MEMBRANE FILTRATION
 
Membrane filtrations are used to concentrate or separate particles of a variety of sizes in the fluid phase, i.e. mostly water. Small particles, colloids, macromolecules, small molecules, or even ions can be separated depending on the pore size and the material of the chosen membrane. Being purely a physical separation method, membrane filtration does not affect the chemical structure or thermal stability of the membrane materials used.
 
Membrane filtration therefore offers the following advantages:

• Energy saving process
• Long lasting membrane products
• No need for auxiliary materials

Definition Membrane
Membranes are thin, semi permeable layers of a thickness of approximate 0.1-0.5 mm. The layers are mounted on a supportive matrix to increase their stability. To prevent clogging membranes are usually asymmetrically composed membranes which layers consist of different materials are termed composite membranes.
The filtrate, also termed permeate, passes unhindered through the membrane.
Particles and molecules are retained dependent of their molecular weight and size and are called retentate.
 
Driving Force
The driven pressure of the membrane filtration is the pressure difference between the feed and the permeate. It is also called trans-membrane pressure.
 
Different Filtration processes
The different separations achieved by membrane filtration are classified on the basis of their separation threshold. This is shown in the following illustration.
 
 

 
 
Transitions between different membrane separation methods are fluent. Dependent on the substance to be separated the pore diameter of the membranes employed can vary between 1 nm for reverse osmosis and 10 mm in the case of microfiltration.
 
Porous membranes and nonporous solution diffusion membranes can be distinguished.
Porous membranes consist of micro-porous layers with a defined pore diameter. They are used in micro-, ultra- and nanofiltrations. Particles and molecules are retained if their pore diameter is larger than the pore diameter of the membrane.
 
Membranes used for reverse osmosis are pore-less (nonporous) solution diffusion membranes. The solvent, usually water, diffuses freely through the membrane. The absorption tendency of salt components to the membrane should preferably be low in order to prevent a blocking of the membrane.
 
Ultra- and Microfiltration
Ultrafiltration is used to retain particles of a size of a few nanometres whereas micro-filtration, which employs porous membranes with pore diameters between 0.05 and 10 µm is able to separate particles in the µm size range. Ultrafiltration is the membrane separation method with the broadest application spectrum. It is used mainly for the concentration and fractionation of solutions that contain macromolecules. The membranes are able to retain oils, fats, waxes, resins, non-polar organic components, as well as solids such as sand, grinding residues and metal hydroxides.
 
However, soluble components such as salts and low molecular organic substances usually cannot be retained with ultra-filtration membranes.
 
Microfiltration represents the border to conventional filtration methods. It is mainly used for the concentration of colloid suspensions, such as the separation of fine hydroxide flakes that remain after precipitation.
 
Membrane modules
For most technical applications membranes are integrated in prefabricated units, the so-called modules. Modules can contain a variety of different membranes structures such as flat foils, tubes, capillaries, and hollow fibres.
 
Tubular modules that contain a large number of tubular membranes are commonly used to purify liquids that contain large amounts of solids. In contrast to other types of modules the flow characteristics of tubular modules can be easily determined and adjusted.
 
Capillary and hollow fibres modules are characterized by their high filtration yield since the filter area is very large relative to their size. However, their mechanical properties are rather poor and they have a high tendency to become blocked (clogged). In order to warrant their effectiveness, the maximal pore size of the contamination therefore should not exceed a tenth of the tube diameter.
 
The performance of membrane modules is evaluated by the following criteria:

• Specific filtration performance
• Retention
• Selectivity
• Mechanical stability
• Resistance against chemicals and solvents
• Life cycle

Operation methods of membrane filtration processes
Two fundamentally different filtration operation methods can be distinguished, the dead-end and the crossflow processes. In addition, novel, so-called submerged systems have recently been established that combine characteristics of both, the dead-end and crossflow systems.
 
Dead-end filtration
This process represents a static membrane filtration where the input is lead vertically onto the filtration area. The retained substances form a layer on top of the membrane. This ("filter cake") sludge can clog the membrane and is responsible for a reduction in the filtration performance. To remove this "filter cake" the membrane is rinsed from the permeate side interrupting the filtration process. The static dead-end process is therefore not suitable for most industrial applications on a large-scale.