On the 27th of April 2011, Mr. Tarek Kamal Abdalla Waly successfully presented and defended his PhD thesis and was awarded with a Doctoral degree. Professor dr. G.L. Amy and Prof. dr. G-J. Witkamp were his promoters. The PhD research focused on 'Minimizing the Use of Chemicals to Control Scaling in SWRO. Improved prediction of the scaling potential of calcium carbonate'.
Water stress in the MENA region has led to the implementation of sea water desalination as one of the options for augmenting freshwater resources. Reducing the amount of chemicals employed in the pre-treatment of seawater reverse osmosis plants will improve the sustainability of the process, and result in further cost savings. The aim of this research was to reduce the consumption of chemicals (acids and antiscalants) employed for calcium carbonate scaling control in sea water reverse osmosis systems.
The approach adopted in the thesis was to improve the accuracy of calcium carbonate scaling prediction, and subsequently prevent overdosing the chemicals employed to control scaling. Firstly, the scaling potential was estimated employing theoretical calculations and experimental data. Secondly, the kinetics of precipitation were investigated employing induction time measurements at different levels of super saturation. The outcome of this study showed that the degree of superstation was lower than commonly expected. An important reason was that the calculated pH value of sea water reverse osmosis concentrate was systematically higher than actually measured in full scale plants. In addition, kinetic studies indicated that vaterite precipitated instead of calcite. Consequently, the supersaturation of sea water reverse osmosis concentrate is significantly lower than expected when calcite is assumed to be the precipitating phase.
Finally, induction time measurements showed rather long induction times in artificial SWRO concentrates. These findings suggest that reducing or even eliminating acid/antiscalant dosing to prevent calcium carbonate scaling may be possible in sea water reverse osmosis plants. This hypothesis is supported by the fact that a limited number of sea water reverse osmosis plants no longer apply acid or antiscalant to control calcium carbonate scaling. Moreover, a sea water reverse osmosis pilot plant in the Netherlands has been in operation for more than 6 months without acid or antiscalant, with no signs of scaling.
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