Membranes: Fouling & Cleaning

Membrane technology is widely accepted as a means of
producing various qualities of water from surface water, well water, brackish
water and seawater. Membrane technology also is used in industrial processes
and industrial wastewater treatment, and lately membrane technology has moved
into the area of treating secondary and tertiary municipal wastewater and oil
field-produced water. In many cases one membrane process is followed by another
with the purpose of producing water of increasing purity and quality for
various purposes. Thus, one type of membrane may enhance the function of
another to meet goals ranging from disposal of wastewater to production of
drinking water from unexpected sources. In this way membrane technology offers
the possibility of managing total water resources. The spiral wound membrane
element configuration is the most widely used due to its high packing density
and relatively low price. This article will describe some technological
advances in the area of innovative new membranes and application concepts for
spiral wound membrane elements.

 

Spiral Wound Elements

Spiral wound elements span the four commonly defined
membrane technologies, which are microfiltration (0.01 to 10 microns),
ultrafiltration (500 to 100,000 Dalton), nanofiltration (100 to 500 Dalton) and
reverse osmosis (up to 100 Dalton). A sandwich consisting of two membrane
sheets with an inserted permeate carrier is glued together and a feed spacer is
inserted between the opposing membrane surfaces to complete the membrane
package. The membrane package is wound around a perforated central tube through
which the permeate exits the element. The physical shape of a membrane element
is secured by applying a suitable outer wrap. The physical and chemical
properties of the various materials including the membrane are chosen according
to the operating parameters. The typical reverse osmosis elements have
limitations with respect to temperature (45° C), pH value (2 to 10), silt
density index (less than 3 SDI), chlorine (dechlorination mandatory) and
several other parameters. This generally is acceptable for conventional pure
water applications, but for more complex membrane applications these
limitations must be diminished or removed.

 

Advanced materials and material science have been applied to
the membranes, materials and construction of spiral wound elements. This effort
has resulted in elements with improved operating parameters and wider areas of
applications. Various new membrane applications have been made possible and
there is no limit in sight, except that new applications must rest on a
profitable foundation for the user. Scarcity of water, environmental
requirements and the simple logic of reusing water instead of discharging it are conditions that call for increased use of membrane technology in a multitude of applications.

 

Membrane Fouling

Fouling of the membrane surface during operation diminishes
the productivity of the membrane and, in the case of a continued fouling
condition, causes the salt rejection to suffer. Fouling mainly stems from three
sources, namely particles in the feed water, buildup of sparsely soluble
minerals and byproducts of microorganism growth. All of these conditions
require frequent cleaning, which is expensive and leads to shorter service life
of the membrane elements. Especially when more than one fouling condition
prevails, the membrane can be irreversibly fouled, in which case correction of the condition and replacement of the membrane elements is the only solution.

In general, the feed to membranes should not contain
suspended solids, and adequate pretreatment of the feed is mandatory to a
well-functioning membrane plant. The most common sparsely soluble minerals
include silica, barium and contributors to hardness. Growth of microorganisms
is most pronounced in the temperature range of 30 to 45° C, which
ironically is the prevailing operating temperature for membrane plants in areas with warm climates.

 

There is only one way to avoid membrane fouling and
resulting frequent cleaning procedures—to ensure adequate pretreatment of
the feed and operate the membrane filtration equipment using parameters that do
not produce fouling. Pretreatment needs to be designed to remove suspended
solids in the feed. Normally, the combination of sand filters and depth filters
will accomplish this goal. The water recovery and flux per area unit of
membrane must be chosen to ensure that the crossflow over the membrane can remove
the boundary layer—which forms at the surface of the membrane—at a
rate faster than it is formed. Last but not least, the bacteriological
conditions of the feed and in the plant must be controlled to prevent growth of
microorganisms

 

Reverse Osmosis Membranes

Reverse osmosis membranes are sensitive to mineral fouling,
which occurs when the solubility product of a material is exceeded in the
boundary layer formed on the membrane surface during operation.

The concentration in this boundary layer can be twice as
high as in the bulk solution, if the crossflow of the feed stream is
inadequate. Several ways have been suggested to diminish mineral fouling
including various post-treatment methods to change the electrical charge of the
membrane surface, which is then believed to repel certain species. However,
only moderate success has been achieved following this avenue. The cellulose
acetate membrane, which has largely been replaced by thin-film polyamide
membrane, still seems to offer the best fouling resistance.1 A recent study2
suggests that the single most important factor in preventing mineral fouling,
as well as fouling caused by microorganisms, is the smoothness of the membrane
surface. The conclusion is that if there are no crevices in the surface of the membrane,
no material will be deposited there, because the boundary layer formation
becomes less pronounced, and the crossflow will be able to remove it at a
faster rate than it can be deposited (see Figures 1 and 2).

 

Most reverse osmosis membranes in use today are thin-film
polyamide membranes formed on top of another membrane, usually a polysulfone
ultrafiltration or microfiltration membrane called a substrate. The surface
morphology of the substrate determines the smoothness of the resulting membrane
sandwich. A smoother and less fouling-prone membrane can be made by inserting
an additional membrane layer between the polysulfone and thin-film layers or by
designing a polysulfone membrane with a smooth surface to begin with. Both
methods have proved their value through results in practical applications.

 

In some cases, naturally occurring water—for instance
from artesian wells—has a temperature exceeding that which normally is
considered feasible for reverse osmosis membranes. The water can be cooled down,
often at great expense, to allow reverse osmosis treatment, but in most cases
such projects fall by the wayside for economical reasons in areas where water
is much needed. Lately, reverse osmosis elements tolerating operating
temperatures up to 80° C have been developed and commercialized. Operating
a reverse osmosis plant at temperatures in the 50° C to 80° C range
also alleviates the risk of microorganism growth, which can cause severe
fouling of the membrane surface. High temperature compatible elements offer the
capability of operating a membrane plant at high temperature or sanitizing a
plant occasionally at 90° C to keep microorganism growth under control.

 

Membrane Cleaning

Even if all possible measures have been taken to prevent
membrane fouling, changes in the composition of the feed, breakthrough in the
sand filter, production upsets and a host of other irregular operating
conditions may result in membrane fouling. The fouling will, in most cases, be detectable as a gradual decrease in plant productivity
and later as a gradual increase in the salt content of the permeate. If
biofouling is the culprit, there almost always will be a marked increase in
bacterial count in the concentrate and, in severe cases, the permeate will show unacceptably high
bacterial counts. It is important to recognize the symptoms and to take action
at the earliest possible time. A detailed log of the plant operation and
analysis of the production parameters helps to identify a problem situation at
an early stage before the problems become severe. If fouling is allowed to
develop, cleaning becomes much more difficult.

 

If fouling occurs in a membrane filtration plant, regularly
or as an exception, two goals must be accomplished. The cause of the problem must be identified and corrected, or the same fouling situation simply will reoccur. Then there is the problem of choosing a suitable cleaning regimen to bring the plant back on line, operating at or close to the design capacity.

 

The most effective cleaning method in the case of biofouling
is an oxidizing agent such as chlorine, hydrogen peroxide and peracetic acid.
Cellulose acetate membranes tolerate oxidizing agents, but the more popular
thin-film polyamide reverse osmosis membranes do not. Of nonoxydizing cleaners
for biofouling situations formaldehyde, glutaraldehyde and quaternary ammonium
compounds may be used for polyamide membranes. If the fouling is caused by
intracellular byproducts from microorganisms, various enzymatic cleaning agents
have proved to be effective.

 

In the case of mineral fouling, chemical cleaning agents are
used. Acid generally will remove inorganic salts. Cellulose acetate membranes
do not tolerate low or high pH values, whereas polyamide membranes generally can
be cleaned down to pH 2. Acetic acid, which also is a complexing agent for some
metals, often is used with good results. Complexing agents like quaternary
ammonium compound operate better at elevated pH values, which renders them of
little use for cellulose acetate membranes.

 

Membrane technology has undergone a rapid development as it
pertains to most applications, in particular, as it pertains to water
purification. The last 20 years have witnessed new membranes working at ever
lower pressures and with increasing salt rejection from the original cellulose
acetate membrane requiring 400 psi (28 bar) to modern polyamide thin-film
membranes requiring only 100 psi (7 bar) net driving pressure. Salt rejection
of the reverse osmosis membranes has increased from 97.0 to 99.5 percent with
some special membrane types exhibiting even higher salt rejections. Are we
nearing the end of the possible, and will we have to accept today’s
standards in the future?

 

The answer is that membrane technology will continue to
develop with huge benefits for the user of membrane filtration equipment for
water purification. Material science and molecular modeling are some of the
tools that are used in the advancement of membrane technology. Net driving
pressures will continue to decrease. Salt rejection will continue to increase,
although it already is close to 100 percent. Resistance of membrane materials
to oxidizing agents will increase to the point that cleaning with chlorine is
possible. The tolerance for solvents and other aggressive chemicals will be
improved. In short, the end is not in sight, and membranes will claim an
increasing role in water purification to the advantage of the thirsty humanity.             

Bjarne Nicolaisen is vice president of business development for the crossflow membrane business at Osmonics, Inc.

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