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PhD Dissertation Defense - Heba Alnajjar

Start Date: May 24, 2017
End Date: May 23, 2017

TITLE: Performance Evaluation of Carbon Dioxide Nucleation as a Novel Cleaning Method for Reverse Osmosis Membranes
ADVISORS: Professors Noreddine Ghaffour & Peng Wang 
DATE: Wednesday, May 24, 2017
TIME: 3:00 - 4:00 p.m.
LOCATION: Building 5 · Level 5 · Room 5209

Reverse osmosis (RO) is being increasingly used in water desalination, and membrane fouling is the main operational problem in RO desalination plants. Among other solutes, dissolved biopolymers, such as polysaccharides can lead to severe membrane fouling especially with the addition of calcium ions due to the formation of complexation between foulants and the membrane surface. However, complexation can also occur in the feed solution (bulk complexation), resulting in the formation of foulants aggregates. Although there are some physical techniques that can maintain the membrane performance without reducing its lifetime, only chemical cleaning methods are still widely used in RO industry. In this study, a novel cleaning method is proposed to restore the membrane performance by removing the deposited foulants without reducing the membrane lifetime. The cleaning method is based on using water saturated with dissolved CO2 gas, and its principle is based on producing spontaneous CO2 bubbles due to local pressure difference leading to nucleation of bubbles throughout the membrane surface, especially at nucleation sites, which improve the cleaning efficiency. Alginic acid sodium salt was used as a model of polysaccharides foulants in presence of different concentrations of NaCl and calcium ions aiming to enhance membrane fouling, and then CO2 cleaning solution efficiency, in terms flux recovery (FR), was tested under different operating conditions and compared to other cleaning methods. Average FR of 20%±3, 25%±3 and 80%±3 for MilliQ water, a cleaning solution at pH4, and CO2 solution at 6 bars, 0.17 m/s, and 23 ̊C ±0.2 for 6 minutes were obtained, respectively. The efficiency of this novel cleaning method was also compared to direct osmosis overnight, and the average flux was comparable (about 60%±3), though that the cleaning time was significantly different. Various calcium concentrations (0-10 mM) were added in the alginate solution to study the fouling behavior in terms of the potential for bulk complexation to form cake alginate layer on the membrane surface rather than a gel layer, and the role of CO2 bubbles nucleation to remove foulants was investigated. This cleaning method can be considered as an alternative more environmentally friendly technique in RO application.