Ph.D Thesis

Ph.D StudentDrak Alexander
SubjectCharacterization of Water Recovery Limits in RO Desalination
DepartmentDepartment of Chemical Engineering
Supervisors PROFESSOR EMERITUS Raphael Semiat


Reverse osmosis desalination has achieved prominence as the most cost effective desalting technology. It is generally recognized that RO technology has not reached maturity and there is a wide scope for process improvements. The objective of the present work was to investigate two largely unresolved design uncertainties: the difficulty in determining threshold scaling limits which govern the allowable product water recovery and the lack of adequate tools for diagnosing mal functioning of the generally adopted spiral wound module design.

In RO desalination it is essential to restrict the fractional recovery of purified water below a critical threshold limit at which there is a risk of scale precipitation. Currently, there is no satisfactory laboratory test for characterization of threshold limits of scaling and for confident selection of the most cost effective anti-scalant. The only reliable technique involves time consuming pilot plant tests.

Major efforts were devoted for developing and consolidating rational laboratory techniques for reliable characterization of scaling threshold limits and evaluation of the performance of anti-scalants used to extend water recovery limits. This study was performed within the framework of an EC funded project involving industrial and academic partners. The techniques studied were of three types: once through flow, batch recycle and intermittent recycle.

The other main direction investigated in this work is related to the problem of characterizing the complex flow patterns prevailing in a commonly used spiral wound modules. Efforts were undertaken to apply residence time distribution (RTD) techniques combined with concentration polarization (CP) determinations for characterizing the degree of uniformity of flow in spiral wound membrane passages and to develop tools for diagnosing flow distribution defects.