters produced in the United States have an annual energy potential equivalent to 0.3 quadrillion BTUs. See Logan, above. By comparison, the total annual electricity generation of the United States is only 13 quadrillion BTUs. It is further believed that significant energy potential exists in industrial wastes and wastewaters, including those produced by pulp and paper processing and by food processing. 
     Various technologies, including methanogenesis, biohydrogen production using fermentative processes, and direct electricity production using biofuel cells or microbial fuel cells, have been demonstrated to be capable of producing energy resources from wastes and wastewaters. However, the efficiencies of the energy generation using these technologies, both in terms of rate and net units generated, remain problematic. For example, while researchers have estimated that hydrogen production from wastewater has the greatest potential for economical production of biohydrogen from renewable resources, fermentative technologies used to produce hydrogen from wastewater have been found to capture only 15% of the available organic energy. See Logan, above. This represents less than half of the estimated conversion efficiency of 33%. 
     SUMMARY OF THE INVENTION 
     In one aspect, a method of converting biological material into energy resources includes transmitting biological material to a pulsed electric field station, and applying a pulsed electric field to the biological material within a treatment zone in the pulse electric field station to generate treated biological material. The method also includes transmitting the treated biological material to a biogenerator, and processing the treated biological material in the biogenerator to produce an energy resource. 
     In another aspect, a converter that converts biological material into energy resources includes a pulsed electric field station, the pulsed electric field station comprising an inlet adapted to receive biological material, a treatment chamber through which biological material received via the inlet passes and including at least two spaced electrodes between which is generated a pulsed electric field and which define at least one treatment zone therebetween, and an outlet adapted to pass treated biological material. The converter also includes a biogenerator, the biogenerator comprising an inlet coupled to the outlet of the pulsed electric field station, at least one chamber in which the treated biological material is processed into an energy resource, a first outlet adapted to pass the energy resource and a second outlet adapted to pass processed treated biological material. 
     In a further aspect, a wastewater treatment system including a primary treatment station that receives a wastewater stream and separates the wastewater stream into primary sludge and a first liquid fraction, a secondary treatment station coupled to the primary treatment station, the secondary treatment station receiving the first liquid fraction and digesting the solids in the liquid fraction to produce activated sludge and a second liquid fraction, and a bioreactor that receives the primary sludge and at least part of the activated sludge and digests the primary sludge and activated sludge to produce a digested product. The system also includes a converter which receives at least part of at least one of the wastewater stream, the primary sludge, the activated sludge, the first liquid fraction, the second liquid fraction and the digested product. The converter includes a pulsed electric field station, the pulsed electric field station comprising an inlet adapted to receive biological material, a treatment chamber through which biological material received via the inlet passes and including at least two spaced electrodes between which is generated a pulsed electric field and which define at least one treatment zone therebetween, and an outlet adapted to pass treated biological material. The converter also includes a biogenerator, the biogenerator comprising an inlet coupled to the outlet of the pulsed electric field station, at least one chamber in which the treated biological material is processed into an energy resource, a first outlet adapted to pass the energy resource and a second outlet adapted to pass processed treated biological material. 
     Additional aspects of the disclosure are defined by the claims of this patent. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a converter according to the present disclosure; 
         FIG. 2  is a schematic view of an embodiment of a pulsed electric field (PEF) station for use in the converter of  FIG. 1 ; 
         FIG. 3A  is a cross-sectional view of an embodiment of a treatment chamber for use in the PEF station of  FIG. 2 ; 
         FIG. 3B  is an end view of the treatment chamber of  FIG. 3A ; 
         FIG. 4  is a cross-sectional view of another embodiment of a treatment chamber for use in the PEF station of  FIG. 2 ; 
         FIG. 5  is a schematic view of a first embodiment of the converter of  FIG. 1 , the generator using methanogenesis; 
         FIG. 6  is a schematic view of a second embodiment of the converter of  FIG. 1 , the generator using fermentive processes to produce hydrogen and methane; 
         FIG. 7  is a schematic view of a third embodiment of the converter of  FIG. 1 , the generator being a two-chamber microbial fuel cell; 
         FIG. 8  is a schematic view of a fourth embodiment of the converter of  FIG. 1 , the generator being a single chamber microbial fuel cell; 
         FIG. 9  is a block diagram of a method of converting biological materials into energy resources according to the present disclosure; 
         FIG. 10  is a schematic view of a wastewater treatment system in which the converter according to  FIG. 1  may be used, exemplar positions for integration of the converter into the system being marked; 
         FIG. 11  is a schematic view of a prokaryotic cell; and 
         FIG. 12  is a schematic view of an eukaryotic cell. 
     
    
    
     DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS 
     Although the following text sets forth a detailed description of numerous different embodiments of the invention, it should be understood that the legal scope of the invention is defined by the words of the claims set forth at the end of this patent. The detailed description is to be construed as exemplary only and does not describe every possible embodiment of the invention since describing every possible embodiment would be impractical, if not impossible. Numerous alternative embodiments could be implemented, using either current technology or technology developed after the filing date of this patent, which would still fall within the scope of the claims defining the invention. 
     It should also be understood that, unless a term is expressly defined in this patent using the sentence “As used herein, the term ‘_’ is hereby defined to mean . . . ” or a similar sentence, there is no intent to limit the meaning of that term, either expressly or by implication, beyond its plain or ordinary meaning, and such term should not be interpreted to be limited in scope based on any statement made in any section of this patent (other than the language of the claims). To the extent that any term recited in the claims at the end of this patent is referred to in this patent in a manner consistent with a single meaning, that is done for sake of clarity only so as to not confuse the reader, and it is not intended that such claim term be limited, by implication or otherwise, to that single meaning. Finally, unless a claim element is defined by reciting the word “means” and a function without the recital of any structure, it is not intended that the scope of any claim element be interpreted based on the application of 35 U.S.C. §112, sixth paragraph. 
       FIG. 1  illustrates an embodiment of a converter  50  according to the present disclosure. The converter  50  may include a pulsed electric field (PEF) station  52  and a biogenerator  54 . Biological materials may flow into the PEF station  52  via an inlet  56 , may be treated, and may then be released via an outlet  58 . The outlet  58  may be coupled to an inlet  60  of the biogenerator  54 , permitting the treated biological materials to flow from the PEF station  52  into the biogenerator  54 . After processing in the biogenerator  54 , the processed, treated biological materials may pass from the biogenerator via a first outlet  62 , while the energy resources generated by the biogenerator  54  may be extracted or harvested via a second outlet  64 . 
     A wide variety of biological materials may be introduced into the converter  50  for conversion into an energy resource. For example, the biological materials may include, for example, organic waste materials, such as municipal wastewater and wastes, industrial wastewater and wastes (such as pulp and paper wastewater and wastes, brewing wastewater and wastes, and food processing wastewater and wastes), and agricultural wastewater and wastes (including animal products and by-products, such as manure). Alternatively, the biological materials may include materials generated by or from waste treatment facilities, such as sludges (including primary and waste activated sludges), active microorganisms from bioreactors (including both the active microorganisms used to carry out digestion in the bioreactors and any excess microorganisms produced as a product of digestion) and effluents. As a further alternative, the biological materials need not be wastes, but may be materials that have value or alternative uses, but for which the decision has been made to convert the biological materials into energy resources instead of putting the biological materials to the alternative use. 
     An embodiment of the PEF station  52  is shown in greater detail in  FIG. 2 . The PEF station  52  may include a pulse generator  80  and a treatment, or PEF, chamber  82 . In particular, materials contained in or passing through the treatment chamber  82  may be subjected to non-arcing electric field pulses generated by the pulse generator  80 . 
     The electric field pulses may be generated by applying a voltage pulse to the electrodes, the pulse having a square-wave shape. However, the pulses may also have an exponentially decaying or oscillatory shape. Further, the pulses may be monopolar, bipolar, or even instant reverse charges. It is presently believed that the bipolar pulses may enhance the release of the cell contents, as is explained in greater detail below, and may improve energy utilization and electrode performance. 
     The electric field pulses may be of an individual duration of 2 to 15 microseconds with a peak field strength of 15 to 100 kV/cm. Preferably, the electric field pulses may be of an individual duration of 2 to 8 microseconds with a peak field strength of 20 to 60 kV/cm. The pulses may repeat at frequencies of between 2,000 and 10,000 pulses per second (or pps, and sometimes expressed in Hertz (Hz)). The resulting duration of treatment may be between 20 and 100 microseconds, which may be a function of the shape of the treatment zone (e.g., electrodes) and the characteristics of the electric field pulses. 
     Turning first to the pulse generator  80 , the generator  80  may be coupled to a power supply  84 , which the pulse generator  80  may use to generate a series of high voltage non-arcing electric field pulses across electrodes  85 ,  86  associated with the treatment chamber  82 . Depending on the power supply  84  used, a voltage transformer may be included, coupled between the power supply  84  and the pulse generator  80 . The pulse generator  80  may include a bank of capacitors  88  and switching circuitry  90  that may connect the bank of capacitors  88  across the electrodes  85 ,  86  to create the pulses within the treatment chamber  82 . The switching circuitry  90  may be controlled by a controller  92  that has as an input a signal from a signal generator  94 . By varying the characteristics of the signal from the signal generator  94 , the characteristics of the pulses in the treatment chamber  82  may be varied. 
     The treatment chamber  82  may be similar or identical to those discussed in any of U.S. Pat. Nos. 5,695,650, 5,893,979, 6,030,538, 6,395,176, 6,491,820, 6,540,919, 6,709,594, each of which are incorporated herein by reference in their entirety. 
     Alternatively, an embodiment of the treatment chamber  82  is shown in  FIGS. 3A and 3B . The treatment chamber  82  may include a housing  100 , which in the present embodiment may be cylindrical in shape, as can be seen in  FIG. 3B , although other geometries are possible. In the treatment chamber  82  may be disposed electrodes  85 ,  86 , one of the electrodes  85 ,  86  coupled to a higher voltage and the other the electrodes  85 ,  86  coupled to ground or a lower voltage. Insulators  102 ,  104 ,  106  may be disposed at either side of the electrodes  85 ,  86  and between the electrodes  85 ,  86 . The insulators  102 ,  104 , as well as the housing  100 , which may be made of an insulating material, isolate the electrodes  85 ,  86  from couplings which may be attached or secured to either end of the housing  100 . Similarly, the insulator  106  and the housing  100  space the electrodes  85 ,  86  to define a treatment zone  108  disposed therebetween. In operation, the biological materials to be treated are passed through the treatment zone  108  as they pass through the treatment chamber  82 . 
     As a further alternative, another embodiment of the treatment chamber  82 , designated  82 ′, is shown in  FIG. 4 . The treatment chamber  82 ′ may include a supporting material  120 , which may be made of a material having insulating properties. The supporting material  120  may also support three electrodes  85 ′,  86 ′, the electrode  85 ′ being coupled to a higher voltage and the electrodes  86 ′ being coupled to ground or a lower voltage. As shown, the electrodes  85 ′,  86 ′ may be cylindrical in shape, although other geometries are possible. According to this embodiment, two treatment zones  122 ,  124  are defined between the electrodes  85 ′,  86 ′. In operation, the biological materials to be treated are passed through the treatment zones  122 ,  124  as they pass through the treatment chamber  82 ′. 
     It will be recognized that access to the substrate in biological materials is a significant threshold that must be resolved if efficient generation of energy resources from biological materials using biological methods (f) r example, methanogenesis) is to be achieved. It is believed that treatment of the biological materials with PEF prior to processing in the biogenerator  54  may enhance the efficiency of the biogenerator, thereby removing a major obstacle to the commercialization of energy generation from biological materials, and in particular biological waste materials. To understand how PEF improves access to the substrate, a brief digression into cell structure and composition is appropriate. 
     A significant source of biological material, especially in wastes and wastewaters, is bacteria. Although not all bacteria have the same cell structure (compare the prokaryotic and eukaryotic microorganisms of  FIGS. 11 and 12 ), most bacteria share certain common structural elements. Generally, bacteria may include a colloidal fluid, referred to as cytoplasm. It is in the cytoplasm that the dissolved nutrients, enzymes, other proteins, nucleic acids and other intracellular materials used in the energy generating reactions discussed below may be found. The cytoplasm may include both organic and inorganic biosolids. Further discussion of the composition of the cell, and in particular, the cytoplasm, can be found in Rittmann et al., Environmental Biotechnology (2d ed. 2001), which is hereby incorporated by reference in its entirety. However, bacteria may also include a cell wall and a cell membrane, which lies just beneath the cell wall, that surround the cytoplasm and limit access to the cytoplasm. Consequently, to obtain access to the cytoplasm, one must first deal with the cell wall and membrane. 
     One way in which access to the cytoplasm may be achieved is by digesting the cell wall and the cell membrane. Unfortunately, digestion is a slow and typically incomplete process. For example, it may take up to 40 days to achieve even incomplete digestion in anaerobic processing of wastewater by methanogenesis. In a similar vein, researchers have shown that with a given microbial fuel cell, electricity generation is almost immediate when using an easily accessible material, such as a solution of glucose and water, while electricity generation requires approximately 80 hours when wastewater is used. See Liu et al., Electricity Generation Using an Air-Cathode Single Chamber Microbial Fuel Cell in the Presence and Absence of a Proton Exchange Membrane, Envtl. Sci. and Tech., vol. 38, pp. 4040-4046 (2004), hereby incorporated by reference in its entirety. 
     The use of PEF treatment prior to processing in the biogenerator  54  looks to overcome the cell wall/membrane obstacle. It is believed that when a voltage gradient of one volt or greater is impressed over a microorganisms cell structure, the structure experiences a change referred to as electroporation. More particularly, in electroporation, it is believed that the high voltage electric field pulses temporarily destabilize the lipid bilayer and proteins of the cell membrane. As a consequence of this destabilization, it is believed that the cell membrane experiences an increase in permeability. As additional material flows into the cell, because of the increased permeability, it is further believed that the cell swells and the cell wall and membrane eventually rupture. With the cell wall and membrane ruptured, the contents of the cell may be released, which may make the cell contents available as a substrate for energy resource generating reactions in the biogenerator  54 . 
     Testing of PEF treatment on biowaste has been conducted, and the following results have been observed. In regard to the pulsed electric field used, the pulses had a field strength of 17.3 to 20.5 kV/cm, a pulse width of 4 to 6 microseconds, and a frequency of 2000 to 2500 pps. The treatment chamber was similar to that shown in  FIGS. 3A and 3B , with the electrodes shaped such that the treatment zone provided a treatment duration of 20 to 100 microseconds. The testing was conducted over a total period of approximately 500 hours. Samples were collected on a daily basis, and analyzed to determine the release of soluble organic and inorganic material from cells relative to the starting materials. A summary of the test results showing the change in soluble material following PEF treatment is shown in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Release of Soluble Cellular Contents 
               
            
           
           
               
               
               
            
               
                   
                 Parameter Measured 
                 Average Percentage Increase 
               
               
                   
                   
               
               
                   
                 Total dissolved solids 
                 10.8% 
               
               
                   
                 Total organic carbon 
                 72.8% 
               
               
                   
                 Soluble chemical oxygen 
                   35% 
               
               
                   
                 Soluble ammonia nitrogen 
                 29.7% 
               
               
                   
                 Soluble orthophosphate 
                 15.4% 
               
               
                   
                 Soluble total phosphorus 
                   65% 
               
               
                   
                 Total kjeldahl nitrogen 
                 34.3% 
               
               
                   
                   
               
            
           
         
       
     
     It is submitted that the data in Table 1 indicates that the cell walls and membranes are sufficiently perforated as a consequence of the PEF treatment, leading to the observed increases in the release of the water-soluble cell contents of the treated biowastes. It is further believed the increased amount of soluble organic material is released and available for more efficient consumption and conversion into energy resources. 
     As stated above, the treated biological material may pass from the PEF station  52  to the biogenerator  54  via the PEF outlet  58  and the biogenerator inlet  60 . While the PEF outlet  58  is shown coupled directly to the biogenerator inlet  60  in  FIG. 1  such that all of the treated biological material flows into the biogenerator  52 , other alternatives are possible. For example, a fraction of the treated biological material may be diverted before entry into the biogenerator. This fraction of the treated biological material may then flow into the inlet  56  of the PEF station  50  for further treatment. Alternatively, the fraction may be diverted to a bioreactor for digestion before being reintroduced into the PEF station  52  or passed to the biogenerator  54 . 
     A variety of biogenerators  54  may be used with the PEF station  52 . Four embodiments of a biogenerator  54  are shown in  FIGS. 5-8 . However, it will be recognized that still further embodiments of biogenerator may be used with and may benefit from use with the PEF station  52 . The embodiments of the biogenerator  54  shown in  FIGS. 5-8  may generate energy resources in the form of fuels, such as methane and/or hydrogen, or electricity. Moreover, where the energy resource generated is a fuel, the fuel may be combusted in an engine, which in turn may be coupled to a conventional generator to convert the kinetic energy into electricity. The embodiments of the biogenerator  54  may include generators  54   a ,  54   b  that use methanogenesis ( FIG. 5 ) and two-stage methane and hydrogen generation ( FIG. 6 ) as well as one- and two-chamber microbial fuel cells  54   c ,  54   d  ( FIGS. 7 and 8 ). 
     As shown in  FIG. 5 , biological materials may enter the PEF station  52  via the inlet  56 , may be treated, and may pass to the biogenerator  54   a  via outlet  58  and inlet  60 . The biogenerator  54   a  according to this embodiment may be an anaerobic bioreactor that generates methane gas from treated biological materials by methanogenesis. It will be recognized that methanogenesis is an anaerobic process in which electron equivalents in the organic matter are used to reduce carbon to its most reduced oxidation state, CH 4 , or methane. See generally, Logan, Extracting Hydrogen and Electricity from Renewable Resources, Envtl. Sci. and Tech., vol. 41, pp. 161-167 (2004), hereby incorporated by reference in its entirety. As an initial step, bacteria may hydrolyze complex organic matter, such as carbohydrates, proteins and fats, into simple carbohydrates, amino acids, and fatty acids. Other bacteria may then use hydrogen as an electron donor and carbon dioxide as an electron acceptor to generate methane gas. One of the byproducts of the methanogenesis process may be water. 
     As shown, there may be three outlets  150 ,  152 ,  154  from the biogenerator  54   a . The first outlet  150  may be used to pass some of the solids materials from the biogenerator  54   a  to the PEF station  52  for further processing along with the biological materials entering the PEF station  52  via the inlet  56 . The processed, treated biological materials from the biogenerator  54   a  may be mixed with the biological materials from the inlet  56  before entry into the PEF station  52  or within the treatment chamber  82  of the PEF station  52 , or may be processed in parallel with the materials biological materials entering via the inlet  56 . The second outlet  152  may be used to pass a liquid fraction (primarily water) released from the processed, treated biological materials as a consequence of the PEF treatment and as a consequence of the methanogenesis process. The third outlet  154  may be used to pass the gaseous methane generated by the biogenerator  54   a.    
     As shown in  FIG. 6 , biological materials may enter the PEF station  52  via the inlet  56 , may be treated, and may pass to the biogenerator  54   b  via outlet  58  and inlet  60 . The biogenerator  54   b  according to this embodiment may include two-stage anaerobic bioreactor that generates methane gas and hydrogen gas from treated biological materials by biohydrogen. In the first stage, hydrogen may be recovered during hydrolysis and fermentation of the biological material. In the second stage, the remaining biological material would be processed using methanogenesis or a similar process. 
     As shown, there may be four outlets  160 ,  162 ,  164 ,  166  from the biogenerator  54   b . The first outlet  160  may be used to pass some of the solids materials from the biogenerator  54   b  to the PEF station  52  for further processing along with the biological materials entering the PEF station  52  via the inlet  56 . The processed, treated biological materials from the biogenerator  52   b  may be mixed with the biological materials from the inlet  56  before entry into the PEF station  52  or within the treatment chamber  82  of the PEF station  52 , or may be processed in parallel with the materials biological materials entering via the inlet  56 . The second outlet  164  may be used to pass a liquid fraction (primarily water) released from the processed, treated biological materials. The third outlet  164  may be used to pass the gaseous methane generated by the biogenerator  54   b , while the fourth outlet  166  may be used to pass the gaseous hydrogen generated by the biogenerator  54   b.    
     As shown in  FIG. 7 , biological materials may enter the PEF station  52  via the inlet  56 , may be treated, and may pass to the biogenerator  54   c  via outlet  58  and inlet  60 . The biogenerator  54   c  according to this embodiment includes two-chamber microbial fuel cell that generates electricity from biological materials, such as is described in U.S. Pat. No. 5,976,719, which is incorporated by reference in its entirety. In particular, the biogenerator  54   c  may include a first chamber  170  that is substantially oxygen-free and a second chamber  172  that is oxygen-rich. The two chambers  170 ,  172  may be separated by a proton exchange membrane  174 . Microorganisms capable of digesting biological materials, such as the treated biological materials from the PEF station  52 , may be disposed in the first chamber  170 . These microorganisms may attach to an anode  176  disposed in the first chamber  170 , may oxidize the treated biological material entering the first chamber  170 , and may transfer electrons to the anode  176 . The released electrons may travel from the anode  176  to a cathode  178 . The electricity generated by the movement of the electrons from anode  176  to cathode  178  may be used by a load  180  disposed between the anode  176  and the cathode  178 . Alternatives are described in Liu et al., Electricity Generation Using an Air-Cathode Single Chamber Microbial Fuel Cell in the Presence and Absence of a Proton Exchange Membrane, Envtl. Sci. and Tech., vol. 38, pp. 4040-4046 (2004), hereby incorporated by reference in its entirety. 
     As shown in  FIG. 8 , biological materials may enter the PEF station  52  via the inlet  56 , may be treated, and may pass to the biogenerator  54   d  via outlet  58  and inlet  60 . The biogenerator  54   d  according to this embodiment may includes a single-chamber microbial fuel cell that generates electricity from biological materials, such as is described in Liu et al., Electricity Generation Using an Air-Cathode Single Chamber Microbial Fuel Cell in the Presence and Absence of a Proton Exchange Membrane, Envtl. Sci. and Tech., vol. 38, pp. 4040-4046 (2004), hereby incorporated by reference in its entirety. In particular, the biogenerator  54   d  may include a chamber  190 . Microorganisms capable of digesting biological materials, such as the treated biological materials from the PEF station  50 , may be disposed on a plurality of anodes  192  arranged in a cylindrical geometry. An air-porous cathode  194  may be disposed centrally relative to the anodes  194 , and an air stream may be passed therethrough. As for alternatively embodiments of this biogenerator, see Liu et al., Electricity Generation Using an Air-Cathode Single Chamber Microbial Fuel Cell in the Presence and Absence of a Proton Exchange Membrane, Envtl. Sci. and Tech., vol. 38, pp. 4040-4046 (2004), hereby incorporated by reference in its entirety. 
     Having thus explained the structure of the converter  50 , the process of converting biological materials to energy resources is now discussed with reference to  FIG. 9 . Starting at block  250 , the biological materials may be received at the PEF station  52 . Pulsed electric fields may then be applied to the biological materials at block  252 , and the treated biological materials may be transferred to the biogenerator  54  at block  254 . The treated materials may be processed at the biogenerator  54  at block  256 , and the energy resources generated by the processing of the treated biological materials may be gathered at block  258 . As shown above, some but not all of the embodiments of the converter  50  may have recycling of at least a fraction of the processed, treated biological materials. In those embodiments of the converter  50  that include recycling, the processed, treated biological materials may be recycled at block  260 . 
     Having thus discussed the structure and operation of the converter  50 , an embodiment of a system wherein one or more of the converters  50  may be used is now discussed. 
     One example of a system in which one or more of the converters  50  according to the present disclosure may be used is a wastewater treatment system, such as is shown in  FIG. 10 . The wastewater treatment methods in use today have changed little in their basic principles since they were first developed in the early 1900&#39;s. Simply stated, microorganisms are used to consume and to oxidize the organic wastes present in wastewater so that the resultant effluent may be discharged into a large body of water, such as a river, lake or ocean. As a consequence, there are a variety of biological materials (wastewater, sludges, microorganisms used by the bioreactors, and effluents, for example) present in such a system that may be converted by the converter  50  in energy resources. Furthermore, there is a large volume of this biological material available; over 33 billion gallons of domestic wastewater are treated each day in the United States. 
     As shown in  FIG. 10 , wastewater may enter the wastewater treatment system  350  at the upper left. The wastewater may flow first into a preliminary treatment station  352 . The preliminary treatment station  352  may include one or more screens  354 , which may be large metal grates that prevent larger objects in the wastewater stream from passing further downstream. After the wastewater stream passes through the preliminary treatment station  352 , the wastewater stream may enter the primary treatment station  356 . 
     The primary treatment station  356 , according to this embodiment of the system  350 , may include a plurality of settling tanks  358 . According to other embodiments, the primary treatment station  356  may include a lagoon. The wastewater may be held in the primary treatment station  356  to permit larger solids, which were not removed in the preliminary treatment station  352 , to separate from the remainder of the wastewater. The wastewater may also be held in the primary treatment station to permit lighter materials, such as oil and grease, to separate from the wastewater and float to the top of the tanks  358 . The liquid fraction (which may still contain up to 5% biosolids, and may be referred to as primary effluent) may then be directed to the secondary treatment station  360 , while the biosolids that settled to the bottom or floated to the top of the tanks  358  may be directed to one or more bioreactors  362 . The materials that are directed to the bioreactors  362  may be referred to as primary treatment biosolids or primary sludge. 
     Leaving discussion of the bioreactors  362  for the moment, the secondary treatment station  360  may include one or more treatment substations. As shown, the secondary treatment station  360  may include a plurality of aeration tanks  364 , which may also be referred to as bioreactors, and a plurality of clarifiers  366 . Alternatively, the bioreactors used in the secondary treatment station may be facultative (able to function with or without oxygen), anoxic (low concentrations of oxygen) or anaerobic (without oxygen). According to the embodiment shown, the materials received from the primary treatment station  356  may be held in the aeration tanks  364  to permit microorganisms within the liquid fraction digest at least some of the biosolids remaining in an oxygen-rich environment. The biosolids and microorganisms may then be separated from the liquid fraction by the clarifiers  366 . The fraction of the wastewater leaving the secondary treatment station  360  and containing a higher percentage of biosolids may be referred to as activated sludge. The fraction of the wastewater leaving the secondary treatment station  360  and containing a lower percentage of biosolids may be referred to as secondary effluent. 
     Some of the waste activated sludge, referred to as return activated sludge, may be returned to the secondary treatment station  360 . The remainder of the activated sludge, referred to as waste activated sludge, may be passed along to the thickeners  368 , where chemicals, such as polymers, are added to the waste activated sludge or gravity is used to increase the solids concentration. The thickened waste activated sludge may be passed along to the bioreactors  362 . 
     The thickened waste activated sludge and the biosolids from the primary treatment station  352  may be mixed in the bioreactor(s)  362 . In the bioreactors  362 , the primary sludge and waste activated sludge may be exposed to microorganisms for anaerobic digestion. At least two product streams may exit the bioreactor  362 : a first stream of gaseous by-products, which may represent energy resources and may be gathered, and a second stream of solids, digested solids, microbiological processors, and liquid fraction, which is passed along to the presses  372 . 
     In the presses  372 , the biosolids exiting the bioreactor(s)  362  may be subjected to pressure to further separate liquids from the biosolids. For example, belt presses and/or centrifuges may be used. The remaining biosolids may be gathered from the presses  372  for disposal, in a landfill, for example, while the liquid fraction may be returned to secondary treatment station  360 . 
     The secondary effluent passes to a final treatment station  370 . The final treatment station  370  may include one or more substations, similar to the secondary treatment station  360 . For example, the final treatment station may include a chlorine disinfection station  374  and an ultraviolet disinfection station  376 . These stations may be concurrent or consecutive. The resultant flow may then be directed to the filtration station  378 . 
     The filtration station  378  is an optional station, and may be included or omitted depending upon the use for which the resultant treated water is intended. One or more filters, such as sand or crushed coal filters, may be used to remove impurities remaining in the treated water stream. Biosolids collected on the filters may be removed, by backwashing the filters, for example, and directed to the bioreactors  362 . The resulting water stream may be discharged into a river, lake or ocean, or put to an alternative use, such as for irrigation or for industrial processes. 
     The converter  50  according to the present disclosure may be used at any of a number of different places within the treatment system  350  just described. For ease of illustration, several junctions within the system  350  have been labeled, A through F. A converter  50  may receive part or all of the stream at these junctions. For example, a converter  50  may receive a fraction of the wastewater stream passing through junction A before it passes to the primary treatment station  356 . Alternatively, a converter  50  may receive the stream from the primary treatment station  356  before it is combined with the return activated sludge (junction B), or after it is combined with the return activated sludge (junction C). As a further alternative, a converter  50  may receive activated sludge after the secondary treatment (junction D). As yet additional alternatives, a converter may receive the product of the bioreactors  362  (junction E) or the presses  372  (junction F). 
     As still further alternatives, more than one converter  50  may be used at any one junction, or at more than one junction (junctions B and D, for example). Moreover, a converter  50  may receive streams from more than one junction (junctions B and D, for example), which streams may be mixed prior to being introduced into the converter  50  (such as at junction C) or within the treatment chamber  82  of the PEF station  52  of the converter  50 . Moreover, a stream from one of the junctions may be passed to a first converter  50 , the product of which is then fed to a second converter  50 . Still other alternatives will be recognized by one skilled in the art.