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REPORT On Reverse Osmosis Desalination Research · November 2015 DOI: 10.13140/RG.2.1.4855.7521

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12/26/2014

CHEMICAL ENGINEERING DEPARTMENT

REPORT ON REVESRE OSMOSIS COURSE: SUPERVISOR:

MASS TRANSFER OPERATIONS LAB ENGR. MUHAMMAD ABDUL QYYUM

SUBMITTED BY: ANSA AMAN ULLAH

CHE-SP12-010(B)

SIDRA YASIN

CHE-SP12-084(B)

MUHAMMAD USMAN JAVED

CHE-SP12-060(B)

COMSATS INSTITUTE OF INFORMATION AND TECHNOLOGY, LAHORE CAMPUS

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CONTENTS 1.

MASS TRANSFER OPERATION ................................................................... 4 1.1 CLASSIFICATION .............................................................................................................. 4 1.2 SIMILARITIES BETWEEN VARIOUS MASS TRANSFER OPERATIONS .................... 4 1.3 COMPARARISON OF VARIOUS MASS TRANSFER TECHNIQUES ............................ 5 1.4 COMPARISON OF REVERSE OSMOSIS WITH OTHER MASS TRANSFER TECHNIQUES .................................................................................................................................. 6

2

WATER ............................................................................................................... 9 2.1 CHEMICAL COMPOSITION: ............................................................................................ 9 2.2 CHEMICAL NATURE: ....................................................................................................... 9 2.3 WATER SOURCES AND CONTAMINANTS: .................................................................. 9 2.4 POTABLE WATER ........................................................................................................... 12 2.5 BOILER-FEED-WATER-[27] ........................................................................................... 12 2.5.1 BFW Contaminants , Their Effect and Possible Treatment : .......................................... 13

3

WATER TREATMENT .................................................................................. 16 3.1 PRETREATMENT[29] ...................................................................................................... 16 3.1.1 PRETREATMENT OF WATER PRIOR TO RO-TREATMENT[30] .......................... 16 3.2 TREATMENT TECHNIQUES[30] .................................................................................... 17

4

RO-MASS TRANSFER TECHNIQUE .......................................................... 20

5 WHEN WILL BE THE RO IS PREFERRED OVER OTHER SEPARATION TECHNIQUES [31]: .................................................................... 22 6

OSMOSIS .......................................................................................................... 22

7

REVERSE OSMOSIS ...................................................................................... 23 7.1 OBJECTIVES ..................................................................................................................... 24 7.2 BLOCK SCHEMATIC DIAGRAM FOR RO-OPERATION............................................. 24 7.3 FLOW MECHANISMS THROUGH RO-MEMBRANES ................................................. 25 7.3.1 CROSS-FLOW FILTRATION ...................................................................................... 25 7.3.2 DEAD-END FILTRATION ........................................................................................... 26 7.4 BASIC DEFINITIONS AND TERMS................................................................................ 27 7.4.1 RECOVERY................................................................................................................... 27 7.4.2 REJECTION ................................................................................................................... 28 7.4.3 SALT PASSAGE ........................................................................................................... 29 7.4.4 FLUX.............................................................................................................................. 29 7.4.5 CONCENTRATION POLARIZATION ........................................................................ 29 7.4.6 BETA.............................................................................................................................. 30 7.4.7 FOULING....................................................................................................................... 31 7.4.8 SCALING ....................................................................................................................... 31 7.5 DESIGN CONSIDERATIONS/FACTORS FFECTING RO ............................................. 32 7.5.1 TOTAL DISSOLVED SOLIDS ..................................................................................... 32 7.5.2 TEMPERATURE ........................................................................................................... 33 7.5.3 PRESSURE .................................................................................................................... 34 7.5.4 FEED WATER FLOW ................................................................................................... 34 7.5.5 CONCENTRATE FLOW ............................................................................................... 34

3 7.5.6 7.5.7 7.5.8

8

BETA.............................................................................................................................. 35 RECOVERY................................................................................................................... 35 FLUX.............................................................................................................................. 36

MEMBRANE[35] ............................................................................................. 36 8.1 MODULES[37] .................................................................................................................. 37 8.1.1 TUBULAR MODULES : ............................................................................................... 37 8.1.2 FLAT SHEET MODULES : .......................................................................................... 38 8.1.2.1 SPIRAL WOUND MODULE .................................................................................... 38 8.1.3 COMPARISON OF CHARACTERISTICS OF VARIOUS MODULES [35] ............. 39 8.2 GENERALLY USED MEMBRANES ............................................................................... 39 8.2.1 Cellulose Acetate Membranes[37].................................................................................. 39 8.2.2 Thin Film Composite Membranes[37] ........................................................................... 40 8.3 FOULING FACTORS AND METHODS TO AVOID : ..................................................... 41

9

APPLICATIONS [34]: ..................................................................................... 42

10 EXPERIMENTAL ANALYSIS [39]............................................................... 42 10.1 PROCEDURE: ................................................................................................................... 42 10.2 OBSERVATIONS AND CALCULATIONS: .................................................................... 42 10.2.1 OBJECTIVE 1 :.......................................................................................................... 42 10.2.2 OBJECTIVE 2:........................................................................................................... 43 10.2.3 OBJECTIVE 3: To Check The Performance Of RO-apparatus ................................. 43 10.2.4 OBJECTIVE 4: TO Study And Compare The RO Feed Water, Permeate And Retentate 43 10.2.5 OBJECTIVE 5:To study the effect of pressure on RO performance by qualitative analysis of permeate and retentate. ............................................................................................ 43 10.3 RESULTS AND DISCUSSIONS: ...................................................................................... 44 10.3.1 Comparison Of Properties Of Various Types Of Water ............................................. 44 10.3.2 RO Water Analysis ..................................................................................................... 45 RO WATER ANALYSIS ........................................................................................................... 45

11 REFERENCES: ................................................................................................ 46

4

REPORT ON REVERSE OSMOSIS TECHNIQUE 1. MASS TRANSFER OPERATION [1] Mass transfer operation is the one in which the following phenomena must exist: 1. Two or more phases must come in contact with each other. 2. Materials should flow from one phase to other. 3. A part of the total flow must be by molecular motion or molecular diffusion.

1.1 CLASSIFICATION The mass transfer operations have been classified according to the phase contact in table below:

1.2 SIMILARITIES BETWEEN VARIOUS MASS TRANSFER OPERATIONS There are many similarities between the various mass transfer operations. They are: 1. Phase equilibrium is reached after a sufficiently long period of contact. 2. Rate of transfer is calculated by deviation from equilibrium concentration. 3. Equilibrium exists at phase interphase or there is no resistance to mass transfer at the interface, with some exceptions. 4. Material transfer is due to combined effect of molecular diffusion and turbulence.

5

1.3 COMPARARISON OF VARIOUS MASS TRANSFER TECHNIQUES Distillation [2]

Evaporation[2]

Mixture of components are separated on the basis of their different volatility

Separate single component (usually water) from nonvolatile component

Vapors consist of at least two components

Vapors consist of one component

Distillation [2] Vapor is produced in each stage by partial vaporization Diffusion is both from liquid to gas and gas to liquid Ratio of Liquid to gas flow rate is low

Absorption[2] Feed is a mixture of gases

Absorption Bulk phenomena Molecule dissolve in bulk of fluid[3] Concentration of adsorbed material is high on surface[4]

Adsorption Surface phenomena Molecule binds on surface of solid[3] Concentration of absorbed material is uniform throughout the bulk[4]

Evaporation It is used when large amount of water/liquid is present in water[6] To obtain saturated solution or recovery of valuable product[6]

Drying It is used when small amount of water/liquid is present in water[6] Final product is free flowing powder of individual particles, agglomerates and granules[5] Vapors may of multicomponent

Vapor is of single component[7]

Liquid Liquid Extraction [8] Liquid extraction LLE is used to separate two miscible liquids by the use of solvent which prefer to dissolve one of them

Diffusion is unidirectional Ratio of Liquid to gas flow rate is high

Leaching[8] Solid extraction Is used to dissolve soluble matter from its mixture with an insoluble solid

6

Distillation [9]

Extraction[9]

Constituent of liquid mixtures are separate by using thermal energy

Constituent of liquid mixtures are separate by using insoluble solvent

Constituent of liquid mixtures are separate due to difference in their boiling point It give almost pure product

Constituent of liquid mixtures are separate due to difference in their solubility It does not give product. further treatment is necessary Require mechanical energy for mixing and separation

Require thermal energy

1.4 COMPARISON OF REVERSE OSMOSIS WITH OTHER MASS TRANSFER TECHNIQUES Reverse Osmosis

Ion Exchange

Continuous process[10] More Sensitive to incoming suspended particles Pretreatment is required[10] Sensitive for temperature variation Salt passage is increased with increase in temperature[10] RO removes compound on basis of their sizes Small ions or molecule (Na,Cl,CO2) are partially removed[10] RO is partially demineralized process[10] High energy efficient Electrical energy is used for pumping [11]

Batch process[10] Less Sensitive to incoming suspended particles(1) Less Sensitive for temperature variation[10]

Reverse Osmosis

Electrodialysis

Driving force is pressure [12] Sensitive for temperature variation (15 C to 25 C) Salt passage is increased with increase in temperature [12] Can remove neutral toxic components such as bacteria and viruses [12]

Driven force is electrical energy [12] Operation at elevated temperature up to 50 C [12]

more sensitive to membrane fouling pretreatment is required[13] No generation of chlorine gas[13]

Less sensitive to membrane fouling Less raw water pretreatment[11] Generation of chlorine gas at anode cause corrosion problem in surrounding of plant[13] Energy loss is caused by friction of ions on their pathway through membrane from dilute to brine solution [12]

Energy loss in RO is caused by friction experienced by water molecule on their pathway through membrane matrix [12]

It removes ions and doesn’t remove nonionic [10] Complete demineralized process[10] Regeneration of chemical is main running cost[11]

It cannot remove toxic components such as bacteria and viruses and may require post treatment[13]

7

Energy loss is independent of feed salt concentration [12]

Both the energy consumption and required membrane area is increased with increase of salt concentration [12]

[10]

Reverse Osmosis

Distillation

Low energy consumption[15] Output is higher than distillation It does not remove dissolved gases in water(CO2 AND O2) [10] Require less time for desalination[15]

High energy consumption[15] Low yield[14] It remove dissolve gases in water(CO2 AND O2)[14] Require more time than RO for desalination[15]

Reverse Osmosis

Evaporation

Is also use to obtain concentrated liquid food/beverages[16] It is used to concentrate heat sensitive liquid(reduce heat damage to food color and taste[16] Energy efficient As only pressure energy is used[16] It particularly use full to obtain pre-concentrator Before evaporation [16] Not suitable for viscous fluids[16] It is useful to concentrate the solution up to the point where osmotic pressure become excessive[17]

To obtain saturated solution or valuable product By evaporation heat sensitive material can damage[16] Use thermal energy to vaporize liquid[16]

Used for evaporation of slurries Evaporation is used to concentrate the solute up to its dryness[17]

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[18]

Reverse Osmosis

Ultrafiltration Both have same working principle[19] The main difference is between them the size of particle that can pass through membrane[19] Allow smallest particle to flow Limits the flow of macromolecule Water molecule, some salts and volatile (proteins, starch and gums) component[19] [19] Osmotic pressure is greater for small solute Osmotic pressure is low for molecule[20] macromolecule [20] Operating pressure is high than Operating pressure is lower than reverse ultrafiltration[20] osmosis[20]

[21]

9

2 WATER 2.1 CHEMICAL COMPOSITION: Water molecule consist of one oxygen atom covalently bonded to two hydrogen atoms. It has the following structure:

Fig. water molecule with hydrogen bonding [22].

2.2 CHEMICAL NATURE: Water is a polar molecule, and although electrically neutral it has a polar nature such that the molecule has a region of negative charge near the O atoms and positive charge near the H atom. This dipolar nature gives water important properties such as its ability to act as a solvent. The polar nature of water leads to H bonding of water molecules and other molecules. Hence due to this chemical nature, many substances (solutes) dissolved in water to varying degrees, depending on specific chemical properties, are found.

2.3 WATER SOURCES AND CONTAMINANTS: The sources of water that are potentially useful to humans fall into four categories, namely, oceans, lakes, surfaces, and sub surfaces. [23] Water, being a universal solvent, normally contains many impurities that it picks up from its surroundings. The common impurities found in fresh water are: [24]

Constituent

Turbidity

Hardness

Chemical Formula

Difficulties Caused

Means of Treatment

imparts unsightly appearance to water; coagulation, non-expressed in deposits in water lines, settling, and analysis as units process equipment, etc.; filtration interferes with most process uses chief source of scale in softening; calcium and magnesium heat exchange equipment, demineralization; salts, expressed as boilers, pipe lines, etc.; internal boiler CaCO3 forms curds with soap, water treatment; interferes with dyeing, surface active

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etc.

Alkalinity

Free Mineral Acid

agents

foam and carryover of lime and lime-soda solids with steam; bicarbonate(HCO3 ), embrittlement of boiler softening; acid treatment; hydrogen carbonate (CO32-), and steel; bicarbonate and zeolite softening; hydroxide(OH ), carbonate produce CO2 in demineralization expressed as CaCO3 steam, a source of dealkalization by corrosion in condensate anion exchange lines H2SO4 , HCI. etc., expressed as CaCO3

corrosion

Carbon Dioxide

CO2

aeration, corrosion in water lines, deaeration, particularly steam and neutralization with condensate lines alkalies

PH

hydrogen ion concentration defined as: 1 pH = log [H+]

pH varies according to acidic or alkaline solids pH can be increased in water; most natural by alkalies and waters have a pH of 6.0- decreased by acids 8.0

SO42-

adds to solids content of water, but in itself is not usually significant, combines with calcium to form calcium sulfate scale

Sulfate

Chloride

Cl -

Nitrate

NO3-

Fluoride

F-

neutralization with alkalies

demineralization, reverse osmosis, electrodialysis, evaporation

demineralization, adds to solids content and reverse osmosis, increases corrosive electrodialysis, character of water evaporation adds to solids content, but is not usually significant industrially: high demineralization, concentrations cause reverse osmosis, methemoglobinemia in electrodialysis, infants; useful for control evaporation of boiler metal embrittlement adsorption with cause of mottled enamel magnesium in teeth; also used for hydroxide, calcium control of dental decay: phosphate, or bone not usually significant black; alum industrially coagulation

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Sodium

Silica

Iron

Na+

adds to solids content of water: when combined with OH-, causes corrosion in boilers under certain conditions

SiO2

scale in boilers and cooling water systems; insoluble turbine blade deposits due to silica vaporization

Fe2+ (ferrous) Fe3+ (ferric)

Discolors water on precipitation; source of deposits in water lines, boilers. etc.; interferes with dyeing, tanning, papermaking, etc.

Manganese Mn2+

Aluminum AI3+

Oxygen

O2

Hydrogen Sulfide

H2S

Ammonia

NH3

Dissolved Solids

none

demineralization, reverse osmosis, electrodialysis, evaporation hot and warm process removal by magnesium salts; adsorption by highly basic anion exchange resins, in conjunction with demineralization, reverse osmosis, evaporation aeration; coagulation and filtration; lime softening; cation exchange; contact filtration; surface active agents for iron retention same as iron

same as iron usually present as a result of floc carryover from clarifier; can cause improved clarifier deposits in cooling and filter operation systems and contribute to complex boiler scales corrosion of water lines, deaeration; sodium heat exchange equipment, sulfite; corrosion boilers, return lines, etc. inhibitors aeration; cause of "rotten egg" chlorination; highly odor; corrosion basic anion exchange cation exchange corrosion of copper and with hydrogen zinc alloys by formation zeolite; of complex soluble ion chlorination; deaeration refers to total amount of lime softening and dissolved matter, cation exchange by determined by hydrogen zeolite; evaporation; high demineralization, concentrations are reverse osmosis, objectionable because of electrodialysis,

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Suspended none Solids

Total Solids none

process interference and as a cause of foaming in boilers refers to the measure of undissolved matter, determined gravimetrically; deposits in heat exchange equipment, boilers, water lines, etc. refers to the sum of dissolved and suspended solids, determined gravimetrically

evaporation

subsidence; filtration, usually preceded by coagulation and settling see "Dissolved Solids" and "Suspended Solids"

2.4 POTABLE WATER 

 



Potable water is water that has been either treated, cleaned or filtered and meets established drinking water standards or is assumed to be reasonably free of harmful bacteria and contaminants, and considered safe to drink or use in cooking and baking.[25] Potable water is fresh water that is sanitized with oxidizing biocides such as chlorine or ozone to kill bacteria and make it safe for drinking purposes.[26] By definition, certain mineral constituents are also restricted. For example, the chlorinity will be not more than 250 ppm chloride ion in the United States or 400 ppm on an international basis. Examples of potable water would be that from treated municipal water systems, water that has been UV filtered or purified by reverse osmosis.[25]

2.5 BOILER-FEED-WATER-[27] Feed-water composition depends on the quality of the make-up water and the amount of condensate returned to the boiler. The principal difficulties caused by water in boiler are:   

Scaling Foaming and priming Corrosion Proper treatment of boiler feed water is an important part of operating and maintaining a boiler system. As steam is produced, dissolved solids become concentrated and form deposits inside the boiler. This leads to poor heat

13

transfer and reduces the efficiency of the boiler. Dissolved gasses such as oxygen and carbon dioxide will react with the metals in the boiler system and lead to boiler corrosion. In order to protect the boiler from these contaminants, they should be controlled or removed, through external or internal treatment. 2.5.1

BFW Contaminants, Their Effect and Possible Treatment:

Following table constitutes the list of the common boiler feed water contaminants, their effect and their possible treatment. IMPURITY

RESULTING IN

GOT RID OF BY

COMMENTS

Soluble Gasses

Water smells like Found mainly in rotten eggs: Tastes Aeration, Filtration, groundwater, and bad, and is corrosive and Chlorination. polluted streams. to most metals. Filming, neutralizing Corrosive, forms Deaeration, amines used to neutralization with Carbon Dioxide (CO2) carbonic acid in prevent condensate condensate. alkalis. line corrosion. Deaeration & Pitting of boiler Corrosion and chemical treatment tubes, and turbine pitting of boiler with (Sodium blades, failure of Oxygen (O2) tubes. Sulphite or steam lines, and Hydrazine) fittings etc. Suspended Solids Tolerance of approx. Sludge and scale Clarification and 5ppm max. For most Sediment & Turbidity carryover. filtration. applications, 10ppm for potable water. Found mostly in surface waters, caused by rotting vegetation, and farm run offs. Organics break down to form Carryover, foaming, Clarification; organic acids. deposits can clog filtration, and Organic Matter piping, and cause Results in low of chemical treatment corrosion. boiler feed-water pH, which then attacks boiler tubes. Includes diatoms, molds, bacterial slimes, Hydrogen Sulphide (H2S)

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iron/manganese bacteria. Suspended particles collect on the surface of the water in the boiler and render difficult the liberation of steam bubbles rising to that surface. Foaming can also be attributed to waters containing carbonates in solution in which a light flocculants precipitate will be formed on the surface of the water. It is usually traced to an excess of sodium carbonate used in treatment for some other difficulty where animal or vegetable oil finds its way into the boiler. DissolvedColloidalSolids Enters boiler with condensate Forms are bicarbonates, sulphates, chlorides, Scale deposits in boiler, inhibits heat and nitrates, in that Hardness, Calcium transfer, and thermal Softening, plus order. Some calcium (Ca), and Magnesium efficiency. In severe internal treatment in salts are reversibly cases can lead to boiler. (Mg) soluble. Magnesium boiler tube burn reacts with thru, and failure. carbonates to form compounds of low Oil & Grease

Foaming, deposits in Coagulation & boiler filtration

15

solubility. Foaming, carbonates form carbonic acid in steam, causes Sodium, alkalinity, condensate return NaOH, NaHCO3, Na2CO3 line, and steam trap corrosion, can cause embrittlement. Sulphates (SO4)

Chlorides, (Cl)

Iron (Fe) and Manganese (Mn)

Silica (Si)

Deaeration of makeup water and condensate return. Ion exchange; deionization, acid treatment of makeup water.

Sodium salts are found in most waters. They are very soluble, and cannot be removed by chemical precipitation. Tolerance limits are Hard scale if Deionization about 100-300ppm calcium is present as CaCO3 Priming, or the passage of steam Priming, i.e. uneven from a boiler in delivery of steam "belches", is caused from the boiler by the concentration (belching), carryover sodium carbonate, of water in steam sodium sulphate, or lowering steam sodium chloride in Deionization efficiency, can solution. Sodium deposit as salts on sulphate is found in super heaters and many waters in the turbine blades. USA, and in waters Foaming if present where calcium or in large amounts. magnesium is precipitated with soda ash. Deposits in boiler, in Most common form Aeration, filtration, large amounts can is ferrous ion exchange. inhibit heat transfer. bicarbonate. Silica combines with many elements to produce silicates. Silicates form very tenacious deposits in Hard scale in boilers Deionization; lime boiler tubing. Very and cooling systems: soda process, hotdifficult to remove, turbine blade lime-zeolite often only by fluoric deposits. treatment. acids. Most critical consideration is volatile carryover to turbine components.

16

3 WATER TREATMENT Water treatment removes constituents through a combination of physical and chemical means and are known as physiochemical unit processes. [28]

It is performed in two steps: 1. Pretreatment 2. Treatment

3.1 PRETREATMENT [29] Pretreatment can be classified into four groups: physical, chemical, biological and electrical strategies. Pretreatment can remove soluble salt (hardness) collides (silt, Fe, Al, silica) solid (suspended solids and particulate organics) biological material and dissolved organic material. .

3.1.1

PRETREATMENT OF WATER PRIOR TO RO-TREATMENT [30]

To control the RO membrane fouling, all the organic, colloidal, and biological matter needs to be removed from feed water to the RO system. Hence a proper pretreatment process capable of producing a substantial reduction in fouling potential of membrane is very important to the functioning of a RO filtration process. Conventional pretreatment methods and their limitations Conventionally disinfection, coagulation, flocculation, sedimentation, and deep bed filter application are used together as a pretreatment approach after which SDI (silt density index) measurements are used as a criterion to evaluate the efficiency of the pretreatment. However, minute changes occurring in a conventional treatment can have adverse effect on the RO filtration process. Factors such as chemical overdose, improper

17

chemical use in pretreatment will result in irreversible fouling, power consumption and increased cleaning operations. RO system-pretreatment techniques Due to these limitations many sea-water reverse osmosis (SWRO) plants are using membrane filtration such as micro-filtration (MF) and ultra-filtration (UF) as pretreatment techniques. Advantages of UF-pretreatment technique over conventional methods Application of UF as a pretreatment technique results in the usage of fewer chemicals, less floor space and higher water recovery than conventional methods. It also eliminates the need for cartridge filter/sludge disposal with similar energy requirements as conventional pretreatment processes.

3.2 TREATMENT TECHNIQUES [30] When the pretreatment of the process fluid (water) is efficiently done, its treatment can now be performed. A list of treatment techniques is given, in which various chemical and physical techniques are employed for the treatment of the process fluid, of one is the reverse osmosis technique which is the most effective and energy efficient technique to be employed.

18

19

20

4 RO-MASS TRANSFER TECHNIQUE Reverse Osmosis is a technique employed in mass transfer operations because of the following reasons or concerns:

21

 













The pressure-driven “transport of water from a solution through a membrane” is known as reverse osmosis.[31] In the reverse osmosis (RO) process, water passes through a membrane, leaving behind a solution with a smaller volume and a higher concentration of solutes.[31] This is the selective mass-transfer as proposed in “Homogeneous Solutiondiffusion Model”[32] which assumes that both solute and the solvent dissolves in the upstream face of the membrane then cross through the membrane by molecular diffusion and are released into the permeate bulk in contact with the downstream face.[31] Membrane separation process:

Figure# 2. Reverse osmosis systems [33] The equilibrium concentrations of components of mixtures often differ across the boundary between one phase and another as in RO the boundary is membrane itself. These differences can be used to effect separations by the enrichment of one phase relative to the other, by ‘differential transfer of mass’ of particular components across the phase boundary. The net driving force for mass transfer in reverse osmosis is the difference between the net applied differential pressure DPa, and the differential osmotic pressure, DPo, which resists the flow in the desired "reverse" direction. Therefore it can be described by the standard rate equation, with the rate of mass transfer being equal to the driving force multiplied by the appropriate mass-transfer coefficient[32]: Dw/dt = KA [DPa- DPo] where dw/dt is the rate of mass transfer, K is the mass transfer coefficient, A the area through which the transfer is taking place. DP is therefore the difference in the applied pressure on the solutions at each side of the membrane and DP is the difference in the osmotic pressures of the two solutions, as in Fig.2. Moreover, membranes used in such techniques also involve mass-transfer in their formation as ultimate membrane structure results as a combination of

22

phase separation and mass transfer, variation of the production conditions giving membranes with different separation characteristics.

5 WHEN WILL BE THE RO IS PREFERRED OVER OTHER SEPARATION TECHNIQUES [31]: 

Whilst effective product separation is crucial to economic operation in the process industries, certain types of materials are inherently difficult and expensive to separate. Important, examples include:

(a)Finely dispersed solids, especially those which are compressible, and which have a density close to that of the liquid phase, have high viscosity, or are gelatinous. (b) Low molecular weight, non-volatile organics or pharmaceuticals and dissolved salts. (c)Biological materials which are very sensitive to their physical and chemical environment.  

Since highly effective and energy efficient. Moreover, they potentially offer the advantages of ambient temperature operation, relatively low capital and running costs, and modular construction.

REVERSE OSMOSIS [34] Reverse osmosis is a demineralization process that relies on a semi permeable membrane to effect the separation of dissolved solids from a liquid. The semi permeable membrane allows liquid and some ions to pass, but retains the bulk of the dissolved solids. To understand how RO works, it is first necessary to understand the natural process of osmosis.

6 OSMOSIS Osmosis is a natural process where water flows through a semi permeable membrane from a solution with a low concentration of dissolved solids to a solution with a high concentration of dissolved solids. Picture a cell divided into 2 compartments by a semi permeable membrane. This membrane allows water and some ions to pass through it, but is impermeable to most dissolved solids. One compartment in the cell has a solution with a high concentration of dissolved solids while the other compartment has a solution with a low concentration of dissolved solids. Osmosis is the natural process where water will flow from the compartment with the low concentration of dissolved solids to the compartment with the high concentration of dissolved solids. Water will continue to flow through the membrane until the concentration is equalized on both sides of the

23

membrane. At equilibrium, the concentration of dissolved solids is the same in both compartments, there is no more net flow from one compartment to the other. However, the compartment that once contained the higher concentration solution now has a higher water level than the other compartment. The difference in height between the 2 compartments corresponds to the osmotic pressure of the solution that is now at equilibrium.

Osmotic pressure (typically represented by n (pi)) is a function of the concentration of dissolved solids. For example:  Brackish water at 1,500 ppm TDS would have an osmotic pressure of about 15psi.  Seawater, at 35,000 ppm TDS, would have an osmotic pressure of about 350 psi.

7 REVERSE OSMOSIS Reverse osmosis is the process by which an applied pressure, greater than the osmotic pressure, is exerted on the compartment that once contained the highconcentration solution. This pressure forces water to pass through the membrane in the direction reverse to that of osmosis. Water now moves from the compartment with the high-concentration solution to that with the low concentration solution. Due to this, relatively pure water passes through membrane into the one compartment while dissolved solids are retained in the other compartment. Hence, the water in the one compartment is purified or “demineralized,” and the solids in the other compartment are concentrated or dewatered.

.

24

Due to the resistance of the membrane, the applied pressures required to achieve reverse osmosis are significantly higher than the osmotic pressure. For example  For 1,500 ppm TDS brackish water, RO operating pressures can range from about 150 psi to 400 psi.  For seawater at 35,000 ppm TDS, RO operating pressures as high as 1,500 psi may be required.

7.1 OBJECTIVES  

Reverse osmosis can be used to either purify water Or to concentrate and recover dissolved solids in the feed water (known as "dewatering").

7.2 BLOCK SCHEMATIC DIAGRAM FOR RO-OPERATION

 



Raw water prior to its treatment is fed to three cartridge filters in series for pretreatment at a flow rate enough for its flow across the filters. Since RO membrane is sensitive to TDS removal, any suspended solids and colloids will damage the membrane. Moreover, these are also undesirable for the feed pump, hence filters are used to remove any undesired suspended solids and colloids. Because an RO feed pump requires a certain volume and pressure of makeup water to the suction side of the RO feed pump so as not to cavitate the

25





 







pump, Low pressure switch along with the Auto-control-valve is installed at the outlet of first filter. This switch helps in maintaining the suction pressure of influent stream to RO feed pump as well as for flow across the filters. When somehow the pressure becomes not sufficient for flow across next filter, low pressure switch yield signal to the controller, it will further send order to auto control valve to be closed, this results in pressure built-up at that end thus maintaining sufficient pressure. The pretreated water from the filters is then fed at required operating pressure to the membrane module (spiral-wound) through the RO-feed pump. Here Diaphragm booster pump is used which acts centrifugally to provide the desired operating pressure safely. The feed water is then treated through Spirally-wounded flat sheet membrane resulting in two effluents streams, one permeate and the other retentate/concentrate. Spiral-wound modules allow the efficient packaging of flat sheet membrane in a convenient cylindrical form. Moreover their other features making the apparatus efficient will be discussed later under the heading of spiral-wound modules. High pressure switch is situated at the permeate stream to check for membrane performance i.e.: senses high pressure at that stream and send information to the controller thus indicating that the membrane efficiency has lowered due to its damage. A Combination valve is installed at the effluent of retentate stream for the protection of diaphragm booster pump and membrane. It is performing two functions simultaneously: measure the flow along with controlling it at this side. Moreover, it is a type of throttling valve which remains partially open at normal operation so that helps in maintaining the pressure at the feed side of the membrane. Its throttling action also provide scouring action to the scales on the membrane thus helps in effective mass transfer across the membrane. Permeate is either stored in the Storage tank or further sent to Taste and Odor removal unit (if necessary for further purification).

7.3 FLOW MECHANISMS THROUGH RO-MEMBRANES Following are the two mechanisms through which the feed stream follow through the membranes: 7.3.1

CROSS-FLOW FILTRATION

In cross-flow filtration, feed water passes tangentially over the membrane surface rather than perpendicularly to it. Water and some dissolved solids pass through the membrane while the majority of dissolved solids and some water do not pass through the membrane. Hence, cross-flow filtration has one influent stream but yields two effluent streams.

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7.3.1.1 Advantage Of Cross Flow Cross-flow helps to minimize fouling or scaling of the RO membrane. In an effort to keep the membrane surface free of solids that may accumulate and foul or scale the membrane, tangential flow across the membrane surface aids in keeping the surface clean by scouring the surface; minimum flow rates across the membrane surface are required to effectively scour the surface. In practice, however, the scouring action of cross flow is not always enough to prevent all fouling and scaling. Periodically, the membranes will need to be taken off line and cleaned free of material that has accumulated at the surface. A simplified block diagram showing how cross-flow RO actually works. The diagonal line inside the rectangle represents the membrane. This diagram shows how the influent stream, with an applied pressure greater than the osmotic pressure of the solution, is separated into two effluent streams. The solution that passes through the membrane is called permeate or product, and the solution retained by the membrane is called the concentrate, reject, waste, brine, or retentate. The flow control valve on the concentrate stream provides the back-pressure needed to cause reverse osmosis to occur. Closing the valve will result in an overall increase in pressure driving force, and a corresponding increase of influent water that passes through the membrane to become permeate.

7.3.2

DEAD-END FILTRATION

Dead end filtration involves all of the feed water passing through the membrane, leaving the solids behind on the membrane.

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Dead end filtration is a batch process. That means that the filter will accumulate and eventually blind off with particulates such that water can no longer pass through. The filtration system will need to be taken off line and the filter will need to be either cleaned or replaced.

7.4 BASIC DEFINITIONS AND TERMS 7.4.1

RECOVERY

“Recovery (referred as “conversion”) is a term used to describe what volume percentage of influent water is “recovered” as permeate.” Equation to calculated recovery: % Recovery = (permeate flow / feed flow) * 100 A system recovery of 75% means that for every 100 gpm influent, 75 gpm will become permeate and 25 gpm will be retained as concentrate.  At 75% recovery, the concentrate volume is one-fourth that of the influent volume. If assumed that the membrane retains all the dissolved solids, they would be contained in one-fourth of the volume of influent water. Hence, the concentration of retained dissolved solids would be four times that of the influent stream. This is called the "concentration factor."  At 50% recovery, the concentrate volume would be one-half that of the influent water. In this case, the dissolved solids would be concentrated by a factor of two, so the concentration factor would be 2. Higher recovery results in the need to dispose of less reject water. Higher recovery also results in lower-purity permeate. At the influent end of the membrane, the influent concentration is 100 ppm, while the recovery is 0%, and the membrane passes 2% total dissolved solids (TDS). The permeate right at this spot would be about 2 ppm. As the influent water passes across more and more membrane area, more water is recovered. At 50% recovery, the concentration factor is 2, so the influent water now has a concentration of about 200 ppm. The permeate water at this point would now have a concentration of 4 ppm. At 75% recovery, the concentration factor is 4, so the influent water now has a concentration of about 400 ppm. The permeate water at this point would have a concentration of 8 ppm. Hence, higher recovery results in lower product purity.

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In practice, the recovery of the RO system is adjusted using the flow control valve located on the RO concentrate stream .Throttling the valve will result in higher operating pressure, which forces more water through the membrane as opposed to down along the feed/concentrate side of the membrane, and results in higher recovery. The recovery of an RO system is fixed by the designer.  Exceeding the design recovery can result in accelerated fouling and scaling of the membranes, because less water is available to scour the membrane on the concentrate side.  Falling below the design recovery will not adversely impact membrane fouling or scaling, but will result in higher volumes of wastewater from the RO system. 7.4.2

REJECTION

“Rejection is a term used to describe what percentage of an influent species a membrane retains.” For example 98% rejection of silica means that the membrane will retain 98% of the influent silica. It also means that 2% of influent silica will pass through the membrane into the permeate (known as "salt passage"). Formula to calculate rejection % Rejection = [(Cf- Cp)/ Cf]* 100 Cf= influent concentration of a specific component Cp= permeate concentration of a specific component Rejection of a particular species is also based on the following characteristics:    

The rejection of multi-valent ions is generally greater than for mono-valent ions Degree of dissociation: the greater the dissociation, the greater the rejection, for example, weak acids are rejected better at higher PH Molecular weight: in general, the greater the molecular weight, the greater the rejection, for example, the rejection of calcium is marginally better than the rejection of magnesium. Polarity: in general, the greater the polarity, the lower the rejection, for example, organics are rejected better than water.

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

Degree of hydration: in general, the greater the degree of hydration, the greater the rejection, for example, chloride is rejecter better than nitrate.  Degree of molecular branching: in general, the more branching, the greater the rejection, for example, isopropanol is rejected better than normal propanol. The rejection of gases is 0%, means that the concentration in the permeate stream will be the same as it is in the influent and concentrate streams. 7.4.3

SALT PASSAGE

It is essentially the opposite of rejection: % Salt Passage = 100 - % Rejection 7.4.4

FLUX

“Flux is defined as the volumetric flow rate of a fluid through a given area.”  In terms of RO: flux is expressed as gallons of water per square foot of membrane area per day (gfd).  The flux of water through an RO membrane is proportional to the net pressure driving force applied to the water. 7.4.5

CONCENTRATION POLARIZATION

The flow of water past an RO membrane is similar to that of the flow of water through a pipe. The flow in the bulk solution is convective, while the flow in the boundary layer is diffusive and is perpendicular to the convective flow of the bulk solution. There is no convective flow in the boundary layer.

30

Consider the flow along the surface of a membrane. The same boundary layer forms as with flow through a pipe. However, with a membrane system, because there is a net flow out through the membrane, there is convective flow to the membrane, but only diffusional flow away from the membrane. Since diffusion is slower than convection, solutes rejected by the membrane tend to build up on the surface and in the boundary layer. Thus, the concentration of solutes at the membrane surface is higher than in the bulk solution. This boundary layer is called "concentration polarization." Concentration polarization has a negative effect on the performance of an RO membrane.  It acts as a hydraulic resistance to water flow through the membrane.  The buildup of solutes increases the osmotic pressure within the boundary layer, effectively reducing the driving force for water through the membrane.  The higher concentration of solutes on the membrane surface than in the bulk solution, leads to higher passage of solutes than would be predicted by the feed water concentration. For example, Assume that the bulk concentration of silica is 10 ppm, while the concentration at the membrane surface is 11.5 ppm. If the rejection is 98%, the silica concentration that would be expected to be in permeate based on the bulk concentration is 0.20 ppm. However, the membrane "sees" 11.5 ppm, so the actual salt passage is 2% of 11.5 ppm, or 0.23 ppm. Actual rejection is still 98%. Apparent rejection is 97.7%. 7.4.6

BETA

"Concentration polarization factor" “It is the ratio of the concentration of a species at the membrane surface to that in the bulk solution.”  The higher the Beta number, the more likely the membranes are to foul or scale. Since Beta measures the ratio of concentration at the surface to that in the bulk solution, the higher the beta number, the higher the relative concentration at the surface. If the concentration at the surface gets high enough, saturation may be reached and scale will begin to form.  Beta is a function of how quickly the influent stream is dewatered through the RO system. If water is removed too quickly from the influent stream, Beta will increase, as a relatively high volume of dissolved solids is

31

left behind on the membrane because of the high volume of water that permeates through the membrane. 7.4.7

FOULING

Membrane fouling is a result of deposition of suspended solids, organics, or microbes on the surface of the membrane, typically on the feed /concentrate side. Fouling is increased by high membrane flux and low cross flow velocity both conditions that increase concentration polarization.  Higher flux translates into water being removed through the membrane at a faster rate, leaving behind solids that now accumulate more rapidly in the concentration polarization boundary layer.  Cross-flow velocity affects the thickness of the boundary layer. Lower crossflow velocity corresponds to a thicker boundary layer. A thicker boundary layer allows for greater accumulation of solids in the layer, and the solids spend more time in the layer due to the increased thickness of the boundary layer, setting up the potential for accelerated fouling of the membrane. A fouled membrane exhibits two major performance issues:  

7.4.8

Higher than normal operating pressure. Higher than normal pressure drop.

SCALING

Scaling of RO membranes is a result of precipitation of saturated salts onto the surface of the membrane.  Calcium scales, including carbonate, sulfate, fluoride, and phosphate,  Reactive silica, which is measured in the RO reject and is a function of temperature and pH  Sulfate-based scales of trace metals, such as barium and strontium. Scaling is increased by high membrane flux and low cross-flow velocity. 

Higher flux brings more solutes into the concentration polarization boundary layer quicker. If the concentration of the solutes in the boundary layer reaches saturation, these solutes will scale the membrane.

32



Lower cross-flow velocity corresponds to a thicker boundary layer. This increases the residence time for solute within the boundary layer, increasing the chance that saturation will be achieved and scale will form. A scaled membrane exhibits three major performance issues:  Higher than normal operating pressure  Higher pressure drop,  Lower than expected salt rejection

7.5 DESIGN CONSIDERATIONS/FACTORS FFECTING RO Operating conditions affect the performance of an RO system. These conditions include:  Total dissolved solids  Temperature  Pressure  Feed water flow  Concentrate flow  Beta  Recovery  Flux  PH 7.5.1

TOTAL DISSOLVED SOLIDS

The total dissolved solids (TDS) concentration affects both the system flux and the salt rejection of an RO system.  As feed TDS increases, the driving force for water decreases, due to the increase in osmotic pressure of the feed. This results in a decrease in system flux.



As the driving force for water decreases, the amount of water passing through the membrane relative to the amount of salt decreases, resulting in a higher TDS concentration in the permeate.

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7.5.2

TEMPERATURE

Temperature influences system flux and rejection performance.  For every 1°C change in temperature, there is a 3% change in water flux. This occurs because the lower viscosity of warmer water allows the water to flow more readily through the membranes.



Salt rejection decreases slightly with increasing temperature. Salt diffusion through the membrane is higher at higher water temperature.

Temperature changes are dealt with by adjusting the operating pressure:  Lower pressure in the warmer summer months.  Higher pressure in the colder winter months.

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7.5.3

PRESSURE

Operating pressure directly affects water flux and indirectly affects salt rejection.  Operating pressure directly affects the driving force for water across the membrane, higher pressure will result in higher flux.



7.5.4

Salt transport is unaffected by pressure. So, the same amount of salt passes through the membrane at low or at high feed water pressure. More water has passed through the membrane at higher pressure, the absolute salt concentration in permeate is lower, so it appears as if the salt passage decreases and the salt rejection increases as pressure increases.

FEED WATER FLOW

At higher feed water flow rates, contaminants such as colloids and bacteria that may be present in the source water, are sent to the membrane more rapidly, resulting in faster fouling of the membrane. This is why lower flow rates are recommended for water sources that contain high concentrations of contaminants.

7.5.5 

CONCENTRATE FLOW At lower concentrate flow rates, good cross-flow velocity is not maintained, and contaminants, such as colloids and scale-formers, have a much greater

35



7.5.6

chance of fouling or scaling a membrane. This is because the concentration polarization boundary layer is thicker at lower cross-flow velocities than it would be at higher concentrate flow rates. The potential for fouling or scaling a membrane can be very high at low concentrate flow rates. BETA

Beta is the ratio of the concentration of a species at the membrane surface to that in the bulk solution Beta affects both the flux through an RO membrane and the salt rejection.  The increase in Beta due to concentration polarization at the membrane surface results in increased osmotic pressure and decrease is water flux and increase in salt passage. 7.5.7

RECOVERY



As the recovery increases, the water flux decreases slowly until the recovery is so high that the osmotic pressure of the feed water is as high as the applied pressure, in which case, the driving force for water through the membrane is lost and the flux ceases.



As the osmotic pressure of the feed /concentrate stream approaches the applied pressure, the driving force for water is decreased, but the driving force for salt is unaffected. Less water passes through the membrane relative to the amount of salt passing through the membrane. Thus, it appears as if the salt passage increases and salt rejection decreases with increasing recovery. Salt rejection becomes 0% at about the same time that the flux ceases.

36

7.5.8     

FLUX Flux is affected by several operating variables: Flux is directly proportional to operating pressure. Flux is directly proportional to water temperature. Flux decreases slightly as recovery increase until the osmotic pressure of the feed water equals the driving pressure, at which point productivity ceases. Flux decreases with increasing feed concentration of dissolved solids. Flux is relatively constant over a range of pH, although for some newer polyamide membranes, flux is also a function of pH.

8 MEMBRANE [35] “Membrane is thin interphase that restricts the passage of different components in a specific mode and over a wide range of particle sizes and molecular weights, from ions to macromolecules.” The efficiency of a membrane is determined by two parameters:  

Permeability(the rate at which a given component is transported through the membrane) Selectivity(the ability to separate in specific way a given component from others)

The transport of different species through membrane is a non-equilibrium process, and separation of the different components is due to the difference in their transport rate. Reverse osmosis membranes are characterized by a high degree of semipermeability, high water flux, mechanical strength, chemical stability and economically acceptable cost. [36]

37

8.1 MODULES [37] Membrane can have two different configurations:  

8.1.1

Tubular Flat sheet

TUBULAR MODULES:

38

8.1.2

FLAT SHEET MODULES:

8.1.2.1 Spiral Wound Module [35]: Is utilized in the apparatus, under consideration, used in laboratory, its general configuration is detailed as: They consist in an arrangement of two rectangular membranes placed back to back and sealed on three sides. They are rolled around a collector tube connected to the fourth side which remains open. The solution to be treated is brought to one end of this cylinder and the product circulates between both membranes to the collector tube. Following figure shows the general flow pattern:

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8.1.3

COMPARISON OF CHARACTERISTICS OF VARIOUS MODULES [35]

8.2 GENERALLY USED MEMBRANES 8.2.1     

CELLULOSE ACETATE MEMBRANES [37]

Lower Cost than Thin Film Membranes. Typical salt rejection of 96%. Typical operating pressures of 400 PSI. Optimum pH operating range of 4.8to 6.5. Good Chlorine Tolerance.

8.2.1.1 Characteristics of Cellulose Acetate RO Membranes: [34]

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8.2.2       8.2.2.1

THIN FILM COMPOSITE MEMBRANES: [37] More expensive than cellulose acetate membranes Typical salt rejection of 97 to 99% Typical operating pressures of 200 PSI Wide pH operating range of 2 -10 Very Low Chlorine Tolerance Less susceptible to compaction due to lower PSI Characteristics of Polyamide Membranes [34]

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8.3 FOULING FACTORS AND METHODS TO AVOID:

[38]

42

9 APPLICATIONS [34]:        

Desalination of seawater and brackish water for potable use. Generation of ultrapure water for the microelectronics industry. Generation of high-purity water for pharmaceuticals. Generation of process water for beverages (fruit juices, beer, bottle water). Processing of dairy products. Concentration of corn sweeteners. Waste treatment for the recovery of process materials such as metals for the metal finishing industries, and dyes used in the manufacture of textiles. To purify water for use as boiler makeup to low- to medium-pressure boilers, as the product quality from an RO can directly meet the boiler make-up requirements for these pressures.

10 EXPERIMENTAL ANALYSIS [39] 10.1 PROCEDURE: 1. Collect the sample of different types of water like distilled water, tap water and filtered water in different sample bottles. 2. Switch on the reverse osmosis plant to collect the sample of reverse osmosis water. 3. Insert pH meter, conductivity meter and TDS meter to determine pH, conductivity and TDS in different samples of water. 4. Keep on running the RO plant and also keep on taking different samples after a specific interval of time and check its properties. 5. Note down all the readings in the table and compare what is the difference. 6. Also note what the effect of time on RO water properties. 7. Draw the graphs between Time VS TDS, conductivity and pH in case of reverse osmosis water. 10.2 OBSERVATIONS AND CALCULATIONS: Room temperature = ᵒC 10.2.1 OBJECTIVE 1: To study and compare the properties of tap water, reverse osmosis water and distilled water. Tap Water TDS (ppm)

Conductivity (mho)

Distilled Water pH

TDS (ppm)

Conductivity (mho)

RO Water pH

Time (min)

TDS (ppm)

Conductivity (mho)

pH

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In addition to this, following objectives can also be achieved by performing further observations and calculations: 10.2.2 OBJECTIVE 2: Same calculations as mentioned above can also be performed for “The Purification of Water for Drinking Purposes" 10.2.3 OBJECTIVE 3: To Check the Performance of RO-apparatus 10.2.4 OBJECTIVE 4: TO Study and Compare the RO Feed Water, Permeate and Retentate RO WATER ANALYSIS NO. Feed of Obs TDS Conductivity (ppm) (mho) .

Permeate p Time H (sec)

TDS (ppm)

Retentate

Conductivity (mho)

p TDS H (ppm)

Conductivity (mho)

10.2.5 OBJECTIVE 5: To study the effect of pressure on RO performance by qualitative analysis of permeate and retentate. RO WATER ANALYSIS

NO. Operatin of g Obs pressure .

permeate TDS (ppm)

Conductivity (mho)

retentate pH

TDS (ppm)

Conductivity (mho)

pH

p H

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10.3 RESULTS AND DISCUSSIONS: The experiment is performed on the RO apparatus in the Mass Transfer Lab, Chemical Engineering Department, ciit Lahore for the qualitative comparison of Tap Water, Reverse Osmosis Water, and Distilled Water. Following results were obtained during the experiment performed under the observation of Muhammad Abdul Qyyum as the resource person: Room temperature = 25 ᵒC 10.3.1 Comparison of Properties of Various Types of Water No. of Obs .

TDS (ppm)

1-

370

56

5.9

TDS (ppm ) 1

2-

-

-

-

-

Tap Water

 



 



Conductivity (mho)

Distilled Water pH

Conductivity (mho) 1.6 -

RO Water pH

Time (sec)

5.7

573

TDS (ppm ) 133

-

600

157

Conductivity (mho)

pH

19.6

5.0

23.3

5.4

It is clearly seen from the readings obtained that the distilled water has the least TDS and conductivity with a pH 5.7. Although the distilled water should have zero TDS and correspondingly the conductivity, but we had a slight increase in the values, the possible reasons for a slight increase in our values may be the human error while taking the readings through respective meters and/or may be due to the in-accuracy of the instruments used. In comparison to distilled water, tap water as well as RO water should have greater values for corresponding parameters, and we have obtained the same results experimentally as well. Tap water fed to the RO apparatus resulted in the decrease in the values of its prescribed parameters as expected from the RO system. The values for the RO water suggest that the conductivity of the water decreases along with the TDS removal through RO membrane since it is a function of TDS. Moreover, the pH value of the RO water has been lowered from the feed pH. The reason is the increase acidic nature of the permeate water due to the presence of CO2.

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In addition to these readings, retentate reading was also obtained so that one can have better understanding of the performance of the apparatus as well as to examined experimentally the how the RO-system works. 10.3.2 RO Water Analysis RO WATER ANALYSIS

NO . of Obs

Raw Water

Permeate

.

TDS (ppm)

Conductivity (mho)

pH

Time (sec)

1-

370

56

5.9

573

    



Retentate

TDS (ppm )

Conductivity (mho)

pH

TDS (ppm )

Conductivity (mho)

pH

133

19.6

5.0

341

58.7

5.9

Above table shows the comparison of various parameters of Raw water, permeate and the retentate. It has been shown that the raw water TDS got distributed in the two effluent streams, permeate and the retentate as per conditions of the process. Permeate is obtained with minimum amount of TDS while the membrane rejecting remaining TDS to the retentate stream as concentrate. As a result of this distribution of TDS over the two streams, the conductivity of the two streams changes accordingly. Now as concerning with the pH, results show that the pH of permeate has been lowered from the feed water pH, while the retentate pH remained same as it would be. The reason for the low pH of the permeate is the presence of CO2 which also passes through the membrane resulting in an increase acidic nature of the permeate thus yielding low pH.

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11

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