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<br />t- <br /> <br />en We have also esti~ated the reverse osmosis costs for t~e concit~ons of <br /> <br /><.0 <br />~ Table 4-7 and find a cost of $0.94/1000 gal (; SJ06/acre-ft) :~r the lower <br /> <br />flow rate and about SO.67/1000 gal (; S218/ac=e-ft) for the h>gher rate. The <br /> <br />cost is essentially independent of concentraeicn for the ci::ere~ces in :nitial <br /> <br />TDS considered, and is about equally divided betwee~ O&~ and debt ser.ice. <br /> <br />The selection of one syste~ over the otter clearly cannot be made on <br /> <br />costs alone since t~ey appear to be sa close. In favor of electrodialysis is <br /> <br />that it is a well tested and reliable system with units now operating with <br /> <br />ca9acities of 4 to 5 mqd. Another L~portant :actor is that modified in sit~ <br /> <br />cgerations envisaqa generating electricity on ~its so that d.c. power could be <br /> <br />prcdu=ec directly. T~is could result in an important reduction in t~e capital <br /> <br />cost 0: the system s~nce the need for rectifiers is eliminated. It might be <br /> <br />noted that =~e origina~ ~ornrr.ercial developmen~ of el=ctrocialysis was fer t~e <br /> <br />trea~~ent of ~i~e dr~i~age ~ater from a gold ~ine i~ Sout~ Af~ica. !n :avor <br /> <br />of reverse osmosis is tha~ if organics are ?rese~t, the system has a rnod~~ate <br /> <br />capabili~1 for organics rejection. r~ also c~, yield a procuct ~ater wit~ a <br />lower dissolved solids :ontent ~~an elec~=odialysis. ~cwever, large scale <br />i~stallacior.s of t~e ~ype :equired are only now being install~d ~~d lcng te~ <br />cperating ~xperience is not as yet available. <br />~e ~ave assumed in our trea~ent schemes that ~o~o~ r~~va~ will be <br />car:ied cut sepa~ate17 ~y means of selective ion exchange. Using Rohm & Haas' <br />~erlit= lRA-743 resi~, which has ~ high sele=tivity :or boren, we fir.d t~at <br /> <br />the t=eatment cost ranges f=om SO.13/1000 gal (= S42/acre-ft) to SO.30/l000 <br />gal (; S9B/acre-ft). The lower figure cor=esponds to ~~e highe= flow rate of <br />9,400 gpm (= 15,230 acre-ft/y~) and ~~e highe~ one to ~~c lower rate of 2,150 <br />gpm (~ 3,460 acre-ft/yr). The cost of ~he ion exchange treatment at the high <br />flow rate is more than 70' ca?ital cost and at ~~e low :low ra~e ~ore than <br />85\. The regenerant waste is small and may be assumed to be disposed wi~h the <br />concentrate from either the reve:se osmosis or electrodialysis systems. <br />A cost no~ so far discussec is that for the concentrate disposal. If <br />we assume ~~ 85\ reco~~rf and 90\ separation of ~~e dissolved solics, then <br />the waste stream has about 6 times the concentration of the mi~~ wate~ and <br />abcu~ one-seventh its volume. The cost of disposal of t~is stream is a major <br /> <br />36 <br />