What Is Reverse Osmosis (RO)?
Reverse osmosis (RO), also called inverse osmosis, is a membrane separation operation driven by a pressure difference that separates solvent from a solution. When pressure exceeding the osmotic pressure is applied to the feed on one side of the membrane, the solvent flows counter to natural osmosis—reverse osmosis. The low-pressure side yields the permeate (the passed-through solvent); the high-pressure side yields the concentrate. Treating seawater this way gives fresh water on the low-pressure side and brine on the high-pressure side.
The solvent permeation rate (volume flux) N is:
N = Kh(Δp − Δπ)
where Kh is the hydraulic permeability coefficient (slightly increasing with temperature), Δp is the hydrostatic pressure difference across the membrane, and Δπ is the osmotic-pressure difference. For a dilute solution, osmotic pressure π = iCRT (i = number of ions produced by dissociation of the solute molecule; C = molar concentration; R = universal gas constant; T = absolute temperature).
RO usually employs asymmetric or composite membranes, and the equipment is mainly hollow-fiber or spiral-wound membrane separation units.
RO membranes reject inorganic ions, colloidal matter, and macromolecular solutes in water, producing purified water; they can also pre-concentrate macromolecular organic solutions. Because the RO process is simple and energy-efficient, it has developed rapidly over the past two decades and is now used at scale for seawater and brackish-water desalination (see brine), boiler-water softening, and wastewater treatment, and in combination with ion exchange for high-purity water. Its scope is expanding into concentration of dairy and fruit juices and separation/concentration of biochemical and biological preparations.
In pretreatment and desalination, RO performs well: it can cut the load on ion-exchange resin by over 90% and reduce regenerant consumption by more than 90%, saving cost and benefiting the environment. It also removes particulates, organic matter, and colloids, protecting the resin from fouling and extending its life.
The principle in plain terms
Place equal volumes of a dilute solution (e.g., fresh water) and a concentrated solution (e.g., seawater or brine) on either side of a container, separated by a semi-permeable membrane. The solvent in the dilute side naturally passes through the membrane toward the concentrated side until the concentrated side's liquid level rises to a height that creates a pressure difference—the osmotic pressure—reaching osmotic equilibrium. Osmotic pressure depends on the concentrated solution's type, concentration, and temperature, not on the membrane. If a pressure greater than the osmotic pressure is applied to the concentrated side, its solvent flows toward the dilute side—the reverse of the original direction—and this is reverse osmosis.
Solution–diffusion model
Lonsdale et al. proposed the solution–diffusion model for RO. The active skin of the RO membrane is treated as a dense, pore-free film; solute and solvent are assumed to dissolve in the homogeneous non-porous surface layer and diffuse through it, driven by chemical-potential differences from concentration or pressure. Differences in solubility and in diffusivity within the membrane phase govern their transport. The process: (1) adsorption and dissolution of solute and solvent at the feed-side surface; (2) independent molecular diffusion through the active layer driven by their respective chemical-potential gradients; (3) desorption at the permeate-side surface. Steps 1 and 3 are fast, so the rate is governed by step 2. Membrane selectivity enables separation of gas or liquid mixtures; permeation ability depends on both the diffusion coefficient and the solubility in the membrane.
Preferred-adsorption / capillary-flow theory
When a liquid dissolves different substances, its surface tension changes differently. Dissolved organics (alcohols, acids, aldehydes, esters) lower surface tension, while some inorganic salts slightly raise it, because the solute distributes unevenly—more concentrated at the surface layer than in the bulk—the surface adsorption phenomenon. When an aqueous solution contacts a polymer porous membrane that negatively adsorbs solute but preferentially adsorbs water, a film of pure water of a certain thickness forms at the membrane–solution interface. Under external pressure this water passes through the membrane's capillary pores, yielding pure water.
