Modeling and Optimization of the Forward Osmosis Process - Parameters Selection, Flux Prediction and Process Applications

Abstract

The forward osmosis (FO) process is a membrane process that makes use of the osmosis phenomenon for the transport of water from a feed solution to a draw solution across a highly-selective FO membrane. Several challenges of the FO process were identified, including limited advancement on theoretical modeling and prediction of FO performance, lack of an ideal FO draw solution and limited data to evaluate the feasibility of FO applications. The objective of this thesis is to investigate these challenges and systematically evaluate the feasibility of the FO process in water and wastewater treatment. In the first part of the study on FO modeling, the mass transfer coefficients derived from the boundary layer concept was used in the film theory model to describe the external concentration polarization (ECP) layer. A modified model for the internal concentration polarization (ICP) layer was proposed. It was shown that the revised models developed in this study could predict water fluxes and model both the ECP and ICP phenomenon for the FO process more accurately than the previous model proposed by other researchers. In the second part of the study on FO process modeling, water fluxes for the FO process using 6 different draw solutes were predicted using revised FO models proposed in this study. Previously modified ECP model (developed in the first part of this study) can predict the flux behavior for the FO process accurately with NaCl or KCl as the draw solute only. When other draw solutes were considered, the effects of dilution/suction and property (diffusivity) variation were included in the revised ECP model, so as to improve the accuracy of prediction. The revised ICP model with the solute specific KS proposed in this study could improve the accuracy of the ICP effect because of the different degree of interactions of the different solutes with the porous matrix membrane material. Further experiments were conducted to select the most appropriate draw solutions for the FO process, and at the same time a complementary reconcentration process was also proposed. Results obtained from laboratory-scale FO and NF tests suggest that both MgSO4 and Na2SO4 could be used as potential draw solutes for the hybrid FO-NF process. With the appropriate draw solutions proposed, results from the laboratory-scale FO and NF tests in the next phase suggested that Na2SO4 could possibly be the most suitable draw solution for the proposed hybrid FO-NF process for seawater desalination. In order to produce good quality product water that meets the recommended TDS of the GDWQ from WHO, a hybrid FO-NF process with two-pass NF regeneration was proposed. Finally in the last phase, feasibility investigations were conducted on a hybrid FO-MBR with NF post treatment process for domestic wastewater treatment. First, CFD simulation was used to understand the draw solution fluid flow within a plate-and-frame FO module and modifications were conducted to fabricate a more effective module. A modified 6-chamber membrane module was designed and fabricated. With the optimized membrane module, two separate tests were conducted to investigate the effect of different mean-cell residence time (MCRT) on FO-MBR operations (3-, 5-, and 10-day MCRT) and the effect of backwash and chemical cleaning to mitigate flux decline. Results from both studies indicated that mixed liquor conductivity increase had a large impact on flux decline as conductivity was linked to solute concentration that was further linked to the osmotic driving force. In addition, final permeate water quality indicated that the hybrid FO-MBR system had high organic removal, largely due to the non-porous FO membrane that was capable of retaining most of the solute within the mixed liquor. However, it was found that the final permeate had high concentrations of nitrate.