1. REVERSE OSMOSIS
ÏU AZOGUE
JAVIER PÉREZ
December 1st 2003

2. WHAT IS REVERSE OSMOSIS? (I)
But, first, what is osmosis?
SOLUTE
C1 < C2
SOLVENT

3. WHAT IS REVERSE OSMOSIS? (II)
This is osmosis:
Osmotic pressure difference Dp
SOLVENT

4. WHAT IS REVERSE OSMOSIS? (III)
P > Dp
SOLVENT

5. 1748. J.A. Nollet discovers osmosis phenomena.
HISTORY
1748. J.A. Nollet discovers osmosis phenomena.
1855. A. Fick enunciates a law to describe membrane diffusion. 1887, J.A. Van’t Hoff expose first theorical explanations.
Gibbs adduce a scientific base these theories are completed. However, osmosis is banished from one’s mind.

6. HISTORY: Desalination
First attempt, J.E. Breton and C.E. Reid who is recognized as inventor of reverse osmosis. Nevertheless, they obtain low volume of drinking water.
1960. S. Loeb and S. Sourirajan prepare cellulose acetate membranes.
1962. Pilot plant in California. 1965, 4th june: Incorporation to water supply.

7. 1963. U. Merten propound mathematical equations which describe solute and solvent flow across membrane.
1968. J. Westmoreland and D.T. Bray design and patent spiral configuration.
1917. Patent of aromatic polyamide.
First industrial plants to produce drinking water appear in the second half of 70’s.

8. There are some process which both solvent and solute are recovered.
WHY IS USEFUL?
There are many applications for reverse osmosis. These can be included in two general groups:
Recovery of solvent
Recovery of solute
There are some process which both solvent and solute are recovered.

9. Production of drinking water Treatment of urban waste water
SOME USES
Production of drinking water
Treatment of urban waste water
Production of water for industrial uses
Treatment of different wastes
Concentration of fruit juices, white of an egg, whey...
Fermentation

10. PRESSURE RETARDED OSMOSIS
This process enables to generate energy from a concentration difference.
The water flow at a pressure water P < 
Power per unit membrane area:
Practical problems:
Salt flux
Concentration polarisation
fresh water
turbine
saline water

11. PRESENT AND FUTURE
Reverse osmosis has have a high increase since 60’s. This increase has been promoted by its application to desalation and waste treatment
Nowadays water market hold majority economic resources for investigation in osmosis field.

12. PRESENT AND FUTURE
Removal and selective concentration of certain substances are less developed than production of clean water
However, their low development may involve espectacular innovations and new applications during next years.

13. PARAMETERS Qa Ca Qp Cp Pa pa Pp pp Qr Cr Pr pr Q - Flux
C - Concentration
P - Pressure
p - Osmotic pressure

14. A, permeability coeficient (for T and salinity fixed) [m3/d·m2·bar]
PARAMETERS
A, permeability coeficient (for T and salinity fixed) [m3/d·m2·bar]
Y, recovery
R, rejection

15. Fc, concentration factor
PARAMETERS
Ps, pass of salts
Fc, concentration factor

16. There are two forces which rule solvent and solute fluxes:
EQUATIONS OF PROCESS
There are two forces which rule solvent and solute fluxes:
Solvent: Pressure gradient
Ja = A(DP - Dp)
Solute: Concentration gradient
Js = B·DC + M·JaCm

17. Ja = A(DP - Dp) Ja, solvent flux
SOLVENT TRANSPORT
Ja = A(DP - Dp)
Ja, solvent flux
A, permeability coeficient [m3/d·m2·bar]
DP, applied pressure difference
Dp, osmotic pressure difference

18. Js = B·DC + M·JaCm Js, solute mass flux B, permeability [m3/d·m2·bar]
SOLUTE TRANSPORT
Js = B·DC + M·JaCm
Js, solute mass flux
B, permeability [m3/d·m2·bar]
DC (= Cm-Cp), concentration difference in solute
M, distribution constant

19. Js = Ja·Cp From this equation, Cp:
Is proportional to concentration gradient
Is inversely proportional to pressure gradient

20. Rejection and solvent flux

21. SOLVENT FLUX AND REJECTION
Solvent flux and rejection are two important parameters
Study of these and their relation with other parameters are useful to optimize the process. Some of these parameters are pressure, Ca, feed temperature, recovery, feed pH.

22. PRESSURE

23. FEED SOLUTE CONCENTRATION

24. FEED TEMPERATURE

25. RECOVERY

26. FEED PH

27. Is the most important application of reverse osmosis
DESALINATION
Is the most important application of reverse osmosis
There are many osmosis desalination plants constructed or in process of construction
In 1987, reverse osmosis represented 25% of total worldwide desalination capacity by all methods

28. FEED CONSIDERATIONS (DESALINATION)
Depending water source:
Superficial
Well
Salinity of water used. Feed of process is usually seawater or brine:
Seawater: g/l Brine: g/l

29. PRINCIPAL SECTIONS: DESALINATION
Water feed
Pretreatment
Reverse osmosis
Complementary treatment

30. SCHEMATIC DIAGRAM DESALINATION

31. REVERSE OSMOSIS PLANTS DESALINATION

32. TECHNOLOGY COMPARISON
There are another technologies as destilation and electrodialysis to desalinate water
Comercialy, destilation is used for seawater, electrodialysis for saltwater and reverse osmosis for both kind of water

33. TECHNOLOGY COMPARISON

34. THECNOLOGY COMPARISON

35. REVERSE OSMOSIS MEMBRANES
ÏU AZOGUE
JAVIER PÉREZ
December 1st 2003

36. Membrane Selection
Membrane accounts for 15 to 40 percent of the price in reverse osmosis.
Membranes must be replaced periodically
CAREFUL MEMBRANE
SELECTION IS ESSENTIAL
SELECTION CRITERIA:
Chemical tolerance
Mechanical suitability
Price
Cleanibility
Separation performance
GOOD DESIGN:
Consistent performance
Needs less frequent membrane cleaning
Reasonable consum of power
Little operational attention

37. Pore size in reverse osmosis
Salts and low molecular weight compounds

38. Pressure and flux range
Membrane process
Pressure range (bar)
Flux range (l/m2·h·bar)
Microfiltration
0,1 - 2,0
>50
Ultrafiltration
1,0 - 5,0
10 – 50
Nanofiltration
5,0 – 20
1,4 - 12
Reverse Osmosis
0,05 - 1,4
The pressures used in reverse osmosis range from 20 to 100 bar and the flux from 0,05 to 1,4 l / m2·h

39. Features of membranes in RO
In contrast to MF and UF the choice of material directly influences the separation efficiency through the constants A and B.
A hydrodynamic permeability coefficient
B solute permeability coefficient
The types of membranes used are porous, and it can be asymmetric or composite. Usually they are formed by a toplayer (about 1 mm) and a sublayer (150 mm)
The flux through the membrane is as important as its selectivity towards various kinds of solute. The flux is approximately inversely proportional to the membrane thickness, and for this reason membranes have an asymmetric structure.
The resistance towards transport is determined mainly by the dense toplayer.
The function of the sublayer is mainly support.

40. Types of membranes (I) INTEGRAL MEMBRANES
Both toplayer and sublayer consist of the same material.
Materials used: cellulose esters (especially diacetate & triacetate), polybenzimidazoles.
Prepared by phase inversion techniques.
COMPOSITE MEMBRANES
Toplayer and sublayer composed of different polymeric materials such as aromatic polyamides
The support material is commonly polysulfones while the thin film is made from various types of polyamides, polyureas, etc
Prepared by dip coating, in-situ and interfacial polymerization.

41. CTA membranes (Cellulose TriAcetate)
Main Membrane Types
CTA membranes (Cellulose TriAcetate)
Low stability against chemicals, temperature and bacteria.
Typically needs a sediment prefilter
Low duration (18-24 months)
High sensibility to hydrolysis
High work pressures
High permeability
Allow contact with chlorine in water.
Low cost
High removal percentage of sales %

42. TFC membranes (Thin Film Composite)
Main Membrane Types
TFC membranes (Thin Film Composite)
Synthetic membranes
Higher rejection to many chemicals than CTA membranes
High duration (3-5 years)
High removal percentage 94-99%
Low work pressures.
High chemical stability
High sensibility to the oxidants and chlorine.
Typically needs a carbon prefilter.
Easy fouling
High cost

43. Membranes comparative

44. Types of membranes (II)
Very low pressure membranes:
Work pressures between 5-10 bars used to desale water with a salt content in a range of mg/l
Designed to compete with the ion exchange resins
Low pressure membranes
Range pressure: bars Salt content ( mg/l)
Desalation of water or removal of compounds such as nitrates, organic substances ..
Medium pressure membranes
Range pressure: bars Salt content ( mg/l)
At the beginning used in desalation of waters with high salt content, but now used in multiple processes of separation and concentration.
High pressure membranes
Conceived to obtain potable water from the sea water in a single pass.
Osmotic pressure of sea water (till 35 bar in the Red Sea)  Range pressure (50-80 bar)
Recommendations of O.M.S (salt < 500 mg/l)  salt reject  99%

45. Prices of membranes CTA Membranes TFC Membranes Model l/m2 · h Size
Mem-S-25
3,94
1.8" x 12"
69$
Mem-S-40
6,30
2.0" x 12"
79$
Mem-S-60
9,50
89$
Mem-S-90
14,2
99$
TFC Membranes
Model
l/m2 · h
Size
Price
XTFCH-150
23,7
2.5" x 14"
112$
XTFCH-350
55,1
2.5" x 21"
174$
XTFCH-750
118,3
2.5" x 40"
247$
XTFCH-1000
157,7
4.0" x 21"
361$
XTFCH-2000
315,4
4.0" x 40"
489$
XTFCH-2600
410
540$

46. How to prepare RO membranes
Phase inversion techniques
Chemical PI is a process whereby a polymer is transformed in a controlled manner from a liquid to solid state. The process is very often initiated from the transition from one liquid state in two liquids. One of the liquid phases (the high polymer one) will solidify forming a matrix.
Immersion Precipitation is the most used PI technique currently specially for flat or tubular membranes. The only requirement is that the polymer must be soluble in a solvent or a solvent mixture.

47. How to prepare RO membranes
Dip coating
Very simple and useful technique for preparing composite membranes with a very thin but dense toplayer.
An asymmetric membrane (often of the type used in UF) is immersed in the coating solution where a thin layer adheres to it
The film is put in a oven where the solvent evaporates and crosslinking occurs.

48. How to prepare RO membranes
Interfacial polimerisation
A polymerisation reaction occurs between two very reactive monomers at the interface of two immiscible solvents.
Support layer (usually a UF membrane) immersed in an aqueous solution. Film immersed in a water immiscible solvent.
Reaction of the two monomers forms a dense polymeric toplayer.
The process takes advantage of the self-inhibiting character of the reaction.

49. Polarisation & Membrane fouling
Performance in RO is diminished by polarization and fouling phenomena.
Polarisation: reversible processes related with the increase of concentration over the bulk when we are closed to the membrane.
Fouling: deposition of retained particles, colloids, macromolecules, salts, etc … on or in the membrane. Typically more important in micro and ultrafiltration, but it becomes important also when we use hollow fiber or spiral wound configurations.

50. Fouling description and tests
The cake layer resistance can be written from a mass balance as:
Now the flux may be written as: or

51. MFI fouling test: Integrating the before expression:
The use of MFI values has the next advantages:
By comparing various solution, different fouling behavior can be observed
A maximum allowable MFI value can be given for a specific plant
Flux decline can be predicted to some extent
Other tests used: silting index (SI), plugging index (PI), silt density index (SDI)

52. Reducing fouling
Pretreatment of the feed solution: ph-adjustement, pre-microfiltration, pre-ultrafiltration, adsorption onto active carbon …
Membrane properties: use of hydrophilic membranes and dense membranes reduces the fouling.
Module and process conditions: fouling decreases as concentration polarization decreases. In this order is convenient to work with high flow velocities.
Cleaning:
Hydraulic
Mechanical
Chemical
Electric

53. Cleaning needs
As a general rule is necessary to realize a cleaning of the membranes when one of the next situations occurs:
Salts filtration increases in more than 15% over the last value.
Production changes (increase or decrease) in more than 10%
Reject flow changes in more than 10%
Drop pressure in the modules increases in more than 20%
Feed pressure increases in more than 10%
Long plant stops (> 1week)
Before applying any reactant to regenerate membranes.

54. Chemical cleaning
Problem identification: the chemical product to be used will depend on the nature of the deposit, so the first thing to do is identify the type of fouling.
Types of fouling
Precipitations: mineral salts and metallic oxids
Deposits: high size particles, colloids, biological developments
Others: colloidal sulfur, oils, organic compounds ..
Fouling effects
We can also identify the effect produced by the effects produced in the pressure drop of the equipment, the flow permeate and the filtrate of salts.

55. Fouling effects

56. Chemical cleaning
The cleaning is realized recirculating through the modules the prepared solutions:

57. TFC membrane formulations

58. CTA membrane formulations

59. Membrane configurations in RO (I)
Spiral-wound configuration
Next logical step from a flat membrane but with higher packing density
300 – 1000 m2/m3
Permeate is collected in the central tube

60. Membrane configurations in RO (III)
Tubular configuration
Not self supporting in contrast to hollow fiber –modules
Permeate crosses the membrane layer to the outside
Low surface-volume ratio
Usually the active layer is inside

61. Membrane configurations in RO (II)
Capillary/hollow fiber configuration
Fibers diameter: <0.5 mm
• Flux velocity: low (up to 2.5 m/s)
• Feed: inside-out or outside-in
• Surface area/volume: high
• Pressure drop: low (up to 1 bar)
• Maintenance: hard
• Cleaning: poor

62. Comparison of module configurations
Tubular
Plate and frame
Spiral-wound
Capillary
Hollow-fiber
Packing density
LOW
VERY HIGH
Investment
HIGH
Fouling tendency
Cleaning
GOOD
POOR
Membrane replacement
YES NO