In “crossflow” membrane filtration, liquid
flowing tangential to the membrane surface inhibits the formation of deposits.
The membrane retains dissolved and suspended solids. The retained fraction is
known as the concentrate or retentate. The fraction passing through the
membrane, driven by pressure, is ther permeate or filtrate. The product may be
the permeate (e.g. Clarification of fruit juices, effluent purification) or the
concentrate (e.g. Concentration of antibiotics) or both. Depending on the size
and type of the particles or solutes containes in a liquid there is wide range
of separation processes available. Crossflow membrane filtration covers a range
from 2 micron down to the molecular level below 0.001 micron (approx.). there
are four main classifications of membranes, Microfiltration (UF),
Nanofiltration (NF) and the finest of these, reverse Osmosis (RO).
Microfiltration
Microfiltration membranes are capable of
retaining particles of about 0.01 microns or larger. Smaller particles (e.g.
Salts, Sugars and proteins) pass through the membrane. The operating pressure
for microfiltration is usually 0.1 – bar.
Ultrafiltration
Particles of about 0.005 microns (MW > 100 Dalton) or larger are retained by
ultrafiltration membranes. Low molecular substances (e.g. Sugars and salts)
pass through the membranes. The operating pressure of ultrafiltration is 0.5 –
10 bar.
Nanofiltration
Water amd the monovalent ions, as well as
low molecular weight organic compounds (MW < 250 Dalton), pass through
nanofiltration m,embranesd. Divalent ions (e.g. MGSO4) are retained or allowed
to pass, depending on the type of membrane. The operating pressure is usually 8
– 40 bar.
Reverse Osmosis
Reverse Osmosis membranes allow only water
to pass through them. Salts and dissolved organic substances with higher
molecular weight than 50 Dalton are retained almost without exception. The
operating pressure used for reverse osmosis usually vary between 20 and 60 bar
and in some cases can be over 80 bar.
