Reverse osmosis, types of membranes, advantages and disadvantages

1. REVERSE OSMOSIS
AISHWARYA. B
D3 BIOTECH

2. OSMOSIS
 Osmosis is the movement of water or other solvent through a semipermeable
membrane from a region of low solute concentration to a region of high solute
concentration.Osmosis is passive transport, and it does not require energy to
be applied.

3. REVERSE OSMOSIS
 Reverse osmosis is the type of osmosis in reverse.
 Water molecules passes from a more concentrated region to a less
concentrated region
 The phenomenon requires a certain pressure to overcome the applied
osmotic pressure
 Pore size of a membrane can range from 1000 to 0.0001 microns.
 Reverse osmosis(RO) has the capability to removes dissolved solids from water
but also imporoves it’s taste, odour and colour of the water.
 It can remove dissolved particles as small as ions.
 Reverse osmosis can remove many types of dissolved and
suspended chemical species as well as biological ones (principally bacteria)
from water, and is used in both industrial processes and the production
of potable water.

4. PRINCIPLE OF REVERSE OSMOSIS
The solution is subjected to pressure and
pressed against the semipermeable
membrane.
Applied pressure > osmotic pressure!
This makes water molecules move from the
higher concentrated to the less concentrated
solution

5. Factors affecting reverse osmosis
Permeate flux and salt retention are the main parameters that determine the
performance of a Reverse Osmosis system.The factors are mainly influenced by
Pressure-When the pressure of the water is increased, the dissolved solids
content of the permeate will decrease, while the permeate flux increases.
Temperature-When temperatures increase and other parameters are continual,
the permeate flux and the salt flow will increase.
Recovery-When recovery increases, the salt retention will decrease.

6. TYPES OF MEMBRANES USED
1. SPIRAL WOUND MEMBRANES
2. THIN FILM COMPOSITE MEMBRANE
3. STAINLESS STEEL MEMBRANE

7. Spiral wound membranes
 Spiral-wound elements come in multiple configurations
and allow it to be used in a wide variety of applications.
 High packing density.
 Easy cleaning
 Low capital and operating costs.
LIMITATION
 Due to the high packing density, total suspended solids
(TSS) must be reduced to prevent plugging of the
membrane.
 Spiral elements also cannot handle mechanical cleaning
like tubular elements and
 contain lower packing density than hollow fiber.

8. THIN-FILM COMPOSITE MEMBRANE
 It is made out of a thin polyamide layer
deposited on top
of polyethersulfone or polysulfone porous layer
on top of a non-woven fabric support sheet.
 The three layer configuration gives the desired
properties of high rejection of undesired
materials (like salts),
 High filtration rate.
 Good mechanical strength.
LIMITATION
 Thin film composites membranes typically
suffer from compaction effects under pressure.
 As the water pressure increases, decrease in
it’s porosity.
 Surface fouling: Colloidal particulates, bacteria
infestation (biofouling).
 Chemical decomposition and oxidation.

9. STAINLESS STEEL MEMBRANES
 High thermal and chemical stability.
 Inertness to microbiological degradation.
 Ease of cleaning.
 The main use of metallic membranes is for
hydrogen separation with palladium.
LIMITATIONS
 Higher capital costs
 Surface poisoning effect

10. ADVANTAGES OF REVERSE OSMOSIS
 The ability to remove many dissolved substances
efficiently.
 Produce a good tasting finished water.
 RO does not add any other chemical to your water.
 Performs a separation without a phase change.
 The energy requirements are low.
 Compact, and space requirements are less.
 RO systems are fully automated and designed to start-up and shutdown
automatically.
 RO plants usually require little labour.
 Easy maintenance.

11. DISADVANTAGES OF REVERSE OSMOSIS
 Expensive process with slow output
 Limited to pre-filtered water, must be combined with another water treatment device
 Does not disinfect water to make it pure and clear
 For each gallon of water produced, between 2-20 gallons of water are lost as waste.
 Reverse osmosis units can be expensive.
 The RO membranes are subject to decay and require periodic replacement. As they decay,
the quality of the treated water becomes poorer.
 Hard water can shorten the life span of the RO membrane. A water softener might be
necessary to keep the membrane working at its best.
 Reverse osmosis units should not be used to treat water that contains harmful
microorganisms.
 Small holes in a worn membrane can allow microorganisms to pass through with the treated
water.