Method and Apparatus for the Electrochemical Treatment of Liquids Using Frequent Polarity Reversal
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.
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
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.
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.
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.
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'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.
Other chemicals such as hydrogen peroxide and peracetic acid are used as disinfectants but typically have high cost and other issues associated with them.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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).
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.
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.
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.
It is thus a feature of at least one embodiment of the invention to space the electrodes greater than 5 mm apart.
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.
It is thus a feature of at least one embodiment of this invention to treat flow rates of five gallons per minute and higher.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Methods for disinfecting example food processing liquid streams are disclosed.
It is thus a feature of at least one embodiment of this invention to disinfect food processing liquids.
A method for oxidizing and destroying trace pharmaceuticals and personal care products is disclosed.
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.
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.
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.
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.
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.
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.
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.
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.
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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.
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.
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 48a and 48b. 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.
Conductors 48a and 48b 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.
The throws of the switching unit 50 are controllable so that one conductor 48a or 48b may be connected to a given voltage (positive or negative) while the other conductor 48a or 48b is connected to the opposite voltage.
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.
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.
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.
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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.
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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.
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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.
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.
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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.
1. A liquid treatment system comprising:
- a treatment cell receiving a liquid to be treated at a rate of at least 5 gallons per minute;
- at least a first and a second electrode positioned within the treatment cell and having a separation of no less than 5 mm permitting the liquid with suspended solids or an organic load to pass therebetween wherein at least one of the electrodes has a surface selected from the group consisting of metals, metal oxides and doped diamond;
- a polarity reversing power supply connected across the at least two electrodes, the power supply providing an alternating positive and negative electrical power; and
- a power supply controller communicating with the polarity reversing power supply to provide a polarity reversal of the electrical power to the at least two electrodes during a cycle period having a length of at least 10 seconds and no more than 60 minutes.
2. The liquid treatment system of claim 1 wherein the electrodes are opposed substantially planar conductive plates.
3. The liquid treatment system of claim 1 wherein the at least two electrodes are a substantially concentric tube and center electrode.
4. The liquid treatment system of claim 1 wherein the at least two electrodes have a surface containing at least one platinum group metal consisting of platinum, palladium, rhodium, iridium, osmium, and ruthenium.
5. The liquid treatment system of claim 1 wherein one of the at least two electrodes has a different surface material composition than the other of the at least two electrodes.
6. The liquid treatment system of claim 1 wherein the cycle period provides different durations of negative and positive electrical power.
7. The liquid treatment system of claim 1 wherein the cycle period provides substantially equal durations of negative and positive power.
8. The liquid treatment system of claim 1 wherein the liquid being treated has a chemical oxygen demand of at least 200 mg/l.
9. The liquid treatment system of claim 1 wherein a flow splitting means delivers a side stream of a circulating liquid to the treatment cell, which side stream after treatment is then blended back into the circulating liquid after treatment.
10. A method of treating a liquid with suspended solids or organic load comprising the steps of:
- passing the liquid through a treatment cell containing at least a first and second electrode positioned within the treatment cell and having a separation of no less than 5 mm, wherein one or more of the electrodes has surfaces selected from the group consisting of metals, metal oxides and doped diamond; wherein said liquid flows through the treatment cell at a rate of at least 5 gallons per minute;
- applying power to the electrodes the power having an alternating negative and positive polarity during a cycle period having a length of at least 10 seconds and no more than 60 minutes.
11. The liquid treatment method of claim 10 wherein the at least first and second electrode have surfaces containing at least one platinum group metal consisting of platinum, palladium, rhodium, iridium, osmium, and ruthenium.
12. The liquid treatment method of claim 10 wherein at least one electrode has a different surface material composition than at least one other electrode.
13. The liquid treatment method of claim 10 wherein the cycle period provides different durations of negative and positive electrical power.
14. The liquid treatment method of claim 10 wherein the cycle period provides substantially equal durations of negative and positive power.
15. The liquid treatment method of claim 10 wherein the liquid being treated has a chemical oxygen demand of at least 200 mg/l.
16. The liquid treatment method of claim 10 wherein a side stream of a circulating liquid is treated and blended back into the circulating liquid.
17. The liquid treatment method of claim 10 wherein a portion of the treated liquid is circulated back into the liquid treatment system inlet.
18. The liquid treatment method of claim 10 wherein the treatment is disinfection of the liquid stream.
19. The liquid treatment method of claim 10 wherein the treatment is disinfection of materials selected from the group consisting of: food processing liquids, processed meat and poultry chiller brine or water, raw poultry processing chiller water, and fruit and vegetable processing flume water.
20. The liquid treatment method of claim 10 wherein the treatment oxidizes organic and inorganic contaminants of the liquid stream being treated.
21. The liquid treatment method of claim 20 wherein the treatment is a destruction of pharmaceutical and personal care product residuals.
22. The liquid treatment method of claim 10 including the step of monitoring a bacteria in the liquid and wherein a disinfection efficacy of bacteria in the liquid is used as a proxy for the efficacy of oxidation of trace residual chemicals.
23. The liquid treatment method of claim 22 including the steps of dosing the liquid with a known concentration of bacteria and testing results of killing of the bacteria to estimate the efficacy of oxidation of trace residual chemicals.
24. The liquid treatment method of claim 10 wherein the treatment is a generation of hydrogen and oxygen and separated by an external means.
Filed: Sep 30, 2010
Publication Date: Apr 7, 2011
Inventors: James A. Tretheway (Madison, WI), Brian R. Hale (Lake Mills, WI), Ajit K. Chowdhury (Madison, WI), Jeremy J. Vogel (Fort Atkinson, WI), Dennis E. Bahr (Madison, WI), Myron F. Miller (Sacramento, CA), Karl W. Marschke (Madison, WI)
Application Number: 12/895,197
International Classification: C02F 1/461 (20060101); C25B 15/00 (20060101);