Introduction to Reverse Osmosis
Training Package
Desalination – A World of Salt
Desalination Technology
Two Types of Desalination Process – Thermal or Membrane
Market Share – Desalination Technologies
Firstly, What Are Membranes?
•
Thin Layer of MATERIAL capable of separating
constituents as a function of their physical and
chemical properties when a DRIVING FORCE is
applied across the membrane.
•
Different Membranes defined by the particle size
they can exclude.
•
Different Membranes defined by the material of
construction.
•
Usual driving force in water is pressure. (In some
specialist applications can be electricity).
Two Types – Microporous and Semi-permeable
•
Transport Mechanisms
– Sieve or Filter – (Microporous Membranes) – Removes Particles
– (Semi-permeable membranes) – Removes Dissolved Substances
Type of Membrane
Micro-porous
Semi-permeable
Pore Size
Removal Mechanism
Microfiltration (MF)
>0.1 micron
Sieve
Ultrafiltration (UF)
>0.01 micron
Sieve
Nanofiltration (NF)
20,000 MW Cutoff
Ion Separation
Reverse Osmosis
(RO)
200 MW Cutoff
Ion Separation
Electrodialysis
Reversal (EDR)
200 MW Cutoff
Charged Ion
Separation
Filtration spectrum
Microfiltration
Ultrafiltration
Nanofiltration
Reverse Osmosis
10 um – 100 nm
100 - 10 nm
10 - 1 nm
< 1 nm
giarda
crypto
bacteria
colloids
viruses
color
hardness
pesticides
salts
water
Courtesy – Norit Membranes
colloids
viruses
colour
hardness
pesticides
salts
water
colour
hardness
pesticides
salts
water
salts
water
Operating Pressure for Different Membranes
Semi Permeable
Microporous
84
70
bar
56
42
28
14
0
SWRO BWRO LP-RO
NF
UF
MF
Microporous Membranes - MF/UF- Membrane Separation Mechanism
Feed
Salt
H2O
Pores
H2O
Particle
Membrane
Virus
H2O
H2O
H2O
H2O
Filtrate
Semi-Permeable
Membranes
RO/ NF Membrane
Separation
Mechanism
RO/ NF Membrane Separation Mechanism
Feed Flow
H 2O
Concentrated
Salts
Na+
-
SO4
Cl
2-
H 2O
Na+
H 2O
H 2O
H 2O
-
Ca
Cl-
HCO3
H 2O
Na+
2+
ClH 2O
H 2O
H 2O
H 2O
Permeate
What is Osmosis?
Fresh Water
Salty Water
Concentrated
Fresh Water
Membrane
(Prevents passage of salt)
Diluted
Salty Water
What is Reverse Osmosis?
Applied
Pressure
Applied
Pressure
Fresh Water
Concentrated
Salty Water
RO Membrane – Configured as Spiral Wound Element
Most common way to package membrane.
Maximises surface area of membrane into plant footprint
Convenient to manage flow streams.
RO Element are Assembled in Pressure Vessels
Seal
Feed
Concentrate
Permeate
Permeate
collection tube
Permeate tube coupling
Pressure vessel
Reverse Osmosis Element Pressure Vessel Assembly
Feed
O-rings
Interconnector
Brine Seal
Permeate
Head Seal
Pressure Vessel
Thrust Cone
Head
End Adapter
R.O. Element
Concentrate
Retaining Ring
Pressure vessels assembled to ASTM requirements and rated by
feed pressure
Part 3
Membrane Terminology
Membrane Terminology – Permeate/Concentrate
•
Permeate – Treated Water
– Permeate is the product produced by RO.
– Reduced concentration of salt.
•
Concentrate – Concentrated Waste from Membrane
– Called brine in seawater applications.
– Also sometimes called reject.
•
Stages – Banks of pressure vessels in series. They treat feed
water, or concentrate from a previous stage.
– Desal plants usually single stage only.
– Wastewater Recycling 2 to 3 stages.
•
Pass – RO Train producing permeate product. A second pass
treats permeate from a first pass RO.
Membrane Terminology – Recovery
Ratio of the permeate flow to the feed flow
Recovery
=
Permeate
Flow
as %
Feed Flow
VFD
Permeate Flow
Water
Feed Feed
RO Feed Pump
With VFD
Concentrate Flow
RO Unit Recovery
•
•
The recovery of the system
increases with the number of
stages.
Recovery is limited by a variety
of factors including the
solubility of salts in the feed,
the osmotic pressure and the
maximum pressure of the
vessels.
Typical RO Recoveries
Feed Water
Typical
Recovery
Tap Water
90 %
Brackish Water
(including
wastewater
recycling)
70% - 85%
Seawater (first
pass)
40% - 50%
RO Process Control
FIT
VFD
Permeate
Feed
RO Feed Pump
With VFD
FIT
Concentrate
Concentrate Flow
Control Valve
RO Improvements - Declining Power Consumption
Source: Water Factory 21 Water Reclamation Plant
Membranes operating at 10.4gfd
Slide courtesy of Rob Huehmer, CH2M Hill
Pressure/Work Exchanger