The membranes can be cast to produce flat sheets, or spun to make hollow fibers and tubular membranes. These membranes are then assembled in a Module or a Cartridge (unit type separation systems), to manage the flow streams in the membrane machines, and to support the required hydraulic pressures.

Modules are designed to achieve two objectives:

• Getting the maximum exchange surface per unit volume (compactness).
• Limiting the circulation polarization (for Reverse Osmosis RO and Ultrafiltration UF), and the particle deposits (for Microfiltration) by providing a sufficient flow of treated liquid.

In addition, modules must be cleaned (hydraulic or chemical unclogging, sterilization) to treat fouling.
The fouling is the process of formation of a deposited layer on a membrane surface. In fact, the membrane process involves the filtration of contaminants from a solution by the action of continuously pumping the liquid through the membrane, the resulting concentration of the contaminants increases their accumulation on the membrane surface.

A balance must be found between cost of the module for producing a given volume of treated liquid, and energy cost of separation (high circulation velocity and small passage cross section with a great head loss as a consequence).

There are four types of modules:

1. Hollow Fiber module
2. Spiral Wound module
3. Plate Type module
4. Tubular module

1. Hollow Fiber module

Fibers are produced by extrusion through annular dies. Diameter varies from a few dozen microns to a few millimeters. They can stand high internal or external pressures and are assembled in a bundle of several thousands fibers.

Liquid flow can take place inside (Inside-Out flow filtration, the concentrate stays inside and the permeate flows outside the fiber) or outside the fibers (Outside-In filtration).

As the exchange surface / volume ratio is inversely proportional to the diameter, these units are very compact (several thousands of square meters per cubic meter).

In addition, these hollow fibers can be back-washed to treat the fouling. This means permeate is placed under a pressure greater than the feed pressure: this allows removal of the Cake of particles deposited on the surface. This cake will be then transported out of the module by the flow.

2. Spiral Wound module

In this case, we will present only the Thin Film Composite (TFC system). A flexible, porous sheet is placed between two flat membranes. This "sandwich" thus produced is sealed on three of its edges. The folded side is sealed to a cylindrical collector tube on both sides of a distributor with holes drilled in it. Several "sandwiches" are thus fastened and separated from one another by a spacer of flexible plastic. The fluid to be treated circulates in the spacer and the porous sheet ensures the drainage of permeate towards the axial collector.
Due to a different configuration, spiral wound module is more sensitive to fouling (flow is slow in the spacer) than plate-type module.

Membrane area per unit volume is large, and fouling is mainly treated by chemical cleaning.

3. Plate-Type module

The plate-type module is made up of piled membranes and support plates (filter press design). The fluid flows between the membranes of two adjacent plates. Thickness of liquid sheet is between 0.5 and a few millimeters.

Plates ensure the mechanical support of membrane and drainage of permeate. This module is easy to remove, thus allowing membrane replacement and complete cleaning if needed. However, due to length and winding configuration, head losses are very high.

Membrane area per unit volume is large, and fouling is mainly treated by chemical cleaning.

4. Tubular module

Membranes are placed inside a support porous tube, and these tubes are placed together in a cylindrical shell to form the unit module.

Diameter of the fiber varies from 0,5 to a few millimeters.

High circulation velocities are possible, and they don’t need fine liquid pre-filtration. They are easy to clean, and well adapted to viscous fluids (with a large volume of matter). Therefore, they’re not compact and have a high cost per square meter installed.

Membrane area per unit volume is small, and fouling is mainly eliminated by mechanical cleaning.