Fundamentals
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Form-No: TM-WW002 // Revision: A // Date: Oct. 10, 17
3
Fundamentals
What is Ultrafiltration?
3.1
Ultrafiltration (UF) is a membrane filtration process using pressure as a driving force to sepa-
rate particulate matter, bacteria, viruses and high molecular substances from a liquid. Water
and low molecular weight solutes pass through the membrane. This separation process is wide-
ly used in water treatment, industry and research for purification and concentration of solutions.
Ultrafiltration is not fundamentally different from microfiltration or nanofiltration, except in terms
of the size of the molecules it retains. The basic process of membrane filtration is shown in Fig-
ure 1 below. The process is pressure driven and typically operates with a feed pump that push-
es the water through the membrane.
Figure 2: Schematic representation of membrane filtration process
The filtration spectrum and the selectivity of various filtration methods are shown in Figure 2.
Typically, UF membranes range from 0.01
–
0.1 micron in pore size. Two of the most common
classification parameters for membranes are pore size and molecular weight cut-off (MWCO):
The pore size is the nominal diameter of the openings or micro pores in the membrane ex-
pressed in microns.
The MWCO is the molecular mass or weight of a solute that rejects greater than 90 percent. The
unit of measurement for MWCO is Dalton (Da).
Retentate
Permeate
Feed
Fundamentals
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Form-No: TM-WW002 // Revision: A // Date: Oct. 10, 17
Figure 3: Membrane filtration spectrum
Definition and Explanation of Common Terms
3.2
The following section describes the most important commonly used terms which are closely re-
lated to UF operations.
Hollow Fiber (HF)
A hollow fiber is one of many forms and shapes in which membranes are produced. The most
common ones in the range of ultrafiltration are: hollow fiber, flat sheet, spiral wound and tubular
membranes.
Fundamentals
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Form-No: TM-WW002 // Revision: A // Date: Oct. 10, 17
Figure 4: Cross section made by scanning electron microscope picture of a hollow fiber membrane
Hollow fiber membranes consist of a very thin (Ø 1-3mm) and long fiber that is fixed at one or
either end in a potting material. At least one end is open to allow for permeate extraction. The
advantages of hollow fiber membranes include high packing density inside the module, back
flush ability, high permeability and good mechanical strength.
There are two options for the flow direction through the membrane during filtration: outside-in
and inside-out. For inside-out the feed water is fed into the lumen of the fibers and the perme-
ate comes out into the shell of the membrane module, while for outside-in the feed water is fed
to the shell and the permeate is collected in the lumen of the fibers. Most commonly used oper-
ation mode is outside-in.
During the outside-in operation, the fibers are surrounded by feed water, facilitating a very even
flow distribution along the entire length of the fiber and among all fibers within a module. The
permeate is collected inside the fibers.
The inside-out operation is the exact opposite; with the feed water distributed to the inside of
the fiber and the permeate surrounding the fibers. This operation mode allows for low pressure
operation and frequent and efficient back flush, as the volume on the feed side is much smaller
than compared with the outside-in operation, but this mode is very sensitive to higher solids
load or peak solids load in the feed water.
Dead-End/Crossflow Filtration Mode
There are two principle ways to operate UF membrane modules: dead-end and crossflow. If
there is no retentate stream and the entire medium either passes through the membrane or it is
accumulated on the feed side, then the process is called dead-end; if the retentate stream is
continuously flowing and only a small fraction of the feed flow is filtered through the membrane,
then the process is called crossflow. See the principle below:
Cross -Flow
Dead-End
Figure 5: Operation Modes
Trans-Membrane-Pressure (TMP)
TMP is the driving force for filtration and is the pressure difference between the feed pressure
and the permeate pressure for dead-end operation. Usually, the TMP is applied by usage of
pumps, but it can also be achieved by gravity flow in certain applications.
The TMP is measured by pressure sensors in the feed and permeate header. Generally, TMP
is calculated as follows:
Permeate
Permeate