The Solution-Diffusion Model
Reverse osmosis is governed by the solution-diffusion model: water dissolves into the dense polymer layer of the membrane, diffuses across under a pressure gradient, and desorbs on the permeate side. The driving force is the net pressure difference — applied transmembrane pressure minus the osmotic pressure difference across the membrane. When applied pressure exceeds osmotic pressure, water flows from the concentrated feed side to the dilute permeate side, reversing the natural osmotic direction.
Membrane Materials & Performance
The workhorse of modern RO is the thin-film composite (TFC) membrane: an ultrathin (100-200 nm) aromatic polyamide active layer on a porous polysulfone support. These membranes achieve remarkable performance — water permeabilities of 3-8 L/m²·h·bar with salt rejections exceeding 99.5%. Recent advances in aquaporin-inspired and graphene oxide membranes promise even higher permeabilities, though commercial deployment remains challenging.
Energy & Recovery Optimization
Energy consumption is the dominant operating cost of RO desalination. The thermodynamic minimum energy for seawater desalination at 50% recovery is about 1.06 kWh/m³, but real plants consume 2.5-4 kWh/m³ due to irreversibilities. Energy recovery devices (pressure exchangers) capture energy from the high-pressure brine reject stream, achieving efficiencies above 95% and reducing energy consumption by 50-60% compared to systems without recovery.
Concentration Polarization & Fouling
In practice, rejected solutes accumulate at the membrane surface, creating a concentration polarization layer that increases local osmotic pressure and reduces effective driving force. Combined with membrane fouling from organic matter, colloids, and biogrowth, this means real-world flux is always lower than the clean-water prediction. Pretreatment, crossflow velocity optimization, and periodic cleaning are essential to maintain long-term performance.