What is a reverse osmosis membrane？

Reverse osmosis membranes are the core component to achieve reverse osmosis and are artificial semi-permeable membranes with certain characteristics made to simulate biological semi-permeable membranes.

It is generally made of polymeric materials. For example, cellulose acetate membranes, aromatic polyhydrazide membranes, aromatic polyamide membranes. The diameter of the surface micropores is generally between 0.5 and 10 nm, and the size of the permeability is related to the chemical structure of the membrane itself.

Some polymer materials have good rejection of salt. Some polymers have a chemical structure with more hydrophilic groups and therefore have a relatively fast water permeation rate. Therefore a satisfactory reverse osmosis membrane should have an appropriate permeation or desalination rate.

Characteristics of reverse osmosis membranes.

(1) Should have a high desalination rate at high flow rates.

(2) High mechanical strength and service life.

(3) The ability to function at low operating pressures.

(4) Resistant to chemical or biochemical action.

(5) Less affected by factors such as pH and temperature.

There are two types of reverse osmosis membrane structures, asymmetric and homogeneous. The membrane materials currently used are mainly cellulose acetate and aromatic polyamides.

The components are hollow fibre, rolled, plate and frame and tubular. They can be used in chemical unit operations such as separation, concentration and purification, and are mainly used in pure water preparation and water treatment industries.

The working principle of reverse osmosis membranes.

Reverse osmosis, also known as reverse osmosis, is a membrane separation operation in which a pressure difference is used as the driving force to separate the solvent from the solution.

Pressure is applied to the liquid on one side of the membrane and when the pressure exceeds its osmotic pressure, the solvent permeates in the reverse direction of natural osmosis. This results in a permeate of solvent on the low pressure side of the membrane.

On the high pressure side, a concentrated solution is obtained, i.e. the concentrate. If seawater is treated by reverse osmosis, fresh water is obtained on the low pressure side of the membrane and brine is obtained on the high pressure side.　

In reverse osmosis, the permeate rate of the solvent, i.e. the liquid flow energy N, is: N = Kh (Δp - Δπ) where Kh is the hydraulic permeability coefficient, which increases slightly with temperature; Δp is the difference in hydrostatic pressure between the two sides of the membrane; Δπ is the difference in osmotic pressure of the solution on both sides of the membrane.

The osmotic pressure π of a dilute solution is: π = iCRT where i is the number of ions generated by ionisation of the solute molecules; C is the molar concentration of the solute; R is the molar gas constant; and T is the absolute temperature.　

Reverse osmosis usually uses asymmetric and composite membranes. The equipment used for reverse osmosis is mainly hollow fibre or rolled membrane separation equipment.　Reverse osmosis membranes retain all kinds of inorganic ions, colloidal substances and macromolecular solutes in water, thus obtaining a clean system of water. It can also be used for the pre-concentration of large organic solutions.

Due to the simplicity of the process and low energy consumption, reverse osmosis has developed rapidly in the last 20 years. It has been used on a large scale for desalination of seawater and brackish water (see brine), softening of boiler water and wastewater treatment, and in combination with ion exchange to produce high purity water, and its range of applications is being expanded and has begun to be used for the concentration of dairy products, fruit juices and the separation and concentration of biochemical and biological preparations.