Types of membranes: What are they and how are they classified?

Categories: Circular economy in the plastics industry, Innovation and trends in plastic materials

Membrane synthesis allows us to offer solutions aligned with the circular economy. Thanks to this technology, we achieve more sustainable processes in order to reduce greenhouse gas emissions, capture microplastics, generate new energy storage methods or produce different compounds.

What is a membrane?

A membrane is a semi-permeable physical interface or barrier that separates two phases and acts as a selective barrier regulating the transport of matter. Membrane technology has been widely developed to carry out separation and concentration processes of liquid or gaseous mixtures. However, membranes have other important applications such as biomaterials, catalysts (including fuel cell systems), energy storage, and CO2 separation, among others.

Classification of the membranes

Different types of criteria exist for the classification of membranes, according to their characteristics and properties. The most common is according to their nature and structure.

1. According to its nature

The membranes are classified as natural and synthetic.

1.1. Natural Membranes: Biological or non-biological

The membranes of original nature are more regular due to their low cost; however, they have little industrial use. These can be classified by either biological or non-biological membranes.

Synthetic membranes are more useful at an industrial level in separation processes in pharmaceutical, food, chemical or automotive sectors. They can be classified according to their source material.

An example would be water or acrylic-based polyurethane membranes capable of being applied as a UV-resistant, waterproof, highly elastic coating with high adhesion to concrete.

2. According to its structure

It can be considered a microscopic or macroscopic structure.

2.1. Microscopic Structure

The microscopic structure of membranes allows them to be classified according to their porosity and configuration.

According to their porosity:
Depending on their configuration:

 2.2. Macroscopic structure

The macroscopic structure refers to the geometry of the membranes and their position in space in relation to the flow of the feed fluid and permeate. Therefore, they can be classified by function in the following configurations:

Hollow fibres

Polymeric hollow fibre membranes are prepared by extruding a polymer solution through an annular spinneret and a perforating fluid flowing in the annular centre, the size of the central channel is less than 1 mm. This synthesis process is complex as it involves many spinning parameters, the thermodynamics of the polymer solution and the phase inversion process, the rheologies of the polymer solution within the spinneret and in the air gap, and other spinning conditions. These membranes are very sensitive to fouling and therefore require very controlled handling of the fibres, but they have a high surface area/volume filtering ratio of up to 30,000 m2/m3. In this type of membrane, the fluid to be treated can circulate inside the hollow fibres or perpendicularly to the fibres.

Laminar or flat sheet

Laminar membranes are prepared by casting or moulding a polymer solution (10-30% by weight). This solution is then subjected to evaporation or inversion in another solvent, usually water, to obtain the laminar membrane. The manufacture of flat sheet membranes is also a complicated process, involving the preparation of the solution, the rheology of the casting solution, the air space and the immersion precipitation in the coagulant bath. These membranes present a low surface/volume relation of filtration (100-400 m2/m3). However, as they can form a laminar membrane module, they can be arranged in series or parallel, increasing the filtering surface area, and they are compact and easy to clean.

Láminas planas

Flat sheet membrane synthesized in AIMPLAS


Tubular modules consist of a set of filter elements of tubular or multi-channel geometry. In this type of configuration, the membranes are arranged inside cylindrical housings that act as supports. These modules can be regenerated chemically, mechanically or with pressurised water, are highly resistant and able to accept almost any fluid without pre-treatment. However, they present a low surface/volume ratio of around 400 m2/m3.

Spiral Wound

The spiral module consists of the winding of various flat sheets or membranes separated from each other by layers of fabrics of different types that function as transporters and generators of turbulence of the feed and permeate solutions, using a central perforated tube. This type of configuration considerably improves the surface/volume ratio, which can reach between 300 and 1000 m2/m3 and reduces energy costs. However, they are easily fouled and difficult to clean.

AIMPLAS experience in membranes synthesis

Due to the great challenges in terms of the circular economy that society has been facing in recent years, the need has arisen to research new methods to reduce greenhouse gas emissions, capture microplastics present in water, generate new methods of energy storage or produce different compounds in a more sustainable way.

AIMPLAS develop projects focused on the synthesis of different types of membranes to achieve these objectives, offering a wide range of possibilities and advantages in the development of more sustainable processes, both in the capture and separation of pollutants in liquid effluents and gases and in the production and storage of hydrogen-type energy.

In these projects we are pursuing different objectives:

Daniela Andrea Ramírez Espinosa Técnico investigación del grupo de Descarbonización en AIMPLAS

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