|Ph.D Student||Lisitsin Dmitry|
|Subject||Study of Techniques for Increasing Water Recovery in|
Brackish Water Desalination
|Department||Department of Chemical Engineering||Supervisors||Professor Emeritus Raphael Semiat|
|Professor Emeritus David Hasson|
|Full Thesis text|
It is widely recognized that reverse osmosis is the most cost effective technique for producing potable water. Being less expensive than sea water desalination, desalination of brackish water has a strong potential for much wider application. However, the widespread use of inland brackish water desalination is limited by the concentrate disposal problem.
The objective of the present work was to investigate fundamental aspects of techniques for augmenting water recovery in brackish water RO desalination. The basic concept for increasing the water recovery was extracting additional permeate from the concentrate stream after precipitating and separating the scaling salts.
The first part of the research work was focused on examination of several promising anti-scalant destabilization techniques in the CaCO3 system.
Ø Experimental data was obtained in a continuous flow precipitation vessel of 21 L operated at retention times ranging from 1 to 4 hr. The most effective destabilization technique was by pH increase. High pH levels induce a high CaCO3 supersaturation level beyond the inhibitory capability of the anti-scalant.
Ø A novel destabilization technique was developed based on aeration of concentrate solution which induces a marked CO2 desorption effect. Effective desorption of CO2 has a strong destabilization effect through a significant pH increase. This was confirmed in experiments carried out with solutions simulating concentrates of different Israeli brackish water desalination plants. Conversion levels as high as 60% to 90% CaCO3 precipitation were achieved at retention times of 1-2 hr.
The second part of the research was dedicated to consolidation of the destabilization technique based on CO2 stripping and quantification of the effect of anti-scalants on continuous CaCO3 precipitation.
Ø A model integrating the kinetics of CO2 desorption and CaCO3 crystallization was developed. The model was confirmed by data measured at temperatures in the range of 25 to 370C in both batch and continuous aerated crystallization, covering a wide range of parameters. The activation energy of 10.4 kcal/mole is in excellent agreement with literature data.
Ø Available kinetic information on CaCO3 crystallization retarded by anti-scalants is very scant. In this work, the kinetic data of retarded CaCO3 crystallization was measured under continuous flow conditions over a wide range of parameters. The data were successfully correlated by a model incorporating crystal growth kinetics and growth retardation due to anti-scalant adsorption on active growth sites.
The basic work conducted in this project played a central role in the consolidation of a practical enhanced water recovery desalination process currently being developed with industry.