Suppression of CaCO₃ scale deposition by anti-scalants

The objective of this study was to compare the degree of CaCO₃ scale suppression provided by various anti-scalants belonging to different chemical categories. The scaling system examined consisted of a freely falling film of a hot aqueous solution of Ca(HCO₃)₂ flowing down the outer surface of a vertical pipe. Evaporative air-cooling of the falling film released gaseous CO₂ from the water, thereby creating a substantial CaCO₃ scaling potential. Scale suppression effectiveness was determined by comparing the amount of scale deposited on a test pipe fed with water dosed with an anti-scalant with the amount deposited on an identical reference pipe, fed with the same water but without anti-scalant dosage. In the absence of an inhibitor, the scale layer was found to grow linearly with time, past an induction period. In the presence of an anti-scalant, the length of the induction period was extended and scale growth was arrested at an asymptotic limiting thickness. Scale suppression was more effective in the case of feed waters having a relatively higher temperature and a somewhat lower scaling potential, as compared to feed waters at a lower temperature and a somewhat higher scaling potential. No major difference could be discerned among the tested anti-scalants in the falling film system; all showed a substantially similar scale suppression effectiveness, within a narrow range of concentrations (0.2 to 0.5 ppm). Preliminary results of tests, aiming to check the applicability of the above conclusions to membrane systems, are also presented.