Abstract:
A liquid purification apparatus which uses ozone to purify water is disclosed. Off-gas from the purification process is passed through an ozone detector that provides an electric signal corresponding to the ozone concentration in the off-gas. The electric signal is monitored and used to control the length of the ozonation process. The detector comprises a MgO 2  and CuO based ozone destroying catalyst which generates an electric potential when used to decompose ozone into oxygen.

Description:
FIELD OF THE INVENTION 
     This invention relates to gas concentration sensors and more particularly to a sensor for measuring the concentration of a gas such as ozone in a gas stream. The invention also relates to water purification systems and more particularly to a method and apparatus for sensing the concentration of a reactable gas (such as ozone) in an off-gas during a water purification process. 
     BACKGROUND OF THE INVENTION 
     In many areas, a reactable gas is used as a processing agent to treat a liquid. Examples of this include water treatment to remove waste or to create potable water and chemical oxidation (i.e. bleaching) processes. 
     In such processes, it is important to ensure that treatment of the liquid with the reactable gas continues for a sufficient period that the desired treatment result is achieved. In water treatment applications, commonly used reactable gases include ozone and hydrogen peroxide. Ozone is used in many water treatment applications to remove impurities. It is important to ensure that ozonation of the water continues until the level of impurities has fallen to an acceptable level. One method of doing this is to fix the volume of water and then ozonate the water for a period that is known to be sufficient to reduce the impurity level, regardless of the initial concentration of impurities in the water. However, this method may waste ozone (if the initial level of impurities was relatively low) as well as requiring a fixed, and possibly lengthy, time for each ozonation process. It is preferable to use a system that monitors the impurity level and stops the ozonation process when the acceptable impurity level is achieved. 
     Accordingly, various different sensors have been developed to measure the level of ozone in water. Some of these sensors operate by passing ultraviolet light through a fluid stream and measuring the ultraviolet light received on a detector. Another type of gas detector is disclosed in U.S. Pat. No. 5,167,927 to Karlson. Karlson discloses a monitor which measures the heat energy released when a gas, e.g. ozone, is catalytically converted into a different compound, e.g. oxygen. A third type of sensor is disclosed in U.S. Pat. No. 5,427,693 to Mausgrover et al. Mausgrover incorporates a meter to measure the oxidation-reduction potential (ORP) of the water being cleaned. The ORP is then equated to an ozone concentration in the water. A fourth type of ozone sensor is disclosed in U.S. Pat. No. 5,683,576 to Olsen. In the system described by Olsen, an ozone containing gas is passed through contaminated water until the concentration of ozone in solution in the water reaches a pre-determined level. Ozonation then continues for a pre-determined period. The objective of this system is to ensure that a specified volume of water will be treated with a specified concentration of ozone for a specified period of time. 
     Although these systems may provide a reliable measure of the concentration of ozone in water, none of them provides an accurate measure of the degree to which impurities have been removed from the water. Continuing ozonation after the desired ozone concentration is reached for a pre-determined period ensures only that a minimum amount of ozone passes through the water over the entire treatment period. Olsen assumes that once the concentration of ozone reaches the predetermined level, it does not subsequently fall. Further, Olsen assumes that simply allowing a selected concentration of ozone to remain in the water for a selected time ensures that the water is suitable for use. However, this will not necessarily be true, especially in the case of highly contaminated water. For example, lake or well water will normally require more treatment than treated water from a municipal supply. 
     SUMMARY OF THE INVENTION 
     Accordingly, there is a need for a method and apparatus to accurately measure, on an ongoing basis, the degree to which impure water has been treated by ozonation. This may be done by measuring the amount of ozone that passes through water in a water purification chamber without being consumed. As the amount of unconsumed ozone exiting the chamber rises, the level of impurities is known to have fallen. When the amount of unconsumed ozone exiting the chamber becomes substantially constant, the water may be reliably considered to be substantially free of impurities that are susceptible to removal by the ozone. 
     In accordance with a first aspect of the present invention, there is provided a sensor for detecting the concentration of ozone in an incoming gas stream, said sensor comprising a sensing element positioned in the flow path of the incoming gas stream, said sensing element being electrically sensitive to the presence of ozone such that an electrical potential corresponding to said concentration is induced across said sensing element; and an electrical circuit coupled to said sensing element for allowing said electrical potential to be measured. 
     In accordance with a second aspect of the present invention, there is provided an apparatus for removing impurities from water, said apparatus comprising: a contact chamber for containing said water, said contact chamber having a head space for containing an off-gas and said contact chamber having an off-gas outlet for allowing said off-gas to exit said contact chamber in an off-gas stream; a closure for providing a substantially gas tight seal between the interior and exterior of said contact chamber; a reactable gas source for providing a reactable gas; a reactable gas control for controllably introducing said reactable gas into said contact chamber; a reactable gas sensor for providing a reactable gas concentration signal corresponding to the concentration of reactable gas in said off-gas stream at a signal node, said reactable gas sensor having an off-gas inlet and said off-gas inlet being in fluid communication with said off-gas outlet; and a controller, said controller being coupled to said signal node for receiving said reactable concentration signal and to said reactable gas control for controlling the introduction of said reactable gas into said contact chamber in response to said reactable gas concentration signal. 
     In accordance with a third aspect of the present invention, there is provided a method of removing impurities from an impure liquid, said method comprising the steps of: providing a quantity of said impure liquid in a contact chamber; providing a controller for controlling the flow of a treatment gas containing a reactable gas into said chamber; initiating the flow of said treatment gas into said contact chamber, wherein said reactable gas flows through said liquid and wherein at least some of said reactable gas reacts with impurities in said liquid consuming at least some of said reactable gas, the remainder of said treatment gas collecting in said chamber as an off-gas; withdrawing some of said off-gas; monitoring the concentration of said reactable gas in said off-gas; and terminating the flow of said treatment gas in response to the rate of change of said concentration of said reactable gas in said off-gas falling below a selected level. 
     In accordance with a fourth aspect of the invention, there is provided a method of removing impurities from an impure liquid, said method comprising the steps of: providing a quantity of said impure liquid in a contact chamber; providing a supply of a reactable gas; providing a controller for controlling the flow of said reactable gas into said contact chamber; providing a sensor for measuring the concentration of said reactable gas in an off-gas stream exiting said chamber and providing an electrical signal corresponding to said concentration; initiating the flow of said reactable gas into said chamber wherein said reactable gas flows through said liquid, and wherein at least some of said reactable gas reacts with impurities in said liquid consuming at least some of said reactable gas, the remainder of said reactable gas exiting said chamber in said off-gas stream; monitoring said signal until the rate of change of said signal falls to a selected level; terminating the flow of said reactable gas. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be explained by way of example only with reference to the drawings, in which: 
     FIG. 1 is a perspective cut-away drawing of a preferred embodiment of an ozone sensor according to the present invention; and 
     FIG. 2 is a schematic drawing of a preferred embodiment of a water purification apparatus incorporating the ozone sensor of FIG. 1; and 
     FIG. 3 is a graph of the output of the ozone sensor used in the water purification apparatus of FIG. 2 over time during a water purification cycle. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference is first made to FIG. 1, which shows a preferred embodiment of an ozone sensor  10  according to the present invention. Ozone sensor  10  comprises a housing  12  with a gas inlet  14  for receiving an incoming gas stream  34  and a gas outlet  16  for expelling an outgoing gas stream  36 . Gas inlet  14  and gas outlet  16  are located at longitudinally opposed ends of housing  12 . A sensing element  18  is positioned within housing  12 . 
     Sensing element  18  is sensitive to the concentration of ozone in the incoming gas stream  34 . An electrical potential is induced across sensing element  18  in the longitudinal direction (with respect to housing  12 ). Electrical lines  20  and  22  are coupled to the longitudinal ends of sensing element  18 . Resistor  24  and capacitor  26  are coupled between lines  20  and  22  outside of housing  12 . 
     Sensor  10  produces an electrical signal corresponding to the concentration of ozone in the incoming gas stream at terminals  28  and  30 . A voltmeter  32  coupled to nodes  28  and  30  will show the magnitude of this electrical signal. The magnitude of the electrical signal may be correlated to the concentration of ozone in incoming gas stream  34  through simple experimentation, which will be within the ability of one skilled in the art. To simplify use of voltmeter  32  for this purpose, the scale of voltmeter  32  may be replaced with a scale indicating ozone concentration, thereby producing an ozone concentration meter. 
     In use, inlet  14  will be coupled to a gas source (not shown) and outlet  16  may be coupled to a gas processor (not shown). In the preferred embodiment, sensing element  18  is an ozone destroying substance comprising magnesium dioxide (MgO 2 ) and copper oxide (CuO). Such a material is commercially available from the Carus Chemical Company, 315 Fifth Street, Peru, Ill., USA 61354 (Telephone: 1-800-435-6856) under the trade name CARULITE® 200. CARULITE 200 is a catalyst that decomposes ozone into oxygen through a catalytic reaction. CARULITE 200 is not consumed in this reaction. In the preferred embodiment, sensing element  18  is a pellet of CARULITE 200. CARULITE 200 is porous and the catalytic reaction takes place as an air stream containing ozone passes through the pellet. Preferably, sensing element  18  is sized such that the outer surface of sensing element  18  is substantially in contact with the inner surface of housing  12  such that air stream  36  flows substantially through sensing element  18 , rather around the outside of sensing element  18 . The inventors have found that an electric potential is created across the CARULITE 200 pellet as ozone is decomposed into oxygen. Lines  20  and  22  carry this potential to terminals  28  and  30 . Resistor  24  discharges the electrical potential across lines  20  and  22 . Resistor  24  is chosen to have a high resistance (1-10 MΩ, and preferably 5-6 MΩ) so that the potential discharges slowly enough to permit voltmeter  30  to display the potential. Capacitor  26  acts as a filter to smooth the electric potential. Although resistor  24  and capacitor  26  are not essential to the operation of ozone sensor  10 , their use is preferred to provide a smoother electrical signal which is responsive to changes in the ozone concentration in air stream  34  at terminals  28  and  30 . 
     When CARULITE 200 is used as sensing element  18 , the electrical signal produced by sensor  10  is a millivolt level signal. The inventors have found that this signal is essentially directly proportional to the concentration of ozone in the incoming gas stream. 
     In an alternate embodiment of ozone sensor  10 , resistor  24  and capacitor  26  may be integrated within housing  12  and voltmeter  32  may be integrated onto the exterior of housing  12 , providing a integral ozone sensor with a concentration meter. In another embodiment, terminals  28  and  30  may be left unconnected, providing an integral sensor which may be electrically coupled to a monitoring device, as is done in the water purification system described below with reference to FIG.  2 . 
     Reference is next made to FIG. 2, which is a schematic diagram of a water purification system  100  for purifying contaminated water by bubbling ozone through the water. 
     Water purification system  100  comprises a contact chamber  102  with a sealing lid  104 , water outlet valve  106 , water pump  108  carbon filter  110  and clean water receptacle  112 , oxygen source  114 , oxygen pump  116 , ozone generator  118 , sparger  120 , ozone sensor  10 , ozone destroyer  126 , controller  128  and a dispensing nozzle  162 . 
     Ozone sensor  10  is identical to the ozone sensor of FIG. 1, except that it is not connected to voltmeter  32 . Instead, nodes  30  and  38  of ozone sensor  10  are coupled to controller  128  by data line  130 . 
     Controller  128  monitors and controls the water purification process. Controller  128  is connected to oxygen pump  116  by control line  132 , to ozone generator  118  by control line  134 , to water outlet valve  106  by control line  136  and to water pump  108  by control line  138 . 
     Controller  128  is also coupled to a “Start Purification” button  142  by data line  143 , to a “Clean” indicator light  144  by control line  145  and to a “Unable to Clean” indicator light  146  by control line  147 . “Start Purification” button  142  may be a typical normally open pushbutton. Indicator lights  144  and  146  may be typical LEDs. In one embodiment, “Clean” indicator light  144  comprises a green LED and “Unable to Clean” indicator light  146  comprises a red LED. In another embodiment, both indicator lights  144 ,  146  are combined using a single combination red/green LED. Controller  128  is also coupled to a “Dispense Water” button  164  by control line  165 . 
     Prior to initiating the purification process, the user of water purification system  100  pours water  148  into contact chamber  102  from impure water source  104  (which may be a municipal water supply). Water  148  contains impurities which may be neutralized by exposing them to ozone. The interior of contact chamber  102  is marked with a maximum water level  150  to indicate the maximum amount of water that may be put into contact chamber  102 . Lid  104  fits onto contact chamber  102  to provide a gas-tight seal, providing a head space  152  between maximum water level mark  150  and lid  104 . Gas inlet  14  of ozone sensor  10  is in fluid communication with head space  152 . 
     Oxygen source  114  contains oxygen  154 . Oxygen source  114  may be ambient air, air or another gas enriched with oxygen or pure oxygen. 
     To initiate the purification process, lid  104  is installed onto contact chamber  102  and the user presses “Start Purification” button  142 . After “Start Purification” button  142  is pressed, controller  128  energizes oxygen pump  116  and ozone generator  118 , which converts some of oxygen  154  into ozone  156 . Ozone generator  118  will, in general, not convert all of the gas in oxygen source  114  into ozone  156  (even if oxygen source  114  is pure oxygen). The concentration of ozone in the output gas of ozone generator  118 , defined here as [O 3 ] gen-out , will depend on the concentration of oxygen  154  in oxygen source  114  and on the efficiency of ozone generator  118  in converting oxygen  154  into ozone  156 . [O 3 ] gen-out  may be calculated for a particular configuration of water purification system  100  (i.e. for a particular oxygen source  114  and a particular ozone generator  118 ). 
     Ozone  156 , along with any gases not converted by ozone generator  118 , is fed into sparger  120 , which is located inside contact chamber  102 . Sparger  120  disperses ozone  156  in finely separated bubbles  158  through water  148 . Some of ozone  156  will react with impurities in water  148  to neutralize the impurities, consuming the ozone. Unreacted ozone  156 , gaseous by-products of the reaction between ozone and the impurities and other gases not converted into ozone by ozone generator  118  will rise into head space  152  and collects as off-gas  160 . 
     As off-gas  160  builds up in head space  152 , some of off-gas  160  will be forced into ozone sensor  10 . Off-gas  160  passes through ozone sensor  10  (where some of the ozone in off-gas  160  is decomposed into oxygen, as described above) to ozone destroyer  126 , where the remainder of the ozone in off-gas  160  is destroyed. The resulting gas, which contains no or only a nominal amount of ozone is released into the ambient environment. 
     An electrical signal V sensor  corresponding to the concentration of ozone in off-gas  160 , defined here as [O 3 ] off-gas , is transmitted by sensor  10  to controller  128  across data line  130  during the entire purification process. Controller  128  monitors V Sensor  to control the purification process. 
     Reference is next made to FIG. 3, which is a graph of V sensor  over time during a typical water purification cycle. 
     The purification process is started at time T 1  by the user pressing “Start Purification” button  142 . Controller  142  records the value of V sensor  at time T 1 . This value is defined as V o  and represents the condition where [O 3 ] off-gas  is equal to zero (0). The inventors have found that V o  varies for different samples of sensing element  18  and can vary for the same sensing element  18  at different times. Accordingly, measuring V o  at the beginning of each purification cycle provides a self-calibration feature to ensure that the specific characteristics of the sensing element  18  do not affect the operation of water purification system  100 . Although the cause of the variability of V o  in different sensing elements  18  made of the same CARULITE 200 material is not fully understood, the inventors believe that this may relate to the sensitivity of the material to ambient temperature, variations in the manufacture of the material and possibly to residual electrical effects remaining from a previous operation of water purification system  100 . 
     At time T 2 , controller  128  energizes oxygen pump  116  and ozone generator  118 . As shown in FIG. 3, time T 2  may be a selected period A after time T 1 . Alternatively, time T 2  may occur immediately after V o  has been recorded. Initially, ambient air that was sealed into head space  152  when lid  104  was placed onto contact chamber  102  will be forced through ozone sensor  10 . Accordingly, [O 3 ] off-gas  and V sensor  will remain flat. At time T 3 , the majority of this ambient air has passed through ozone sensor  10 . 
     Starting at time T 3 , some of ozone  156  will begin to pass through ozone sensor  10 . Initially, a relatively large proportion of ozone  156  generated by ozone generator  118  will be consumed in removing impurities from water  148 . As a result, [O 3 ] off-gas  will be relatively low and V sensor  will correspondingly be relatively low. 
     After time T 3 , [O 3 ] off-gas  will rise as the number of impurities remaining in water  148  falls. Generally, after some time, V sensor  will exceed a selected voltage level V min , which corresponds to a selected minimum increase in off-gas ozone concentration level [O 3 ] off-gas(min) . This point is defined as time T 4 . When V sensor  exceeds V min , it is assumed that the water purification process has been successfully commenced. V min  is defined as a selected voltage V Δ  greater than V o . V Δ  is selected to ensure that a non-nominal change in [O 3 ] off-gas  must occur before it is assumed that the water purification process has been started properly. If V sensor  does not exceed V min , this may indicate that there is a problem with oxygen supply  114  (i.e. it does not contain oxygen  154 ), oxygen pump  116 , ozone generator  118  or with the tubing connecting these elements to one another or to sparger  120 . Controller  128  may record this information for use in maintaining or repairing water purification system  100 . 
     After time T 4 , controller  128  monitors V sensor  until the rate of change of V sensor  is approximately zero (i.e. the absolute value of the average of the derivative of V sensor  over a selected period is less than a selected voltage). This is defined as time T 5  and the value of V sensor  at time T 5  is defined as V C . When V sensor  levels off, as shown in FIG. 3 immediately prior to time T 5 , this indicates that [O 3 ] off-gas  has levelled off, indicating that almost no ozone is being consumed to remove impurities from water  148 . Accordingly, it is assumed that most impurities in water  148  susceptible to removal by ozonation have been removed or neutralized. The inventors have found that at time T 5 , [O 3 ] off-gas  is approximately equal to [O 3 ] gen-out . 
     Time T 6  is a selected time period B after time T 5 . Period B is chosen to ensure that sufficient ozonation of water  148  is performed to remove almost all remaining impurities in water  148  that are susceptible to removal by ozonation are removed without unduly extending the length of the water purification process. 
     At time T 6 , controller  128  de-energizes ozone generator  118 . The operation of oxygen pump  116  is continued. The result is that gas from oxygen source  114  is bubbled directly through water  148 , into head space  152  and into ozone sensor  10 . Ozone dissolved in water  148  will be removed by the gas from oxygen source  114  and [O 3 ] off-gas  will fall, as shown after time T 6 . 
     Time T 7  is a selected time period C after time T 6 . Period C is chosen to ensure that any ozone dissolved in water  148  prior to time T 6  is removed and that head space  152  is also free of ozone. 
     At time T 7 , controller  128  will de-energize oxygen pump  116 . Controller  128  will then open water outlet valve  106  and energize water pump  108  if clean water receptacle  112  has been positioned to receive water  148 . Water  148 , which is now relatively free of impurities subject to removal by ozonation, is pumped from contact chamber  102 , through carbon filter  110  (which may remove other impurities from water  148 ) and dispensing nozzle  162  into clean water receptacle  112 . One skilled in the art will be capable of configuring a detection device such as a microswitch to detect the presence of water receptacle  112 . If clean water receptacle  112  is not correctly positioned at time T 7 , water  148  remains in contact chamber  102 . 
     At this point, the water purification cycle is complete. Controller  128  will then energize “Clean” indicator  144  and the user of water purification system  100  may use the clean water from clean water receptacle  112 , if it was properly positioned at time T 7 . 
     If water receptacle  112  was not properly positioned at time T 7 , the clean water  148  may be dispensed through dispensing nozzle  162  by positioning a clean water receptacle  112  under dispensing nozzle  162  and then pressing “Dispense Water” button  164 . Water will only be dispensed by “Dispense Water” button  164  is held pressed. 
     If time T 5  does not occur for a selected period of time T max  (not shown) after the water purification cycle is initiated at time T 1  (i.e. V sensor  does not flatten out as shown in FIG.  3 ), then controller  128  will terminate the water purification cycle and energize “Unable to Clean” indicator  146 . The user may then attempt to clean water  148  again by pressing “Start Purification” pushbutton  142  or may discard and replace water  148  prior to commencing a new water purification cycle. 
     In some processes, such as the water purification process disclosed above, ozone is used for a specific purpose and then must be destroyed as is done using ozone destroyer  126 . As described above, CARULITE 200 is an ozone destroying substance. If CARULITE 200 or another ozone destroying substance with the same electrical properties as CARULITE 200 is used as the sensing element  18  of ozone sensor  10 , it may be possible to combine ozone sensor  20  and ozone destroyer  126  in water purification system  100  by positioning electrodes  20  and  22  on opposite longitudinal ends of the ozone destroyer. 
     Voltage and time values during a typical water purification cycle may have the following values: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Name 
                 Typical Value 
               
               
                   
                   
               
             
             
               
                   
                 V o   
                 −50 mV to 50 mV 
               
               
                   
                 V Δ   
                 10 mV 
               
               
                   
                 V c   
                 V o  + (70 to 100 mV) 
               
               
                   
                 A 
                 0 to 10 seconds 
               
               
                   
                 B 
                 60 seconds 
               
               
                   
                 C 
                 10 seconds 
               
               
                   
                   
               
             
          
         
       
     
     Although the present invention has been described with reference to removing impurities from water by ozonating the water, the invention is equally applicable to any purification process where a different reactable gas is used to clean impurities from a liquid and the active component in the reactable gas is consumed as impurities are removed. In particular, the present invention may be used to monitor the progress of a water purification process using hydrogen peroxide (H 2 O 2 ) as the reactable gas rather than ozone. It will be necessary to use a different sensing element  58  in this case, however, the structure and operation of the invention will remain the same in such an embodiment. Various other changes may be made to the invention without departing from its scope, which is limited only by the appended claims.