GB918215A - Water treatment - Google Patents

Abstract

918,215. Electrodialysis apparatus for treating brackish water. AMERICAN MACHINE & FOUNDRY CO. Aug. 3, 1961 [Aug. 19, 1960], No. 28226/60. Class 41. A compact water treatment electrodialysis apparatus for purifying brackish water comprising an electrodialysis unit which delivers a product and concentrate stream. The apparatus is enclosed in a water-tight cabinet 20, Fig. 1, fitted with a drainage device 32, 34 to discharge the waste stream. Brackish water enters via pipe 36 to flow into the filter 37 for removal of any solids. Exit water from the filter 37 passes through the solenoid valve 38, a pressure-regulating valve 39 and a low-pressure cut-off switch 40 to enter the electrodialysis stack 100 via tube 45. Two outflow tubes 50 and 51 convey product and concentrate streams from the electrodialysis stack 100 to the reversal and diverter assembly 60. The product stream flows into the product storage tank or reservoir 52 from the diverter and electrical reversal unit 60 via a tube which conducts treated water from an aperture in the base of the diverter drain pan 61. Treated water is drawn off from the reservoir 52 by the valve 84 (Fig. 4). Alternating current enters the unit via plug 80 and line switch 81 (Fig. 4) and passes to the level switch 56. This switch is activated by an air trap extending downward into the product storage reservoir 52 and is closed whenever the water level falls below a predetermined height in the reservoir 52. When level switch 56 is closed solenoid valve 38 is activated so that brackish water may flow into the apparatus. Should the pressure in pipe 36 become inadequate the low-pressure switch 40 opens to prevent electrical energization of and damage to the electrodialysis stack 100. With pressure switch 40 closed the primary coil 73 of transformer 72 is energized, together with the timing motor 76 which operates the diverter 60 via shaft 82. From the secondary coil 74 a suitable voltage tapping 75 extends to the four rectifiers 71 to produce direct current which is transmitted to the two electrodes 78 and 79 in the stack 100 across the double-pole doublethrow reversing switch 77, which switch is controlled by the timing motor 76 via shaft 82 and which reverses the direct current flow to the stack 100. A unit 85 may be connected directly across the alternating current supply from switch 81; it may be an aerator or coding unit for rendering the treated water in the reservoir more suitable for consumption. The waste or concentrate stream from the stack 100 drains into the bottom of the drain pan 61 of the diverter 60 and discharges through drain pipe 67 ; it is joined in the drain pan 61 by the electrode washing streams flowing out from extensions 65 and 66 connected to tubes 151 and 160. The elements forming the electrodialysis stack 100 comprise alternate gaskets 102 interspersed between membranes 101A and 101K. The membranes 101A are anion permeable, the membranes 101K are cation permeable. The gaskets 102 are 0.04 inch thick, the cut-out portion 109 containing a screen spacer 116 of expanded plastic to space the adjacent membranes 101 and to promote turbulent flow within each gasket. Items 118 and 119 are end spacers. The gaskets 102 and membranes 101 are assembled in alignment to form the fluid flow aperture 125. The two electrode compartment forming gaskets 130 are 0.08 inch thick and the two electrodes 133 are thin strips of platinum foil. Each electrode compartment contains two screen spacers 116. Plastic inlet and outlet plates 140 and 141 admit fluid by tube 142 and discharge product and concentrate at tubes 155 and 156. Metal plates 165 and 166, fitted with holding bolts, hold the stack together. Some of the brackish water entering via tube 142 flows into the first electrode compartment via slot 145 to wash electrode 133 and exhaust through tube 151 and some flows into the second electrode compartment via fluid flow aperture 125 and slot 158 to exhaust through tube 160. The remainder of the brackish water flows through the stack 100. It has been found that the most effective period of time between current reversal is from 15 minutes to 3 hours. Hydrogen gas liberated from the stack 100 escapes into the cabinet 20 and is removed in the convection currents created by the heat liberated from the transformer 72 and the rectifiers 71 in conjunction with the cabinet vent apertures 180.