Abstract:
A silver recovery and monitoring system including a sensor for transmitting a silver concentration signal in response to the concentration of silver in a fluid, means for determining whether the silver concentration signal indicates the concentration of silver in the fluid to be above a predetermined silver level and for initiating a silver alert signal in response to a determination that the silver concentration signal indicates the concentration of silver in the fluid to be above the predetermined silver level, and means for transmitting a notification signal to a silver monitoring station in response to the silver alert signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a nonprovisional U.S. patent application that relates to and claims the benefit of the filing date of U.S. Provisional Patent Application Ser. No. 60/298,648, filed on Jun. 15, 2001. 
    
    
     REFERENCE TO COMPUTER PROGRAM LISTING APPENDIX 
     The source code of an embodiment of the software described in this patent application is provided in a computer program listing appendix stored as a text file on a recordable compact disc (CD-R) filed with this application. The file is named TFXCODE.TXT and is incorporated herein by reference. The file has a date of creation of Jun. 12, 2002 and a size of 53 kilobytes. 
     BACKGROUND OF THE INVENTION 
     Silver is contained in a fluid waste product generated during traditional photographic film development processes. Such processes are currently used in a variety of industries, including but not limited to the health care industry, as in the development of radiological film, the print media industry, as in the development of photographs on film to be printed in publications such as newspapers, and in the commercial development of photographs taken on film by the general public. Regulations promulgated and enforced by the U.S. Environmental Protection Agency require that the concentration of silver in fluids drained into the environment as waste be limited to 5 parts per million. However, industry routinely fails to comply with this regulation, frequently draining fluids into the environment that contain hundreds and even thousands of parts per million of silver. 
     This compliance failure is typically caused by use of primitive silver recovery and monitoring systems. One such primitive system is a bucket of steel wool in which raw photographic fixer fluid containing silver is collected and filtered before being drained into the environment. In the ideal case, in such systems the iron atoms in the steel wool react with the silver ions in the fixer fluid to replace the steel wool with solids of silver and silver compounds, causing the resulting iron ions to flow out of the system into the environment with fluid containing no more than 5 parts per million of silver. However, in practice, such systems rarely if ever result in compliant drain fluid because the actual silver concentration in the fluid drained into the environment is never monitored, and the steel wool is spent very quickly, thereby causing unfiltered, high-silver-concentration, noncompliant fixer fluid to be drained into the environment. 
     A system and method for silver recovery and monitoring is disclosed below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a system according to an embodiment of the invention; 
         FIG. 2  is a perspective view of the interior of a unit box of the system illustrated in  FIG. 1 ; 
         FIG. 3  is a perspective view of a sensor block housed in the unit box illustrated in  FIG. 2 ; 
         FIG. 3A  is a plan view of the top surface of the sensor block illustrated in  FIG. 3 , with the sensors removed; 
         FIG. 3B  is a plan view of the bottom surface of the sensor block illustrated in  FIG. 3 , with the sensors removed; 
         FIG. 3C  is an elevational view of the right-hand surface of the sensor block illustrated in  FIG. 3 , with the taps and hoses removed; 
         FIG. 3D  is an elevational view of the left-hand surface of the sensor block illustrated in  FIG. 3 , with the taps and hoses removed; 
         FIG. 3E  is an elevational view of the front surface of the sensor block illustrated in  FIG. 3 ; 
         FIG. 3F  is an elevational view of the rear surface of the sensor block illustrated in  FIG. 3 ; 
         FIG. 4  is a cross-sectional view of the sensor block illustrated in  FIG. 3 , taken through line  4 — 4  in  FIGS. 3C and 3D ; 
         FIG. 5  is a cross-sectional view of the sensor block illustrated in  FIG. 3 , taken through line  5 — 5  in  FIGS. 3C and 3D ; 
         FIG. 6  is a cross-sectional view of the sensor block illustrated in  FIG. 3 , taken through line  6 — 6  in  FIGS. 3C and 3D ; 
         FIG. 7  is a fragmentary perspective view of the receptacle illustrated in  FIG. 1 , with the interior of the receptacle shown to illustrate a fluid level sensor; 
         FIG. 8  is an elevational view of the front panel of the unit box illustrated in  FIG. 1 ; 
         FIG. 9  is an elevational view of the right wall of the unit box illustrated in  FIG. 1 ; 
         FIG. 10  is an elevational view of the left wall of the unit box illustrated in  FIG. 1 ; 
         FIG. 11  is a perspective view of a leak detector printed circuit board housed in the unit box illustrated in  FIG. 2 ; 
         FIG. 12  is a cutaway perspective view of one of the filter canisters illustrated in  FIG. 1 ; 
         FIG. 13  is a perspective view of iron wool fibers of an iron fragment mix contained in the filter canister illustrated in  FIG. 12 ; 
         FIG. 14  is a perspective view of one of the corrugated iron strips of the iron fragment mix contained in the filter canister illustrated in  FIG. 12 ; 
         FIG. 15  is a block diagram generally illustrating the flow of electrical signals among various elements of the system; 
         FIGS. 15A–15T  are interrelated portions of a schematic diagram of the circuitry on a main printed circuit board housed in the unit box illustrated in  FIG. 2 ; 
         FIG. 16  is a flow chart generally illustrating the flow of fluid among various elements of the system; and 
         FIGS. 17 ,  17 A,  17 B,  17 C,  17 D,  17 E,  17 F,  17 G, and  17 H are flow charts generally illustrating an embodiment of a silver monitoring method executed by software programmed into a microprocessor utilized in an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, an embodiment of a silver recovery and monitoring system in accordance with the invention is shown broadly at reference numeral  10  in  FIG. 1 . The system  10  includes a receptacle  11  in which silver-laden raw fluid (not shown) is collected, three filter canisters  12 A,  12 B,  12 C and a unit box  13 . A fluid level sensor  17  ( FIG. 7 ) is mounted through a lid  11 A of the receptacle  11 . The receptacle lid  11 A also carries a main pump outlet hose  18 , an auxiliary pump outlet hose  19 , and a drain pipe  26 . As shown in  FIG. 2 , the unit box  13  contains a main fluid pump  14 , an auxiliary fluid pump  15 , a sensor block  16 , a main printed circuit board  20  (“the main PCB”) with multiple components mounted thereon, and a leak detector printed circuit board  21  (“the leak detector PCB”) oriented beneath the sensor block  16  and wired to the main PCB  20 . As shown in  FIG. 3 , four double-junction, silver- selective probes  22  are mounted on a top surface  23  of the sensor block  16 . Four corresponding silver probes  24  are mounted on a bottom surface  25  of the sensor block  16  in registration with the double-junction probes  22 . 
     As shown in  FIG. 1 , the fluid connections among the elements of the system  10 , including the connections between the receptacle  11 , the pumps  14 ,  15 , the sensor block  16 , and the filter canisters  12 A,  12 B,  12 C are achieved with hoses formed of flexible plastic tubing  29 . In particular,  FIG. 1  illustrates the main pump outlet hose  18 , the auxiliary pump outlet hose  19 , six hoses  27  connecting the filter canisters  12 A,  12 B,  12 C to the sensor block  16  ( FIG. 2 ) in the unit box  13 , and a drain hose  28  that connects the sensor block  16  ( FIG. 2 ) to the drain (not shown). 
     Looking at the basic elements of the system  10  shown in  FIGS. 1 ,  2 , and  3 , the pumps  14 ,  15  are model 16200-011×-112 T-011 pumps manufactured by Gorman-Rupp Industries, 180 Hines Avenue, Bellville, OH 44813 (www.gripumps.com). The double-junction probes  22  are model 5731403CT-X18T probes manufactured by Phoenix Electrode, 6103 Glenmont, Houston, Tex. 77081 (www.phoenixelectrode.com). The corresponding silver probes  24  are model 6520141-X18T probes, also manufactured by Phoenix Electrode. The components mounted on the main PCB  20  are readily available from a wide range of electronics manufacturers and suppliers, except as otherwise discussed below. The filter canisters  12 A,  12 B,  12 C, the unit box  13 , the sensor block  16 , the fluid level sensor  17 , the main PCB  20 , the plastic tubing  29  and the leak detector PCB  21  are proprietary elements of the illustrated embodiment of the invention and are described below. 
     Turning now to  FIG. 3 , a perspective view of the sensor block  16  of the system  10  is shown. The sensor block  16  is a substantially rectangular prism attached to an interior surface of a left-hand wall  46  of the unit box  13  ( FIG. 2 ) and is manufactured out of white acetal copolymer plastic or similar plastic. A plurality of passages ( FIGS. 4 ,  5 ,  6 ) are drilled into the interior of the sensor block  16  to accommodate the necessary fluid flow within the sensor block  16 . These passages are accessed by a plurality of ports drilled into four external surfaces  23 ,  25 ,  64 ,  70  of the sensor block  16 . Referring now to  FIG. 3A , a top surface  23  of the sensor block  16  defines four ports  50  for receiving the four double-junction probes  22  ( FIG. 3 ). As shown in  FIG. 3B , a bottom surface  25  of the sensor block defines four ports  51  for receiving the four silver probes  24  ( FIG. 3 ). An externally threaded plastic fitting  53  ( FIG. 3 ) is inserted into each of the top surface ports  50  ( FIG. 3A ) and the bottom surface ports  51  ( FIG. 3B ) of the sensor block  16  and are sealed to the respective top and bottom surfaces  23 ,  25  of the sensor block  16 . The fittings  53  are each provided with a mating internally threaded and gasketed nut  54 . The double-junction probes  22  and the silver probes  24  are installed into the sensor block  16  by being inserted through the fittings  53  carried by the top surface ports  50  and the bottom surface ports  51 , respectively, of the sensor block  16 . The nuts  54  are then tightened into place on the fittings  53  surrounding the probes  22 ,  24 . 
     The top surface ports  50  are aligned in registration with the bottom surface ports  51  such that each of the four double-junction probes  22  is coaxial with one of the four silver probes  24 . This orientation of the double-junction probes  22  relative to the silver probes  24  enables the double-junction probes  22  and the silver probes  24  to contact substantially the same portion of fluid substantially simultaneously within the sensor block  16 , as is necessary to obtain accurate signals transmitted by the double-junction probes  22  and the silver probes  24  to the main PCB  20  ( FIG. 2 ). In this way, the four double-junction probes  22  and the four silver probes  24  act as four probe pairs, with each probe pair acting as a silver sensor. Therefore, the illustrated embodiment includes four silver sensors, a first silver sensor  60 , a second silver sensor  61 , a third silver sensor  62 , and a fourth silver sensor  63  ( FIG. 3 ). As discussed below, the invention encompasses embodiments that include as few as one silver sensor. 
     Turning now to  FIG. 3C , the right-hand surface  64  of the sensor block  16  defines three ports: outlet ports  65 ,  66  and an inlet port  67 . As shown in  FIG. 3D , the left-hand surface  70  of the sensor block  16  defines nine ports: outlet ports  71 ,  72 ,  73 ,  74 , and inlet ports  80 ,  81 ,  82 ,  83 ,  84 . A hose-barb tap  85  ( FIG. 3 ) is inserted and sealed into each of the right-hand surface and left-hand surface ports  65 – 67 ,  71 – 74 ,  80 – 84 . The taps  85  are configured such that the flexible plastic tubing  29  ( FIG. 3 ) may be slid over each tap  85  and clamped thereto to direct the fluid flowing into and out of the sensor block  16 . When the sensor block  16  is properly installed into the unit box  13  ( FIG. 2 ), the taps  85  in the nine left-hand surface ports  71 – 74 ,  80 – 84  of the sensor block  16  each extend through one of nine openings in the left-hand wall  46  ( FIG. 10 ) of the unit box  13 . 
     As shown in  FIGS. 3E and 6 , the front surface  90  of the sensor block  16  defines two ports  91 ,  92 . These ports  91 ,  92  are drilled into the sensor block  16  to help define the passages within the sensor block  16 , as further described below, and are plugged and sealed with plastic rods  93 ,  94  in order to prevent undesired fluid leakage from the passages in the sensor block  16 . Similarly, as shown in  FIGS. 3F and 6 , the rear surface  95  of the sensor block  16  defines one port  96  that is also drilled into the sensor block  16  to help define the passages within the sensor block  16 , but is also plugged and sealed with a plastic rod  100  to prevent leakage. Finally, as shown in  FIGS. 3B and 5 , the bottom surface  25  of the sensor block  16  defines one port  101  that is drilled into the sensor block to help define the passages within the sensor block  16  but is plugged and sealed with a plastic rod  102  to prevent leakage. 
     The passages through the interior of the sensor block  16  are illustrated in  FIGS. 4 ,  5 , and  6 , and are configured as follows. As shown in  FIG. 4 , the inlet port  80  defined by the left-hand surface  70  and the outlet port  65  defined by the right-hand surface  64  are connected by a first passage  103  through the sensor block  16 . The inlet port  81  defined by the left-hand surface  70  and the outlet port  66  defined by the right-hand surface  64  are connected by a second passage  104  through the sensor block  16 . Referring now to  FIG. 5 , the inlet port  67  defined by the right-hand surface  64  and the outlet port  71  defined by the left-hand surface  70  are connected by a third passage  105  through the sensor block  16  that contains the first silver sensor  60 . Portions of the third passage  105  are also shown in  FIGS. 4 and 6 . As shown in  FIG. 6 , the remaining passages through the sensor block  16  connect pairs of ports defined by the left-hand surface  70  of the sensor block  16 , and each such passage contains a silver sensor. Specifically, the inlet port  82  and the outlet port  72  defined by the left-hand surface  70  are connected by a fourth passage  106  through the sensor block  16  that contains the second silver sensor  61 . The inlet port  83  and the outlet port  73  defined by the left-hand surface  70  are connected by a fifth passage  110  through the sensor block  16  that contains the third silver sensor  62 . The inlet port  84  and the outlet port  74  defined by the left-hand surface  70  are connected by a sixth passage  111  through the sensor block  16  that contains the fourth silver sensor  63 . 
     Turning now to  FIG. 7 , the fluid level sensor  17  is shown in the receptacle  11 . The fluid level sensor  17  is comprised of seven electrically conductive metal rods  30 - 36 , including a ground rod  30 , a main pump shutoff rod  31 , an auxiliary pump shutoff rod  32 , a main pump activation rod  33 , an auxiliary pump activation rod  34 , an overflow indicator shutoff rod  35 , and an overflow indicator activation rod  36 . The rods  30 – 36  are comprised of 316 stainless steel and are mounted to a connector  40  which interfaces the rods  30 – 36  with a seven-conductor cable  41 . The interior of the connector  40  is protected from the raw fluid in the receptacle  11  by a hard plastic shield  42  molded and adhered to the connector  40 . 
     Each of the rods  30 – 36  includes an exposed, fluid-contacting portion  43 . The remaining portion of each rod  30 – 36  is covered by an electrically insulating plastic sheath  44 . The fluid-contacting portion  43  of each rod  30 – 36  is a unique distance from the connector  40  such that no two fluid-contacting portions  43  of the rods  30 – 36  are aligned. As the level of raw fluid in the receptacle  11  rises, the raw fluid comes into contact with the successively higher fluid-contacting portions  43  of the rods  30 – 36 . The first fluid-contacting portion  43  contacted by the raw fluid in the receptacle  11  is the fluid contacting portion  43  of the ground rod  30 . As the raw fluid continues to rise in the receptacle  11  and contacts the successively higher fluid- contacting portions  43  of the rods  30 – 36 , the electrically conductive nature of the raw fluid completes an electrical circuit by electrically coupling the ground rod  30  to the successive rods  31 – 36  having fluid-contacting portions  43  above the fluid-contacting portion  43  of the ground rod  30 . As the raw fluid contacts the fluid-contacting portions  43  of the rods  30 – 36 , electrical pump control signals are transmitted to the main PCB  20  ( FIG. 2 ), thereby activating and deactivating the pumps  14 ,  15  and an overflow indicator  45  ( FIG. 8 ) as follows. 
     When the fluid level in the receptacle  11  rises to contact the main pump activation rod  33 , the fluid level sensor  17  transmits an electrical pump control signal to the main PCB  20  ( FIG. 2 ), which in turn activates the main pump  14  ( FIG. 2 ). If the fluid level in the receptacle  11  continues to rise despite the continued operation of the main pump  14 , and the fluid level rises to contact the auxiliary pump activation rod  34 , the fluid level sensor  17  transmits an electrical pump control signal to the main PCB  20 , which in turn activates the auxiliary pump  15  ( FIG. 2 ). If the fluid level in the receptacle  11  still continues to rise despite the continued operation of both the main pump  14  and the auxiliary pump  15 , and the fluid level rises to contact the overflow indicator activation rod  36 , the fluid level sensor  17  transmits an electrical pump control signal to the main PCB  20 , which in turn illuminates the overflow indicator  45  ( FIG. 8 ) and informs a remote monitoring station  210  ( FIG. 15 ) of the overflow. 
     When the fluid level in the receptacle  11  falls below the overflow indicator shutoff rod  35  after the fluid has contacted the overflow indicator activation rod  36 , the main PCB deactivates the overflow indicator  45 . Similarly, when the fluid level in the receptacle  11  falls below the auxiliary pump shutoff rod  33  after the fluid has contacted the auxiliary pump activation rod  34 , the main PCB deactivates the auxiliary pump  15 . Finally, when the fluid level in the receptacle  11  falls below the the main pump shutoff rod  31  after the fluid has contacted the main pump activation rod  32 , the main PCB deactivates the main pump  14 . In each of these instances, the particular element (overflow indicator, auxiliary pump, main pump) begins to operate when the respective activation rod has been contacted by the fluid and continues to operate until the fluid falls below the respective shutoff rod. 
     Referring now to  FIG. 8 , an exterior surface  112  of a front panel  113  of the unit box  13  is shown. The front panel  113  of the unit box  13  is removably attached to the unitary remainder of the unit box  13  with fasteners to permit access to the interior of the unit box  13  for assembly and maintenance purposes. Eleven light-emitting diodes (“LEDs”) are visible through the exterior surface  113  of the front panel  112  of the unit box  13 . These LEDs are controlled by the main PCB  20  ( FIG. 2 ), as discussed below, through a front panel LED printed circuit board (“the front panel LED PCB”) (not shown). The front panel LED PCB is mounted on the interior surface (not shown) of the front panel  113  of the unit box  13  and is electrically connected to the main PCB  20  with a ribbon cable (not shown) and connector (not shown). The front panel LED PCB merely contains electrical traces necessary to carry LED activation and deactivation signals from the main PCB  20  to the LEDs themselves, which are each mounted on a pair of leads (not shown) such that the LEDs are visible through openings in the front panel  112  of the unit box  13 . 
     The LEDs on the front panel  112  notify a user of the status of the system  10  ( FIG. 1 ). In particular, “monitoring” LEDs  114 ,  115 ,  116  are provided for each of the three filter canisters  12 A,  12 B,  12 C ( FIG. 1 ), respectively, to indicate when the silver concentration in the fluid leaving each of the filter canisters  12 A,  12 B,  12 C is being monitored by the system  10 . In addition, LEDs are provided for indicating when filter canisters  12 A,  12 B,  12 C reach maximum capacity. For filter canisters  12 A and  12 B, these LEDs are designated “max capacity” LEDs  120 ,  121 , respectively, on the exterior surface  112  of the front panel  113  of the unit box  13 . The maximum capacity LED for the filter canister  12 C is designated a “check drain” LED  122 , as the drain must be checked when the filter canister  12 C reaches maximum capacity in order to prevent fluid with an unacceptably high silver concentration from draining out of the system  10 . When the modem  204  ( FIGS. 15 ,  150 ; discussed below), the main pump  14  ( FIG. 2 ), or the auxiliary pump  15  ( FIG. 2 ) is operating, the respective “modem” LED  123 , “main pump” LED  124 , or “aux pump” LED  125  will illuminate. The remaining LEDs on the front panel  112  of the unit box  13  are the overflow indicator  45 , which is an LED designated with the words “check system,” and a “power on” LED  126 , which remains illuminated while the system  10  is receiving power. 
     Turning now to  FIG. 9 , an exterior surface  130  of a right-hand wall  131  of the unit box  13  is shown. A pump power switch  132  (designated “pump power” with “on” and “off” positions marked), an AC power cord  133  for powering the system, a seven-conductor fluid level sensor input jack  134  designated “level sensor” for receiving a seven-pin connector carrying the electrical signals from the various rods of the fluid level sensor ( FIG. 7 ), and a quarter-inch phone plug jack  135  through which a microprocessor  190  ( FIGS. 15 ,  15 Q–R, discussed below) may be externally programmed and controlled are mounted on the exterior surface  130  of the right-hand wall  131  of the unit box  13 . 
       FIG. 10  illustrates an exterior surface  140  of the left-hand wall  46  of the unit box  13 . As discussed above, the sensor block  16  is attached to the interior surface of the left- hand wall  46  of the unit box  13  such that the hose-barb taps  85  protruding from the left-hand surface ports  71 – 74 ,  80 – 84  of the sensor block  16  extend through openings in the left-hand wall  46  of the unit box  13 . As shown in  FIG. 10 , the taps  85  extending from outlet ports  71 ,  72 ,  73 ,  74  are respectively designated on the exterior surface  140  of the left-hand wall  46  of the unit box  13  as “To “A”,” “To “B”,” “To “C”,” and “Drain.” Similarly, the taps  85  extending from inlet ports  80 ,  81 ,  82 ,  83 ,  84  are respectively designated on the exterior surface  140  of the left-hand wall  46  of the unit box  13  as “Aux Pump,” “Main Pump,” “From “A”,” “From “B”,” and “From “C”.” The meanings of these designations are clarified in the discussion of the fluid flow in the system, as discussed below. 
     A rear wall (not shown) of the unit box defines an opening through which a telephone cable (not shown) extends to be connected to a telephone jack (not shown) for facilitating telecommunication between the modem  204  ( FIGS. 15 ,  150 ) and the remote monitoring station  210  ( FIG. 15 ). 
       FIG. 11  illustrates the leak detector PCB  21 , which, as shown in  FIG. 2 , is loosely oriented on the floor of the interior of the unit box  13  below the sensor block  16  ( FIG. 2 ). Two sets of electrically conductive traces  141 ,  142  are provided on the leak detector PCB  21 , with the traces of each set  141 ,  142  alternating along the surface of the leak detector PCB  21 . Wires  143 ,  144  are respectively connected to the trace sets  141 ,  142  and to the main PCB  20  ( FIG. 2 ). One of the wires  143  consistently carries an electrical signal from the main PCB  20  to the leak detector PCB  21 . If electrically conductive fluid leaks from the sensor block  16  or the pumps  14 ,  15  ( FIG. 2 ) and contacts the leak detector PCB  21 , such fluid will electrically connect the two trace sets  141 ,  142  on the leak detector PCB  21 , causing the leak detector PCB  21  to pass an electrical signal along the other of the wires  144  back to the main PCB  20 . As discussed below, the main PCB  20  will then automatically shut down the system until it is manually reset, thereby giving a user an opportunity to troubleshoot the leak before further fluid is pumped. A second, similar leak detector PCB (not shown) may be included in the system to detect leaks from the filter canisters  12 A,  12 B,  12 C ( FIG. 1 ). 
     The three filter canisters  12 A,  12 B,  12 C ( FIG. 1 ) are substantially identically structured and operate in a substantially identical manner. The following description therefore applies to all three filter canisters  12 A,  12 B,  12 C. Referring now to  FIG. 12 , a cutaway view of one of the filter canisters  12 A,  12 B,  12 C is shown generally at reference numeral  145 . The canister  145  includes an elongate tube  150  with a top end  151  covered by a top end cap  152  and a bottom end  153  covered by a bottom end cap  154 . The top end cap  152  defines a substantially centrally disposed fluid inlet  155  and a fluid outlet  156 . The fluid inlet and outlet  155 ,  156  are each sealingly provided with fittings  160  on the top end cap  152  for receiving hoses ( FIG. 1 ) carrying fluid. In the interior of the tube  150 , a substantially centrally disposed downspout  161  is sealingly attached to the fluid inlet  155 . The downspout  161  is carried through substantially centrally disposed openings  162 ,  163  in a citric acid toroid  164  and an upper settling chamber  165  adjacent to the top end  151  of the tube  150 . The downspout  161  then extends along the length of the tube  150  until the downspout is adjacent to the bottom end  153  of the tube  150 , where the downspout  161  is carried through a substantially centrally disposed opening  166  in a lower settling chamber  170  before terminating near the bottom end  153  of the tube  150 . 
     A loose mix of iron fragments  171  is contained in the tube  150  between the upper and lower settling chambers  165 ,  170 . The iron fragment mix  171  comprises two types of iron fragments, which are shown in  FIGS. 13 and 14 . As shown in  FIG. 13 , fibers of iron wool  172  are one type of iron fragment in the iron fragment mix  171  ( FIG. 12 ).  FIG. 14  illustrates a corrugated iron strip  173 , the other type of iron fragment in the iron fragment mix  171 . The iron wool fibers  172  and the corrugated iron strips  173  are mixed according to a predetermined ratio to form the iron fragment mix  171  used in the tube  150 . Specifically, the iron fragment mix  171  is comprised of a 2:3 ratio by volume of iron wool fibers  172  to corrugated iron strips  173 . 
     Referring again to  FIG. 12 , the filter canister  145  functions as follows. Fluid (not shown) enters the fluid inlet  155  at the top end  151  of the tube  150  and flows down through the downspout  161  and into the lower settling chamber  170 . The lower settling chamber  170  is provided with a plurality of openings  174  that facilitate fluid flow at the bottom end  153  of the tube. However, the openings  174  in the lower settling chamber  170  are sufficiently small to prevent the coarse iron fragment mix  171  from impinging upon fluid flow within the lower settling chamber  170 . As fluid flow at the bottom end  153  of the tube increases, the fluid begins to flow up through the iron fragment mix  171  toward the top end  151  of the tube  150 . 
     When the fluid contacts the iron fragment mix  171 , a chemical reaction occurs between silver ions in the fluid and iron atoms in the iron fragment mix  171 . Specifically, as is known to those of ordinary skill in the art, the iron atoms “donate” electrons to the silver ions to cause the silver ions to become complete silver atoms. The silver atoms then form solids of silver and silver compounds such as silver sulfides and silver oxide. The iron ions resulting from the reaction are then entrained in the fluid passing through the iron fragment mix  171  and silver solids have replaced the iron fragment mix. The silver is thereby removed or “filtered” from the fluid passing through the tube  150 , leaving silver solids in the tube  150 . 
     The filtered fluid flows toward the top end  151  of the tube  150 , where it flows into the upper settling chamber  165 , which substantially separates the iron fragment mix  171  (and the silver solids resulting from the reaction) from the filtered fluid, thereby facilitating the flow of filtered fluid at the top end  151  of the tube  150  in a manner similar to that described above relative to the lower settling chamber  170  at the bottom end  153  of the tube  150 . The fluid then flows over the citric acid toroid  164 , which reacidifies the fluid to prevent buildup of iron hydroxide and other residues in the system. The manufacture and operation of the citric acid toroid  164  is described in greater detail in commonly-owned U.S. Provisional Patent Application Ser. No. 60/375,142, filed Apr. 24, 2002 and incorporated herein by reference. After flowing over the citric acid toroid  164 , the filtered fluid exits the canister  145  through the fluid outlet  156 . 
       FIG. 15  is a block diagram illustrating the flow of electrical signals through the system  10  ( FIG. 1 ), including a general depiction of the various components on the main PCB  20  ( FIG. 1 ) and the electrical components of the system not included on the main PCB  20 . Detailed schematic diagrams of the main PCB  20  are included as  FIGS. 15A–15T . The reference numerals used to describe the elements of the main PCB  20  shown in the block diagram in  FIG. 15  are also shown on the schematic diagrams in  FIGS. 15A–15T . However, on the schematic diagrams in  FIGS. 15A–15T , the reference numerals usually refer to collections of components enclosed by dotted lines rather than to individual components. 
     Referring now to  FIGS. 15 ,  15 C,  15 G,  15 J, and  15 N, when the main PCB  20  receives signals from the fluid level sensor  17  that one of the activation rods (main pump, auxiliary pump, overflow indicator) of the fluid level sensor  17  has been contacted by the fluid in the receptacle  11 , a latch circuit activates the respective element (main pump, auxiliary pump, overflow indicator) and illuminates the respective LED on the front panel of the unit box. The latch circuits on the main PCB comprise a main pump latch circuit  180  ( FIG. 15C ), an auxiliary pump latch circuit  181  ( FIG. 15G ), and an overflow indicator latch circuit  182  ( FIG. 15J ), and the respective LEDs ( FIG. 15N ) comprise a main pump LED  124 , an auxiliary pump LED  125 , and a “check system” LED  45  (overflow indicator). When the fluid in the receptacle  11  falls below a shutoff rod (main pump, auxiliary pump, overflow indicator) of the fluid level sensor  17  after having contacted the respective activation rod of the shutoff rod, the respective latch circuit  180 ,  181 ,  182  on the main PCB deactivates the element and LED associated with the shutoff rod. Therefore, the latch circuits  180 ,  181 ,  182  activate their respective elements and LEDs (main pump, auxiliary pump, overflow indicator) when the respective activation rods are contacted by fluid in the receptacle and the latch circuits  180 ,  181 ,  182  do not deactivate those elements and LEDs until the fluid falls below the respective shutoff rod. 
     The silver sensors  60 ,  61 ,  62 ,  63  take readings of the silver concentration of the fluid passing through the system when a microprocessor  190  on the main PCB  20  instructs the silver sensors  60 – 63  to do so in accordance with the programming of the microprocessor  190 . The microprocessor  190  used in an embodiment of the invention is a Model TFX-11 manufactured by Onset Computer Corporation, P.O. Box 3450, Pocasset, MA 02559-3450 (www.onsetcomp.com). The source code of an embodiment of the software for programming the microprocessor  190  is provided in a computer program listing appendix filed with this application on recordable compact disc (CD-R). A flow chart illustrating a silver monitoring method according to an embodiment of the invention, as implemented by an embodiment of the software of the invention, is shown in  FIGS. 17–17H  and discussed below. 
     The silver sensors  60 – 63  are each controlled by the microprocessor  190  via an optical switch  191 ,  192 ,  193 ,  194  ( FIGS. 15A ,  15 E). When they are activated by the optical switches  191 – 194 , each of the silver sensors  60 – 63  transmit a silver concentration signal, the voltage of which varies inversely with the silver concentration of the fluid. These silver concentration signals are transmitted by the silver sensors  60 – 63  to the main PCB  20  and are amplified by respective amplifier circuits  200 ,  201 ,  202 ,  203  ( FIGS. 15B ,  15 F,  151 ) on the main PCB  20  before being transmitted to the microprocessor  190 . The microprocessor  190  has been programmed with a set of predetermined acceptable silver concentration signal voltages. Using the method such as the embodiment illustrated in  FIGS. 17–17H , the microprocessor  190  analyzes the silver concentration signal voltages from the silver sensors  60 – 63  to determine if the silver concentration in the fluid is above the predetermined acceptable levels. 
     If a silver concentration signal voltage received from a silver sensor  60 – 63  is determined by the microprocessor  190  to be below a predetermined acceptable level, the microprocessor  190  transmits a silver alert signal in the form of an error code to activate the modem  204 , which utilizes a telecommunications network (not shown) to transmit a notification signal (the error code received from the microprocessor  190 ) to the remote monitoring station  210  ( FIG. 15 ) to alert the remote monitoring station  210  that the silver concentration in the fluid is too high. As discussed further below, depending on which silver sensor  60 – 63  has transmitted the silver concentration signal with the unacceptably low voltage (indicating an unacceptably high silver concentration), these error codes indicate that one of the filter canisters  12 A,  12 B,  12 C ( FIG. 1 ) has either failed or reached maximum capacity and therefore needs to be serviced by a technician. As shown in  FIGS. 17–17H , the microprocessor  190  transmits a different error code depending on which silver sensor  60 – 63  transmitted the unacceptably low silver concentration signal voltage to the microprocessor  190 . Error code  1  is transmitted by the microprocessor  190  when the second silver sensor  61  has transmitted a silver concentration signal determined by the microprocessor  190  to indicate the failure or maximum capacity state of the filter canister  12 A. Error code  2  is transmitted by the microprocessor  190  upon determination that the third silver sensor  62  has transmitted a silver concentration signal determined by the microprocessor  190  to indicate the failure or maximum capacity state of the filter canister  12 B. Error code  3  is transmitted by the microprocessor  190  upon determination that the fourth silver sensor  62  has transmitted a silver concentration signal determined by the microprocessor  190  to indicate the failure or maximum capacity state of the filter canister  12 C. 
     In addition, as shown in  FIGS. 17–17H , the microprocessor  190  may send error codes via the modem  204  to the remote monitoring station  210  ( FIG. 15 ) that are not triggered by silver concentration signals from the silver sensors  60 – 63 , but by other conditions or events occurring in the system, such as the powering on of the system (error code  0 ), the switching of the pump power switch  132  to the “off” position (error code  4 ), the activation of the overflow indicator  45  with the pumps  14 ,  15  operating (error code  5 ), the activation of the overflow indicator  45  without the pumps  14 ,  15  operating (error code  6 ), the deactivation of the overflow indicator  45  (error code  7 ), the passage of a predetermined time after the powering on of the system (error code  8 ), and the detection of a fluid leak within the unit box by the leak detector PCB  21  (error code  9 ). As can be seen from the descriptions of these various error-code- triggering conditions and events, although the term “error codes” is used to denote the signals sent to the remote monitoring station  210  by the microprocessor  190  via the modem  204 , the conditions and events being monitored do not always constitute failures or errors in the system. All the above error codes are translated at the remote monitoring station  210  to communicate the occurrence of the respective conditions or events. 
     In the illustrated embodiment, the microprocessor  190  also transmits the silver alert signal to one or more of the LEDs on the front panel  113  of the unit box  13 , and the one or more LEDs then provide a visual notification signal to a user utilizing the system as an onsite silver monitoring station. Alternatively, the microprocessor  190  may transmit the silver alert signal to either the modem  204  or the one or more front panel LEDs, thereby triggering only one notification signal at either the remote or onsite silver monitoring station. 
     Turning now to  FIG. 16 , a block diagram generally illustrating the flow of fluid in an embodiment of the invention is shown. Raw silver-laden fluid is expelled by an independent system  207  (e.g., a system for developing photographic film) for discarding. The fluid is deposited in the receptacle  11  to be processed by the silver recovery and monitoring system  10  ( FIG. 1 ). Upon activation of the main pump  14  or the auxiliary pump  15  by the fluid level sensor  17  ( FIG. 7 ) in the receptacle  11 , the raw silver-laden fluid in the receptacle  11  is urged out of the receptacle  11 . If the main pump  14  is operating, the fluid is pumped out of the receptacle  11  into the sensor block  16  through the inlet port  81  ( FIGS. 3D ,  10 ) on the left-hand surface  70  ( FIG. 3D ) of the sensor block  16 , through the first passage  103  ( FIG. 4 ) of the sensor block  16 , and out the outlet port  66  ( FIGS. 3C ,  10 ) on the right-hand surface  64  ( FIG. 3C ) of the sensor block  16  into the main pump  14 . Similarly, if the auxiliary pump  15  is operating, the fluid is pumped out of the receptacle  11  into the sensor block  16  through the inlet port  80  ( FIGS. 3D ,  10 ) on the left-hand surface  70  ( FIG. 3D ) of the sensor block  16 , through the second passage  104  ( FIG. 4 ) of the sensor block  16 , and out the outlet port  66  ( FIGS. 3C ,  10 ) on the right-hand surface  64  ( FIG. 3C ) of the sensor block  16  into the auxiliary pump  15 . 
     The fluid exiting the main pump  14  and the auxiliary pump  15  is combined in a “Y” or “T” shaped fitting  205  ( FIG. 2 ) and flows back into the sensor block  16  through the inlet port  67  ( FIG. 3C ) on the right-hand surface  64  of the sensor block  16  and into the third passage  105  ( FIG. 5 ) of the sensor block  16 . While in the third passage  105 , the fluid contacts the first silver sensor  60 , from which, depending on the current status of the silver monitoring method being executed by the microprocessor  190  ( FIGS. 15 ,  15 Q-R), a reading of the silver concentration in the fluid may be taken. After flowing through the third passage  105  of the sensor block  16 , the fluid flows out the outlet port  71  ( FIGS. 3D ,  10 ) on the left-hand surface  70  of the sensor block  16  and into the filter canister  12 A. Within the system, the filter canister  12 A is occasionally referred to only as “A” (e.g., the ports designated “To ‘A’” and “From ‘A’”). 
     After passing through the filter canister  12 A, the fluid flows back into the sensor block  16  through the inlet port  82  ( FIGS. 3D ,  10 ) on the left-hand surface  70  of the sensor block  16  and into the fourth passage  106  ( FIG. 6 ) of the sensor block  16 . While in the fourth passage  106 , the fluid contacts the second silver sensor  61 , from which, depending on the current status of the silver monitoring method being executed by the microprocessor  190 , a reading of the silver concentration in the fluid may be taken. After flowing through the fourth passage  106  of the sensor block  16 , the fluid flows out the outlet port  72  ( FIGS. 3D ,  10 ) on the left-hand surface  70  of the sensor block  16  and into the filter canister  12 B. Within the system, the filter canister  12 B is occasionally referred to only as “B” (e.g., the ports designated “To ‘B’” and “From ‘B’”). 
     After passing through the filter canister  12 B, the fluid flows back into the sensor block  16  through the inlet port  83  ( FIGS. 3D ,  10 ) on the left-hand surface  70  of the sensor block  16  and into the fifth passage  110  ( FIG. 6 ) of the sensor block  16 . While in the fifth passage  110 , the fluid contacts the third silver sensor  62 , from which, depending on the current status of the silver monitoring method being executed by the microprocessor  190 , a reading of the silver concentration in the fluid may be taken. After flowing through the fifth passage  110  of the sensor block  16 , the fluid flows out the outlet port  73  ( FIGS. 3D ,  10 ) on the left-hand surface  70  of the sensor block  16  and into the filter canister  12 C. Within the system, the filter canister  12 C is occasionally referred to only as “C” (e.g., the ports designated “To ‘C’” and “From ‘C’”). 
     After passing through the filter canister  12 C, the fluid flows back into the sensor block  16  through the inlet port  84  on the left-hand surface  70  of the sensor block  16  and into the sixth passage  111  ( FIG. 6 ) of the sensor block  16 . While in the sixth passage  111 , the fluid contacts the fourth silver sensor  63 , from which, depending on the current status of the silver monitoring method being executed by the microprocessor  190 , a reading of the silver concentration in the fluid may be taken. After flowing through the sixth passage  111  of the sensor block  16 , the fluid flows out the outlet port  74  ( FIGS. 3D ,  10 ) on the left-hand surface  70  of the sensor block  16  and through the drain hose  28  ( FIG. 1 ) into a drain  206 . 
     If the volume of raw silver-laden fluid being expelled by the independent system  207  into the silver recovery and monitoring system  10  ( FIG. 1 ) exceeds the pumping capacity of the main pump  14  and the auxiliary pump  15  of the system  10 , or if some other failure of the system  10  prevents the pumps  14 ,  15  from being able to keep pace with the volume of fluid being expelled by the independent system  207  into the receptacle  11 , the raw silver-laden fluid flows out of the receptacle  11  through the drain pipe  26  ( FIG. 1 ) and into the drain  206  without being filtered and without the silver in the fluid being recovered or monitored. This aspect of the system  10  prevents the fluid from overflowing out of the receptacle  11  into the worksite where the system  10  is located. When this condition occurs, referred to as a “drain violation” or a “violation” on the flow chart describing the “check drain” subroutine of the silver monitoring software ( FIG. 17C ), the overflow indicator activation rod  36  of the fluid level sensor  17  ( FIG. 7 ) is contacted by the fluid and transmits a signal to the microprocessor  190 , which transmits an error code (either error code  5  if the pumps are running or error code  6  if the pumps are not running; see  FIG. 17C ) to the modem  204  ( FIGS. 15 ,  150 ) for transmission to the remote monitoring station  210 . In addition, the overflow indicator latch circuit  182  ( FIGS. 15 ,  15 J) causes the “check system” LED  45  (overflow indicator) ( FIG. 8 ) to illuminate on the front panel  113  of the unit box  13  of the system  10 . 
     The fluid flow connections between the receptacle  11 , the pumps  14 ,  15 , the sensor block  16 , and the filter canisters  12 A,  12 B,  12 C, as well as the drain hose  28 , are achieved with a flexible plastic tubing having restricted air permeability. Specifically, the tubing is comprised of a semi-rigid polyethylene interior extruded with a flexible polyvinyl exterior. 
       FIGS. 17-17H  illustrate flow charts that generally depict a plurality of nested routines and subroutines programmed into the microprocessor  190  with software according to an embodiment of the invention.  FIG. 17  is a master flow chart generally illustrating the overall silver monitoring method of an embodiment of the invention. However,  FIG. 17  incorporates subroutines illustrated on other flow charts by reference. In the blocks designated “Execute Modem Call,” “Check Raw,” “Check Raw,” “Check A,” “Check B,” “C Phase  1 ,” and “C Phase  2 ,” the flow chart in  FIG. 17  incorporates subroutines generally illustrated by flow charts in  FIG. 17A  (modem call subroutine),  FIG. 17G  (raw check subroutine),  FIG. 17B  (“A” cartridge check subroutine),  FIG. 17D  (“B” cartridge check subroutine),  FIG. 17E  (C Phase  1  subroutine), and  FIG. 17F  (C Phase  2  subroutine), respectively. In turn, the “A” cartridge check subroutine ( FIG. 17B ), the “B” cartridge check subroutine ( FIG. 17D ), the C Phase  1  subroutine ( FIG. 17E ), and the C Phase  2  subroutine ( FIG. 17F ) incorporate the modem call subroutine ( FIG. 17A ) by using the “Execute Modem Call” designation and a check drain subroutine ( FIG. 17C ) by using the “Execute Drain Check” or “Check Drain” designations. Finally, the check drain subroutine ( FIG. 17C ) incorporates the leak detect subroutine generally illustrated by the flow chart in  FIG. 17H  by using the “Execute Check Leaks” designation. 
     In these flow charts ( FIGS. 17–17H ), the terms “Raw Sensor,” “‘A’ Sensor,” “‘B’ Sensor,” and “‘C’ Sensor” refer to the first, second, third, and fourth silver sensors  60 - 63 , respectively. The terms “RawAB Thrsh” and “CLEAKTRSH” refer to fixed voltage values that are incorporated into the software to help ensure that the system does not allow the amount of silver being passed through the system to surpass an acceptable level without providing adequate notification signals to the remote monitoring station  210  via the modem  204 . The terms “RawAvg” and “AAvg” denote the results of averaging multiple readings from the “Raw Sensor” and the “‘A’ Sensor,” respectively, to help ensure that anomalous spikes or dips in the voltages transmitted by these sensors do not affect proper operation of the system. The incorporation of a “strikes”-based notification system further protects the system from overreacting to ephemeral anomalies in sensor readings. The references to “the First Call” and the “Time to Call” indicate that the modem  204  is instructed to re-send certain error codes to the remote monitoring station  210  at certain predetermined time intervals until the problem causing the error code to be sent is rectified. This prevents the system from unnecessarily contacting the remote monitoring station  210  every time the same occurrence of a certain problem is detected by the system. Finally, the microprocessor  190  may be enabled to transmit (via the modem  204 ) at least one daily error code (error code  8 ; “Regular Calls”) to the remote monitoring station  210  to inform the remote monitoring station  210  that the system is operating, and will transmit error codes to the remote monitoring station  210  whenever the system is reset (error code  0 ; “New Power Up”), and a predetermined time interval after a reset (error code  8 ; “3 Day Calls”). 
     In addition to sending error codes to the remote monitoring station  210  via the modem  204 , the microprocessor  190  causes the “monitoring” LEDs  114 ,  115 ,  116  ( FIGS. 8 ,  15 ) to illuminate when signals are being received by the microprocessor  190  from the second, third, and fourth silver sensors  61 ,  62 ,  63 , respectively. The microprocessor  190  causes the “max capacity” LEDs  120 ,  121  ( FIGS. 8 ,  15 ) and the “check drain” LED  122  ( FIGS. 8 ,  15 ) to illuminate when the microprocessor  190  has determined that the filter canisters  12 A,  12 B,  12 C, respectively, are no longer reducing the silver concentration in the fluid passing through them. 
     The source code of an embodiment of the software used to program the microprocessor  190  is provided with this application as a computer program listing appendix stored on a recordable compact disc (CD-R) filed with the application. The code is written in the TF BASIC programming language, a proprietary language used to program the Tattletale TFX-  11  microprocessor used in the illustrated embodiment of the invention. The operation manual for the Tattletale TFX-11 microprocessor, including a description of the TF BASIC language used to program the Tattletale TFX-11, may be accessed as an Adobe Personal Document Format (PDF) file on the Internet at ftp://ftp.onsetcomp.com/Public/TattleTale/Manuals/TFX11MAN.pdf. The Tattletale TFX-11 microprocessor, the TF BASIC programming language, and the operation manual for the Tattletale TFX-11 are proprietary to Onset Computer Corporation, P.O. Box 3450, Pocasset, MA 02559-3450, which operates a site on the World Wide Web at http://www.onsetcomp.com. 
     Via the microprocessor  190  and the modem  204 , the remote monitoring station  210  ( FIG. 15 ) receives routine updates of the signal transmitted by the fourth silver sensor  63 , the furthest downstream silver sensor in the system, and converts these periodic signals into a report of the parts per million of silver being passed through the drain  206  ( FIG. 16 ) into the environment. This report may then be accessed by the user of the system via the World Wide Web in order that the user may monitor its own compliance with the government regulation that imposes a 5 parts per million maximum on the amount of silver that may be discharged into the environment. However, as long as the system is functioning properly, it is intended for the operator of the remote monitoring station  210 , pursuant to service agreements with the user of the system, to perform any maintenance of the user&#39;s system that is necessary to ensure the user&#39;s compliance with the government regulation. Such maintenance would likely include servicing the filter canisters within a reasonable time after receiving a notification signal from the modem in the user&#39;s system that one or more of the filter canisters are spent (i.e., that the silver concentration in the fluid entering a filter canister is greater than or equal to the silver concentration in the fluid exiting the same canister). Therefore, in the absence of unforeseeable malfunctions in the system, the system and the operator of the remote monitoring station  210  should be able to keep the user of the system in consistent compliance with the government regulation without the need for intervention by the user, beyond the need for the user to allow maintenance personnel to gain access to the system for maintenance purposes. 
     The embodiment of the invention disclosed herein is currently marketed by Chemtronix Inc. 144 Industrial Park Drive, Waynesville, NC 28786, as Model CTX- 2000 (standard receptacle size) or Model CTX-3000 (larger receptacle size). Another embodiment of the invention is as described herein but with only two silver sensors and one pump, with appropriate alterations to the sensor block, the tubing, the receptacle, the software, and other elements of the system to accommodate the reduced capabilities of this embodiment, which is currently marketed by Chemtronix Inc. as Model CTX-1000. Further embodiments of the invention, either with capabilities lesser or greater that those described herein, may be practiced without departing from the scope of the invention. For instance, as shown on  FIGS. 15J and 15M  in conjunction with the model number CTX-500, the system may use only the LEDs on the front panel  113  of the unit box  13  to provide the notification signals contemplated herein, without incorporating a microprocessor or a modem and with only one silver sensor and one pump. Of course, such a system would require the user of the system to function as the silver monitoring station, as no automated computerized connections to the remote monitoring station  210  would be present. 
     A silver recovery and monitoring system is described above. Various details of the invention may be changed without departing from its scope. Furthermore, the foregoing description of an embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation—the invention being defined by the claims.