An evaluation of membrane materials for the treatment of highly concentrated suspended salt solutions in reverse osmosis and nanofiltration processes for desalination

This thesis presents a study to enhance and improve a zero liquid discharge (ZLD) reverse osmosis process that uses seed crystals to promote crystallization of the dissolved salts in the residual brine while it is being treated by identifying those membrane materials that are most suitable for the process. In the study, a one plate SEPA Cell module by GE Osmonics was used to determine which membranes were most susceptible to fouling and/or membrane hydrolysis. A cellulose acetate (CA), polyamide (PA) low MWCO, and PA high MWCO membrane were tested under reverse osmosis conditions. The CA and thin film (TF) membranes were also tested for nanofiltration. The cell was operated under conditions that were determined to be optimum for each membrane by the manufacturer, GE Osmonics. A high pressure, low flow, positive displacement diaphragm pump circulated the saturated calcium sulfate solution with 2 % suspended solids through the cell while the reject and permeate were recycled back to the feed, thereby preserving a saturated solution to promote crystal growth and simulate the seeded reverse osmosis process. The temperature was maintained constant by adding an ice pack to the feed vessel when necessary. The transmembrane pressure differential was maintained constant by adjusting a back pressure valve on the concentrate outlet. The results illustrate that if potable drinking water is the intended use, then the nanofiltration cellulose acetate membrane should be used. If irrigation is the desired use, then the nanofiltration thin film membrane should be used. Overall, the reverse osmosis cellulose acetate membrane was observed to outperform all membranes when all performance parameters were normalized. However, this membrane was observed to be prone to degradation in a seeded slurry and therefore its lifetime should be analyzed further. The polyamide membrane initially had a high water transport coefficient, but fouling led to its rapid decline which was attributed to the membrane?s rough and protrusive surface. A lifetime test on the thin film and cellulose acetate revealed that when operated at their maximum pressure specified by GE Osmonics for a duration of 8 hours that no decrease in rejection occurred.