Water treatment - membrane filtration
Membrane filtration techniques are used for very specific applications in the field of industrial waste water treatment.
Membrane filtration is often applied as final step in the treatment of wastewater, usually
whithin the scope of water recuperation or reuse.
However, this technology is also used for the treatment of very specific wastewater and processwater flows, such as for the treatment of oil-emulsions and other specific emulsions.
Based on the separative capacity of the membranes, there are mainly four different steps to be distinguished in the field of membrane filtration :
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microfiltration :
separation of approx. 0,1 to 1 μm size particles (big proteins, yeast cells, microorganisms, …). The principle
of micro-filtration is physical separation. The extend, to which particles are seperated depends on the membrane pore size.
Particles bigger than the membrane pore size will be completely separated, particles smaller than the membrane pore size will only be partly removed, depending
on the dirt layer on the membranes. Conventional pressures between 0,1 and 3 bar.
Possible applications in the field of wastewater treatment are e.g. : microorganism separation from wastewater,
separation of oil/water emulsions, pretreatment of waste and process water prior to nanofiltration or reverse osmosis.
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ultrafiltration :
separation of approx. 0,01 tot 0,1 μm size particles
(organic substances, oil emulsions, protein molecules such as als gelatin,
bacteria, etc.). Conventional pressures between 3 and 5 bar.
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nanofiltration :
separation of approx. 0,001 tot 0,01μm size particles
(bacteria, viruses, sugars, pigments, sulfur,
water softening).
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reverse osmosis : separation of approx. 0,0001 tot 0,001 μm size particles
(salt solutions, metal ions, nitrates, etc.). Conventional pressures between 10 and 60 bar.
Most filtration techniques can only separate undissolved particles from the water.
Microfiltration and ultrafiltration can only separate the suspended solids from the pretreated wastewater.
This way for instance, dissolved salts, pigments, metal ions (influencing the conductivity of the wastewater)
and sugars remain present in the wastewater. Consequently the COD/BOD values of the wastewater treated with ultrafiltration
and microfiltration techniques will remain too high to be reused in the production process.
Only the last step, reverse osmosis, will allow to effectively separate salts, metal ions etc.
During the reverse osmosis process the membranes will have to support very high pressures. Only high-technological and pressure resistant membranes
are having the right features for this kind of application.
Membrane filtration - reverse osmosis
This technology is allowing to obtain the highest degree of purification : physical, chemical, organical and
bacteriological. For this technique a semipermeable membrane is used.
In the area of wastewater treatment, the reverse osmosis technology is used for the production of ultrapure water
for industrial processwater, for nitrate separation, desalination of briny water and seawater, …
Working principle reverse osmosis technology
Osmosis is based on a natural phenomenon.
When two liquids with a different concentration are divided by a semipermeable membrane, the pure water will flow through the membrane
from the least concentrated liquid to the most concentrated liquid. This natural action restores the osmotic
balance of the two solutions.
The reverse osmosis technology involves the application of mechanical pressure on the most concentrated solution, this pressure is to be higher than the osmotical pressure.
This way, the osmotic pressure redistribution phenomenon through the membrane will reverse itself, allowing to obtain pure water.
Different types of membranes
To counteract the fouling problem of the membranes (surface and pores)
, every manufacturer has its own range of high-tech
features (ao. asymmetric membranes, special
properties of the membrane surface, hydrophilic
versus hydrophobic membranes, crossflow filtration).
Moreover, the membranes often have to be permanently
resistant to solvents, high temperatures and very
diverse pH degrees. That's the reason why the new generation membranes have become much more performant in comparison to
the old generation membranes; this way the necessary replacement of the membranes can be postponed for longer and longer periods,
allowing to compensate increasingly for the high purchase price of the membranes.
There are 3 main types of membranes : tubular membranes, hollow fiber membranes and spiral wound membranes. Each type has it's own preferential application area.
Advantages of the use of membrane techniques in wastewater
and process water treatment
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material reliability
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less chemicals required
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relatively simple follow-up,
once the installation is correctly set
-
efficient use of energy
-
no change in state of aggregation necessary
Disadvantages of the use of membrane technology in wastewater
and process water treatment
Possible application areas of membrane
filtration in wastewater and process water treatment
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oil/water separation,
treatment of lyophilic substances
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recycling of car wash water
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treatment of liquid manure
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treatment of wastewater from the cosmetics industry
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treatment of wastewater from the
food and beverage industry
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filtration of suspended solids out of wastewater
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biomass separation
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cooling oil separation
-
ink separation in the flexographic printing industry
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reuse of process water
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rain water recycling
-
reuse of several kinds of
effluent
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