9  US 6,537,456 B2  10  I have found that by reliable hardness and non-hydroxide  alkalinity removal, to levels which effectively avoid forma› tion of scale at a selected pH for RO operation, the concen› tration of SiO$_{2 }$ in the RO reject stream can be safely  increased to 450 ppm or more. This is accomplished by  increasing the pH of the feedwater to the RO system, and  without use of scale-inhibition chemicals. Moreover, even  with this increase of silica in the RO reject, the level of silica  The first step is to adjust the hardness-to-alkalinity ratio of  the feedwater, if needed. Optimizing this ratio, which is 60  typically done by alkali addition, makes complete hardness  removal feasible in the next process step.  The second step in the RO process train involves the  utilization of a weak acid cation (WAC) resin (e.g. DOWEX  (R) MAC-3, or Lewatit CNP-80, Amberlite(R) IRC-86).  Operated in hydrogen form, the WAC resin removes hard› ness quantitatively, given the proper hardness-to-alkalinity  The selection of specific operations and control points to  fulfill the above process condition requirements is influ› enced by the characteristics of each specific feedwater. The  percent recovery needed ( or desired for a specific  application) also affects the operations and control point  criteria as well. FIG. 2 represents a highly cost effective RO  unit process sequence.  Scaling due to various scale forming compounds, such as  calcium sulfate, calcium carbonate, and the like, can be  predicted by those of ordinary skill in the art and to whom  this specification is directed, by use of the Langlier Satura- tion Index, as discussed above, or other available solubility  data. Operating parameters, including temperature, pH, per› meate and reject flow rates, must be properly accounted for,  as well as the various species of ions in the raw feedwater,  and those species added during pretreatment.  Feedwaters utilized for production of high purity water, as  well as those encountered in wastewater treatment, include  the presence of silicon dioxide ( also known as silica or  Si0$_{2}$ )  in one form or another, depending upon  pH  and the other  species present in the water. For membrane separation  systems, and in particular for RO type membrane separation  systems, scaling of the membrane due to silica is to be  religiously avoided. This is because (a) silica forms rela› tively hard scale that reduces productivity of the membrane,  (b) is usually rather difficult to remove, (c) the scale removal  process produces undesirable quantities of spent cleaning  chemicals, and (d) cleaning cycles result in undesirable and  unproductive off-line periods for the equipment. Therefore,  regardless of the level of silica in the incoming raw  feedwater, operation of conventional membrane separation  processes generally involves concentration of  Si0$_{2 }$ in the  high total dissolved solids  ("TDS")  stream to a level not  appreciably in excess of 150 ppm of SiO$_{2 }$ (as SiO$_{2}$ ).  Typically, RO systems are operated at lowered recovery  rates, where necessary, to prevent silica concentration in the  reject stream from exceeding roughly 150 ppm.  In order to operate an  RO  system with reject up to near  pH  11, or at about  pH  11, or above, several process conditions  must be met in order to effectively eliminate the potential for  scale formation on the  RO  membrane. Some of those  process conditions are also necessary for operating an  RO  system at very high recovery rate. Such process conditions  are as follows:  The next step consists of operating the RO system in such  a manner that the pH of the reject is approximately, but  preferably not appreciably higher than, 11.0. Note that this  pH 11 limitation is applicable simply with respect to cur› rently available RO membranes.  An exemplary membrane,  with the highest pH tolerance capability, is a FILMTEC type  FT30 membrane.  If RO membranes with a higher pH  tolerance capability become available in the future, then the  maximum allowable RO reject pH can be raised accordingly,  with concomitant benefits from the higher pH, in excess of  11.0.  Because of the very high concentration factors (i.e. per- cent recovery) allowed by my HERO brand RO process, the  RO feed pH is correspondingly lower. For example, in a  system operating at ninety percent (90%) recovery, a feed  pH of 10.0 will produce a reject stream at an approximate pH  of 11, provided that the RO feed is only slightly buffered by  the presence of carbonate, phosphate, etc. Unlike conven› tional RO systems, typically operated at about seventy five  percent (75%) recovery, a HERO brand RO system can be  routinely operated at ninety percent (90%) or greater  recovery, limited only by osmotic pressure of the RO reject.  The pH increase from RO feed to reject is magnified at very  high recoveries. Thus, the maximum allowable pH is spe› cifically applicable for the  RO  reject conditions.  The HERO brand RO system is highly site-specific.  Individual process steps are customized to fit the specific  feedwater at a specific site. Regardless of the difference in  pretreatment process for different sites, one process param› eter is common for all applications, namely that the RO  system is operated at the highest feasible reject pH. Con› sistent with the highest allowable  pH  limit for currently  available  RO  membranes (for example,  pH  11.0 for  FILMTEC(R)  brand  RO  elements), a typical  HERO  brand  RO  system is designed to operate at  pH  of up to approxi› mately 11, as measured in the  RO  reject stream.  In a fourth step, the acidified effluent, containing virtually  zero hardness and alkalinity, is then treated for carbon  dioxide removal. This removal can be accomplished in a  forced/induced draft decarbonator or in an existing vacuum  degasifier of either packed bed or gas permeable membrane  barrier design. The decarbonated, essentially zero hardness,  essentially zero alkalinity water, is then injected with a  soluble alkali, preferably for adjusting pH to 10.0 or higher,  and most preferably to the pH as needed to achieve pH up  to at or near 11.0 in the RO reject.  The third step involves adding acid to the WAC effluent  to destroy the remaining alkalinity, if any such alkalinity is  present. Total alkalinity removal at this step is important in  order to achieve very high recovery across the RO system.  ratio of the influent. The hydrogen ions liberated in the  cation exchange process react with the alkalinity and pro› duce carbonic acid (H$_{2}$ CO$_{3}$ ) 2 ,  which is dissolved in the  WAC effluent.  (8) Substantially higher flux is achieved.  (9) Reduced overall capital cost, compared to conven› tional RO systems.  (10) Reduced overall operating cost, compared to con- ventional RO systems.  (11) The complexity of an ultrapure water system is  significantly reduced.  (1) Calcium, magnesium, strontium, and barium concen› tration in the RO feed must be substantially eliminated,  preferably to near zero, and most preferably, to essen› tially zero.  (2) Aluminum, iron, and manganese content including  organically bound species, as well as the presence of  colloidal particles containing such materials, should be  substantially eliminated, and preferably to near zero.  (3) Buffering anions (specifically bicarbonate, or  carbonate, and/or phosphate species) should be reduced  to as low of a level as can be practically achieved. 