Oct. 24, 2002  8  US 2002/0153319 Al  method, as illustrated in  FIG. 2,  are quite efficient in the  removal of hardness associated with alkalinity. Such a  reaction proceeds as follows:  [0102]  Then, the hydrogen combines with the bicarbonate  to form carbonic acid, which when depressurized, forms  water and carbon dioxide, as follows:  [0103]  Regeneration of the resin is accomplished by use of  conveniently available and cost effective acid. It is well  known by those in the art that regeneration of  WAC  ion› exchange resins may proceed quite efficiently, at near sto› ichiometric levels (generally, not more than about one  hundred and twenty percent (120%) of ideal levels). Pref› erably, hydrochloric acid may be used, since in such cases  highly soluble calcium chloride would be produced, and the  regeneration process would not pose the potential danger of  formation of insoluble sulfate precipitates, such as calcium  sulfate, even with high strength acids. However, by use of a  staged regeneration procedures, i.e., by using a low concen› tration acid followed by a higher concentration acid, it is  possible to reliably utilize other acids, including sulfuric  acid  (H$_{2}$SO $_{4}$ ),  while still avoiding undesirable precipitates  on the resin. In this manner, hardness ions are solubilized to  form soluble salts, which are eluted from the resin bed and  are typically sewered. Use of sulfuric acid is particularly  advantageous in semiconductor manufacturing operations,  since such plants typically use large quantities of such acid,  and waste or spent acid may be advantageously utilized for  regeneration of a weak acid cation exchange bed.  [0104]  Other polyvalent cations, most commonly iron  (Fe++/Fe+++), magnesium (Mg++), barium (Ba++), strontium  (Sr++), aluminum (Al+++), and manganese (Mn++;Mn++++),  are also removed by the WAC resin. Since the presence of  even very small quantities of hardness or other species of  decreasing solubility at increasing  pH  will result in precipi› tation of sparingly soluble salts under the process conditions  present in my process, care must be taken to prevent  precipitation on the membrane of the substances such as of  calcium carbonate, calcium hydroxide, magnesium hydrox› ide, and magnesium silicate. One precaution that should be  observed is that both hardness and non-hydroxide forms of  alkalinity should be aggressively reduced in the feedwater,  prior to upward  pH  adjustment to selected  RO  operating  conditions. Once hardness and non-hydroxide forms of  alkalinity have been removed, then the desired  pH  increase  may be accomplished with any convenient alkali source,  such as sodium or potassium alkali, or by anion exchange.  Once this pretreatment has been thoroughly accomplished,  then an  RO  system can be safely operated at very high  pH  levels, in order to take advantage of the aforementioned  silica solubility.  [0105]  In cases where raw water composition is such that  sodium zeolite softening is advantageous, as is depicted in  FIG. 3,  elimination of calcium hardness proceeds as fol› lows:  [0106]  Then, bicarbonate alkalinity is converted to carbon  dioxide, with a selected acid, in a manner similar to the  following:  [0107]  For those waters where lime softening may be an  acceptable or preferred method for initial hardness and  alkalinity reduction, the addition of lime to the water reduces  calcium and magnesium hardness, and associated bicarbon› ate alkalinity, as follows:  [0108]  This process configuration is depicted as an alter› nate embodiment of my method, as illustrated in  FIGS. 3  and 4.  In the cases where lime or lime/soda softening is  used, however, extreme care must be used in evaluating the  performance of the remainder of the pre-treatment system,  since the solubility of hardness ions remains appreciable in  the softener  52  effluent stream  54.  [0109]  For most feedwaters, particularly where lime or  lime/soda softening is not employed, the use of a carbon  dioxide removal step significantly enhances cost-effective› ness of the process when carried out prior to the pH increase.  This also helps to maintain a lower total alkalinity level in  the feed to the RO, thus providing a greater margin of safety  against scaling due to hardness leakage from the cation  removal step. Dealkalization by carbon dioxide removal also  helps to enhance silica rejection, due to the lack of compet› ing species. This is because the rejection of one weakly  ionized anion is affected by the presence and concentration  of other weakly ionized anions in the feedwater; this applies  to weakly ionized anions such as boron, organic acids  (TOC), cyanide, fluoride, and certain arsenic and selenium  compounds.  [0110]  Since the high pH operation also increases ioniza› tion of other weakly ionized anions, including borate,  organic acids (TOC), cyanide, fluoride, and certain arsenic  and selenium compounds, their rejection rates are enhanced  in an RO membrane system. Consequently, in general, my  method may be advantageously applied to reject across the  membrane most weak acids with a pKal of about 7.5 or  higher. Silica rejection can be increased to about 99.95%, or  more, from a conventional baseline of about 99% rejection;  this amounts to at least one order of magnitude decrease in  the amount of silica escaping into the permeate, thus pro› viding a ten plus (10+) fold increase in running life for the  silica scavenging ion-exchange resin bed, namely anion  exchanger  42  and the mixed bed units.  [0111]  In the case of cyanide, rejections in a first pass RO  of in excess of ninety percent (90%) can be attained, in  contrast with a more typical range of about fifty percent  (50%) or so with conventional RO processes.  [0112]  Similar to the case for silica, boron rejection can be  increased from a conventional baseline from a range of  about 60-70% to 99% and higher, by operation at a suitably  high pH. The beneficial effects on rejection percentage due  to higher pH operation start at a slightly lower pH in the case  of boron, since the pKa for boron is 9.14, roughly one-half  pH unit higher than that for orthosilic acid, namely 9.7. The  beneficial effects of high pH operation are much more  pronounced in the case of boron, however, because ortho› silic acid (H$_{2}$SiO$_{4}$ ) in aqueous solution typically includes six  ca+ $^{2}$+Na$_{2}$X  -caX+2Na+  NaHC0$_{3}$+HCl-NaCl+H$_{2}$0+C0$_{2 }$ ca+++2RCOOH -(RC00)$_{2}$Ca+2H+  H+ +HC0$_{3 }$ - -H$_{2}$C0$_{3}$-H$_{2}$0+C0$_{2 }$ Ca(HC0 3 )2 + Ca(OH)2 - 2 CaC03t + 2 H20  Mg(HC0 3 ) 2  + 2Ca(OH) 2  - Mg(OH)2t + 2 CaC03 + 2 H20 