January/February 2010 World Water
32 Membrane Technology
The use of microfiltration (MF) and
ultrafiltration (UF) membranes with pore sizes
greater than .01 microns are simply a hydraulic
barrier that adds little filtration value with
significant energy losses and increased levels of
maintenance due to fouling. This conclusion is
based on a literature review by the author and
work undertaken at the University of
Washington (USA).
Instead, the use of a thin film (20 to 200
microns) of positive charge media on a
one-micron or larger screen will provide
significantly better operational results than a
MF or UF membrane. Therefore, the membrane
industry should focus on the best method of
applying the precoat and replacing it when it is
exhausted. The major benefit of the precoat
concept is that larger size membranes can be
used to significantly reduce the amount of head
loss and maintenance issues.
First, an understanding of basic membrane
technology reveals the key challenges of its
applications. Membranes can be categorized as
low-pressure membranes (MF and UF) and
high-pressure membranes such as nanofiltration
(NF) and reverse osmosis (RO). MF and UF
membranes remove suspended solids and
colloidal solids. NF and RO membranes
separate molecules, but are fouled easily by
scaling, biofouling, and colloidal fouling.
One design technique used to prevent fouling
is to position MF or UF membranes before NF
or RO membranes, but this type of prefiltration
is costly and moves fouling upstream to the MF
and UF membranes. Instead, a sand filter
followed by a disposable five-micron cartridge
filter is the method most typically applied.
In addition, the Silt Density Index (SDI) for
prefiltration level is required to be less than five,
but the European Commission-funded scientific
organization known as Techneau 1 reviewed
fouling issues with NF and RO membranes and
indicated that a SDI level of 5 is ineffective.
Recent work undertaken by the author has
indicated that pre-filtration levels must ensure
that the water being treated by RO or NF has
turbidity less than 0.1 NTU (approximate
SDI of 2 to 3) Techneau 1 recommends that
prefiltration be in the 0.1 micron range as
typical prefiltration of 5 microns or 100 microns
makes little difference. In fact both levels of
prefiltration resulted in 31 to 37 percent
linear decline in flux versus water treated.
Organic and colloidal fouling
Much of the research has focused on the effect of
fouling from Natural Organic Matter (NOM),
specifically polysaccharides and proteins 1 and
2. Organic and colloidal fouling are considered
different problems that must be dealt with by
pretreatment; however confusion is common in
regards to the types of fouling for LP versus HP
membranes. In general, the literature seems to
focus on organic/biofouling as the problem with
LP membranes and colloidal, predominantly
inorganic, fouling for HP membranes. The
author deviates from this hypothesis and
suggests that non-scaling fouling for both
membranes is colloidal in nature and that
colloidal particles can be organic or inorganic.
As a result, pretreatment of colloids is a key
treatment process to eliminate non-scaling
fouling in any membrane.
Membrane manufacturers have acknowledged
for many years that inorganic foulants affect the
performance of low-pressure membranes. Yet a
significant amount of research has focused on
the use of metal hydroxide coagulants and
polymers as a pretreatment method to remove
organic fouling on LP membranes.
Two professors of engineering have
undertaken comprehensive work in this area.
University of New Mexico's Associate
Professor in Civil Engineering Kerry J. Howe
Studies question
value of MF, UF
membranes
A review of literature published in the past five years on the best methods
to provide prefiltration methods led David E. G. Bromley, M Eng, PE to
question the value of using microfiltration (MF) and ultrafiltration (UF)
membranes. In this article, the author also examines non-scaling fouling
agents of MF and UF membranes for low-pressure applications, and NF
and RO membranes for high-pressure applications.

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