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Membrane separation is the most advanced filtration technology utilized
for removal of micron, sub-micron and ionic species that pollute any
water stream. Semi-permeable membrane of different materials, pore
sizes and configurations are typically utilized to "filter"
out the undesirable impurities. Membrane materials
such as Cellulose Tri-acetate (CTA), Thin Film Composite (TFC),
Polyamide (PA), Polysulfone, etc. are most common. Membrane Pore
Size range from 1-10 Angstrom for Reverse Osmosis (RO) membrane to 103
- 104 Angstrom for the Micro Filtration (MF) membrane.
There are several membrane element configurations commercially
available, such as Spiral Wound (SP), Hollow Fine Fiber (HFF) and
Tubular type. |
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This is a process in which organics and dissolved salts
are removed using Crossflow
(also called Tangential Flow) Filtration. Crossflow filtration is
achieved using a semi-permeable membrane in which feedwater flows under pressure,
parallel to the membrane surface . A portion of the feedwater permeates (or
Filters) through the membrane, forming the Permeate (or Filtrate) stream,
which represents the Product Water, and
leaving the majority of dissolved solids and organics filtered behind to form higher
concentrations in the feedwater stream. The balance of feedwater becomes enriched
with dissolved solids and organics as more permeate is formed. The balance
of concentrated feed flows tangential to the membrane surface, forming the Concentrate
(Reject) stream. |
Crossflow Filtration
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For the separation process to take place, Feedwater Pressure should be greater than the
Sum
of Feed-Concentrate stream Osmotic Pressure, the Permeate Backpressure
and any System Pressure Drops. |
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Because the feed and concentrate flow parallel to membrane surface instead of
perpendicular to it, the process is called "Crossflow" or
"Tangential Flow". Depending on the membrane pore size, Crossflow
filters are effective in the range of high ions rejection as in the case of Reverse Osmosis,
selective ion rejection for Nanofiltration, low ion rejection for
Ultrafiltration, or no ion rejection but suspended solids removal
for Microfiltration. |
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Crossflow Filtration includes the following Membrane Separation
Processes:
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Also known as "Hyperfiltration" process. It
employs the tightest skin membranes with smallest pore size, where 90-99% of all
Ions are rejected, more than 99.9% of Viruses, Bacteria and Pyrogens are
removed, and virtually all Organics are eliminated. Pressure in
the order of magnitude of 200 to 1,200 psig is normally required, for
applications ranging from brackish water purification to seawater
desalination. |
Reverse Osmosis (RO)
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Also known as "Membrane Softening" process. It
employs less tight skin membranes with larger pore size, where 60-80% of
all ions are rejected, 90-95% of Divalent Ions are removed, and Organic
compounds in the 300 to 1000 molecular weight range are
eliminated. In the Nanofiltration process more water normally
passes at a lower pressure, compared to the RO process. Pressure in
the in the order of magnitude of 100 to 200 psig is typically required to
economically soften water without the pollution of
salt-regeneration experienced with the resin softening process. |
Nanofiltration (NF)
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Is similar to the RO and NF processes, but is defined as a cross flow
process that has no significant rejection of dissolved solids. It
employs loose skin membranes with a relatively large pore size, where virtually
no ions are rejected, but contaminants in the range of 10 Angstrom (0.1
Micron) such as Organics, Bacteria, and Pyrogens size are rejected,
while most ions and small organics such as glucose are allowed to pass
the membrane porous structure. UF process requires a much lower
operating pressure in the range of 10 to 100 psig for the separation
process to take place. |
Nanofiltration (NF)
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Two types of Micro filters are available, Normal Flow and Crossflow
MF. In the Normal Flow MF the direction of flow is perpendicular
to the membrane surface and the entire water volume penetrates the
membrane surface, leaving the contaminant behind. Crossflow MF is
similar to the RO, NF and UF processes but the MF work as absolute
filter that has no rejection of dissolved solids. Pore size of MF
membranes are typically rated in the 0.1 to 2 micron range, and only
rejects particulates and suspended solids with sizes larger than the
membrane pore size. MF process requires extremely low operating
pressure, in the range of 1 to 25 psig for the separation process to
take place. |
Microfiltration (MF)
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