Osmosis is typically defined as the net movement of water across a selectively permeable membrane driven by an across-membrane osmotic pressure gap.

Extended reading: What is Forward Osmosis?

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Figure 1. Water flow pattern in forward osmosis (FO), pressure retarded osmosis (PRO) and reverse osmosis (RO). High salinity water, such as brine, presents higher osmotic pressure (π) than low salinity ones (feed). For a typical FO system, it directly exploits this osmotic pressure difference (Δπ) to generate water flux without external hydraulic pressure (ΔP=P1-P2=0). For PRO and RO systems, an external hydraulic pressure (ΔP) is applied at high salinity side with different magnitudes. As the increase of ΔP, the water flux first decreases under PRO till it reaches the flux reversal point where ΔP=Δπ, followed by an increase with reversed flux direction (RO) if this trend continues.

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Figure 2. Typical configurations of forward osmosis system: submerged FO and cross-flow FO. For a submerged FO, this one-chamber system is filled with draw solution, and will continuously harvest water from the external feed vessel. A pump is implemented for providing proper recirculation. For a cross-flow FO, both draw and feed solutions are pumped separately into a sealed two-chamber module with embedded semi-permeable membrane. A higher water flux can usually be obtained under cross-flow FO system. However, more energy is required by providing elevated cross-flow rate, rendering a larger footprint.

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Figure 3. Configurations of coupled systems targeting at recovery and regeneration of draw solutes: (A) coupled RO system for draw concentration, (B) coupled membrane distillation (MD) for draw isolation with investment of waste heat, (C) coupled electrodialysis (ED) with ion exchange membrane, (D) coupled heat separation and subsequent adsorption for NH3+CO2 regeneration.

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Figure 4. Enhancing wastewater reuse through FO system by utilizing commercial solid fertilizers as draw solutes. Fertilizers are suggested as draw solutes to eliminate the need of regeneration. The self-diluted fertilizer can be directly used for agricultural irrigation, providing an alternative to bypass energy–intensive draw solute recovery process.