Abstract:
An electrolytic method and apparatus for treating liquids using a flow cell with widely spaced electrodes and polarity reversing power designed to prevent electrode fouling and provide for long continuous liquid treatment running times.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application 61/248,077 filed Oct. 2, 2009 hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to an apparatus for treating liquids to disinfect and oxidize contaminants and in particular to a method and apparatus of this type using frequent polarity reversal of the power applied to the electrodes. 
         [0003]    Liquids, including water and solvent based ones, are capable of being treated by many different methods known the in art. Biological methods can disinfect and oxidize, but require long treatment times, a large equipment footprint and other complications. Such methods are commonly used to treat municipal wastewater but have had limited application in liquid treatment at the industrial scale. 
         [0004]    The treatment of liquid streams through the addition of disinfecting chemicals such as chlorine or bromine is also well-known. The class of chlorine disinfectants includes chlorine gas, hypochlorous acid, sodium hypochlorite, and chlorine dioxide. These chemicals typically are purchased in bulk, with attendant cost, storage and dosing issues. Chlorine gas has significant safety and security issues associated with it. 
         [0005]    Chlorine is often used because of its low cost. But in some applications, for example in chiller water, cold brine and flume water used in poultry, meat and vegetable processing, the addition of these chemicals may be disfavored because of concerns about the generation of off-taste in the product and chemical byproducts that may adversely affect product quality, plant personnel or the environment. Chlorine&#39;s efficacy is also dramatically affected by pH levels, oftentimes requiring additional pH adjustment that adds to cost and operating complications. As another disadvantage, chlorine dosage requirements in some applications can be very high due to high organic loads and pathogen levels. High dosages of these chemicals make the process difficult to control to maintain reliable disinfection without generating excessive unwanted chlorinated byproducts, such as trichloramines. They also make it difficult to maintain residual chlorine levels within the levels that may be permitted by the United States Department of Agriculture (USDA) or other application-specific regulations. 
         [0006]    Other chemicals such as hydrogen peroxide and peracetic acid are used as disinfectants but typically have high cost and other issues associated with them. 
         [0007]    High temperature pasteurization can be used for disinfection but requires high energy use and post-treatment cooling. Ultraviolet light is sometimes used for water and wastewater final effluent disinfection. A significant drawback to ultraviolet systems is their inability to work in turbid liquids with suspended solids or color. 
         [0008]    It also is known that electrical methods can be used to disinfect liquids. Electroporation is a high voltage, low current process that disinfects by penetrating the cell walls of pathogens and either destroying or inactivating them. Electroporation has many laboratory scale uses, including for the insertion of genes in cells, but has not scaled up well to industrial use due to the high voltages used and other reasons. 
         [0009]    Research has also been done on disinfection at various alternating current frequencies, including those in the low kilohertz to microwave range. However, it has been shown that the primary method of disinfection at these frequencies comes through the heating of liquid to low pasteurization temperatures, an energy inefficient process. 
         [0010]    Direct current electrolytic processes have also been demonstrated to disinfect contaminants in liquids. In electrolysis, comparatively low voltage, high current electrical power is passed through the liquid, breaking bonds in chemical compounds and causing destructive changes to biological cells. Depending on the application, this can be a more energy efficient method. 
         [0011]    Electrolytic systems called hypochlorite generators have been used commercially for indirect disinfection of liquids. Electrodes are immersed in a relatively pure salt water solution and direct current is applied to generate hypochlorite. The hypochlorite-enhanced solution is then injected into the liquid stream to be treated. Such systems typically use different anode and cathode materials. They have the same issues with pH control as with the direct addition of hypochlorite. 
         [0012]    Another electrolytic method uses an ion exchange membrane in a salt solution between direct current electrodes to create separate acidic anolyte and alkaline catholyte solutions with purported electrochemically activated properties. Sometimes these two liquids are used separately for cleaning, or sometimes may be combined and marketed under rather fanciful names implying health and longevity effects. 
         [0013]    Another electrolytic process is used to directly treat swimming pool water to which salt is added. Since swimming pool liquid is less controlled regarding contaminants, the polarity is reversed on the electrodes, typically every few hours, to reduce electrode fouling caused by the electrochemical deposition of materials onto the electrodes. 
         [0014]    These electrolytic systems use plate or expanded metal electrodes that typically are spaced 0.5-2 mm apart. Any polarity reversal is done only very infrequently, typically 2-6 hours between reversals. The anodes typically use one or more metal oxides from the platinum group metals as a coating over a valve metal such as titanium. It has generally been observed that polarity reversing power is highly destructive to these coatings with various mechanisms of failure being described including hydrogen embrittlement when the electrode is operated as a cathode. 
         [0015]    Lab scale research has been done with electrolytic systems, typically with static treatment cells or very low volume flow cells, to demonstrate disinfection and also the oxidation of trace levels of contaminants such as endocrine disruptors in water and wastewater. This research generally has been done with liquids containing low chemical oxygen demand (COD) and biological oxygen demand (BOD) loads, an unrealistic situation for many industrial liquid treatment applications. Some of this research has used conventional metals like 300 series stainless steel for their electrodes. These test results are not relevant to industrial applications where the liquid flowing through a treatment cell may have significant COD and BOD loads, with these loads being continuously replaced by a new influx of organic and inorganic contaminants. In addition, these tests typically have not analyzed the treated solution for an increase in dissolved heavy metal ions. Under such electrolytic treatment, these heavy metal concentrations can quickly rise to levels above those permitted by the EPA, USDA, FDA or other relevant regulatory agency. 
       SUMMARY OF THE INVENTION 
       [0016]    The present invention provides direct, high flow rate, electrical treatment of liquids with significant organic loads and suspended solids, including but not limited to poultry chiller water and processed meats chiller brine. While the inventors do not wish to be bound by a particular theory, it is believed that such direct treatment may provide significant advantages in exposing the water to short-lived chemical species. The possibility of such direct treatment required a determination that treatment effectivity could be maintained for relatively large electrode gaps (5 mm or larger) and without debilitating electrodes fouling, both empirically confirmed by the present inventors. 
         [0017]    Specifically then, the present invention provides a liquid treatment system with a first and second electrode having a comparatively large separation between them that permits the passage of a significantly large volume of liquid that may contain small solids. A polarity reversing power supply is connected across the first and second electrodes, the power supply switching the polarity of the voltage at a period determined empirically for each liquid being treated within a specified range. 
         [0018]    The inventors have determined that there is an optimum narrow frequency range around 0.03 Hz for high performance duplex stainless steel electrodes above or below which a substantial reduction of treatment performance occurs for a particular liquid stream. This equates to a polarity reversal approximately every 17 seconds. In addition, the disinfection falls off rapidly when the period between polarity reversals is less than 5 seconds (0.1 Hz) or greater than 50 seconds (0.01 Hz). 
         [0019]    In addition, the inventors have determined that for catalytic platinum group metal electrodes, the detrimental effects of frequent polarity reversal on electrode life can be balanced against the need to change polarities to prevent electrode fouling and that the optimal time between polarity reversals is approximately between 10 seconds and 60 minutes, depending on the composition of the liquid stream. 
         [0020]    It is thus a feature of at least one embodiment of the invention to maximize treatment efficacy while balancing electrode lifetimes by performing a polarity reversal in the range of approximately 10 seconds to 60 minutes. 
         [0021]    Laboratory research results with electrochemical treatment of liquids has heretofor been difficult or impossible to scale up to commercially useful liquid treatment due to the need to separate out any solids plus organic or inorganic load that could physically plug the electrodes with their narrow spacing or otherwise cause fouling. Separation processes are capital intensive, require regular cleaning and maintenance, and are not warranted or desirable in many applications. 
         [0022]    It is thus a feature of at least one embodiment of the invention to space the electrodes greater than 5 mm apart. 
         [0023]    In electrochemistry things that work at the lab scale with closely spaced electrodes, low flow rates, and very short running times typically cannot be duplicated even at low commercial flow rates treating liquids with varying composition and with a necessarily wider gap between the electrodes to provide the required higher flow rates, acceptable pressure drops and to permit passage of small solids. The inventors have successfully treated very challenging liquid streams at flow rates up to 650 gallons per minute. 
         [0024]    It is thus a feature of at least one embodiment of this invention to treat flow rates of five gallons per minute and higher. 
         [0025]    The inventors have also found that electrolysis degrades many metals and their oxides when used for electrolysis. Very high levels of metal ions, such as chromium, nickel, iron, and tin are found in liquids that have been treated with electrode materials containing them. This prevents conventional electrolysis with such electrodes from being used in applications where metal toxicity is a concern. They remain quite appropriate for various other treatment applications, such as electrowinning and electro-flocculation. The inventors have also found that catalytic electrodes, for example those from the platinum group metals and metal oxides, do not dissolve into the liquid under polarity reversing electrolysis to any measurable extent. 
         [0026]    It is thus a feature of at least one embodiment of the invention to use catalytic electrode surfaces including those from the platinum group metals and metal oxides, and from the doped diamond category. 
         [0027]    It is known in the art that various catalytic metals and metal oxides in liquids containing water and salt generate differing proportions of reactive oxygen species and/or chlorine species that may be useful for liquid treatment, such as disinfection and oxidation. For example, under electrolysis, ruthenium oxide is known to generate a high proportion of chlorine species and fewer reactive oxygen species when used as an anode. Boron doped diamond is known to produce a much higher ratio of reactive oxygen species when used as an anode. 
         [0028]    It is thus a feature of at least one embodiment of the invention to provide electrode pairs of opposite polarity with surfaces of different metals or metal oxides. Polarity reversal can be controlled to provide different times between polarity reversal for each specific electrode surface, enabling the control system to tailor the reactive species being generated to meet the needs of a particular liquid stream being treated. 
         [0029]    Lab scale testing has typically been done on liquids where such contaminants that affect process performance, like organic loads and bacteria levels are not replaced during the test cycle, further contributing to the inability to extrapolate test results to commercial reality. 
         [0030]    It is thus a feature of at least one embodiment of this invention to treat liquids that have a chemical oxygen demand of 200 mg/l or higher with a continuing influx of organic and inorganic loads. 
         [0031]    In many commercial applications, for example the disinfection of processed meats chilling brine, a fluid is continuously recirculated at high flow rates for another purpose, such as cooling a product and rechilling the liquid to maintain its desired temperature. The inventors have shown that in many cases a proportionally smaller liquid flow is all that is required to maintain the desired level of treatment in the liquid, reducing the size of the electrolytic treatment cell, associated piping and other components. 
         [0032]    It is thus a feature of at least one embodiment of this invention to treat a side stream or smaller volume of a main flow. 
         [0033]    In certain commercial applications the liquid stream is directly discharged and is not recycled for another reason. In such situations the desired treatment level may be difficult to achieve at reasonable equipment cost in a single pass. The inventors have determined that recycling a portion of the treated liquid back to the treatment cell can have a synergistic effect on process efficacy. 
         [0034]    It is thus a feature of at least one embodiment of this invention to directly treat a liquid stream on a one-pass basis, but to recycle a portion of this liquid back through the treatment cell to obtain the overall treatment efficacy desired. 
         [0035]    Methods for disinfecting example food processing liquid streams are disclosed. 
         [0036]    It is thus a feature of at least one embodiment of this invention to disinfect food processing liquids. 
         [0037]    A method for oxidizing and destroying trace pharmaceuticals and personal care products is disclosed. 
         [0038]    It is thus a feature of at least one embodiment of this invention to provide a method to remove pharmaceutical and personal care product (PPCP) residuals from a liquid stream. 
         [0039]    In addition, the inventors have found that disinfection efficacy, a low cost, easily measured value, serves as a robust surrogate for the efficacy of removal of trace pharmaceuticals and personal care product residuals. 
         [0040]    It is thus a feature of at least one embodiment of this invention to use the results from tests for disinfection efficacy as a practical means to estimate the efficacy of PPCP residual oxidation. 
         [0041]    In addition, the inventors have determined that for liquid streams without significant bacterial load, a safe, food grade bacteria like Lactobacillus Acidophilus, used to make yogurt, can be added to the liquid to provide this surrogate disinfection measure. 
         [0042]    It is thus a feature of at least one embodiment of this invention to add bacteria to a liquid stream and use the results of tests for disinfection efficacy of this bacteria as a practical means to estimate the efficacy of PPCP residual oxidation. 
         [0043]    This electrochemical process produces oxygen at the anode and hydrogen at the cathode. With the polarity reversal of this process, each electrode in a pair alternates between generating hydrogen and oxygen. 
         [0044]    It is thus a feature of at least one embodiment of this invention to generate hydrogen and oxygen both as a mixed species and as separate elements, the latter achieved by an external separation means such as a selective membrane. 
         [0045]    These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0046]      FIG. 1  is a perspective view of a liquid treatment system in one embodiment of the present invention showing a main housing holding opposed planar electrodes between liquid inlets and outlets, a power distribution module, and a control unit; 
           [0047]      FIG. 2  is a detailed block diagram of the components of  FIG. 1  showing the electrodes as flat plates; 
           [0048]      FIGS. 3   a  and  3   b  are graphs of disinfection versus frequency showing a preferred range of operation of the present invention for stainless steel and catalytic electrodes; 
           [0049]      FIG. 4  is a simplified representation of a method to disinfect processed meat and processed poultry products immediately after a cooking cycle to rapidly cool them for further processing, packaging or storage; 
           [0050]      FIG. 5  is a simplified representation of a method to disinfect raw poultry chiller water to rapidly cool the birds for further processing, packaging or storage; 
           [0051]      FIG. 6  is a simplified representation of a method to disinfect water used to wash and chill vegetables and fruits, such as cut leafy greens, to help ensure the safety of the product and permit extended reuse of the water; 
           [0052]      FIG. 7  is a graph of performance achieved with the method of the patent for removing trace pharmaceuticals from wastewater showing the strong correlation of trace pharmaceutical destruction with bacterial disinfection performance on this same liquid; 
           [0053]      FIG. 8  is a graph of performance achieved with the method of the patent for removing trace personal care product residuals from wastewater showing the strong correlation of trace personal care product destruction with bacterial disinfection performance on this same liquid; and 
           [0054]      FIG. 9  is a simplified representation of an alternate electrode arrangement using a rod and cylinder configuration; 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0055]    Referring now to  FIGS. 1 and 2 , a liquid treatment system  10  per the present invention may include a treatment unit  12  providing a liquid inlet  14  and outlet  16  to conduct liquid across internal electrodes  28 . The electrodes  28  are contained in an insulating housing  18  supported on frame  20 . A power distribution module  22  provides electrical connections  24  to the internally contained electrodes  28  for power received from a control unit  26 . The control unit  26  has a touchscreen user interface  27  for the display and entry of data including critical operation parameters. 
         [0056]    Referring now to  FIG. 2 , the treatment unit  12  includes two or more generally planar and parallel electrodes  28  held in a channel  36  between the inlet  14  and the outlet  16 . The electrodes  28  are separated along an axis  30  generally perpendicular to the flow of liquid by gaps  32  to receive the influent liquid  34  therethrough. The separation of the electrodes  28  will be greater than 5 mm to permit the passage of influent liquid  34  without undue risk of clogging. 
         [0057]    One or more chemical sensors  40  may be positioned in sensor fitting  38  downstream from the electrodes  28  and channel  36  to measure chemical properties of the liquid and/or a flow sensor  42  may be positioned in the influent liquid  34  or effluent liquid  35  to measure the flow across the electrodes  28 . The chemical sensors  40  may include those measuring pH, oxidation-reduction potential, chlorine level, free chlorine level, or total chlorine level. 
         [0058]    The amount of flow through the channel  36  may be controlled by an electrically driven pump  44  and/or valve  46  alone or in combination. 
         [0059]    The electrodes  28  are electrically isolated from each other as held by the housing  18  but may be joined by the connections  24  from power distribution module  22  so that some or all of the electrodes  28  are electrically connected to electrical conductors  48   a  and  48   b . In some configurations alternating electrodes may be connected to opposite power polarities, in others some electrodes may not be directly connected to the power supply but instead become electrically activated by the ionic currents in the liquids being treated, resulting in each side of such intermediate electrodes having opposite polarities. 
         [0060]    Conductors  48   a  and  48   b  are connected to a switching unit  50  contained in the control unit  26  that may alternate the electrical polarity of alternate electrodes  28 . The switch is depicted logically as a double pole, triple throw electrical switch and will be typically implemented by solid-state electronics controllable by control line  51 . One pole connects to a positive voltage line  52  from a voltage controllable DC power supply  58  and the other pole connects to a negative voltage line  53  from the voltage controllable DC power supply  58 . The voltage controllable DC power supply  58  receives power from electrical mains  62 . 
         [0061]    The throws of the switching unit  50  are controllable so that one conductor  48   a  or  48   b  may be connected to a given voltage (positive or negative) while the other conductor  48   a  or  48   b  is connected to the opposite voltage. 
         [0062]    The positive voltage line  52  may connect to a current sensor  54  and voltage sensing point  56 , both of which are connected to inputs of a controller  60 , the latter being a special-purpose computer, for example, a programmable logic controller executing a stored program to control of the process as will be described. A similar current sensor  54  and voltage sensing point  56  (not shown) may be provided on negative voltage line  53 . Sensors  54  and  56  may also be built into the power supply  58 . The programmable controller  60  also receives signals from the chemical sensors  40  and flow sensor  42  and may provide control signals to the pump  44  and valve  46 . In addition, the controller  60  communicates with the touchscreen  27  or alternative user input device which may be a keyboard or other means known in the art. 
         [0063]    The controller  60  includes a processor  70  and a control program  72 , the latter contained in the memory  81  communicating with the processor  70  as is generally understood in the art. In operation, the program  72  will read various parameters of the process including the electrode current from current sensors  54 , the electrode voltage from voltage sensing points  56 , user entered parameters through touchscreen  27 , chemical environment sensing from the chemical sensors  40 , and/or the flow rate from the flow sensor  42 , and will provide output signals on control line  51  controlling the switching unit  50  and the power supply  58 . In addition, output signals controlling the pump  44  and valve  46  and providing information on the touchscreen  27  may be provided. 
         [0064]    Pump  44  or the valve  46  may be used as the flow controller, Pump  44  may be a variable speed pump and valve  46  may be a continuously adjustable valve. 
         [0065]    Referring now to  FIG. 3   a , the present inventors have determined that the quality of disinfection  82  of the liquid (for example, measured by log kills of test bacteria) peaks when the period between polarity reversals is approximately 17 seconds (0.03 Hz) in duration for high performance duplex stainless steel electrodes  28 . In addition, the disinfection falls off rapidly when the period between polarity reversals is less than 5 seconds (0.01 Hz) or greater than 50 seconds (0.1 Hz). This measurement was produced on a laboratory scale in a 12 mL cell volume with electrodes spaced 1 cm apart, and a flow rate of 750 mL/min in replicated experiments. 
         [0066]    Referring now to  FIG. 3   b , for platinum-group catalytic electrodes  28  the performance peak appears to be occur the closer the electrodes approach direct current. This measurement does not consider the counteracting issues of electrode fouling due to organic and inorganic loads, which occur the closer the electrodes are run to pure unswitched direct current. In commercial scale operations with organic and inorganic loads of 200 mg/l and more of measured chemical oxygen demand, the inventors have shown that disinfection performance degrades and electrode fouling occurs when the time between current reversals exceeds 60 minutes and at even shorter current reversal periods for very high organic and inorganic loads. 
         [0067]    Referring now to  FIG. 4 , the diagram illustrates one configuration of a system to disinfect cold food processing liquids. Housing  400  contains liquid outlets or spray nozzles  414  through which a cooling liquid  404 , normally salt brine or water, flows to impinge on food products (not shown) to cool them down from a higher temperature to a lower one for further processing or storage. A main flow stream  406  is drawn from the sump  402  at the bottom of this chamber and provides a source of cooling liquid  404  which may flow through a pump  408  and a strainer  410  to filter out larger particles and a heat exchanger  412  which chills the liquid prior to discharge through the liquid outlets  414 . 
         [0068]    A side stream  416  is taken from the sump  402  through a pump  418  and strainer  420  to the electrolytic cell  422  where treatment occurs and is then discharged back to the sump  402 . Alternatively, this flow may be a side stream of the main flow stream  406  taken after strainer  410  eliminating the need for a second pump  418  and strainer  420  but removing the capability of operating these two liquid circuits independently. Makeup liquid  424  is added as required to keep the sump full. 
         [0069]    Referring now to  FIG. 5 , the diagram illustrates one configuration of a system to chill solid food products with a liquid, normally water, that is disinfected by the invention disclosed herein. A water tank  500  containing water  502  a conveying means  504  for moving products from one end to the other receives food products  506 , such as recently slaughtered and eviscerated poultry which are then conveyed through the water  502 . Chilled product is removed by unloading means  508 . Makeup water  510  replaces water lost due to carry-off on the product and additional flow may be provided to freshen the water, which then overflows to drain  512 . 
         [0070]    Temperature rises in the water  502  due to the heat removed from the food product. A pump  514  connected with the water tank  500  propels a stream of water  516  into a rechiller  518  which removes heat from the water stream exits back into the chiller tank  500 . Flow control valve  520  redirects some or all of the water stream  522  through the electrode cell  524  where electrolytic disinfection takes place. The side stream  526  exits the electrode cell  524  and is recombined with the main rechiller water stream  516  to go through the rechiller  518  and back to the water tank  500 . 
         [0071]    Referring now to  FIG. 6 , this block diagram represents a flume water system designed to wash food products such as vegetables and fruits. Product to be treated  600  enters the flume  602  where washing and conveying water  604  moves the product under the shower header  608  where shower water  606  is distributed. Washed product exits the flume  610  and enters a strainer  612 , oftentimes a shaking one, and the drained product  614  is transported for further processing or packaging. 
         [0072]    The drain water enters a fine strainer  616  where smaller solids and impurities are removed via outlet  618 . A pump  620  propels the drained water through a flow control valve  622  a rechiller  624  and then back into the flume  602  or shower header  608 . 
         [0073]    Flow control valve  622  redirects some or all of the strainer water  626  through the electrode cell  628  for disinfection with the discharge water  630  being blended back into the main flow. 
         [0074]    Referring now to  FIG. 7 , this graph shows the percentage removal  700  of a pharmaceutical, the estrogen 17-alpha-ethinylestradiol, using the disclosed electrochemical treatment process with the treatment fluid being final wastewater effluent with pharmaceutical and personal care product residuals at their normal levels for such liquids. The graph compares this with disinfection efficacy  702  achieved during each test number, with these tests being conducted at varying power levels and treatment times. The tests show the high pharmaceutical removal efficacy of the process even when operated to achieve relatively low disinfection levels. There is a strong correlation between contaminant destruction  700  and disinfection efficacy  702 . 
         [0075]    Referring now to  FIG. 8 , this graph shows the percentage removal  800  of a personal care product residual, the antibiotic triclosan, using the disclosed electrochemical treatment process with the treatment fluid being final wastewater effluent with pharmaceutical and personal care product residuals at their normal levels for such liquids. The graph compares this with disinfection efficacy  802  achieved during each test number, with these tests being conducted at varying power levels and treatment times. The tests show the high pharmaceutical removal efficacy of the process even when operated to achieve relatively low disinfection levels. There is a strong correlation between contaminant destruction  800  and disinfection efficacy  802 . 
         [0076]    Referring now to  FIGS. 7 and 8 , the present inventors have discovered that for the electrochemical method of this patent, that disinfection efficacy, a low cost, easily measured value, serves as a robust surrogate for the efficacy of removal of trace pharmaceuticals and personal care product residuals. In addition, the inventors have determined that for liquid streams without significant bacterial load, a safe, food grade bacteria like  Lactobacillus Acidophilus , used to make yoghurt, can be added to the liquid to provide this surrogate disinfection measure. This is an inexpensive alternative to the expensive, time-consuming, analysis required to measure trace pharmaceuticals and personal care product oxidation performance. 
         [0077]    Referring now to  FIG. 9 , in an alternate configuration electrode  28   a  may be a conductive tube or rod surrounded by a concentric conductive tube electrode  28   b  wherein an annular space is created for passage of the liquid being treated  34  and  35 . 
         [0078]    The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.