Classification of Reverse Osmosis Membranes

Reverse osmosis membranes are used in the water treatment industry where the chemical structure of the polymeric material has more hydrophilic groups, resulting in a relatively fast water permeation rate. Therefore, reverse osmosis membranes should have an appropriate permeation amount. So what are the classifications of reverse osmosis membranes?

Reverse osmosis membranes are roughly divided into the following three categories.

The first type is cellulose acetate, which is also known as acetylcellulose or cellulose acetate. Cellulose acetate is often made from cellulose-containing cotton, wood and other raw materials, and is then processed into reverse osmosis membranes after an esterification and hydrolysis reaction.

The second category is polyamides, which comprise two major groups: aliphatic polyamides and aromatic polyamides. 1970s applications were mainly for aliphatic polyamides; currently the most used are aromatic polyamide membranes. Membrane materials are aromatic polyamides, aromatic polyamide-hydrazides and some nitrogenous aromatic polymers. Aromatic polyamide membranes can be adapted to a wide pH range of 2 to 11, but are sensitive to free chlorine in water.

The final classification is composite membranes, which are characterised by being made mainly from the two materials mentioned above, and are made by laminating a very thin dense layer with a porous support layer. The porous support layer, also known as the base film, enhances the mechanical strength; the dense layer, also known as the epidermal layer, acts as a desalination layer and is therefore also known as the desalination layer. The thickness of the desalination layer is generally 50nm, with the thinnest being 30nm.

Asymmetric membranes made from a single material have the following shortcomings.

1. The existence of a dense transition layer between the dense layer and the support layer.

2. The thinnest limit of the skin layer thickness is 100nm, which makes it difficult to reduce the push pressure by reducing the thickness of the membrane.

3. The desalination rate and water permeability rate are mutually constrained, as it is difficult to have both desalination and support in the same material.

Composite membranes are a good solution to these problems, as they can select materials with good desalination performance and high mechanical strength for the dense and support layers respectively. As a result, the dense layer of the composite membrane can be made very thin, which helps to reduce the drag pressure, while eliminating the transition zone and providing good resistance to compression and dense properties.

In addition to this, the low pressure side of the reverse osmosis membrane gives the permeate, the permeate, and the high pressure side gives the concentrated solution, the concentrate.

In terms of the structure of reverse osmosis membranes, there are two types of membranes: asymmetric and homogeneous. Current membrane materials of the cellulose acetate and aromatic polyamide type are predominant. 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. 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 various inorganic ions, colloidal substances and macromolecular solutes in water, resulting in clean water.

With the development of the global economic level and the geometric growth of the population, modern industrial wastewater discharges, urban waste, rural pesticide spraying and so on have caused serious water pollution, so reverse osmosis technology is becoming more and more important and the application of reverse osmosis membranes is becoming more and more widespread.