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<br />l- <br />(J) <br />w <br />0) <br /> <br />Most makeup waters contain carbon dioxide in excess of the concentraticn <br />in equilibrium with the atmosphere. Quite high alkalinities occur, nearly all <br />as bicarbonate (HCO;). In.the cooling tower there is a tendencJ for CO2 to be <br />~ <br />driven off converting some of the alkalinity to carbonate (CO)) and causing <br />precipitation. This is common and is usually prevented by adding sulfuric <br />acid which replaces some of the alkalinity with equivalent sulfate. <br />Scale by calcium sulfate is sometimes prevented by softening the makeup <br />wate~. Lime and soda ash are added to precipitate calcium. This has the <br />added advantage of removing some carbonate alkalinity as well. If desired, <br />t~e softener can be operated to precipitate magnesium as hycroxide. Silica <br />quite readily adsorbs onto the surface of magnesium hydroxide and is partly <br />removed with the maqnesi~, wh1ch is advantageous. <br />Gasification wastewate~ can be useful as makeup water to the cooling <br />system because it is usually lower in non-volatile dissolved so lies than river <br />water. <br />Suspended solids, if not sufficiently controlled by blowdcwn, can be ana <br />have been cont=olled by a side stre~ clarifier or filter. <br />Corrosion is con~rolled by operating at as high a pH as is acceptable <br />(just at the point of scale formatio~) and by dosing the ci:culdt:ng ~ater <br />with anti-corrosion chemicals. The best anti-corrosion chemical is hexavalen~ <br />chromium. Fortunately, it is not the only anti-corrosion chemical and it is <br />not essential in alkaline waters. Mixtures of such chemicals as hydroxyethilide~e <br />diphosphonate, sodium molybdate, benzotriazole and sodium gluconate have been <br />14 to 17 <br />demonstrated to be adequate . If chromium is used, it must be removed <br />from the blowdown as its return to a river is not permitted under present water <br />quality standards. Removal of chromium ==orn blowdown is par~ of the cost of <br />using chromium and need not be considered in this study. <br />When treated wastewater is added to the cooling system, the ccn~rol of <br />biological growth can become a serious problem, since the water contains <br />nutrients. P~levant experience has been obtained in the petrOleum incusty and <br />it has been shown that, within lLmits, organic contamination is per.nissable in <br />cooling water makeup. <br /> <br />31. <br />