Advances in Water Treatment and Enviromental Management

194 WATER TREATMENTFIGURE 1 NITRATE CAPACITYThe graph shows the resin’s working capacity with respect to nitrate only. The conventional resin normallyhas a low nitrate capacity because it uses up much of its working capacity in removing the sulphate in thewater as well. The conventional resin performs relatively badly at the low regeneration level, which is themost economical in terms of regenerant usage.This water calls for brine and bicarbonate regeneration to give the final blended product the desiredchloridetbicarbonate ratio. Two points have been superimposed which show the reduced regeneration efficiencydue to lower effectiveness of bicarbonate as a regenerant as compared with chloride. On these points thebicarbonate wash represented 10% and 25% respectively of the total regeneration levelThe graph also shows two “Washout” results for brine/bicarbonate regeneration in which the bicarbonateaccounted for 40% of the total regeneration level. Washout was performed with 10 bed volumes of 8 eq/1sulphate solution. Their results suggest that on this raw water analysis the process brings the conventionalresin’s performance with bicarbonate regeneration to the same level as that of the selective resins, at leastat the low regeneration levels which have so far been explored.This set of process details was determined by an exhaustive computer simulation. Its predictions weretested first on a made-up water in the laboratory and then on the actual water in the field. The resultssuggested that the Washout process with co-flow regeneration of conventional resin has a lower operatingcost (in terms of regenerant usage) than conventional operation of the nitrate-selective resin.In this case the eluates from the actual regeneration by chloride and bicarbonate are both completelyretained for use as washout in the next cycle. Apart from relatively clean streams such as backwash andrinse, only the washout step eluate has to be disposed of, and this contains mostly nitrate and sulphate.The results shown in Figure 1 are field results. The field trials varied considerably from run to run (as ionexchange processes tend to do) but when averaged, the results confirm the computer’s predictions shown inTable 2. Table 2 also compares the use of a conventional resin with Washout, with that of a selective resinin the normal manner. The criteria for these tests where that the nitrate level of the blended product shouldnot exceed 0.8 mg equiv 1 -1 , and that its chloride:bicarbonate ratio should not rise above 0.5.Figure 2 shows the analyses of blended product water produced by these two processes. The last two linesin each half of Table 2 give the waste volume and the total ions discharged by the respective processes.While the Washout process produces a far greater volume, its total salt content, and especially its chloridecontent, is much smaller.