Patent Publication Number: US-2011076519-A1

Title: Systems and Methods for Sustainable Wastewater and Biosolids Treatment

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of U.S. Provisional Application No. 60/977,419, filed Oct. 4, 2007, which is incorporated by reference as if disclosed herein in its entirety. 
    
    
     BACKGROUND 
     Many current wastewater treatments include the use activated sludge. The use of activated sludge is a century-old, energy intensive, aerobic process, which requires pumping oxygen into a reactor. Processes including activated sludge are costly. The annual costs of treating U.S. wastewater alone are $25 billion and escalating. It is estimated that many more billions will be needed in future decades to maintain and replace ageing infrastructure. Furthermore, expanding wastewater infrastructure to accommodate an increasing population adds to this cost. Globally, there is an urgent need for low-cost water treatment technologies in developing countries and rural areas. In recent years, numerous studies have examined the feasibility of new, energy-saving, anaerobic treatment technologies. These include biogas reactors and bio-electrochemical systems. Microbial fuel cells (MFC), a type of bio-electrochemical system, directly capture electrons produced by microbial catabolism. MFCs utilize bacteria in a bioreactor to generate electricity from organic material, including wastewater. Biogas reactors convert biomass into a gaseous intermediate molecule, such as methane or hydrogen, which reduces the efficiency of the system. 
     Although the principles behind MFC technology were discovered approximately 100 years ago, only in the past decade has the technology received renewed attention as a promising source of alternative energy. Recent MFC research has yielded many experimental designs and intriguing results. Some configurations use carbon rods as anodes and carbon paper exposed directly to air as a cathode. Other designs incorporate platinum catalysts into the cathode, employ a proton exchange membrane for ion transfer, and or use electron mediator molecules to shuttle electrons between the microorganisms and the anode. However, all MFCs include substantially similar operating principles: the oxidation of a carbon source occurs at the anode while the reduction of oxygen to water occurs at the cathode. Much research still needs to be done with current MFCs to make them practical and cost efficient. Platinum catalysts and proton-exchange membranes are commonly used in experiments, but both are expensive and would be impractical to implement on a large scale. Electron mediator molecules can dramatically increase power output, but many of these molecules are toxic and non-renewable, detracting from the environmental benefits of the system. Current MFC technologies produce little energy per fuel cell and thus have limited use. 
     SUMMARY 
     Methods of sustainable wastewater and biosolids treatment using a bioreactor including a microbial fuel cell are disclosed. In some embodiments, the methods include the following: enriching an anode of the microbial fuel cell in the bioreactor with a substantially soluble electron acceptor; growing the bacteria in the presence of the anode enriched with a substantially soluble electron acceptor; oxidizing a substrate using the bacteria to produce free electrons; channeling the free electrons away from a terminal electron acceptor and to the enriched anode, the enriched anode serving as an electron acceptor; and carrying the free electrons from the enriched anode to a cathode of the microbial fuel cell to generate electricity. 
     Systems for producing a microbial fuel cell having improved electricity generating capabilities are disclosed. In some embodiments, the systems include the following: a bioreactor module including the following: a bioreactor having a microbial fuel cell; and a substantially soluble electron acceptor for enriching an anode of the microbial fuel cell in the bioreactor; a transfer module including means for serially transferring bacteria grown in the presence of the anode enriched with a substantially soluble electron acceptor from the bioreactor to a second bioreactor having a microbial fuel cell thereby seeding the second bioreactor; a treatment module including the second bioreactor having a microbial fuel cell means for oxidizing elements of domestic wastewater, biosolids, and combinations thereof using primarily the serially transferred bacteria, and means for generating electricity. 
     Methods of sustainable wastewater and biosolids treatment using a bioreactor including a microbial fuel cell are disclosed. In some embodiments, the methods include the following: enriching an anode of the microbial fuel cell in the bioreactor with iron (iii) chloride; growing the bacteria in the presence of the anode enriched iron (iii) chloride; oxidizing a substrate using the bacteria to produce free electrons; channeling the free electrons away from a terminal electron acceptor and to the enriched anode, the enriched anode serving as an electron acceptor; and carrying the free electrons from the enriched anode to a cathode of the microbial fuel cell to generate electricity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings show embodiments of the disclosed subject matter for the purpose of illustrating the invention. However, it should be understood that the present application is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: 
         FIG. 1  is a schematic diagram of a system according to some embodiments of the disclosed subject matter; 
         FIG. 2  is a side section view of a microbial fuel cell according to some embodiments of the disclosed subject matter; 
         FIG. 3  is a top plan view of a microbial fuel cell take along line  3 - 3  of  FIG. 2 ; 
         FIG. 4  is a diagram of a method according to some embodiments of the disclosed subject matter; 
         FIG. 5  is a graph of voltage (and consequently power) production over time before and 20 hours after a nutrient spike for systems and methods according to some embodiments of the disclosed subject matter; 
         FIG. 6  is a graph of ammonium concentrations in two reactors according to some embodiments of the disclosed subject matter before and after a glucose-ammonium spike solution was added; 
         FIG. 7  is a graph of voltage (and consequently power) production over time for systems and methods according to some embodiments of the disclosed subject matter; and 
         FIG. 8  is a graph of voltage production over time for systems and methods according to some embodiments of the disclosed subject matter. 
     
    
    
     DETAILED DESCRIPTION 
     A microbial fuel cell is an anaerobic bioreactor in which bacteria oxidize various substrates to produce free electrons. The electrons are channeled away from the terminal electron acceptor to an anode. A conductive wire carries the electrons from the anode to the cathode, creating electricity that can be captured and used as a source of energy. If wastewater and biosolids is used as the substrate, operation of a microbial fuel cell can be used to treat the wastewater and biosolids and generate electricity. 
     Generally, the disclosed subject matter relates to systems and methods for sustainable treatment of wastewater and biosolids using improved microbial fuel cells. Referring now to  FIGS. 1-3 , some embodiments include a system  100  for producing a microbial fuel cell  102  having improved electricity generating capabilities. In some embodiments, system  100  includes a bioreactor module  104 , a transfer module  106 , and a treatment module  108 . 
     As best shown in  FIGS. 2 and 3 , bioreactor module  104  includes combined bioreactor/microbial fuel cell  112 . Microbial fuel cell  112  includes an anode  114  and a cathode  116  that are in electrical communication with one another via a wire  117 . Anode  114  is typically defined by a plurality of anode panels  118  that are enriched with iron (iii) chloride or another substantially soluble electron acceptor. Cathode  116  is positioned in a central cathode chamber  120  defined by a porous tubular structure  121  that is surrounded by plurality of anode panels  118 . 
     Referring again to  FIG. 1 , bioreactor module  104  includes seed material  122  for seeding bioreactor  112  with material containing bacteria for oxidizing a substrate. A feed material  124  is included to serve as a principal electron donor to encourage the growth of the bacteria in bioreactor  112 . A substantially soluble electron acceptor  126  is included for enriching anode  114  of microbial fuel cell  112 . Again, substantially soluble electron acceptor  126  is typically iron (iii) chloride, but can be other substantially soluble electron acceptors. 
     Transfer module  106  includes standard apparatus and equipment (not shown) for serially transferring bacteria grown in the presence of anode  114  enriched with substantially soluble electron acceptor  126  from bioreactor  112  to a second bioreactor  128  having microbial fuel cell  102  thereby the seeding second bioreactor. 
     Treatment module  108  includes second bioreactor  128  and microbial fuel cell  102  and standard apparatus and equipment (not shown) for introducing a flow of domestic wastewater and biosolids  132  to the second bioreactor. Similar to bioreactor  112  and as discussed above, second bioreactor  128  is configured to oxidize elements of the domestic wastewater and biosolids using primarily the serially transferred bacteria. Operation of system  100  and microbial fuel cell  130  causes the production of free electrons. Enriched anode  114  of microbial fuel cell  102  channels the free electrons away from a terminal electron acceptor and to the enriched anode, which serves as an electron acceptor. Wire  117  carries the free electrons from enriched anode  114  to cathode  116  to generate the electricity. The electricity is typically captured and stored to be used as an energy source  134 . 
     Referring now to  FIG. 4 , some embodiments of the disclosed subject matter include a method  200  of sustainable wastewater and biosolids treatment using a bioreactor including a microbial fuel cell. At  202 , method  200  includes providing a bioreactor having a microbial fuel cell. The microbial fuel cell includes an anode and a cathode that are in electrical communication with one another. At  204 , a substrate that is to be oxidized is provided in the bioreactor. The substrate typically includes domestic wastewater, but can be any other material such as biosolids produced in wastewater treatment plant. Typically, and particularly when used to treat domestic wastewater, the substrate is provided via a continuous flow or refillable batch. At  206 , the bioreactor is seeded with material containing bacteria for oxidizing the substrate. In some embodiments, seeding includes adding an amount of a nitrifying biomass to the bioreactor. At  208 , a feed material is provided to the bioreactor to serve as a principal electron donor, which encourages the growth of the bacteria in the bioreactor. In some embodiments, the feed material includes acetate but can also include any other substances that encourage the growth of the bacteria. At  210 , the anode of the microbial fuel cell is enriched with iron (iii) chloride or another substantially soluble electron acceptor. At  212 , the bacteria are grown in the presence of the anode enriched with iron (iii) chloride, which facilitates propagation of a community of bacteria with iron-reducing capabilities. At  214 , the substrate oxidized by the bacteria to produce free electrons. At  216 , the free electrons are channeled away from a terminal electron acceptor and to the enriched anode, which serves as an electron acceptor. At  218 , the free electrons are carried from the enriched anode to the cathode of the microbial fuel cell to generate electricity. The electricity is typically captured and stored for use as a source of energy. At  220 , bacteria grown in the presence of the anode enriched with a substantially soluble electron acceptor is serially transferring from a first bioreactor to a second bioreactor thereby seeding the second bioreactor. 
     Laboratory scale systems and methods according to the disclosed subject matter were tested. Kinetics tests to determine general consumption rates were designed around a nutrient spike. These tests monitored biomass, ammonia concentration, pH, chemical oxygen demand (COD) concentration, and voltage. Ammonia tests were performed every hour, while COD and biomass collection tests were taken every 2 hours. Voltage was measured every 10 seconds with the data recording device. For each sample removed, an equal volume of tap water was added to the reactor. 
     Tests were performed to determine the voltage generated during operation of MFCs including anodes enriched with various electron acceptors. A first MFC (“F reactor”) included an anode enriched with iron (iii) chloride, a second MFC (“FS reactor”) included an anode enriched with iron (iii) sulfate, and a third MFC (“S reactor”) included an anode enriched with sodium sulfate. As shown in  FIG. 5 , the largest increase in voltage over time, and consequently, the best performing community, was in the F reactor, which was enriched with iron (iii) chloride. The FS reactor, which was enriched with iron (iii) sulfate, also experienced an increase, although a smaller one, and the S reactor, which was enriched with sodium sulfate showed no increase in voltage. 
     Qualitatively, a thick, orange biofilm was observed on the anode of the FS reactor and a thin, red-orange biofilm was observed on the F reactor anode. A few gray strands were observed on the anode of the S reactor, although this reactor had the most turbid bulk phase medium. 
     Calculations of typical power received from the voltage data are as follows: 
     
       
         
           
             
               
                 
                   P 
                   = 
                   
                     
                       
                         V 
                         2 
                       
                       R 
                     
                     = 
                     
                       
                         
                           
                             ( 
                             
                               0.40 
                                
                               V 
                             
                             ) 
                           
                           2 
                         
                         
                           10.0 
                            
                           Ω 
                         
                       
                       = 
                       
                         
                           
                             16 
                              
                             
                                 
                             
                              
                             mW 
                           
                           
                             0.0377 
                              
                             
                               m 
                               2 
                             
                           
                         
                         = 
                         
                           424 
                            
                           
                             
                               mW 
                               
                                 m 
                                 2 
                               
                             
                             . 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     Measurements of ammonium concentrations were taken in two reactors, the “FMG reactor” and the “FMW reactor” before and after a glucose-ammonium spike solution was added. As shown in  FIG. 6 , ammonium concentrations after a spike show a steady decrease in concentration. Approximately one day after the spike, ammonium concentrations returned to baseline levels. The baseline is most likely sustained by endogenous decay in the reactor. 
     To analyze the importance of a biofilm in electricity production, tests were performed to compare results from the biofilm community (“biofilm-phosphate reactor”) and from microorganisms in the bulk phase-liquid or planktonic state (“control reactor”). In a first test, the biofilm-phosphate reactor had its media drained away and the anode was submerged in a phosphate buffer of pH 7.1. The control reactor retained both its anode and media. Following this, both reactors were spiked with the glucose/ammonia solution and voltage was monitored for three days. As shown in  FIG. 7 , the nutrient spike given to biofilm-phosphate reactor, which included a thick, gray biofilm in a phosphate buffer, resulted in a logarithmic increase in voltage. In the control reactor, the nutrient spike caused a slow and short increase in voltage followed by a decrease in voltage to below baseline levels. 
     Referring now to  FIG. 8 , a second test was performed to analyze whether an increase in voltage was attributed to a new phosphate buffer or to a spike of glucose-ammonium solution and a third test analyzed how keeping the bulk phase media in the control reactor while adding a fresh anode with no biofilm on it affected electricity output. The voltage was monitored for three days. 
     In the second test, the biofilm-covered anode from the control reactor was submerged into a new phosphate buffer (“biofilm-phosphate reactor”), yet the reactor was not given a nutrient spike for one day. A delayed spike in the biofilm-phosphate reactor demonstrates that the logarithmic growth in voltage is caused by the addition of glucose-ammonium solution itself and not by the phosphate buffer. 
     In the third test, the anode of the control cell was replaced with a fresh anode that had no biofilm. The new anode was submerged and a spike was immediately given. Still referring to  FIG. 8 , in the control reactor, a slow logarithmic increase in voltage was observed. The R 2  constant is not as high as the ones associated with the biofilm-phosphate reactors. A possible explanation for this is the lack of a biofilm at the beginning of the test, followed by the acquisition of a thick gray biofilm toward the end of the test. 
     Maximum power was generated during the second phase of the experiment in the FM reactor. These calculations are shown here: 
     
       
         
           
             
               
                 
                   P 
                   = 
                   
                     
                       
                         V 
                         2 
                       
                       R 
                     
                     = 
                     
                       
                         
                           
                             ( 
                             
                               0.67 
                                
                               V 
                             
                             ) 
                           
                           2 
                         
                         
                           10.0 
                            
                           Ω 
                         
                       
                       = 
                       
                         
                           
                             44.89 
                              
                             
                                 
                             
                              
                             mW 
                           
                           
                             0.0377 
                              
                             
                               m 
                               2 
                             
                           
                         
                         = 
                         
                           1190 
                            
                           
                             
                               mW 
                               
                                 m 
                                 2 
                               
                             
                             . 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     Efficiency measurements assess how well the microbial community is oxidizing substrate. The Nernst equation relates the free energy of a particular reaction to the voltaic potential difference of the reaction. This equation can then be modified for the particular concentrations of reactants and products present in the reactor. The standard potential for a reactor according to the disclosed subject matter was calculated to be 1.244 V for the oxidation of glucose to carbon dioxide coupled with the reduction of oxygen to water. The following is a calculation of the Nernst Equation for this standard potential with the concentration of glucose added into each nutrient spike: 
     
       
         
           
             
               
                 
                   
                     ∴ 
                     
                       ξ 
                       cell 
                     
                   
                   = 
                   
                     
                       1.24 
                        
                       V 
                     
                     - 
                     
                       
                         ( 
                         
                           2.46 
                           * 
                           
                             10 
                             
                               - 
                               3 
                             
                           
                         
                         ) 
                       
                        
                       
                         
                           log 
                           ( 
                           
                             
                               
                                 [ 
                                 
                                   CO 
                                   2 
                                 
                                 ] 
                               
                               6 
                             
                             
                               
                                 
                                   [ 
                                   0.0056 
                                   ] 
                                 
                                  
                                 
                                   [ 
                                   
                                     O 
                                     2 
                                   
                                   ] 
                                 
                               
                               6 
                             
                           
                           ) 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     At the start of each spike, a bubbler was passed through the cathode chamber, e.g., the second iteration of tests, to saturate the solution with air, which created an oxygen concentration of 7.0 parts per million (ppm). From this, as well as the proportion of oxygen to carbon dioxide in air, the aqueous concentration of carbon dioxide can be calculated from Henry&#39;s Law. The following is a calculation of the Nernst equation while including these values: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           ∴ 
                           
                             ξ 
                             cell 
                           
                         
                         = 
                           
                          
                         
                           
                             1.24 
                              
                             V 
                           
                           - 
                           
                             
                               ( 
                               
                                 2.46 
                                 * 
                                 
                                   10 
                                   
                                     - 
                                     3 
                                   
                                 
                               
                               ) 
                             
                              
                             
                               log 
                               ( 
                               
                                 
                                   
                                     [ 
                                     
                                       5.14 
                                       · 
                                       
                                         10 
                                         
                                           - 
                                           7 
                                         
                                       
                                     
                                     ] 
                                   
                                   6 
                                 
                                 
                                   
                                     
                                       [ 
                                       0.0056 
                                       ] 
                                     
                                      
                                     
                                       [ 
                                       
                                         2.19 
                                         · 
                                         
                                           10 
                                           
                                             - 
                                             4 
                                           
                                         
                                       
                                       ] 
                                     
                                   
                                   6 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           1.27 
                            
                           V 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     As a result, the maximum possible voltage attainable was calculated to be 1.27 V. Comparing this to the maximum observed voltage, simple efficiency calculations yield the following: 
     
       
         
           
             
               
                 
                   
                     
                       
                         Efficiency 
                         = 
                           
                          
                         
                           
                             
                               V 
                               act 
                             
                             
                               V 
                               th 
                             
                           
                           · 
                           100 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               0.784 
                                
                               V 
                             
                             
                               1.27 
                                
                               V 
                             
                           
                           · 
                           100 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           61.7 
                            
                           % 
                            
                           
                               
                           
                            
                           
                             Efficiency 
                             . 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     5 
                   
                   ] 
                 
               
             
           
         
       
     
     Thus, the cell is producing approximately 62% of the voltage it could possibly produce if it were an inorganic reaction operating at 100% efficiency. 
     As shown in the test results, systems and methods including microbial fuel cell according to the disclosed subject matter succeeded in simultaneously generating power and degrading organic nitrogen and carbon in wastewater. As shown in Equation 5, the Nernst equation and efficiency calculations yielded an efficiency of nearly 62. 
     Referring to Equation 2, it was shown in preliminary tests that the reactors according the disclosed subject matter produced approximately 1.2 W/m 2  across a 10Ωresistor. This power density is on the high end of those in known systems. As shown in Equations 4 and 5, in the absence of a resistor, voltaic efficiencies are consistent with the energy theoretically produced by the reaction and consumed by the microorganisms. 
     The high voltaic efficiency reveals that the microbial community is properly carrying out the oxidation half-reaction. This indicates that that the MFCs according to the disclosed subject matter can be effective for bioremediation. Fast consumption kinetics and high efficiency rates mean more wastewater or biosolids can be processed for a given MFC volume. 
     Data from the phosphate buffer experiment, in which biofilm bacteria were shown to be responsible for most of the energy production, were highly reproducible. As shown in  FIGS. 5 and 6 , the logarithmic regression curves for each of the graphs of voltage growth over time show a high R 2  correlation coefficient indicating a high conformation to a mathematical model. This test showed the biofilm-phosphate buffered reactor produced voltage at a greater rate than did the control reactor. Qualitatively, the fact that the control reactor, which included a fresh anode, grew a biofilm spontaneously suggests this is a preferred state for these electricity-producing bacteria to grow in. It can also mean equilibrium exists between the two types of bacteria. 
     SO 4   2−  reduction to H 2 S plays a role in inhibiting electron transfer to the cathode. The high concentration of sulfate makes it a more convenient electron acceptor. As shown in  FIG. 5 , the reactor with the least sulfate in it, i.e., with an iron (iii) chloride-enriched anode, produced the most power. 
     However, the presence of iron likely played a more significant role than the lack of sulfate did in selecting for an electricity-producing community. Species have been discovered that reduce iron (iii) to iron (ii) in their natural environment. Insoluble iron and soluble iron compounds present in a reactor select for organisms with this capability. Supporting this argument is the observation that the population was changing in terms of color and odor. Thus, over time it is reasonable to expect the population will become more productive. 
     Extracting energy from a system treating wastewater and biosolids cuts down on treatment costs and is a step towards sustainable wastewater treatment. This system can be of value in both developed and undeveloped areas of the world as well as for a variety of isolated, small-scale applications, including those at sea or in space. 
     Systems and methods according to the disclosed subject matter provide advantages and benefits over known systems and methods. Systems and methods according to the disclosed subject matter allow for production of electricity using bacteria from wastewater and biosolids. At the same time, systems and methods according to the disclosed subject matter can be used for wastewater treatment, as energy production uses the organic wastes as a substrate in energy production. Technology according to the disclosed subject matter can be used as a convenient power source for portable electronics and can be used for power generation for developing countries that don&#39;t have well established power grids. 
     Although the disclosed subject matter has been described and illustrated with respect to embodiments thereof, it should be understood by those skilled in the art that features of the disclosed embodiments can be combined, rearranged, etc., to produce additional embodiments within the scope of the invention, and that various other changes, omissions, and additions may be made therein and thereto, without parting from the spirit and scope of the present invention.