A membrane is a selective barrier; it allows some things to pass through but
stops others. Such things may be molecules, ions, or other small
particles. Biological membranes include cell membranes; nuclear membranes, which
cover a cell nucleus; and tissue membranes, such as mucosae and serosae.
Synthetic membranes are made by humans for use in laboratories and industry.
The influent of an artificial membrane is known as the feed-stream, the liquid
that passes through the membrane is known as the permeate, and the liquid
containing the retained constituents is the retentate or concentrate.
The concept of a membrane has been known since the eighteenth century, but was
used little outside of the laboratory until the end of World War II. Drinking
water supplies in Europe had been compromised by the war and membrane
filters were used to test for water safety. However, due to the lack of
reliability, slow operation, reduced selectivity and elevated costs,
membranes were not widely exploited. The first use of membranes on a large scale
was with micro-filtration and ultra-filtration technologies. Since the
1980s, these separation processes, along with electrodialysis, are employed in
large plants and, today, a number of experienced companies serve the market.
The degree of selectivity of a membrane depends on the membrane pore size.
Depending on the pore size, they can be classified as microfiltration,
ultrafiltration, nanofiltration and reverse osmosis membranes. Membranes can
also be of various thickness, with homogeneous or heterogeneous structure.
Membranes can be neutral or charged, and particle transport can be active or
passive. The latter can be facilitated by pressure, concentration, chemical or
electrical gradients of the membrane process. Membranes can be generally
classified into synthetic membranes and biological membranes.
Membrane processes classification = Microfiltration =
Microfiltration removes particles higher than 0,08-2 µm and operates within a
range of 7-100 kPa. Microfiltration is used to remove residual suspended
solids, to remove bacteria in order to condition the water for effective
disinfection and as a pre-treatment step for reverse osmosis.
Relatively recent developments are membrane bioreactors which combine
microfiltration and a bioreactor for biological treatment.
= Ultrafiltration = Ultrafiltration removes particles higher
than 0,005-2 µm and operates within a range of 70-700kPa. Ultrafiltration is
used for many of the same applications as microfiltration. Some ultrafiltration
membranes have also been used to remove dissolved compounds with high molecular
weight, such as proteins and carbohydrates. In addition, they are
able to remove viruses and some endotoxins.
= Nanofiltration = Nanofiltration is also known as “loose”
RO and can reject particles smaller than 0,001 µm. Nanofiltration is used for the
removal of selected dissolved constituents from wastewater. NF is
primarily developed as a membrane softening process which offers an
alternative to chemical softening. Likewise, nanofiltration can be used as
a pre-treatment before directed reverse osmosis. The main objectives of NF
pre-treatment are:. minimize particulate and microbial fouling of the RO
membranes by removal of turbidity and bacteria, prevent scaling by removal of
the hardness ions, lower the operating pressure of the RO process by reducing
the feed-water total dissolved solids concentration.
= Reverse osmosis = Reverse osmosis is commonly used for
desalination. As well, RO is commonly used for the removal of dissolved
constituents from wastewater remaining after advanced treatment with
microfiltration. RO excludes ions but requires high pressures to produce
deionized water. Membrane configurations
In the membrane field, the term module is used to describe a complete unit
composed of the membranes, the pressure support structure, the feed inlet, the
outlet permeate and retentate streams, and an overall support structure. The
principal types of membrane modules are: Tubular, where membranes are placed
inside a support porous tubes, and these tubes are placed together in a
cylindrical shell to form the unit module. Tubular devices are primarily
used in micro and ultra filtration applications because of their ability to
handle process streams with high solids and high viscosity properties, as well
as for their relative ease of cleaning. Hollow fiber membrane, consists of a
bundle of hundreds to thousands of hollow fibers. The entire assembly is
inserted into a pressure vessel. The feed can be applied to the inside of the
fiber or the outside of the fiber. Spiral Wound, where a flexible permeate
spacer is placed between two flat membranes sheet. A flexible feed spacer
is added and the flat sheets are rolled into a circular configuration.
Plate and frame consist of a series of flat membrane sheets and support plates.
The water to be treated passes between the membranes of two adjacent membrane
assemblies. The plate supports the membranes and provides a channel for the
permeate to flow out of the unit module. Ceramic and polymeric Flat Sheet
Membranes and modules. Flat sheet membranes are typically built-into a
submerged vacuum driven filtration systems which consist of stacks of
modules each with a number of sheets. Filtration mode is outside-in where the
water passes through the membrane and is collected in permeate channels. Cleaning
can be performed by aeration, back wash and CIP.
Membrane process operation The key elements of any membrane process
relate to the influence of the following parameters on the overall permeate flux
are: The membrane permeability
The operational driving force per unit membrane area
The fouling and subsequent cleaning of the membrane surface.
= Flux, pressure, permeability = The total permeate flow from a membrane
system is given by following equation: Where Qp is the permeate stream flowrate
[kg·s−1], Fw is the water flux rate [kg·m−2·s−1] and A is the membrane area
[m2] The permeability [m·s−2·bar−1] of a
membrane is given by the next equation: The transmembrane pressure is given by
the following expression: where PTMP is the Transmembrane Pressure
[kPa], Pf the inlet pressure of feed stream [kPa]; Pc the pressure of
concentrate stream [kPa]; Pp the pressure if permeate stream [kPa].
The rejection could be defined as the amount of particles that have been
removed from the feedwater. The corresponding mass balance equations
are: In order to control the operation of a
membrane process, two modes, with respect to the flux and to the TMP, can
be used. These modes are: constant TMP and constant flux.
The operation modes will be affected when the rejected materials and
particles in the retentate tend to accumulate in the membrane. At a given
TMP, the flux of water through the membrane will decrease and at a given
flux, the TMP will increase, reducing the permeability. This phenomenon is
known as fouling, and it is the main limitation to membrane process
operation. = Dead-end and cross-flow operation
modes = Two operation modes for membranes can be
used. These modes are: Dead end filtration where all the feed
applied to the membrane passes through it, obtaining a permeate. Since there is
no concentrate stream, all the particles are retained in the membrane. Raw
feed-water is sometimes used to flush the accumulated material from the
membrane surface. Cross-flow filtration where the feed
water is pumped with a cross flow tangential to the membrane and a
concentrate and permeate streams are obtained. This model implies that for a
flow of feed-water across the membrane, only a fraction is converted to permeate
product. This parameter is termed “conversion” or “recovery”. The recovery
will be reduced if the permeate is further used for maintaining processes
operation, usually for membrane cleaning.
Filtration leads to an increase of the resistance against the flow. In the case
of dead-end filtration process, the resistance increases according to the
thickness of the cake formed on the membrane. As a consequence, the
permeability and the flux rapidly decrease, proportionally to the solids
concentration [1] and, thus, requiring a periodic cleaning.
For cross-flow processes, the deposition of material will continue until the
forces of the binding cake to the membrane will be balanced by the forces
of the fluid. At this point, cross-flow filtration will reach a steady-state
condition [2], and thus, the flux will remain constant with time. Therefore,
this configuration will demand less periodic cleaning.
Fouling Fouling can be defined as the potential
deposition and accumulation of constituents in the feed stream on the
membrane. Fouling can take place through a number
of physicochemical and biological mechanisms which are related to the
increase deposition of solid material onto the membrane surface. The main
mechanisms by which fouling can occur, are:
Build-up of constituents of the feedwater on the membrane which causes a
resistance to flow. This build-up can be divided into different types:
Pore narrowing, which consists of solid material that it has been attached to
the interior surface of the pores. Pore blocking occurs when the particles
of the feed-water become stuck in the pores of the membrane.
Gel/cake layer formation takes places when the solid matter in the feed is
larger than the pore sizes of the membrane.
Formation of chemical precipitates known as scaling
Colonization of the membrane or biofouling takes place when
microorganisms grow on the membrane surface.
= Fouling control and mitigation = Since fouling is an important
consideration in the design and operation of membrane systems, as it
affects pre-treatment needs, cleaning requirements, operating conditions, cost
and performance, it should prevented, and if necessary, removed. Optimizing
the operation conditions is important to prevent fouling. However, if fouling
have already take place, it should be removed by using physical or chemical
cleaning. Physical cleaning techniques for
membrane include membrane relaxation and membrane backwashing.
Back-washing or back-flushing consist of pumping the permeate in the reverse
direction through the membrane. Back-washing removes successfully most
of the reversible fouling caused by pore blocking. Backwashing can also be
enhanced by flushing air through the membrane. Backwashing increase the
operating costs since energy is required to achieve a pressure suitable for
permeate flow reversion. Membrane relaxation consists of pausing
the filtration during a period of time, and thus, there is no need for permeate
flow reversion. Relaxation allows filtration to be maintained for longer
period of time before the chemical cleaning of the membrane.
Back pulsing high frequency back pulsing resulting in efficient removal of dirt
layer. This method is most commonly used for ceramic membranes [3]
Recent studies have assessed to combine relaxation and backwashing for optimum
results,. Chemical cleaning. Relaxation and
backwashing effectiveness will decrease with operation time as more irreversible
fouling accumulates on the membrane surface. Therefore, besides the physical
cleaning, chemical cleaning may also be recommended. They include:
Chemical enhanced backwash, that is, a low concentration of chemical cleaning
agent is added during the backwashing period.
Chemical cleaning, where the main cleaning agents are sodium hypochlorite
and citric acid. It should be point out, though, that every membrane supplier
proposes their own chemical cleaning recipes, which differ mainly in terms of
concentration and methods. Optimizing the operation condition.
Several mechanisms can be carried out to optimize the operation conditions of the
membrane to prevent fouling, for instance:
Reducing flux. The flux always reduces fouling but obviously it impacts on
capital cost since it demands more membrane area. It consists of working at
sustainable flux which can be defined as the flux for which the TMP increases
gradually at an acceptable rate, such that chemical cleaning is not necessary.
Using cross-flow filtration instead of dead-end. In cross-flow filtration only
a thin layer is deposited on the membrane since not all the particles are
retained on the membrane, but the concentrate removes them.
Pre-treatment of the feed water is used to reduce the suspended solids and
bacterial content of the feed-water. Flocculants and coagulants are also
used, like ferric chloride and aluminum sulfate that, once dissolved in the
water, adsorbs materials such as suspended solids, colloids and soluble
organic. Applications
Distinct features of membranes are responsible for the interest in using
them as additional unit operation for separation processes in fluid processes.
Some advantages noted include: Less energy-intensive, since they do not
require major phase changes Do not demand adsorbents or solvents,
which may be expensive or difficult to handle
Equipment simplicity and modularity, which facilitates the incorporation of
more efficient membranes Membranes are used with pressure as the
driving processes in membrane filtration of solutes and in reverse osmosis. In
dialysis and pervaporation the chemical potential along a concentration gradient
is the driving force. Also pertraction as a membrane assisted extraction
process relies on the gradient in chemical potential.
However, their overwhelming success in biological systems is not matched by
their application. The main reasons for this are named
Fouling—the decrease of function with use
Prohibitive cost per membrane area Lack of solvent resistant materials
Scale up risks References
External Links KLM Technology Group. Membrane
Technology Bibliography
Metcalf and Eddy. Wastewater Engineering, Treatment and Reuse.
McGraw-Hill Book Company, New York. Fourth Edition, 2004.
Paula van den Brink, Frank Vergeldt, Henk Van As, Arie Zwijnenburg, Hardy
Temmink, Mark C.M.van Loosdrecht. "Potential of mechanical cleaning of
membranes from a membrane bioreactor". Journal of membrane science. 429, 2013.
259-267. Simon Judd. The Memrabne Bioreactor
Book: Principles and Applications of Membrane Bioreactors for Water and
Wastewater Treatment. Elsevier, 2010.