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Gas-Sparged Ultrafiltration: Recent Trends, Applications and Future Challenges 671
2. ULTRAFILTRATION BASICS
Ultrafiltration (UF) fits between nanofiltration and microfiltration in the filtration spectrum
and involves separation of species ranging from about 1 to 100 nm in size, or about 500 to
500,000 kg/kg mole in molecular weight. Ultrafiltration is basically used for the concentration,
diafiltration, clarification and fractionation of macromolecules. Separation primarily
depends on species size, but several other factors are also known to affect separation (6, 14).
Ultrafiltration is usually carried out in the cross-flow mode where the feed flows parallel to
the membrane surface, thereby being split into two product streams. The stream that goes
through the membrane is called permeate or the filtrate, while that remaining on the feed side
is called the retentate. The volume, type and amount of species present in the permeate
depend on the characteristics of the membrane, the operating conditions, and the quality of
feed. The composition of the retentate depends on what goes out along with the permeate.
Ultrafiltration is used to separate large molecules like proteins, DNA and starch, colloidal
dispersions such as clays, paints, pigments and latex particles and fine suspensions such as
bacteria and virus. The osmotic backpressure in ultrafiltration is typically low. Hence the
applied pressure is mostly utilized for generating permeate unlike in reverse osmosis where a
high percentage of the applied pressure is used for overcoming osmotic pressure. The
permeate flux of a membrane is defined as the amount of permeate produced per unit area
of membrane surface per unit time. The permeate flux is directly related to the productivity of
an ultrafiltration process. Different parameters that affect the permeate flux are: the properties
of the membrane, the properties of the feed, the TMP and the system hydrodynamics.
2.1. Applications of Ultrafiltration
Ultrafiltration was initially developed for concentrating the macromolecules such as
proteins. Its current range of applications includes water and wastewater treatment, food
processing, chemical processing and bioprocessing. Ultrafiltration is now playing a major
role in making the downstream processing of proteins and nucleic acids cost effective.
Current downstream processing cost for such products using conventional separation technology
is in the range of 60–80% to total production cost. Hence there is significant room for
cost cutting.
In the water industry there are two major applications of ultrafiltration: production of
process water for the manufacturing industries and production of potable water for human
consumption. In the pharmaceutical and biotechnology industries there is a need for ultrapure
water for a range of applications: preparation of tissue culture and fermentation media, buffer
solutions, analytical solvents, drug and intravenous solutions etc. Ultrafiltration is extensively
being used for producing ultrapure water. Ultrafiltration is able to remove particulate matter
from water without any use of additives.
The manufacturing and service sectors produce large quantities of wastewater. Municipal
and industrial wastewaters are increasingly being treated in MBRs which use ultrafiltration
membranes. The MBR based effluent treatment plant is smaller and the effluent is free from
pathogens (5). Current applications of MBR include water recycling in buildings (15–16),