STUDIES ON WATER DESALINATION USING OSMOTIC PRESSURE-DRIVEN PROCESSES

Abstract

The global population growth has intensified the demand on finite freshwater resources. Membrane separation technologies offer economically viable solutions for seawater desalination. Forward osmosis (FO) has emerged as a promising alternative to reverse osmosis (RO) due to its operation without external hydraulic pressure and inherently lower membrane fouling propensity. However, the commercialization of FO remains limited due to the lack of optimal draw solutions and energy-intensive regeneration processes. This thesis investigates the development of novel phase- separating binary and ternary organic draw solutions for brackish and seawater desalination using FO, exploiting their lower critical solution temperature (LCST) behaviours to enable cost-effective draw solution regeneration. The research establishes theoretical foundations through data-driven techniques using Artificial Neural Networks (ANN) to address the limitations of traditional solution-diffusion models, particularly for multi-component and neutral draw solutions. Through systematic experimental procedures, the study evaluates 4 binary and 6 ternary systems incorporating sodium carboxymethyl cellulose (NaCMC) and propylene glycol propyl ether (PGPE). This is followed by sixteen distinct draw solution compositions of hydroxypropyl cellulose (HPC) and PGPE, including single-solute and ternary mixtures with varying HPC (0.25 – 2 wt.%) and PGPE (1.25 - 3.75 M) concentrations. The research employs comprehensive characterization techniques to analyze physico-chemical properties and osmotic performance, including measurements of osmotic pressure, viscosity, concentration, pH, density, cloud point determinations and membrane compatibility. The experiments were conducted using a custom FO setup with a harvested HTI CTA membrane in AL-FS mode. An ANN model incorporating nine input parameters - FO run details, temperatures, concentrations, flow rates and draw solution molecular weights - predicts permeate fluxes, developed using 312 experimental data points collected during 120-minute FO runs with various draw solutions. The work includes a thorough techno-economic assessment evaluating both operating expenditures (OPEX) and capital expenditures (CAPEX) for the FO process and phase separation (PS) draw regeneration, vii ultimately providing significant research outcomes and recommendations for advancing the study of phase-separating organic draw solutions in FO desalination.