Modeling Water Flux in Forward Osmosis: Implications for Improved Membrane Design

Abstract

In efforts to make fresh water available to all people, researchers are dedicated to establish a water treatment method that will reduce the cost of production and impact on the environment. Forward osmosis desalination has been under the spotlight as a candidate of being a revolutionary water treatment method. Nevertheless, forward osmosis is faced with obstacles that hinder it from being commercially available. One of the main forward osmosis problems is low flux induced by concentration polarization and inadequate membrane design. To examine the problem, a commercially available forward osmosis membrane was tested using two different draw solutions. Using different feed solutions, experimental flux was determined and flux modeling was performed for the system in hand. The flux model selected was a good fit to experimental data for all draw solutions used; NaCl, magnesium sulfate and copper sulfate. The model was tested on our experimental data and other researchers' data. The flux model was found to be in line with experimental data for all systems at various operating conditions. It was found that dilutive internal concentration polarization (ICP) had a significant impact on flux. The overall driving force was reduced by dilutive ICP which caused a substantial reduction in the flux. It was determined that to reduce Dilutive ICP, solute resistance to diffusion (K) had to be minimized. Results also indicated that minimizing solute resistance to diffusion (K) achieved higher flux. It was also concluded that concentrative external concentration polarization (ECP) had a minor impact on flux. Varying feed mass transfer coefficient, a factor controlling concentrative ECP, had a small effect on flux. Magnesium sulfate and copper sulfate draw solutions were compared in terms of flux; it was found that coppers sulfate generates higher flux.