REVERSE OSMOSIS SYESTEM

(University of Engineering and Technology, Lahore)

Presenters

• Awais Yaqoob(2011-ch-32)

• Mujahad Ali(2011-ch-24)1

• Engineer Haq Nawaz(2011-ch-44)

• Adeel Matloob(2011-ch-46)2

• Rizwan Liaquat(2011-ch-72)

• Hafeez-ur-Rehman(2011-ch-80)3

Synopsis

Ion-Exchange Unit

Reactions Involved Merits and Demerits

Activated Carbon Bed

Factors Affecting its Performance Regeneration of Bed

Introduction to Reverse osmosis

Key Terms RO arrangements

Advantages And Disadvantages of RO

Comparison b/w RO and other purification techniques Conclusion

Basic Equations for RO Calculations

RO System Design Guidelines Steps to Design RO Membrane System

Cartridge Filter

Flow Meters Conductivity Meters

Reverse Osmosis

• Reverse Osmosis is a technology used to remove majority of

contaminants from water by pushing water under high pressure

through a semi permeable membrane

• It is a process where you demineralize or deionize water

Osmosis

• To understand purpose and process of RO, you must first

understand naturally occurring process of Osmosis

• It is a process where a weaker saline solution will tend to

migrate to a stronger saline solution

• For example, Plant root absorbs water from soil & Kidney

absorbs water from blood

• In diagram, salts are more concentrated in salty water so natural

flow of salts will be from right side to left side and water will

flow from right side to left side

Reverse Osmosis

• RO is other way round of naturally occurring Osmosis

• Water is pushed through semi permeable membrane under high

pressure, thus leaving behind contaminants

• Pressure depends upon salt concentration of feed water.

• More the salts, more will be the applied pressure

Cont.…

• Two types of water coming out of RO

– Permeate (containing less contaminants)

– Concentrate, reject or brine (containing more contaminants)

• Ro system employs cross filtration rather than standard

filtration where the Contaminants are collected within the filter

media.

Salt Rejection %

• It tells you how effective the RO membranes are removingcontaminants.

• A well designed RO system with properly functioning ROmembranes will reject 95% to 99% of most feed watercontaminants

s𝑎𝑙𝑡 𝑅𝑒𝑗𝑒𝑐𝑡𝑖𝑜𝑛 %

=𝐶𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝐹𝑒𝑒𝑑 − 𝐶𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒

𝐶𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝐹𝑒𝑒𝑑

• The higher the salt rejection, the better the system isperforming

• The lower the salt rejection meaning either filter needs to becleaned or replaced

Salt Passage %

• It is the inverse of Salt rejection.

• It tells the amount of salt passing through RO system

Salt Passage % = (1-Salt rejection%)

Recovery %

• It is the amount of water that is being

recovered as permeate

Recovery % = 𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 (𝑔𝑝𝑚)

𝐹𝑒𝑒𝑑 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 (𝑔𝑚𝑝)* 100

• Industrial RO typically run anywhere from

50% to 85%

Concentration Factor

• The concentration factor is related to RO system recovery

• The more water you collect as permeate, the more concentrated

salts you collect in concentrate stream which may lead to scaling

and fouling

Concentration Factor = 1

1−𝑅𝑒𝑐𝑜𝑣𝑒𝑟𝑦 %

1 & 2 Stage RO System

• In 1 stage RO system, feed enters RO system as one stream

and leaves as concentrate and permeate.

• In 2 stage, concentrate from 1st becomes the feed water to the

2nd stage. Permeate water from 1st stage is collected and mixed

with permeate water from 2nd stage.

1 stage RO

2 Stage RO

Single Pass RO & Double Pass RO

• In double pass RO, permeate from 1st pass becomes the feed to

2nd pass

• By going through 2 RO systems, a much higher quality

permeate can be achieved

• It also removes Carbon Dioxide by injecting Caustic Soda

between 1st and 2nd pass

• By adding Caustic Soda, we convert CO2 to carbonates and

bicarbonates which are removed in 2nd pass.

Single Pass RO

Double Pass RO

Activated Carbon Bed

Presented By:

Mujahad Ali(2011-ch-24)

Activated Carbon Bed

• Activated carbon is a form of carbon processed to have small,

low-volume pores that increase the surface area available

for adsorption.

• Due to its high degree of micro-porosity, just one gram of

activated carbon has a surface area in excess of 500 m2

Activated Carbon Bed

• Active charcoal carbon filters are most effective at

removing chlorine, sediment, volatile organic

compounds (VOCs), taste and odor from water.

• They are not effective at removing minerals, salts, and

dissolved inorganic compounds.

• Typical particle sizes that can be removed by carbon filters

range from 0.5 to 50 micrometers.

Factors Affecting the Operation

• Molecular weight

• pH

• Contaminant concentration

• Particle size

• Flow rate

• Temperature

Molecular Weight

• As the molecular weight increases, the activated carbon

adsorbs more effectively because the molecules are less

soluble in water.

• The pore structure of the carbon must be large enough to allow

the molecules to migrate within.

• A mixture of high and low molecular weight molecules should

be designed for the removal of the more difficult species.

pH

• Most organics are less soluble and more readily adsorbed at a

lower pH

• As the pH increases, removal decreases.

• Increase the size of the carbon bed by twenty percent for every

pH unit above neutral.

Contaminant Concentration

• The higher the contaminant concentration, the greater the

removal capacity of activated carbon.

• The contaminant molecule is more likely to diffuse into a pore

and become adsorbed.

• Higher contaminant concentration may require more contact

time with the activated carbon.

Particle Size

• Activated carbon bed is commonly available in 8 by 30 mesh

(largest), 12 by 40 mesh (most common), and 20 by 50 mesh

(finest).

• The finer mesh gives the best contact and better removal, but

at the expense of higher pressure drop.

• 8 by 30 mesh gives two to three times better removal than the

12 by 40, and 10 to 20 times better kinetic removal than the 8

by 30 mesh.

Flow Rate

• The lower the flow rate, the more time the contaminant will

have to diffuse into a pore and be adsorbed.

• Adsorption by activated carbon is almost always improved by

a longer contact time.

• Whenever considering higher flow rates with finer mesh

carbons, watch for an increased pressure drop.

Temperature

• Higher water temperatures decrease the solution viscosity and

increase diffusion rate.

• Higher temperatures can also disrupt the adsorptive bond.

• Generally, lower temperatures seem to favor adsorption.

Regeneration of Bed

• The economics of the adsorption process greatly depend on the

reuse of activated carbon.

Various regeneration techniques are used such as;

• Thermal regeneration

• Chemical regeneration

• Wet air oxidation

Presented By:

2011-ch-44

32

Water Softener

33

• What is ion exchanger?

• Applications

• Formation of ion exchanger

• Hardness types and removal

• Resins types

• Regeneration

• Advantages & Disadvantages of ion exchanger

Water Softener/Ion Exchanger

34

Ion exchange is an adsorption phenomenon where the mechanism

of adsorption is electrostatic.

What is ION Exchange?

35

• Ca , Mg (hardness removal) exchange with Na or H.

• Fe, Mn removal from groundwater.

• Demineralization (exchange all Cations for H all anions for

OH)

Applications

36

Resins are formed by polymerization process .

• Typical procedures involve heating aqueous solutions

of alumina and silica with sodium hydroxide.

• Equivalent reagents include sodium aluminate and sodium

silicate.

Formation of Resins

37

1) Temporary hardness

2) Permanent hardness

Hardness

Removal of Temporary Hardness

38

(1)Boiling Method:

Ca ( HCo3)2 ---> CaCo3 + Co2 + H2o

Mg ( HCo3)2 ---> MgCo3 + Co2 + H2o

(2)Clark’s Method:

Ca (HCo3)2+ (Ca(OH)2 ---> 2CaCo3+ 2 H2o

Removal of Permanent Hardness

39

(1)Reaction with Washing Soda

Na2Co3 + Caso4 ---> CaCo3 + Na2so4

(2) Ion Exchange Method:

Caso4 + Na2 – zeolite ---> Ca - zeolite + Na2so4

Resin Classification

40

Cationic resin

Anionic resin

Cationic Resin Vs. Anionic Resins

41

• Those that exchange positive ions, called cation exchange

resins.e.g Ca+2, Mg+2

• Those that exchange negative ions, called anion exchange

resins.e.g Cl-1, SO4-2,

42

Regeneration and its Types

Counter-current

Regeneration

Co-current Regeneration

Co-current Regeneration

43

Counter-current Regeneration

44

Regeneration Reaction

45

Ca- zeolite + NaCl ---> Na2 – zeolite + CaCl2

Advantages of Soft Water

46

• Longer appliance life for washing machines

• Less use of household cleaning products, such as detergents.

• Your clothes last longer and remain brighter longer if they are

washed in soft water.

• Your plumbing will last longer. Hard water can cause a build

up of scale from mineral deposits

• Your skin is softer when you bathe with soft water

Advantages of Ion Exchange Column

47

• It is compact and has a low capital cost

• The chemicals used are safer for the operator to handle and

operation

• It can be almost totally automated

• It can be used for production of good quality water

Disadvantages

48

• Adsorption of Organic Matter

• Iron Fouling

• Bacterial Contamination

Cartridge Filter

Cartridge Filter

• Fabric or Polymer-Based

• To remove Particulate material

• Designed along a central core

• Pleats/ Foldings

Top Cap

Bottom Cap

Pleated Media

Working of Cartridge

Filter

• Pressurized Fluid

• Passage through the Pore

• Suspended Solid material

• Clogging

• Pressure drop

Application

• Filtration of surface water or ground water under the influence of surface water.

• Prefiltration prior to subsequent treatment.

• Solids removal.

Rotameter

Rotameter

• Variable area meters

• Cross Sectional area

• Floating

Working of Rotameter

• Volumetric Flow rate increases drag force

• Cone shaped area decreases the buoyancy force

• Equilibrium with the float weight

Why to use Cartridge Filter and Rotameter

Cartridge Filters

Cartridge Filters are less expensive then the other sediment filters

Minimum maintenance is involved

Can filter out anything about 5 to 10 micrometre in size

Can be both the surface and depth type filters

Ease in cleaning

Rota Meter

Relatively Simple Device to manufacture

Requires no External power or fuel to run

Scale of rotameter is almost linear

Clear glass is used which is highly resistant to shock

RO System

REVERSE OSMOSIS SYSTEM DESIGN

Presented By:

Rizwan Liaquat(2011-ch-72)

Basic Equations for RO Calculations

Water Transport

Solute Transport

Correlation of Operating Conditions

Water Transport

• Water transport through the membrane is expressed as a

permeate flux.

• The permeate flux is proportional to the net driving pressure

(NDP).

Where

CONTD…

• The product flow rate can be obtained by multiplying the

permeate flux by total membrane area.

• The pressure drop is calculated by the average flow rate

(feed and concentrate) as follows:

In which a and b are coefficients, specific for element and

feed spacer configuration. The values for these coefficients are

obtained experimentally.

Solute Transport

• Solute transport through an RO membrane is expressed as a

solute flux.

• This solute flux is proportional to the concentration difference

across the membrane.

• The average feed concentration (feed and concentrate) is

used in the feed side to calculate solute transport.

• And the rate of solute transport is defined by rejection or salt

passage as follows:

In which: B, solute permeability, R, rejection, SP, salt passage

Correlation of Operating Conditions

• RO membrane system performance (flux and rejection or salt passage) is influenced by operating conditions such as operating pressure, temperature, feed concentration etc.

• AS = Specific flux at operating conditions,

• An= Specific flux at nominal conditions

• SPS= Salt passage at operating conditions,

• SPn= Salt passage at nominal conditions

• (JV) S= Permeate flux at operating conditions, (JV) n,= Permeate flux at nominal conditions

• TCF = Correlation factor of temperature (1; on specific flux, 2; on salt passage)

• SCF= Correlation factor of feed concentration (1; on specific flux, 2; on salt passage)

• FF= Fouling factor

RO System Design Guidelines

Fouling Tendency with Operating Conditions

Recommended Range of Element Operating Conditions (Design

Guideline)

Fouling Tendency with Operating Conditions

• Membrane fouling is caused by particles and colloidal

materials which are present in the feed water and become

concentrated at the membrane surface.

• The Silt Density Index (SDI) of pretreated feed water is an

index of the fouling potential of particle or colloidal materials

in the RO system.

• The concentration of the fouling materials at the membrane

surface increases with increasing permeate flux, increasing

element recovery and decreasing concentrate flow rate.

• Therefore the average permeate flux of the RO system

should be low if a strong fouling environment is

anticipated.

Recommended Range of Element Operating

Conditions

The maximum lead element permeate flux

The maximum average permeate flux

The maximum recovery (system and element)

The maximum feed flow rate

The minimum concentrate flow rate

Steps to Design RO Membrane System

System Design Information and Feed Water

Selection of Element Type and Average Permeate Flux

Calculation of Number of Total RO Elements

Decision of Recovery Rate

Decision of Number of Stages

Decision of Number of RO Elements per Pressure Vessel

Decision of Element Arrangement

Relations between Nominal Performances and Field Results

System Design Information and Feed Water

Water source and pretreatment required

Customer/process required product flow rate

Application of water being treated

Expected recovery rate

Annual water temperature

Required product water quality, operating pressure limit, etc.

CONTD…

• The RO membrane system highly depends on the available

feed water.

• Therefor, the System design information should be

thoroughly studied and considered in selection of the RO

system design.

• If the required permeate water quality is so high that the

quality cannot be achieved by 1pass RO system, and then

a 2 pass RO system should be considered.

• As an alternative to the 2 pass RO, an ion exchange resin

system may also be a viable design option

Selection of Element Type and Average

Permeate Flux

• According to the feed water source, pretreatment and feed

water salinity, the type of RO membrane element is

selected.

• Once the water source, pretreatment and RO element type

are fixed by the designer, the recommended value of the

average permeate flux (also called “design flux”) is

determined by pilot experiment data or customer’s

experience.

Calculation of Number of Total RO Elements

• The relationship between the number of total elements, the

product flow rate and the average permeate flux is expressed

as follow equation:

• In Which:

• NE= Total element numbers

• Qp= Product flow rate

• JV,ave= Average permeate flux

• (MA)E= Membrane area of element

Decision of Recovery Rate

• In an RO membrane system, a recovery rate as high as

possible is desirable

• A high recovery rate can also cause some problems as

follows:

Possibility of scale formation increase because of the increase

of concentration factor

Osmotic pressure increase because of the increase of

concentration factor

Concentrate flow rate decrease

Permeate water quality deterioration because of average feed

concentration increase

CONTD…

• Generally recovery rate is decided by scale formation and

by feed pressure limit.

Decision of Number of Stages

• The number of RO stages defines how many pressure

vessels are in series in the RO membrane system.

• Every stage consists of a certain number of pressure vessels in

parallel.

• The number of stages is a function of the system recovery

rate, the number of elements per vessel, and the feed water

quality

CONTD…

Decision of Number of RO Elements per

Pressure Vessel

• RO membrane elements can be coupled together in series

in the pressure vessel, typically 1-8 elements per one

pressure vessel.

• In deciding the number of RO elements per pressure

vessel, plant size is usually considered first.

• In a large-scale plant (> 40 m3/h), 6-8 elements per pressure

vessel are usually adopted, and in a smaller plant, 3-5

elements per pressure vessel.

• In all cases, the space required to install or remove the RO

elements should be considered in the plant design.

CONTD…

• By increasing the number of RO elements per pressure

vessel, almost all RO design parameters will change.

Decision of Element Arrangement

• For the decision of element arrangement, the system

design parameters should be consistent with the design

flux guideline.

• To decide the array, several calculations for case study

should be done by computer program and these results

should be compared.

Relations between Nominal Performances and

Field Results

• A higher nominal flow rate element will require lower

feed pressure.

• At different test conditions and /or different membrane

area, feed pressure will be defined by water permeability.

• A higher salt rejection element will produce a permeate

of lower salinity.

• A lower relative salt passage element (multiplier of

nominal permeate flux by nominal salt passage) will

produce a permeate of lower salinity.

Advantages And Disadvantages of RO

Advantages:

1. Low Energy Requirements.RO performs a separation without a phase change. Thus, the energy requirements are low

2. Less Space requirements.

RO systems are compact, and space requirements are less than with other desalting systems, e.g. distillation

3. Easy to Understand

RO equipment is standardized - pumps, motors, valves, flowmeters, pressure gages, etc. Thus, the learning curve for unskilled labor is short.

4. Little labor required:-

Many RO systems are fully automated and designed to start-up and shutdown

automatically through interlocks. Thus, RO plants usually require little labor.

5. Easy maintenance:-

Due to their modular design, maintenance is easy. maintenance can be

performed without shutting down the plant.

Also the expansion of plant is an easy option.

6. Remove unpleasant smell:-

RO removes dissolved minerals and other contaminants that cause to smell

unpleasant.

7. Efficient for plumbing system:-

Removal of dissolved minerals, metals and other particles benefits plumbing

systems, Nothing in the water to corrode pipes.

8. Removes bacteria and pathogens in drinking water

Biological contaminants present in tap water are harmful bacteria that can

cause diseases. And if you happen to drink the water, you may acquire fatigue,

diarrhea, excessive gas, bloating, loss of interest in food and weight loss.

9. Better taste and smell

In this process, 98% of chemicals are removed from your drinking water so it

will not taste of chlorine anymore.

10. Reduces the risk of having diseases and illnesses

Chlorine, asbestos, mercury and lead are some of the toxins that can be

found in tap water. Reverse osmosis offers defense between the body

and the other 2100 known toxins.

11. Children- friendly

Drinking pure water is very important to children’s developing immune

systems. Water filters such as reverse osmosis systems provide the healthiest

water for them.

Disadvantages

1. Purification Limits

Many RO systems come with carbon pre-filters. That’s because

chlorine and Volatile Organic Chemicals (VOC’s) are smaller than

water molecules so they can’t be filtered on the reverse osmosis

membrane.

2. Speed and Efficiency

These systems can only produce 15 (gpd). It works against

standard osmotic pressure so the reverse osmosis process is fairly

slow. And requires 3 to 10 gallons of untreated water to make a

single gallon of purified water, which is wasteful and expensive.

3. Maintenance:-

The maintenance of RO system must be done regularly. its filters must be

cleaned to avoid the fouling of the membranes. The pre-filters must be

changed annually, while the RO membrane should be replaced every 2-3 years.

4. Pressure limitations:-

The applied pressure must exceed the osmotic pressure to separate the solute

from the solvent.

The max pressure for seawater devices is 800 - 1000 psig,

For brackish water varies from 400 - 600 psig.

Due to the high pressure requirement RO is usually not applicable for

concentrated solutions.

5. Pretreatment Required:-

Because all RO membranes and devices are susceptible to

fouling, the RO process usually cannot be applied without

pretreatment.

6. Compatibility of feed Stream:-

RO feed streams must be compatible with the membrane and

other materials of construction used in the devices.

7. High Temperature

Usually at high temperature the RO process is favorable but the

problem at high temperature is the increase in pore size of the

membrane so it requires optimum conditions for temperature.

8. High Conductivity

As Conductivity is directly proportional to total dissolved solids

so It means high conductivity liquids require more treatment and

hence more time required for the RO purification system.

• Comparison b/w RO and other purification techniques

The given below are some techniques comparable with Reverse

Osmosis

1. Ion Exchange

2. Distillation

3. Activated Carbon

4. Precipitation

5. Ultraviolet Radiation

6. Boiling

DiseaseElement Causing

Disease

Reverse Osmosis

ROIon Exchange Distillation

Activated

CarbonPrecipitation

Ultra-

VioletBoiling

Cancer Initiation Chlorine # x B # x x X

pH In-equilibrium Alkali Fume # A # x x x X

Cancer Initiation Chloroform # x B # x x X

Bacteria Infections

DiseaseBacteria # x # B x # #

Virus Infections Disease Viruses # x # x x # #

Intoxication, Liver

DiseaseAgricultural Chemicals # x # # x x X

Hepatitis Dioxin # x # B x x X

Cancer Radioactive Material A A B x x x X

Anorexia Taste and Odor # x x # x x X

Calculus, Enteritis Precipitate # x # B # x X

Poisoning Organic Substances # x B # X x X

Cancer Initiation Fluoride A A # x x x X

Cancer Initiation Fluorescence # x x x x x X

Neuritis Arsenic A A A x x x X

Calculus Calcium A A A x x x X

Notalgia Cadmium A A A x x x X

Nephrosis, Leading

PoisoningLead A A # X x x X

Organic Phosphorus

PoisoningPhosphorus A A # X x x X

Electrolyte In-equilibrium Potassium A A A X x x X

Hypersensitive Heart

DiseaseSodium A A # X x x X

Digestive System Disease Sulphur A A A X x x X

Digestive System Disease Magnesium A A # # x x X

Where #=98-99% Removal ; A =96-99% Removal; B =Partial Removal; x =Can not Remove