Abstract:
A modular sysTem for the demineralization of aqueous liquids comprising a plurality of modular units, each of the modular units being encapsulated and having a cathode proximate a first end of the modular unit and an anode proximate the opposite end of said modular, a plurality of alternating diluting compartments and concentrating compartments positioned between the cathode and the anode, and ion exchange material positioned within the diluting compartments. Each of the diluting compartments has a compartment spacer with an elongated central cavity and a plurality of fine slit openings at each end adjacent the cavity. The ion exchange means comprise a porous and permeable continuous phase of cation or anion exchange resin particles and a porous and permeable dispersed phase of clusters of the other of the cation or anion exchange resin particles. Releasable connecting means are provided to interconnect the modular units in the system to allow for facile substitution of modular units for servicing and to permit modification of flow capacity requirements by increasing or decreasing the total number of modular units in the system.

Description:
This application is a 371 of PCT/CA97/00088 filed, Feb. 10,1997. 
    
    
     FILED OF INVENTION 
     This invention relates to an apparatus for the demineralization of liquids and, more particularly, relates to an apparatus comprised of modular units for the demineralization of liquids. 
     BACKGROUND OF THE INVENTION 
     The purification of liquid has become of great interest in many industries. In particular, pure water is used for many industrial purposes rather than merely as drinking water. For example, pure water is used in processes for producing semiconductor chips, in power plants, in the petro chemical industry and for many other purposes. 
     Ion exchange resins, reverse osmosis filtration and electrodialysis techniques have been used to reduce the concentration of particular ions in a liquid. 
     Electrodeionization apparatus have recently been used with more frequency to reduce the concentration of ions in a liquid. The term “electrodeionization” generally refers to an apparatus and a process for purifying liquids which combine ion exchange resins, ion exchange membranes and electricity to purify the liquids. An electrodeionization module comprises alternating arrangements of cation permeable membranes and anion permeable membranes defining compartments therebetween. In alternating compartments, there is provided ion exchange resin beads. Those compartments are known as diluting compartments. The compartments which generally do not contain ion exchange resin are known as the concentrating compartments. Ions migrate from the diluting compartments through ion exchange beads and ion permeable membranes into the concentrating compartments by the introduction of current. The liquid flowing through the concentrating compartments is discarded or partially recycled and the purified liquid flowing through the diluting compartments is recovered as demineralized liquid product. 
     Electrodialysis apparatus are similar in configuration to electrodeionization apparatus. The main difference between electrodialysis apparatus and electrodeionization apparatus is that electrodialysis apparatus do not use ion exchange resin to aid in the removal of ions in the liquid passed through the diluting compartment. Often electrodialysis apparatus utilize membrane structures extending into the diluting compartments to aid in the removal of ions from a liquid. 
     There are two general configurations for electrodeionization and electrodialysis apparatus: first, a plate and frame configuration, and second, a spiral-wound configuration. 
     U.S. Pat. No. 4,925,541 which issued May 15, 1990 to Giuffrida et al. discloses a plate and frame electrodeionization apparatus and method. The method for removing ions from a liquid in an electrodeionization apparatus is carried out in an electrodeionization apparatus which has a number of subcompartments in the diluting compartments. A mixture of anion exchange resin and cation exchange resin is contained within the subcompartments. The subcompartments are formed by a plurality of ribs extending along the length of the diluting or ion depletion compartments. 
     U.S. Pat. No. 4,636,296 which issued Jan. 13, 1987 to Kunz discloses another embodiment of plate and frame apparatus and method for the demineralization of aqueous solutions in which an aqueous liquid is passed through alternating separate layers of cation exchange resin and anion exchange resin. 
     Plate and frame apparatus are large in size and typically suffer from leaks because of the difficulty of sealing large vessels. Also, the units often are oversize because of inflexibility in designing for capacity, necessitating undesirably high capital and operating costs. 
     U.S. Pat. No. 5,376,253 which issued Dec. 27, 1994 to Rychen et al. discloses an apparatus for the electrochemical desalination of aqueous solutions. The apparatus has a wound or spiral arrangement of anion and cation permeable membranes. Such apparatus are prone to leakage and are relatively difficult to manufacture. 
     It is tedious to increase or vary the total output capacity of purified liquid for plate and frame configurations because it involves disassembly, insertion of additional ion permeable membranes, and installation of longer tie-bars to assemble the apparatus together. It is also tedious if not impossible to increase or vary the total output capacity of purified liquid for spiral configurations because it involves disassembly and the insertion of a longer or shorter arrangement of anion and cation permeable membranes. 
     It is desirable to easily vary the total output capacity for pure liquid in apparatus for the demineralization of liquids. It is also desired to have an electrochemical cell for electrodialysis and electrodeionization apparatus which is relatively easy to situate in an existing water treatment system. 
     SUMMARY OF THE INVENTION 
     The disadvantages of the prior art may be overcome by providing a modular system apparatus for the demineralization of liquids which has a plurality of modular units for the demineralization of liquids and which is relatively easily assembled and disassembled for replacement of modular units or for increasing or decreasing design flow capacity by adding or deleting modular units in the system. 
     In its broad aspect, the apparatus for the demineralization of liquids of the present invention comprises a plurality of modular units for the demineralization of aqueous liquids arranged in parallel with the flow of a liquid and adapted to remove ions from the liquid. The apparatus is a modular system comprised of functional building blocks which can be readily increased or decreased in size and volumetric capacity by increasing or decreasing the number of these building blocks, i.e. modular units. Each of the modular units or cells has a cathode and an anode and means for applying an electrical voltage between the anode and the cathode. A plurality of alternating diluting or demineralizing compartments and concentrating compartments are positioned between the cathode and the anode. Ion exchange material is positioned within the diluting compartments and may be positioned within the concentrating compartments. The apparatus has means for passing a first liquid to be purified through the diluting compartments and means for passing a second liquid through the concentrating compartments for accepting ions from the first liquid. Each modular unit also has means for passing an electrolyte to and from the cathode and anode, means for recovering the purified liquid from the diluting compartments and means for removal of the concentrated liquid from the unit. 
     In another aspect of the invention, each of the modular units is an electrodeionization apparatus. In another aspect of the invention, each of the modular units is an electrodialysis apparatus. The modular units are in parallel with each other and have quick release securement means to allow facile release of the modular units from the system. 
     In a preferred embodiment, the portable modular unit for use in a modular system for demineralizing aqueous liquids comprises a rigid, compact housing, said housing having a pair of opposite end plates, a pair of opposite side plates, a top plate and a bottom plate, and connector means for joining said end plates to the side and plates and for securing the top and bottom plates thereto to form a liquid-tight encapsulating enclosure; said housing containing an anode compartment having an anode and a cathode compartment having a cathode, a plurality of cation exchange membranes and anion exchange membranes which are alternately arranged between the anode compartment and the cathode compartment to form demineralizing compartments each defined by a demineralizing compartment spacer having an anion exchange membrane on the anode side and by a cation exchange membrane on the cathode side, and concentrating compartments each defined by a concentrating compartment spacer having a cation exchange membrane on the anode side and by an anion exchange membrane on the cathode side, and a porous and permeable ion exchanger filling said demineralizing compartments, and means for releasably connecting the modular unit to a piping system in a modular system whereby the modular unit can be removed from or added to the modular system. 
     Each demineralizing compartment comprises a demineralizing compartment spacer having an elongated central cavity for receiving the porous and permeable ion exchanger, said spacer having a liquid inlet port at one end and a liquid outlet port at the opposite end, a plurality of fine slit openings formed in the spacer at each end adjacent the cavity, and at least one channel in the spacer at each end for interconnecting the liquid inlet port to the fine slit openings adjacent the cavity and for connecting the liquid outlet port to the fine slit openings, whereby an aqueous liquid can be flowed through the porous and permeable ion exchanger filling the demineralizing compartment. The ion exchanger preferably a porous and permeable continuous phase of one of cation exchange resin particles or anion exchange resin particles and a porous and permeable dispersed phase of clusters of the other of the cation exchange resin particles or the anion exchange resin particles in the continuous phase. 
     Each of the end plates and the side plates of the modular unit has an outer surface and has a plurality of transverse upstanding reinforcing ribs equispaced along the said outer surface formed integral therewith, and a cover plate substantially co-extensive with an attached to the distal edges of the reinforcing ribs to form a rigid box structure therewith for stiffening and reinforcing the plates from internal pressure. 
     Each said side plate has a socket formed integral therewith on the outer surface adjacent opposite side edges thereof as an extension of a transverse rib at each end thereof, each said socket having a longitudinal hole therein for loosely receiving a threaded bolt shank and a slot intersecting the hole adapted to receive a nut compatible with the threaded bolt shank, said slot having an interior shape such as a part hexagonal shape for receiving the nut in axial alignment with the bolt for threading the bolt into the nut. 
     Each said end plate has a boss formed on the outer surface adjacent opposite sides thereof at each end of a transverse rib, each said boss having a hole for receiving a bolt in alignment with a mating socket in a side plate. 
     The modular system for demineralizing aqueous liquids comprises a plurality of said portable modular units in which the portable modular units are arranged in parallel, a piping system for feeding an aqueous liquid to be demineralized in parallel to the modular units and for removing a demineralized aqueous liquid and a concentrated waste liquid in parallel from the modular units, means for applying an electrical voltage between the anode and the cathode, and means for removably connecting the modular units to the piping system for facile adding of a modular unit to the system or removal of the modular unit from the system. 
     The apparatus of the invention provides a number of advantages including the following: 1. the electrical connections between the modular units allow for simple wiring of the apparatus; 2. the quick disconnection of the modular units enables the modular units to be easily serviced or replaced; 3. the modular units simplify assembly and disassembly of the entire apparatus; 4. the relatively small size of the modular units allows for encapsulation of the units, thereby enhancing the integrity of the units and minimizing leakage; and 5. the total output capacity of purified liquid is easily increased or decreased to suit design flow requirements by adding or removing modular units in a system assembly of the units. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described with reference to the accompanying drawings, in which: 
     FIG. 1 is a perspective view of a prior art electrodeionization  5  apparatus; 
     FIG. 2 is a fragmentary sectional view taken along line  2 — 2  of FIG. 1; 
     FIG. 3 is a perspective view of an embodiment of the apparatus of the invention for the demineralization of liquids; with a modular unit removed for clarity of illustrations; 
     FIG. 4 is a perspective view of a preferred arrangement of ion exchange material of the invention; 
     FIG. 5 is a top plan view, partially in schematic, of the apparatus of FIG. 3; 
     FIG. 6 is a perspective view of a second embodiment of the present invention; 
     FIG. 7 is a perspective view of the apparatus of FIG. 6 with a row of modules removed to more clearly show the liquid manifolds; 
     FIG. 8 is a sectional view, partly in elevation, of a manifold connector embodiment of the invention shown in FIG. 6; 
     FIG. 9 is a perspective view of the housing of another embodiment of the invention; 
     FIG. 10 is an exploded perspective view of the component of the embodiment shown in FIG. 9; 
     FIG. 11 is an enlarged perspective view of a preferred diluting compartment spacer of the invention shown in FIG. 10; 
     FIG. 12 is a perspective view of modular system of the invention showing the stacks of modular units arranged in racks; 
     FIG. 13 is a perspective view of an embodiment of flow piping of the invention; 
     FIG. 14 is a perspective view, partly cut away, of a side plate of the module housing shown in FIG. 11; and 
     FIG. 15 is a perspective view, partly cut away, of an end plate of the module housing shown in FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a prior art plate and frame electrodeionization apparatus  10  is shown whereby ions may be removed from a liquid. In the preferred embodiment, ions such as sodium and chloride are removed from water. 
     The electrodeionization apparatus  10  has a rectangular frame  12 . The frame  12  comprises a rigid front plate  14  and a rigid back plate  16  formed of metal. The front plate  14  and the back plate  16  are joined together by a number of tie-bars or bolts  18 . Each tie-bar  18  is inserted into a hole  20  located equispaced about the periphery of the front plate  14  and inserted into corresponding holes  18   a  in back plate  16 . A cathode depicted by numeral  22  (FIG. 2) is located proximate the front plate  14  in a cathode compartment  23  and an anode depicted by numeral  24  is located proximate the back plate  16  in an anode compartment  25 . 
     Openings  26  are located in the front plate  14  to allow liquid to enter the electrodeionization apparatus  10  for treatment. Insulating electrode block  28  forming and electrode compartment abuts the perimeter of the front plate  14  and insulating electrode block  30  forming an electrode compartment continuously abuts the perimeter of the back plate  20 . The electrodeionization apparatus  10  has a plurality of alternating cation permeable membranes and anion permeable membranes depicted by numeral  32  between the insulating electrode blocks  28  and  30 . The cation permeable membranes and anion permeable membranes  32  define the boundaries of alternating concentrating and diluting compartments, to be described. 
     FIG. 2 shows representative concentrating compartments  44 ,  46  and a representative diluting compartment  48 , between the concentrating compartments, in further detail. Cation permeable membranes  36  and  38  and anion permeable membranes  40  and  42  define the concentrating compartments and diluting compartments. Spacers (not shown) are placed between the membranes in the diluting compartments and concentrating compartments. The spacers in the diluting compartments  48  have openings for placement of ion exchange material such as ion exchange resin beads  49 . It will be understood that ion exchange resin may also be placed within the concentrating compartments. 
     FIG. 4 shows a preferred arrangement of ion exchange material of the present invention to be used within the diluting compartment  48  shown in FIG. 2. A bed  40  of porous and permeable continuous phase, i.e. matrix, of ion exchange material  50  has a plurality of spaced-apart cylinders of porous and permeable clusters of second ion exchange material  52  dispersed within matrix  50  transversely of the bed plane. The ion exchange materials  50  and  52  preferably are ion exchange resin particles in the form of beads. The ion exchange material  50  and ion exchange material  52  exchange oppositely charged ions. For example, if continuous phase ion exchange material  50  is a cation exchange material, which will have fixed negative charges to capture cations, dispersed phase ion exchange material  52  is an anion exchange material which will have fixed positive charges to capture anions. The transverse arrangement of clusters of the dispersed phased ion exchange material straddling or bridging the diluting compartments ensures that the aqueous liquid which flows within the diluting compartments  48  comes into contact with both forms of ion exchange resins to effectively exchange cations and anions. Referring to FIGS. 1,  2  and  4 , aqueous liquid to be treated flows through the openings  26  and through the concentrating compartments  44  and  46  and the diluting compartment  48 . Streams of liquid depicted by arrows  54  and  56  flow through the concentrating compartments  44  and  46  respectively and a stream of liquid depicted by arrow  58  flows through the diluting compartment  48 . The aqueous liquid contains ions such as sodium and chloride ions. 
     Electric current flows between the cathode  22  in cathode compartment  23  and the anode  24  in anode compartment  25 . The current across cathode  22  and anode  24  may be varied to control the overall efficiency of the electrodeionization process. 
     As the liquid to be purified flows through the diluting compartment  48  as depicted by arrow  58 , it comes into contact with ion exchange resin beads, as in the arrangement such as shown in FIG.  4 . Cation exchange resin  50  has fixed negative charges and captures cations such as sodium ions present in the liquid. Anion exchange resin  52  has fixed positive charges and captures anions such as chloride ions present in the liquid. As the ion exchange takes place between the liquid to be purified and the cation exchange resin beads  50  and the anion exchange resin beads  52 , the voltage induces the non-desired cations and anions typified by sodium ions and chloride ions respectively to travel through membranes  38  and  40  and into the adjacent concentrating compartments  46  and  44 . The ion exchange resin is disposed in a transverse arrangement relative to the flow of liquid by arrows  53  as shown in FIG.  4 . This arrangement ensures that most of the liquid flowing through the diluting compartment  48  comes into contact with ion exchange material  50  and  52 . 
     In the preferred embodiment for purifing water, the current induces some splitting of water into hydrogen and hydroxyl ions. The hydrogen ions are transported through the cation exchange resin  50  towards the cation exchange membrane  38 , and through cation exchange membrane  38  into the concentrating compartment  46 , as shown by arrows  66 . The hydroxyl ions are transported through the anion exchange resin  52 , towards anion permeable membrane  40 , and through anion permeable membrane  40  into the concentrating compartment  44 , as shown by arrows  62 . Thus, the ion exchange resin material  50  and ion exchange resin material  52  are continuously regenerated. 
     Anionic impurities, for example chloride ions in the water to be purified in diluting chamber  48 , are taken up by the anion exchange resin material  52 , by the usual ion exchange mechanism, and are then transported along with hydroxyl ions through the anion exchange resin up to, and through anion permeable membrane  40 , into concentrating compartment  44  as shown by arrows  60 . At the same time, an equivalent amount of hydrogen ions and impurity cations is transported from an adjacent diluting compartment into concentrating chamber  44 , as shown by arrows  70 . 
     Cationic impurities, for example sodium ions, in the water to be purified in diluting chamber  48  are taken up by the cation exchange resin material  50 , by the usual ion exchange mechanism, and are then transported along with the hydrogen ions through the cation exchange resin up to, and through cation permeable membrane  38 , into concentrating compartment  46  as shown by arrows  64 . At the same time, an equivalent amount of hydroxyl ions and impurity anions is transported from an adjacent diluting compartment into concentrating chamber  46 , as shown by arrows  68 . 
     The water flows through the concentrating compartments  44  and  46  to a waste tank (not shown) or is recycled. The purified water flowing through the diluting compartment  48  is recovered as product. 
     Referring now to FIGS. 3 and 5, the embodiment of the apparatus  74  of the present invention for the demineralization of a liquid such as water comprises a plurality of either electrodeionization or electrodialysis module units  76 . In this embodiment, the modules  76  are arranged in a spaced-apart rows or racks  77  and  79 . 
     Liquid to be treated flows through a feed conduit  80  in the direction as depicted by arrow  82  (FIGS. 3 and 5) between module rows  77  and  79 . The feed conduit has a number of lateral connector conduits  84  which allow the liquid to flow in parallel into each of modules  76  in rows  77  and  79 . The flow of liquid from the feed conduit into the modules  76  is depicted by arrows  86  in FIG.  5 . At the same time, waste liquid flows through a waste conduit  81  in the direction as depicted by arrow  83  between rows  77  and  79  of modules  76 . The waste conduit  81  has a number of lateral connector conduits  85  which allow the liquid to flow in parallel into the modules  76  in the direction as depicted by arrow  87 . 
     After the liquid has been purified in the modules  76  as described above, it flows out of the modules  76  in rows  77  and  79  in parallel as depicted by arrows  88  in FIG.  5  through lateral conduits  90  into a product collection conduit  92 . This is depicted by arrow  102  in FIG.  5 . Waste from the diluting compartments flows out of modules  76  in parallel through conduits  96  shown by arrow  98  into a waste collection conduit for flow as depicted by arrow  100 . 
     An electrolyte is passed through the compartments which contain the cathode and the anode. The electrolyte flows through a conduit  104  and through a number of lateral connector conduits  106  from the modules  76  in the rows  77  and  79  in the direction as depicted by arrows  108 . 
     The modules in rows  77  and  79  preferably are separately electrically fused. 
     FIGS. 6-8 show another embodiment of the apparatus  120  of the present invention. The apparatus  120  comprises a plurality of electrodionization modules or electrodialysis modules  122  arranged in rows  123  and  125 . 
     FIG. 6 shows typical module  122  separated from the rack of modules. Module  122  has openings in one end plate  125  to allow for the flow of liquid from the modules  122  to manifolds  130  and  132 . Openings  134 ,  136  and  138  allow for the streams of waste (concentrate), electrolyte and purified liquid respectively to flow from the modules into respective conduits in manifold  130 . Opening  140  allows for the introduction of liquid to be purified and opening  142  permits introduction of liquid to pick up waste (concentrate) liquid. The manifolds  130  and  132  have connectors  144  for connection to the modules  122 . 
     With reference now to FIG. 8, connector  144  is a short pipe with o-ring  146  which friction fits within the openings  134 ,  136 ,  138 , 140  and  142  of the module  122  and maintains a liquid seal with manifolds  130  and  132 . FIG. 8 shows a cross-section of the manifold  130  which has conduits  148 ,  150  and  152  corresponding to the flow of streams of purified liquid, electrolyte and waste, respectively. 
     FIGS. 9-15 show another embodiment of the modular unit of the apparatus of the present invention. With reference to FIGS. 9 and 10, an embodiment of module housing  160  is shown having side plates  162  and end plates  164  joined by a plurality of bolts  166 . Top and bottom plates  168 ,  170  seated into recesses in plates  168 ,  170  close the module. The housing plates are made of a material such as stainless steel or an aluminum alloy configured in box-like structures to be described to provide an assembly for a liquid-tight housing which encapsulates the interior components. A PVC insulating electrode block  172  having inlet and outlet pipes adjacent an end gasket  174  at one end houses a platinum coated titanium anode  176  and a PVC insulating electrode block  178  at the opposite end adjacent an end gasket  180  houses a stainless steel cathode  182 . A polypropylene mesh electrode spacer  184 , an electrode compartment spacer  185  and a cation permeable membrane  186  are located at the anode end of the module. Next, a concentrating compartment spacer  188  is adjacent an anion permeable membrane  190  which abuts a demineralizing or diluting compartment spacer  192  which houses ion exchange material i.e. ion exchanger  40 , such as shown in FIG.  4 . Spacers  188  and  192  may be injection molded polypropylene. 
     A plurality of diluting/concentrating pairs of compartments  196  comprise the central portion of the module. A cation permeable membrane  198  adjacent a concentrating compartment spacer  200 , next to a cation permeable membrane  202  and an electrode compartment spacer  204 , abut stainless steel cathode  182 . 
     FIG. 11 illustrates a diluent spacer  192  containing within a cavity  199  defined by sides  201 ,  203  and ends  205 ,  206  an ion exchanger bed  40  having continuous phase of ion exchange material  50  and discrete spaced-apart cylinders or island clusters of a second ion exchange material  52 , the cylinders  52  extending through bed  40  to be exposed on both sides thereof. The discrete island or clusters  52  may be formed from a shallow bed or sheet of a continuous phase of ion exchange resin particles of a first or second ion exchange material, preferably bonded by a polymeric binder, by die cutting clusters of the desired size and shape from the sheet. A sheet of a continuous phase of ion exchange resin particles of an ion exchange material having an opposite charge bonded by a polymeric resin having a plurality of holes corresponding in size and shape to the clusters  52  die cut therefrom, can receive the cut-out clusters  52  having the opposite charge in tight-fitting frictional engagement to form the ion exchangers. A thermoplastic polymeric binder such as a low density polyethylene, linear low density polyethylene, or the like, in an amount sufficient to form a cohesive sheet or bed structure suitable for handling, while retaining good porosity, liquid permeability and ion exchange capacity, can be used to form the starting sheets of the first and second ion exchange material. A liquid inlet port  208  is connected to cavity  198  by channels  210  terminating in a plurality of fine slit openings  212 , openings  212  having a width smaller than the average size of the particles, e.g., ion exchange resin beads, which constitute the bed  40 . The liquid discharge port  214  is connected to cavity  198  by channels  216  and a plurality of fine slit openings  218 . Covers  220  close channels  210  and  216 . Migration of the resin material thus is inhibited and the resin material is effectively contained within the diluent spacer during liquid flow. 
     FIG. 12 illustrates a typical rack of modular units  160  mounted in a frame  230 . FIG. 13 shows the plumbing; conduit  232  for aqueous liquid to be purified, conduit  234  for liquid to carry away impurities, conduit  236  for purified liquid, conduit  238  for waste liquid and conduit  240  for electrolyte. Junction boxes  242  provide the electrical connection to the anodes and cathodes by wires  244 ,  246  (FIG. 12) individually fused. 
     Turning to FIG. 14, each side plate  162  is shown in more detail to comprise inner planar wall  230  and a plurality of transverse upstanding reinforcing ribs  232 ,  234  equispaced along the length of plate  162  on the outer surface  236  and formed integral therewith. Thin ribs  232  and thick central ribs  234  interconnect sockets  238 ,  240  formed at opposite side edges  242 ,  244  of plate  162 . A rectangular cover plate  246  substantially co-extensive with and attached to the distal edges  248  of ribs  232 ,  234  forms a rigid box structure to effectively stiffen and reinforce side plate  162  from internal pressure. 
     Each of sockets  238 ,  240  comprises a slightly oversize hole  248  adapted to receive the shank  250  of bolt  166  (FIG. 9) and a slot  252  intersecting hole  248  adapted to receive a nut  254 , typically a hexagonal nut, which is compatible with and receives bolt shank  250  in threaded engagement. The interior of slot  252  is shaped to include four sides of hexagon to receive and to centre nut  254  in axial alignment with hole  248  and to prevent rotation of nut  254  to allow bolt shank  250  to be threaded therein. 
     Each end plate  164 , shown in more detail in FIG. 15, has transverse upstanding reinforcing ribs  260  equispaced along the length of the plate on the outer surface  262  formed integral therewith to interconnect bosses  264  having holes for receiving bolts  166 . A rectangular cover plate  266  substantially co-extensive therewith and attached to the distal edges  268  of ribs  260  forms a rigid box structure to effectively stiffen and reinforce end plate  164  from internal pressure. 
     The plurality of bolts  166  tightened to the desired torque level effectively secures end plates  164  to side plates  162  and locks top and bottom plates  168  in inner wall slots to provide an encapsulated, liquid-tight housing capable of effectively withstanding internal pressures of 150 psig, or more without leakage of liquid. 
     The modular system of the present invention provides a number of important advantages. The modular units are compact and can be carried by two people for installation or replacement. The compact units typically are liquid tight and provide effective encapsulation. The compact size allows for facile replacement, obviating the need for field servicing. The parallel arrangement of units allows increase or decrease of capacity by adding or deleting modular units. Failure of one unit does not shut down the system. Each configuration can be serviced by common piping, valves, pumps and the like for minimum capital expenditure and servicing costs. A system containing eight units, each producing nominally 12.5 U.S. gallons per minute (gpm), produces 100 gpm. Stacking of eight units on top of eight units would double production to 200 gpm. Configurations of 100, 300 and 600 U.S. gpm and larger are standard. 
     It will be understood, of course, that modifications can be made in the embodiment of the invention illustrated and described herein without departing from the scope and purview of the invention as defined by the appended claims.