Ph.D Thesis

Ph.D StudentSegev Raviv
SubjectImprovement of Brackish water Desalination Process
DepartmentDepartment of Chemical Engineering
Supervisors PROFESSOR EMERITUS Raphael Semiat
Full Thesis textFull thesis text - English Version


Brackish water desalination has a vast untapped potential for alleviating water scarcity. However, most brackish water sources are located inland. The difficulty of environmentally safe disposal of the concentrate waste stream renders inland brackish water desalination uneconomic in many cases.  There is therefore considerable incentive for increasing the water recovery fraction in brackish water desalination.

A viable process allowing increased recovery augments the volume of the desalinated water, thus acting to lower the unit cost of the water product. More important, by increasing the water recovery fraction the concentrate volume is reduced thus simplifying the disposal difficulty.

An attractive possibility which is the main subject of this thesis is to achieve a high recovery desalination process without the use of any chemicals.  Dosage of chemicals inducing a pH increase for precipitating alkaline scale components in the concentrate can be completely eliminated in the case of commonly encountered concentrates having relatively high carbonate contents. Air stripping of carbonate rich solutions can provide very high pH levels enabling precipitation of the scaling species.

A desalination process based on air stripping precipitation was developed and tested in a pilot plant constructed at a brackish water desalination plant in Atlit, Israel. An overall water recovery of 90% was achieved at a CaCO3 crystallizers retention time of 2 hr.  Further laboratory work indicated that the above high water recovery process can be considerably improved by using a fluidized bed (FBR) air stripping system. The fluidized bed unit investigated was coupled with a CO2 air stripping column generating high pH precipitation conditions. Precipitation occurred on the fluidized bed sand particles. Overall precipitation conversions as high as 98% were achieved in the FBR system as compared to conversions of about 70% in MSMPR crystallization at similar retention times of about 15 minutes.

Another major contribution of this research was the development of a theoretical precipitation models. Two papers provide a rigorous kinetic model for CaCO3 precipitation in pipe flow taking into account both mass transport and surface reaction aspects. These models were extended to the case of CaCO3 precipitation under FBR conditions taking into account the accompanying air stripping process.