This document summarizes reverse osmosis (RO) and nanofiltration (NF) membrane processes. Both RO and NF are pressure-driven membrane processes that separate low molecular weight solutes from water. The main difference is that NF membranes allow for the separation of some low molecular weight non-ionic molecules in addition to ionic solutes. The document discusses similarities and applications of RO and NF, as well as membrane materials, transport mechanisms, and challenges like fouling.
2. SIMILARITES AND DIFFRENCES
• Both pressure driven processes.
• Boundary between the two processes is non
precise
• The distinction is made because
– slight difference in membrane permeability
– the ability of nanofiltration to separate low
molecular weight non-ionic molecules
3. SIMILARITIES AND DIFFERENCES
• Both processes are used to separate low molecular
weight solutes from a solvent.
– Solvent is typically water
– Solutes are typically salts or amino acids
• Both have applications on an industrial and domestic
scale
• Nanofiltration more open polymer structure
• Pure solvent flux much greater in Reverse Osmosis
• Transport mechanisms are considered the same and
generally thought of as primary solution diffusion
4. DEVELOPMENT OF REVERSE OSMOSIS
• Originally developed by the US army to produce potable
water from brackish sources.
• Research leading to commercial application began in the
early 1950’s at the University of Florida.
• Advanced by Loeb and Sourirajan at University of California
– first high flux asymmetric reverse osmosis membranes
• Original membranes made of cellulose acetate highly
susceptible to chemical and biological degradation
• Further research funded by US government resulted in
cellulose ester membranes
– Higher flux and rejection of ionic species
– Still susceptible to biological degradation
5. REVERSE OSMOSIS APPLICATIONS
• Potable water from brackish sources
– Recent advances have made the production of potable water from sea water
possible
• Used in conjunction with ultrafiltration and ion exchange to produce ultra
pure water
• Extensively used to clean waste waters with small solutes and high BOD
– Starch recovery from potato processing
• Concentration of fruit and vegetable juices
– Superior flavour to those produced by heat concentration
– Lower energy requirements than evaporation processes
– Smaller plant cost and size
– Very high dissolved salt concentration
– High operating pressures
– High fouling
– Shortens membrane life
6. NANOFILTRATION APPLICATIONS
• Advantages over reverse osmosis when
retention of mono – valent salts are not
required
– Much more permeable membrane
– Higher solvent fluxes, often a factor of 10
– Lower energy and capital costs
7. NANOFILTRATION APPLICATIONS
• Potable Water
– Remove colour from water drawn from peat lands
– Remove humic and fulvic acids
– Conventional treatments labour and chemical intensive
– Suitable for small domestic supplies in isolated communities
– Can operate unattended for long periods of time
– Does not remove minerals
• Sugar Concentration
– Concentration of lactose from deproteinised whey stream
– Concentrates lactose while allowing mono-valent ions to pass
through
– Concentrated lactose stream used as feed for fermentation
8. NANOFILTRATION APPLICATIONS
• Dye Production
– Improved by using nanofiltration
– 10% NaCl reduced to 0.2%
– Concentrate product by 30%
9. MATERIALS
• Originally derived from modified cellulose
precursor
• Aromatic polyamides no used
– Advantage – Significant lower tendency to
biological degradation
– Disadvantage – Susceptible to damage from free
chlorine
• Drawn as hollow fibres by melt or dry spinning
– Typical diameter – 100 µm
– Wall thickness – 20 µm
10. MATERIALS
• Significant advance in RO/NF technology due to
composite membrane
• Casting active layer on surface of UF membrane
• First membrane of this type utilised the addition
of polyethylenimine (PEI) to a polysulfone UF
membrane
• Addition of toluene 2,4 diisocynate followed by
heat drying improved salt rejection to > 95%
11. MATERIALS
• FilmTec (Dow subsidiary) developed the first
composite membrane by interfacial
polymerisation
• The membrane is wetted with polymer precursor
and brought into contact with co-reactant
• Polymerisation takes place at the interface
• Formation of the polymer prevents further
reaction taking place
• m-phenylenediamine (wetting agent) & trimesoyl
chloride (non-aqueous co-reactant)
13. TRANSPORT METHOD
• Both RO & NF separation achieved by
application of pressure gradient
• Small solutes therefore membranes are
considered non-porous
• Molecular volume of water and salts are
similar therefore not a sieving process
• Exact nature of transport is under debate
– Preferential sorption-pore flow
– Solution diffusion
14. PREFERENTIAL SORPTION-PORE
FLOW
• Assumes that
transported species
forms an adsorbed
layer at membrane
interface
• layer occludes the
pores preventing the
passage of certain
molecules
15. SOLUTION DIFFUSION
• Assumes that molecules
are not convected
through pores but
preferentially dissolve in
the membrane
• The transported species
diffuses through
polymer down a
chemical potential
gradient
16. SOLUTION DIFFUSION
When 2 solutions of different concentration are separated by
a semi-permeable the solvent will permeate through the
membrane from the dilute to the concentrated phase (HWC
to LWC)
17. SOLUTION DIFFUSION
If a pressure is applied to the concentrated phase so that a
pressure difference is generated across the membrane the
net transport of the solvent will be reduced. The pressure
difference required to reduce the net transport of solvent to
zero is known as the osmotic pressure difference.
18. SOLUTION DIFFUSION
To relate the flux of a species to the chemical
potential we need to establish the differences in
chemical potential for the species across a
membrane
For isothermal conditions the chemical potential
of the solvent in the concentrated phase is
expressed as
19. SOLUTION DIFFUSION
The chemical potential of the solvent in the
dilute phase is expressed similarly as
The difference in chemical potential can thus
be expressed as
20. SOLUTION DIFFUSION
For dilute solutions the activity of the solvent
will be approximately equal on both sides.
Therefore the potential is highly dependant on
the pressure difference
21. SOLUTION DIFFUSION
There will be a net flow of solvent through the
membrane until the chemical potentials in
both phases are equal
Which gives:
22. If the dilute phase consists of pure solvent then the osmotic pressure of
the concentrated phase can be expressed as
For dilute solutions
If the solute is denoted as component j
Therefore
SOLUTION DIFFUSION
23. SOLUTION DIFFUSION
Also for dilute solutions
Therefore
If the salt dissociates then the above relationship has to be modified, as
the number of moles of solute will be increased. Therefore:
24. Osmotic Pressure of Real Solutions
• Previous expressions assume that solution
behaviour is ideal
• π of solutions departs significantly from ideal
behaviour at high concentrations
• It is possible to estimate π using a virial expansion
• Unfortunately coefficients are not always readily
available over the range of concentrations
required
25. Osmotic Pressure of Real Solutions
• The vapour pressure of the solution can also
be used to estimate the osmotic pressure with
reasonable accuracy
26. Trans-membrane flux of solvent
From Fick’s law
For isothermal operation the expression can be
reduced to
i.e. for constant retentate conditions the flux will
be linear w.r.t. the trans-membrane pressure
28. TRANS-MEMBRANE FLUX OF SOLUTE
From Fick’s law:
As before this reduces to
Independent of pressure as is the rejection
coefficient
29. TRANS-MEMBRANE FLUX OF SOLUTE
The concentration of the solute in the permeate can
be written as
the rejection coefficient becomes
we can see that as the pressure drop across the
membrane increases so does the rejection
coefficient
30. REVERSE OSMOSIS SYSTEM DESIGN AND
FUTURE CHALLENGES
• Membrane Fouling Control
• Future $
31. MEMBRANE FOULING
• Membrane fouling is the main cause of
permanent flux decline and loss of product
quality in Reverse Osmosis systems, so fouling
control determines system design and
operation.
32. SOURCES OF FOULING
• There are four Categories
1. Scale
2. Silt
3. Bio Fouling
4. Organic Fouling
33. SCALE
• Cause
– Precipitation of dissolved metal salts in the feed
water on the membrane surface.
• Salts that form Scale
– Calcium Carbonate
– Calcium Sulphate
– Silica Complexes
– Barium Sulphate
34. METHOD TO DETERMINE
• Scaling is determined by calculating Expected
Concentration Factor in the feed Brine
solution.
Where
35. • Scaling is not a problem if and only if
–Concentration Factor < 2 or
–Recovery Rate = 50%
• Most systems are operated at Recovery Rate of
80 – 90%, the Concentration Factor Exceeds 2
36. CONTROL OF SCALE
• Calcium Carbonate Scale is a common
problem and is controlled by the acidifying the
feed.
• Others can be reduced by adding Antiscalant
Chemicals such as Sodium
Hexametaphosphate.
37. SILT
• Cause
– It is caused by the suspended particulates of all
types that accumulate on the membrane surface.
• Typical Sources
– Particulates
– Iron Corrosion Products
– Precipitated Iron Hydroxide
– Algae
– Fine Particulate Matter
38. METHOD TO DETERMINE SILT
• SILT DENSITY INDEX (SDI) of the Feed Water
– The time required to filter a fixed volume of water
through a standard 0.45µm pore microfiltration
membrane.
Ti = Total Elapsed Test Time
Tf = Time required to collect 500ml Water
Tt = Total Elapsed Test Time
39. If:-
• SDI <1 ---- RO can run several years without
colloidal fouling
• SDI< 3 ---- RO can run several months without
cleaning
• SDI 3-5 ---- RO need Cleaning on regular basis
• SDI > 5 ---- Unacceptable and additional pre-
treatment is required.
40. BIOFOULING
• Cause
– Growth of Bacteria on Membrane Surface.
– Membrane itself becomes nutrient for bacteria
• Industrial examples are
– Cellulose Acetate Membrane
– Ployamide Fibres
• Solution is sterilization by heat, chlorine or
chemicals.
41. ORGANIC FOULING
• Causes
– Attachment of materials such as oil or grease onto
the membrane surface.
• Such fouling may occur accidentally in
municipal drinking water system, but is more
common in industrial applications in which RO
is used to treat a process or effluent stream.
