How Does RO Membrane Achieve 99.9% Salt Rejection?

Reverse osmosis membrane (RO membrane) operates on the core principle of countering the natural osmotic process. Here is an in-depth analysis of its working mechanism:

The Physical comparison of Natural Osmosis and Reverse Osmosis

Natural Osmosis Phenomenon is when there is a concentration difference on both sides of a semipermeable membrane (such as fresh water and salt water), water molecules will spontaneously flow from the low concentration side to the high concentration side until the concentrations on both sides are balanced. At this point, the pressure generated by the height difference of the liquid surface is the osmotic pressure.

The osmotic pressure π of a dilute solution is: π = iCRT (where i is the number of ions generated by the electrolysis of the solute molecules; C is the molar concentration of the solute; R is the molar gas constant; T is the absolute temperature.)
For example, the osmotic pressure of seawater is as high as 28 bar, requiring greater pressure to reverse the water flow.

 "High-Pressure Counter-Flow" of Reverse Osmosis is applying pressure to the saline side (>osmotic pressure) forces water molecules to flow in reverse, achieving reverse concentration gradient migration from "saline water→pure water."

During reverse osmosis, the osmotic rate of the solvent, i.e., the liquid flow energy N, is: N=Kh(Δp－Δπ) (where Kh is the hydraulic permeability coefficient, which slightly increases with temperature; Δp is the static pressure difference across the membrane; Δπ is the osmotic pressure difference across the membrane).

At this point: On the pure water side: Collecting the water molecules that pass through (permeate), On the concentrated water side: Retaining salt, heavy metals, and other impurities (concentrate).

Extreme Precision of Membrane Structure

Sandwich structure:

Support layer: Non-woven fabric (thickness ≈ 120 μm) - the "steel frame" that withstands high pressure.

Porous layer: Polysulfone material (pore size 0.01-0.1 μm) - the "first line of defense" that intercepts mud, sand, and rust.

Active layer: Polyamide film (thickness ≈ 200 nm) - the true "molecular referee," with a pore size of only 0.1-1 nm (≈ twice the size of water molecules).

Comparison data:

Virus diameter (0.02-0.3 μm) > Membrane pore size 3000 times.

Bacteria size (0.5-5 μm) > Membrane pore size 5000 times.

Classical model theory

• Solution-Diffusion Model

Core Assumption: The membrane is a homogeneous structure without pores, and the solute and solvent are transferred through dissolution into the membrane material and diffusion.

Transmission Process:

Dissolution: Water molecules and solutes adsorb on the membrane surface and dissolve into the membrane material;
Diffusion: Under the driving force of the chemical potential gradient (pressure difference), water molecules preferentially diffuse through the membrane;

Desorption: Released as pure water on the other side of the membrane.

Mathematical expression: Water flux Jw = A(ΔP − Δπ), where A is the membrane permeability coefficient, ΔP is the applied pressure, and Δπ is the osmotic pressure difference.

• Preferential Adsorption-Capillary Flow Model

Core assumption: The membrane surface has nanoscale channels, which preferentially adsorb water molecules to form a monolayer of "bound water," while solutes are repelled.

Transmission mechanism:

Selective adsorption: The membrane surface selectively adsorbs water molecules via hydrogen bonds (e.g., carbonyl oxygen in cellulose acetate);

Channel transport: Under high pressure, bound water passes through the channels via "hydrogen bond jumping," while solutes are retained due to size exclusion.

Performance:

Salt rejection rate: 98%-99.9% (TDS < 500 mg/L after desalination)

Heavy metal removal rate: >99.9% (lead, arsenic, etc.).

Key influencing factors

Operating Pressure
Must be > solution osmotic pressure, typically 60-80 bar for industrial applications (desalination), and 2-4 bar for household use.

Temperature: 5-45℃ (high temperature causes membrane pore deformation)

pH range: 4-11 (polyamide membrane), 5-6 (cellulose acetate membrane).

Type of pollutants

Inorganic scaling (e.g., CaCO₃): Requires citric acid cleaning at pH=2

Biological contamination (e.g., bacterial films): Treated with DBNPA sanitizer.

Application Scenarios and Membrane Type Selection

Seawater desalination   Polyamide composite membrane (TFC)   High-pressure resistance (＞60 bar), 99% desalination rate

Home water purification   Spiral composite membrane   Low pressure operation (2-4 bar), energy saving 30%.

Industrial pure water (electronic chips) Ultra-low pressure composite membrane   Product water resistivity 18 MΩ·cm.