The Development Of High PH Water Treatment

This paper introduces a new idea of treating circulating cooling waterwith high pH method. It discusses corrosion of carbon steel anddeposition of CaCO3 in high pH water. And the influence of high pH onthe growth and reproduction of bacteria is also investigated.The results of weight-losing corrosion test show that the higher thewater pH, the lower the corrosion rate of carbon steel is, when pH is9~11. Nevertheless, if pH is 11~13, the corrosion rate of carbon steelwill increase as raising pH in water. When pH is 11, the corrosion rateof carbon steel is the lowest, only 0.003mm/a. This method has betterefficient than using PBTC with zinc salt.Moreover, the results of test demonstrate that the corrosion rate ofcarbon steel in Beijing tap water containing Cl-which ranges from17mg?L-1 to 3000 mg? L-1 are less than 0.125mm/a when the water pH is10 to 13. Whereas, when [Cl-] is equal to 3000 mg? L-1 and pH is 11, thecorrosion rate is only 0.089mm/a. Therefore, it shows that by increasingwater pH, the influence of [Cl-] on the corrosion of carbon steel will beameliorated.The mechanism of corrosion in the high pH water is studied withelectrochemical method and the result demonstrates that the corrosionis dominated by cathode when pH is equal to 9. When pH ≥ 10, thecorrosion is controlled by anode, and the rate of corrosion decreases.However, if the water contains a large amount of Cl-, pH of thecathode-control turning to anode-control will increase. For example,when [Cl-] is 17 mg? L-1, its pH is 10. Otherwise, when [Cl-] is 3000 mg?L-1, the pH increases to 12, and the corrosion turns to anode-control.The results obtained by deposition and SEM indicate when pH isequal to 9, there is no CaCO3 crystal. when pH is 10 to 12, CaCO3crystal depositing from tested water mainly consists of polymorphicphases and calcite, and amount of the CaCO3 crystal and the depositingrate increases as water pH raises, but when pH is 13, the CaCO3 crystalis mainly composed of vaterite, its amount is less than that of pH 12.The test result of inhibiting heterotroph bacteria and (SRB) sulfatereducing bacteria in high pH water indicates that bactericidal effect isbetter as pH raises and bactericidal time goes on. For example, when pHis 11 and acting time is 48 hours, the sterilizing rate of heterotrophbacteria is 92.7%. High pH water is more effective on sterilizing sulfatereducing bacteria, for instance, when pH is 9 and acting time is 4 hours,the bactericidal rate of sulfate reducing bacteria is 99.2%.