Slide 45:

So, up to now, you learned the first step of the membrane selection process.
You learned how to determine the necessary molecular separation size of the
membrane to be chosen. membrane range size, the pressure applied, the
substances that can be removed with each of which, and some applications.
Let's start the second step.
Determination of the membrane material and structure.
Slide 46:
A range of membrane materials are available from different manufacturers is
wide, and each has unique characteristics. The kay to the success and efficiency
of any membrane separation plant is to choose a membrane material that is
compatible with the application.
For example, for an application, the MF and UF have selected. In the market,
manufacturers produce MF and UF with different materials, different prices and
properties. That's why you have to have a good knowledge of these materials
to optimize your selection and increase the membrane life.
The selection of a membrane material depends on some parameters such as
raw water quality, required permeate quality, and membrane cost. For example,
if there are organic solvents in the raw water, some of the polymeric
membranes are not suitable due to the low resistance of the material.
There are also some factors that have to be considered in selecting a membrane
material. such as: performance, cost, easy to fabrication, resistance to
environmental factors such as PH, temperature, pressure and so on.
Performance of a material is affected strongly by the physical and chemical
properties of the material. The ideal membrane material is one that can produce
a high flux without clogging or fouling and is physically durable, chemically
stable, nonbiodegradable, chemically resistant, and inexpensive.
Temperature also affects the fluid viscosity and the membrane material. The
relationship between membrane material, temperature, and flux is specific to
individual products.
Slide 47:
Membranes are classified according to different features as you can see in this
figure include Origin, Material, Morphology and structure, and Manufacturing
process which I'm going to explain briefly.

Origin and Materials:

Membranes can be of biological and synthetic origin and differ according to
structure, functionality and material transfer. While biological membranes, e. g.
cell membranes, are indispensable for human and animal existence, in
wastewater purification only synthetic, solid membranes are used.
Depending on wastewater composition and characteristics, as well as
operational requirements, different materials, are used for membranes.
Membrane materials are organic or inorganic.
Morphology:
Concerning the morphology of membranes, we distinguish between pore
membranes and solution-diffusion membranes. Inorganic membranes are
always pore membranes.
Now, I'm going to explain types of membrane materials, organic and inorganic
membranes, advantages and disadvantages, properties, and the materials
utilized for MF, UF, NF, and RO.
Slide 48:
Organic membranes or Polymeric membrane:
At present synthetic polymer membranes are used predominantly because it is
possible to select a polymer suitable for the specific separation problem from
the existing huge number of synthetic polymers. Moreover, compared to other
materials, polymer membranes are often cheaper.
For the separation of a constituent, the structural characteristics of the
polymers used, like thermal, chemical and mechanical stability, and the
permeability are decisive. Examples of organic polymer membranes are such as:
polysulfone (PS), polyacrylonitrile (PAN), polyether sulfone (PES), polypropylene
(PP), polyvinylidene fluoride (PVDF), polyamide (PA), some aromatic
polyamides, cellulose acetate.
The most common synthetic organic polymers currently used in water
treatment membranes are polyvinylidene fluoride (PVDF), polysulfone (PS), and
polyethersulfone (PES). These materials have very good resistance to harsh
cleaning chemicals, chlorine, and moderately high temperatures, and tolerate a
wide pH range for cleaning solutions. But for example, Polypropylene does not
have good resistance to chlorine, which is often used as a disinfectant in water
treatment. Some membrane manufacturers consider the composition of their
membranes to be proprietary and do not release information on their material
chemistry.
Slide 49:
Polymeric membranes are thin films of 10–100 μm thickness.
Some of the common advantages of polymeric membranes are given
below:
a. Availability in a wide range of pore sizes varying from those of MF to those of
RO.
b. Availability of both hydrophobic as well as hydrophilic polymeric membranes
so as to minimize fouling.
c. Easy to fabricate and use.
d. Feasibility of scale-up.
e. Inexpensive compared to ceramic membranes.

However, there are some basic disadvantages associated with polymeric

membranes, such as:
a. Low resistance toward organic solvents.
b. Narrower pH range of applicability.
c. Low resistance to high temperatures.
d. Fouling.
e. Low life span (12–18 months).

Recently, modified polymeric membranes are developed to perform under

wider pH and solvent containing media. However, their resistance against
corrosion and organic solvents are yet to be solved to the extent of providing
confidence for their successful use in industrial applications.
Slide 50:
Inorganic membranes
In the recent past, inorganic membranes have gained more and more
importance. They are used especially if the employment of polymer membranes
is excluded because of the characteristics of the raw wastewater or if the
organic membrane surfaces have to be cleaned frequently and intensively due
to the wastewater composition.
Inorganic membrane materials are ceramics, aluminum, high-grade steel, glass,
fiber-reinforced carbon, and zeolite of which ceramic membranes at present
have the greatest importance in wastewater purification.
Inorganic membranes include metallic, ceramic and zeolitic membranes, which
are synthesized by sintering a metal/metal oxide followed by subsequent
deposition on a porous substrate. They are inert at extreme pH and can be used
for catalysis, hydrogen adsorption, etc. However, the major limitations are
surface poisoning, membrane cracking at high temperature and low flux.
Slide 51:
Ceramic membranes:
Among the inorganic materials, the Ceramic membrane due to the special
properties is gaining popularity. For example, as you can see in this graph, the
U.S. Ceramic membrane market, by technology from 2015 has increased and is
predicted to grow in MF, UF, and NF.
Ceramic membranes are a type of artificial membranes made from inorganic
materials (such as alumina, titania, zirconia oxides, silicon carbide or some
glassy materials). They are used in membrane operations for liquid filtration.
By contrast with polymeric membranes, they can be used in separations where
aggressive media are present such as acids and strong solvents. They also have
excellent thermal stability which makes them usable in high-temperature
membrane operations. Ceramic membranes are configured as tubular
membranes. The material is also hydrophilic, rough, and can withstand the high
operating pressure.
Like polymeric membranes, they are either dense or porous. They generally are
composed of an asymmetrical structure and usually have two to three different
levels of porosity. An intermediate level of the layer is often applied to decrease
the overall surface roughness.
In this figure, you see a porous asymmetrical structure include a microporous
layer and mesoporous layer as main layers that incorporated with a
macroporous layer as support.
Configurations in Ceramic membranes include tubular cross-flow and dead-end
membranes as well as flat sheet membranes.
Ceramic membranes come in a variety of shapes, sizes, and configurations,
Manufacturers provide customized products having different channel
diameters, as well as multi-channel construction to provide a higher membrane
density. These product offerings vary from each other and are in line with the
client requirements. The ability of the material to comply with customization is
one of the important factors for the broadening application portfolio of the
product.
Slide 52:
Ceramic membrane (Advantages and disadvantages):
Some common advantages for ceramic membranes are:
• High resistance against heat and chemicals. Except for very few
chemicals like hydrofluoric acid and phosphoric acid, ceramic
membranes show a very high degree of tolerance to strong doses of
acids, bases, solvents and other chemicals like chlorine (up to 2000 mg/L
in certain cases).
• High regeneration capacity
 • long service lives (5–10 years). There are many examples available
where a ceramic membrane system is operational even after 10–14
years of installation.
• Applicability to wider pH ranges (0.5–14).
• Higher mechanical strength.
• Resistance to vast temperature ranges (up to 500°C). Therefore, they
can be utilized for industrial-scale separations without the need for any
feed pre-conditioning steps.
• Low fouling propensities.
However, there exist a few drawbacks of ceramic membranes—they are given
below:
• Generally, ceramic membranes are not applicable for the separation
schemes related to NF and RO, since most of the ceramic membranes are
available with pore diameters within the MF and UF range (0.10–10 μm).
• They are comparatively costly due to the requirement of higher sintering
temperatures, materials, and conditions with high purity, higher amount
of inorganic precursor compared to very less amount of polymeric raw
materials required in the fabrication of polymeric membranes and
involvement of tedious membrane fabrication procedures.
• They are very brittle or fragile in nature. If dropped or subjected to undue
vibrations, they may get damaged.

Slide 53:
In this table, Properties of different membrane materials have presented. The
properties such as hydrophilicity, range of PH value, temperature, molecular
weight cut-off, and oxidant tolerance.
If you compare the organic materials with inorganic materials, you observe that
inorganics have a high tolerance than organics.
For instance, the PH range for the inorganic materials is between (0 - 14) and
there is no limitation for these materials but you can observe the limitation for
the organic types. There is also a significant temperature difference between
them. Most organic materials operate in temperature of less than 100 degree
but inorganic materials can operate to more than 300-degree celsius. The
oxidant tolerance for the inorganic is very high but there some limitations for
the organics.
From the point of view of hydrophilicity (From hydrophilicity point of view,),
inorganic materials are hydrophobic but organic materials having different
hydrophilicity properties. At first, let's have a description of hydrophilicity
property for those who are a lack of knowledge in this concept.

Slide 54:
Hydrophilicity:
One of the most important characteristics for the membrane materials is
hydrophobicity. Hydrophilic materials, which like contact with water, tend to
have low fouling tendencies, whereas hydrophobic materials may foul
extensively.
Hydrophobicity is quantified by contact angle measurements in which a droplet
of water or bubble of air is placed against a membrane surface, and the angle
between the surface and water is measured. Hydrophobic surfaces have a high
contact angle (the water beads like on a freshly waxed car), whereas hydrophilic
surfaces have a low contact angle (the water droplets spread out).
Hydrophobicity is affected strongly by the chemical composition of the polymer
comprising the material. Polymers that have ionized functional groups, polar
groups (water is very polar), or oxygen-containing and hydroxyl groups (for
hydrogen bonding) tend to be very hydrophilic.
Slide 55:
Membrane structure:
The structure of a membrane may be symmetric or asymmetric. While
symmetric membranes have a nearly homogeneous structure all over the
thickness of the membrane, asymmetric membranes are made up of two layers.
The layer on the side of the feed (active layer) determines the separation
behavior of the membrane, while the porous layer below serves as support. The
supporting layer ensures the mechanical stability of the membrane and hinders
the permeate flow only a little. The aim of asymmetric membrane design is to
keep the active layer as thin as possible and, with this, minimize the filtration
resistance of the membrane. With solution-diffusion membranes, it is,
therefore, possible to obtain flows that are 50 to 100 times higher than with
comparable symmetric membranes.
Today asymmetric organic membranes are usually manufactured as phase
inversion or composite membranes.
The active layer and supporting layer of the phase inversion membranes are
made from the same material.
However, in the case of composite membranes, the active layer and supporting
layer consist of different materials, so that both layers can be optimized with a
view to customizing the characteristics required in each case.
Most MF membranes have a homogenous structure, which means that the
structure, porosity, and transport properties are relatively constant throughout
their depth. In contrast, UF membranes have an asymmetric structure (also
called anisotropic or ‘‘skinned’’), which means that the morphology varies
significantly across the depth of the membrane.
Slide 56:
The most popular RO membrane materials are cellulose acetate and thin-film
composite polyamides.
Cellulosic Membranes such as Cellulose acetate that was the first high-
performance reverse osmosis membrane material discovered.
Cellulose acetate membranes still maintain a small fraction of the market
because they are easy to make, mechanically tough, and resistant to
degradation by chlorine and other oxidants, a problem with interfacial
composite membranes.
Cellulose acetate membranes can tolerate up to 1 ppm chlorine,

Noncellulosic Polymer Membranes

Is a membrane system without any cellulosic units
Typically include inorganic membranes, synthetic polymer membranes, and
natural polymer (excluded cellulosic materials) membranes.

Interfacial Composite Membranes

some advantages and disadvantage such as:
Higher salt rejections and fluxes than cellulose acetate membranes
The rejection of low-molecular-weight dissolved organic solutes is also far
better than cellulose acetate.
The only drawback is the rapid, permanent loss in selectivity that results from
exposure to even ppb levels of chlorine or hypochlorite disinfectants.
Slide 57:
In this table, a summary of the main RO membrane materials provided
according to my description from the previous slide.
As I have already mentioned, the most popular RO membrane materials are
cellulose acetate and TFC or thin-film composite polyamides.
The materials most widely used in RO are cellulose acetate (CA) and polyamide
(PA) derivatives. Cellulose acetate membranes are typically asymmetric.
Polyamide membranes are typically of thin-film construction. Polyamide
membranes are chemically and physically more stable than CA membranes.
Under similar pressure and temperature conditions, they typically produce
higher water flux and higher salt rejection than CA membranes. However, PA
membranes are more hydrophobic and susceptible to fouling than CA
membranes and are not tolerant of free chlorine in any concentration.
In this figure, a schematic and scanning electron micrograph of a multilayer
composite membrane on a microporous support has illustrated. As you can see,
the selective layer which the material is polyamide, with a thickness of between
0.05 - 0.2 micrometers located between the high-flux protective surface layer
and high-flux gutter layer with a thickness of 1 - 2 micrometers.
Slide 58:
Let's have an overview of the most current membrane materials for the
different membrane processes.
As you can see, most of the inorganic materials such as aluminum oxide, High-
grade steel, titanium, and Zirconium oxides are applied for both microfiltration
and ultrafiltration and they are not suitable for NF and RO. Because as I said,
ceramic membranes are not applicable for the separation schemes related to
NF and RO, since most of the ceramic membranes are available with pore
diameters within the MF and UF range (0.10–10 μm).
polyvinylidene fluoride (PVDF), and polysulfone (PS) as organic materials
applied for both MF and UF, and polyacrylonitrile (PAN), polyether sulfone (PES)
in most applications are applied only for UF.
The most popular NF and RO membrane materials are cellulose acetate and TFC
or thin-film composite polyamides that I explained it in detail.
Slide 59:
If you are going to select a membrane for water reuse applications, this table
can help you to find a suitable material.
Types of the polymeric materials, most common use, advantages and
disadvantages, mechanical strength, hydrophilicity, PH range, and chlorine
resistance summarized in this table.