Everything you need to know about concentration polarization in reverse osmosis

During the reverse osmosis membrane separation process, when water flows through the membrane surface, water molecules pass through the reverse osmosis membrane, while solutes (such as salt, organic matter, etc.) are retained. These retained solutes will gradually accumulate near the membrane surface, forming a high-concentration boundary layer, resulting in a phenomenon in which the solute concentration on the membrane surface is significantly higher than the concentration of the bulk solution. This phenomenon is called concentration polarization.

Intuitive imagination: Imagine a cup of salt solution is filtered through an RO membrane, and the salt "accumulates" on the membrane surface to form a layer of high-concentration area. Just like when there is a traffic jam, vehicles gather at the intersection.

Solute Rejection Mechanism

The essence of membrane permselectivity: RO membranes achieve selective separation through the molecular sieve effect of dense polyamide layers. Water molecules (diameter of about 0.28 nm) can pass through the membrane pores, while most ions (such as Na⁺ with a diameter of 0.19 nm but 0.72 nm after hydration) and organic matter are retained. This "semi-permeability" causes solutes to continue to accumulate on the membrane surface.

Industry pain points: Although high-density membranes (>99%) improve water quality, they will aggravate polarization and require a "flux-retention rate optimization design" to balance performance.

Hydrodynamic Boundary Layer

- Velocity distribution characteristics: Near the membrane surface, the fluid velocity drops sharply from the main flow velocity (such as 0.3 m/s) to near zero, forming a velocity boundary layer (usually 50-200 μm thick). The solute in this layer relies solely on molecular diffusion to return to the main solution, and the diffusion rate is 3-5 orders of magnitude lower than the convective migration.

- The influence of turbulence and laminar flow:

• Laminar state (Reynolds number Re<2000): The boundary layer is stable, and the solute is seriously accumulated
• Turbulent state (Re>4000): Eddy disturbance destroys the boundary layer, and the polarization rate is reduced by 40-70%

- Engineering case: After a spiral turbulent flow device was installed in the membrane shell of a seawater desalination project, the polarization factor (β) dropped from 1.25 to 1.08, and the system recovery rate increased by 12%.

Osmotic Pressure Gradient

The osmotic pressure of the high-concentration solution on the membrane surface increases, counteracting the operating pressure and reducing the effective driving force.

Concentration polarization significantly impacts reverse osmosis (RO) membrane performance, leading to several adverse effects on both efficiency and longevity. The main effects include:

1. Reduced Water Flux: Concentration polarization results in the accumulation of solutes at the membrane surface, creating a concentration gradient that opposes the osmotic driving force. This reduction in the effective concentration of water at the membrane surface leads to decreased water flux, meaning less potable water is produced over time.
2. Increased Operating Pressure: As the concentration polarization occurs, the effective osmotic pressure differential across the membrane increases. To maintain the desired water flux, operators may need to increase the feed pressure, leading to higher energy consumption and operational costs.
3. Membrane Fouling: The accumulation of solutes at the membrane surface from concentration polarization can lead to fouling. As the concentration increases, more solutes may adhere to the membrane surface, forming a foulant layer that further reduces permeability and increases resistance to flow.
4. Compromised Membrane Selectivity: Changes in concentration at the surface can alter the diffusion behavior of solutes. For instance, higher concentrations of specific ions may influence the transport characteristics, potentially reducing the membrane's selectivity for desired ions and allowing more unwanted species to permeate through.
5. Increased Temperature and Pressure: Since concentration polarization raises solute concentration near the membrane surface, it can lead to localized increases in temperature due to osmotic heating, which may impact the membrane materials' stability and performance over time.
6. Chemical Stability of the Membrane: The elevated concentrations of certain solutes due to concentration polarization may lead to localized chemical reactions or degradation of the membrane material. This can result in diminished membrane life and increased maintenance needs.
7. Decreased Recovery Rates: As concentration polarization affects water flux and efficiency, it can lead to decreased recovery rates of the RO system. This means a smaller percentage of feed water is converted to permeate, increasing the volume of brine produced and potentially complicating waste management.
8. Altered Operating Conditions: To mitigate the effects of concentration polarization, system operators may adjust flow rates, pressure, and other operating conditions, which can lead to suboptimal performance if not done carefully.

1. Increase stirring and turbulence to reduce the accumulation of solutes on the membrane surface;
2. Rationally select membrane design and operating parameters to avoid excessively high membrane flux and too low operating pressure, and slow down the formation of concentration layers;
3. Regularly perform chemical and physical cleaning;
4. Perform appropriate pretreatment before entering the membrane separation system, such as fine filtration, hollow fiber filtration, etc., which can reduce suspended matter and particulate matter entering the membrane system and reduce the degree of concentration polarization.