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Membrane technology has become a dignified separation
technology over the past decennia. The main force of membrane technology is
the fact that it works without the addition of chemicals, with a relatively
low energy use and easy and well-arranged process conductions.
Membrane technology is a generic term for a number of different, very
characteristic separation processes. These processes are of the same kind,
because in each of them a membrane is used. Membranes are used more and more
often for the creation of process water from groundwater, surface water or
wastewater. Membranes are now competitive for conventional techniques. The
membrane separation process is based on the presence of semi permeable
membranes.
The principle is quite simple: the membrane acts as a very specific filter
that will let water flow through, while it catches suspended solids and
other substances.
There are various methods to enable substances to penetrate a membrane.
Examples of these methods are the applications of high pressure, the
maintenance of a concentration gradient on both sides of the membrane and
the introduction of an electric potential.
Membranes occupy through a selective separation wall. Certain substances can
pass through the membrane, while other substances are caught.
Membrane filtration can be used as an alternative for
flocculation,
sediment purification techniques,
adsorption (sand
filters and active carbon filters,
ion exchangers),
extraction and distillation.
There are two factors that determine the affectivity of a membrane
filtration process; selectivity and productivity. Selectivity is expressed
as a parameter called retention or separation factor (expressed by the unit
l/m2·h). Productivity is expressed as a parameter called flux (expressed by
the unit l/m2·h). Selectivity and productivity are membrane-dependent.
Membrane filtration can be divided
up between
micro and ultra filtration on the one hand and
nano filtration and Reverse Osmosis (RO or hyper filtration) on
the other hand.
When membrane filtration is used for the removal of larger
particles,
micro filtration and
ultra
filtration are applied. Because of the open character of the
membranes the productivity is high while the pressure differences
are low.
When salts need to be removed from water,
nano filtration
and Reverse Osmosis
are applied. Nano filtration and RO membranes do not work according
to the principle of pores; separation takes place by diffusion
through the membrane. The pressure that is required to perform nano
filtration and Reverse Osmosis is much higher than the pressure
required for micro and ultra filtration, while productivity is much
lower. |
Membrane filtration has a number of
benefits over the existing water purification techniques:
· It is a process that can take place while temperatures are low.
This is mainly important because it enables the treatment of
heat-sensitive matter. That is why these applications are widely
used for food production.
· It is a process with low energy cost. Most of the energy that is
required is used to pump liquids through the membrane. The total
amount of energy that is used is minor, compared to alternative
techniques, such as evaporation.
· The process can easily be expanded.
Process management of membrane filtration systems
Membrane filtration systems can be managed in
either
dead-end flow or cross-flow. The purpose of the optimisation of
the membrane techniques is the achievement of the highest possible
production for a long period of time, with acceptable pollution
levels.
Membrane systems
The choice for a certain kind of membrane system
is determined by a great number of aspects, such as costs, risks of
plugging of the membranes, packing density and cleaning
opportunities. Membranes are never applied as one flat plate,
because this large surface often results in high investing costs.
That is why systems are built densely to enable a large membrane
surface to be put in the smallest possible volume. Membranes are
implemented in several types of modules. There are two main types,
called the
tubular membrane system and the
plate & frame membrane system. Tubular membrane systems are
divided up in tubular, capillary and hollow fiber membranes. Plate &
frame membranes are divided up in spiral membranes and pillow-shaped
membranes.
Membrane fouling
During membrane filtration processes
membrane
fouling is inevitable, even with a sufficient pre-treatment. The
types and amounts of fouling are dependent on many different
factors, such as feed water quality, membrane type, membrane
materials and process design and control.
Particles, biofouling and scaling are the three main types of
fouling on a membrane. These contaminants cause that a higher
workload is required, to be able to guarantee a continuous capacity
of the membranes. At a certain point the pressure will rise so much
that it is no longer economically and technically accountable.
Seawater desalinationMembrane cleaning
There are a number of cleaning techniques for the
removal of membrane fouling. These techniques are
forward
flushing, backward flushing, air flushing and chemical cleaning,
and any combination of the methods.
Membrane systems management
Tubular-shaped membranes
Plate & frame membranes |
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