11.2 Draw solutions and membrane development for FO

Forward osmosis is gaining attention as an emerging membrane technology for water treatment. This section focuses on draw solutions and membrane development, two crucial aspects that determine FO efficiency and performance.

Draw solutions create the osmotic pressure gradient needed for FO, while membrane materials and structures affect water flux and solute rejection. Understanding these components is key to optimizing FO systems for various applications.

Draw Solutions

Osmotic Agents and Thermolytic Solutes

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• Osmotic agents create the osmotic pressure gradient required for forward osmosis (FO) processes by having a higher concentration than the feed solution
• Thermolytic solutes are draw solutes that can be easily regenerated using thermal methods such as heating or evaporation (ammonium bicarbonate, sulfur dioxide)
• Ammonium bicarbonate ($NH_4HCO_3$) decomposes into ammonia ($NH_3$) and carbon dioxide ($CO_2$) gases when heated, allowing for easy separation and recovery of the draw solution
• Sulfur dioxide ($SO_2$) can be removed from the draw solution by heating, as it has a relatively low boiling point and high solubility in water

Magnetic Nanoparticles and Hydrogels

• Magnetic nanoparticles can be used as draw solutes in FO processes and can be easily separated from the product water using an external magnetic field
• Nanoparticles are typically coated with hydrophilic materials to increase their solubility and prevent aggregation in the draw solution (polyethylene glycol, dextran)
• Hydrogels are cross-linked polymer networks that can absorb large amounts of water and be used as draw agents in FO processes
• The swelling and deswelling of hydrogels in response to external stimuli (temperature, pH, light) can be exploited for easy regeneration of the draw solution
• Hydrogels can be designed to have high osmotic pressures and low reverse solute flux, making them suitable for FO applications

Membrane Materials

Thin-Film Composite and Cellulose Triacetate Membranes

• Thin-film composite (TFC) membranes consist of a thin, selective layer (polyamide) deposited on a porous support layer (polysulfone)
• TFC membranes offer high water permeability, salt rejection, and stability in a wide range of pH and temperature conditions
• Cellulose triacetate (CTA) membranes are asymmetric membranes made from cellulose acetate polymer
• CTA membranes have lower water permeability and salt rejection compared to TFC membranes but exhibit better chlorine resistance and lower fouling propensity

Membrane Support Layer

• The support layer provides mechanical strength and stability to the thin selective layer in TFC membranes
• Support layers are typically made from porous polymeric materials (polysulfone, polyethersulfone) with high porosity and low tortuosity
• The structure and properties of the support layer can significantly impact the performance of the FO membrane by affecting internal concentration polarization (ICP) and mass transfer resistance
• Ideal support layers should have high porosity, low tortuosity, and minimal thickness to reduce ICP and enhance water flux

Membrane Performance Challenges

Membrane Fouling

• Membrane fouling occurs when suspended solids, organic matter, or microorganisms accumulate on the membrane surface or within its pores, reducing water flux and membrane performance
• Fouling can be classified into four main types: organic fouling (natural organic matter), inorganic scaling (mineral precipitation), biofouling (microbial growth), and colloidal fouling (deposition of colloidal particles)
• Strategies to mitigate fouling include pretreatment of feed water, modification of membrane surface properties (hydrophilicity, charge, roughness), and optimization of operating conditions (cross-flow velocity, temperature)
• Periodic cleaning of membranes using physical (backwashing, air scouring) or chemical methods (acid, base, or oxidant solutions) is necessary to restore membrane performance

Internal Concentration Polarization

• Internal concentration polarization (ICP) is a phenomenon unique to FO processes, where the concentration of solutes within the porous support layer differs from that in the bulk draw solution
• ICP occurs due to the hindered diffusion of solutes within the support layer, leading to a reduced effective osmotic pressure gradient across the active layer
• Dilutive ICP occurs on the feed side of the membrane, where the solute concentration at the active layer-support layer interface is lower than that in the bulk feed solution
• Concentrative ICP occurs on the draw side of the membrane, where the solute concentration at the active layer-support layer interface is higher than that in the bulk draw solution
• ICP can significantly reduce water flux in FO processes and is influenced by factors such as membrane properties (porosity, tortuosity, thickness) and operating conditions (draw solution concentration, cross-flow velocity)
• Strategies to mitigate ICP include optimizing the structure and properties of the support layer, using high cross-flow velocities, and employing ultrathin or double-skinned membrane designs