Water Treatment Processes

WATER TREATMENT PROCESSES

WATER TREATMENT

SUBMITTED TO: SIR KAZMI HUSSAIN

SUBMITTED BY: 2012-CH-12 2012-CH-30 2012-CH-36 2012-CH-58

UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

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Water is a great blessing for us. It is very important in every field of life. Without it the life of any creature is impossible. The water that we obtain from rivers, seas and other sources cannot be used directly for drinking purposes. Water treatment is necessary to meet the requirement of the public and to save our people from lethal scourges. Life is possible on this earth because of the presence of water all around us. Nearly two -third of the surface is covered with water. There are many ways in which we can change the water properties with respect to the desired process. These process are studied under the heading of WATER TREATMENT

“WATER IS THE DRIVER OF NATURE”

Leonardo da Vinci “WATER IS LIFE’S MATTER AND MATRIX, MOTHER AND MEDIUM.THERE IS NO LIFE WITHOUT WATER”

Albert szent gyorgy


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CONTENTS When we hear the word water treatment many man y questions arise in our mind that what does it mean??? Which parameters deal with it??? Following assignment is the answer to all the questions.

             

What is WATER TREATMENT? Why Water Treatment is important? Water Sources. Industrial water supplies. Impurities and their types. Pure Water and Its properties. Water Quality Parameters (for domestic, boiler feed water and cooling tower) Comparison of national and international standards of quality of water. Treatment steps for domestic use. Water treatment methods. Membrane Filtration. Chemical Precipitations Ion Exchange method. Conclusion.


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WATER TREATMENT PROCESSES WHAT IS MEANT BY WATER TREATMEN?

.

The process to make water more drinkable, by following process 

Decontamination



Clarifying



Softening



Deodorizing

Basically, Water treatment characterizes those industrial-scale operations used to make water more adequate for a covet end-use. These can involve use for:   

Drinking water Industry Medical etc…

WHY WATER TREATMENT IS IMPORTANT? Water treatment is very important because we required different type of water for different purposes e.g. for domestic purposes, industrial industrial use, medical use etc. this water may be waste water or sea water or may ma y be fresh water, we adjust the concentration of minerals and other impurities in water for desired process. For example for drinking water there must be some amount of minerals otherwise this water will be harmful for human beings and if there is some amount of hard metals is present then it will also cause many diseases like this water treatment is also very important for industrial processes for example most of the processes required distilled water to perform. The efficiency of work done of different machines also depends upon water used in it. For example, the impurity free water is send into the boiler if there is some amount of mineral is present in water then it decreases the efficiency and also the life of boiler because the solids present in water can melt on high temperature and start deposit on the wall of boiler it may rust the boiler material and decrease its life. Same is the case in cooling tower also als o the raw water purify before sending it into i nto tower, and for domestic purpose. That’s why, water treatment of raw water have a great importance on domestic as well as industrial level. UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

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WATER TREATMENT PROCESSES There is some process to purify water but before this we discuss different terms related to this process and impurities and their properties present in water step by step. We also discuss the sources of raw water or industrial waste water. WATER SOURCES: There are two sources of water supply s upply i.e. 

Surface water



Ground water

Both can use for the water treatment. Surface water means the rain water or canal or pool water. Ground water means that water which aquifer (le vel of water) by the use of well. Ground water has fewer amounts of impurities then the surface water. INDUSTRIAL WATER SUPPLIES:

There are different sources for industrial water supplies but no source provide us pure water, the three basic sources of it are following: 

Municipal



Private



Hybrid

MUNICIPAL: Simply municipal means controlled by local authority. This source provides municipal water which has some minerals and hard metals in some quantity. This water can be used for water treatment for boiler or cooling tower and for domestic use. This is ground water. PRIVATE: private source means there is any mechanical mean(e.g. motor) for getting a water so this water also use for the water treatment but at high level it become very difficult to get water by this mean because in industries metric tons of water is required for this purpose we use hybrid source anyways this source can also use but onl y at laboratory level. This is ground water source. HYBRID: Hybrid source means there is any pool or canal is used for getting a water for desired purpose. This source is used on industrial level because metric tons of water are used for water treatment. This is surface water source.

These all sources use for water treatment but with respect to the level of water treatment if we required water treatment at industrial i ndustrial level so use that source which fulfil f ulfil the quantity of water.


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WATER TREATMENT PROCESSES IMPURITIES: Impurities are that entities in the confined amount of matter (liquid, gas or solid) which have different chemical composition from the chemical composition of that matter (liquid, gas or solid). Each source has different impurities and concentration of impurities from others. This impurity variation is because of two reasons: Character Amount CHARACTER : In different sources of water different impurities are present these are may be salts in one source and may be hard metals in other source. AMOUNT: In different sources may be same character of impurity is present but it can be different with respect to the concentration may be in one source the amount of impurity is 23% with respect to the total amount of pure water and in other source the same impurity is present in less than (or greater than) with respect to the total amount of the pure water. GENERAL IMPURITIES: The general impurities in the water are following: 

Dissolved solids



Suspended solids



Matter that cause turbidity



Sediments



Coloured matters



Organic matters



Matter that have taste



Odour causing compounds



Microorganisms

These are some impurities present in i n raw water but the question rises what is the pure water/fresh water??????? Which type of properties it have???????? PURE WATER The pure or fresh water is that which is colourless, tasteless and odourless. PROPERTIES OF FRESH WATER : 

Free from pathogenic organisms.



Colourless.



It is not saline.



Free from displeasing or smell/odourless.



Free from elements that may be harmful.



Do not damage any place.


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WATER TREATMENT PROCESSES The fresh water has some minerals which are ar e important for human being but for boiler the fresh water is that which is free from all type of solids or impurities. the fresh water has specific amount of each compound present in it before explaining these amounts first defined and explained the some important terms that are following : WATER QUALITY PARAMETERS That parameters on which the quality of water depends known as water quality parameters . These parameters are following: 



PHYSICAL PARAMETERS: Colour Turbidity PH Hardness TDS CHEMICALS: Nitrates Chloride Fluoride Arsenic Aluminium etc…

BACTERIOLOGICAL: Harmful bacteria The explanation of each parameters are following: 

PHYSICAL PARAMETERS: COLOUR: As the pure water is colourless so the colour present in the water is due to the coloured compounds present in it. It may be pigments present in raw water, many metals also have colour so because of these impurities there is a colour of water. The limits of amount of colour for different purposes are: Drinking water is <15 TSU (max. permissible limit) in national standards and in international standards is 5TSU. Boiler feed water is no objectionable. Cooling tower is no objectionable. TASTE: The pure water has no taste. The raw water has a taste due to the presence of organic substances and some biological activities can be the cause of taste of water. Industrial pollution is the big reason of the taste of water because of this many gases (may ( may be liquid or solid) mix with water and become the cause of taste.


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WATER TREATMENT PROCESSES ODOUR: The fresh or pure water is odourless. The odour of raw wate r is due to the presence of some organic compounds and may be due to t o bacteriological compounds. The water which is send to the boiler, cooling tower and domestic use is free of odour. TURBIDITY: Turbidity is the haziness of the liquid caused by the suspended solids which are invisible to human eye. In raw water as many impurities or compounds are present so t he turbidity of that water is very ver y high we should decrease the turbidity of water before any use of it. Surface water has high turbidity than ground water. The maximum permissible limit of turbidity in Drinking water is 5 NTU in Pakistani standards and in international standards is 0.5NTU. Boiler feed water is 0.2-1kg/cm2 and international is 0.03-0.1kg/cm2. Cooling tower is <1kg/cm2and international is 0.02-0.1kg/cm 2. PH: The PH is basically the measurement of the acidity of water. The neutral PH is 7 for water is PH is great than this value the water is basic and if the PH of water is less than this value water is called to be acidic. The PH of water is changes due to the presence of any acidic components (e.g. Hcl etc.) or basic components (e.g. NaoH). The desirable limit for all (domestic use, boiler feed water, cooling tower) 6.5 to 8.5.The PH control is very essential for persuasive control of dethroning and corrosion in many water systems. ALKALINITY: Alkalinity is basically the quantitative measure of acid neutralization in water. It determines the following: 

Efficiency of wastewater processes.



Contamination by anthropogenic compounds.

It gives the measurements of following: 

Acid neutralizing capacity.



Basic PH.



Anions.

ACID NEUTRALIZING CAPACITY: The ability of water sample to neutralize the strong acids known as acid neutralizing capacity. BASIC PH: The PH greater than the 7 is known as Basic PH. ANIONS: Any molecule or an element that has negative charge on it known as anion. UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

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WATER TREATMENT PROCESSES Some anions that are mostly present in the raw water are following: 

Hydroxide.



Carbonates.



Bicarbonates.

MEASUREMENT OF ALKALINITY: The alkalinity is measure by the double titration method that is addition of an acid to a water sample to the phenolphthalein whose end point (P H 8.3) and the methyl orange o range whose end point (PH 4.4). The titration to the phenolphthalein measures hydroxides and carbonates and titration to methyl orange measures hydroxide, carbonates and bicarbonates. Now question raises that if anions neutralize the acids then what is the function of cations of any impurity…????

So, the answer is that cations (calcium, small amount of iron and magnesium ions) produce hardness in water . Let’s see what is hardness??? HARDNESS: Hardness in water is the measure of total concentration of magnesium and calcium and sometimes iron. There are two types of hardness i.e. 

Permanent Hardness



Temporary Hardness.


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WATER TREATMENT PROCESSES PERMANENT HARDNESS: Permanent hardness in water is due to the presence of calcium and magnesium salts (sulphates, chlorides and nitrates of calcium and magnesium other than the hydrogencarbonates hydrogencarbonates and it is not affected by heating known as Permanent Hardness. HYDROGENCARBONATES: In permanent hardness there is salts of calcium and magnesium other than the hydrogencarbonate. That’s not affected by heat. LE CHATELIER’S PRINCIPLE: It states that If a constraint (e.g. change in pressure, temperature and concentration of a reactant) is enforced enforced to a system in equilibrium, then the equilibrium will wil l shift such as tend to cancel out the effect of the constraint. So, in permanent hardness this principle is not held. THERMAL STABILITY: Permanent hardness is thermally stable. FORMATION OF PRECIPITATE: There is no generation of precipitates on heating. SOLUBILITY CURVES: The solubility of chlorides, sulphates and nitrates is increases with the increase in temperature.


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WATER TREATMENT PROCESSES TEMPORARY HSARDNESS: Temporary hardness is that type of hardness where negative ion is hydrogencarbonate (bicarbonates (bicarbonates and carbonates of calcium and magnesium) ion is called Temporary Hardness. The maximum permissible limit of hardness in drinking water is <500mg/l. HYDROGENCARBONATES: There is hydrogencarbonates means carbonates and bicarbonates and that’s affected by heating and easily removed from water. LE CHATELIER’S PRINCIPLE: This principle is stated above and is held in temporary hardness. THERMAL STABILITY: This hardness is thermally unstable. FORMATION OF PRECIPITATE: There is the formation of precipitates on heating heat ing when we heat water then carbonate ions are separated that are insoluble in water and appear as a precipitates. SOLUBILITY CURVES: The solubility of carbonates and bicarbonates is decreases with increase in temperature.

The water is characterized in four categories with respect to the hardness that are soft water, moderately hard water, hard water and very hard water. The range of hardness for: Soft water is 0-60mg/l. Moderately hard water is 60-120mg/l. Hard water is 120-180mg/l. Very hard water is >180mg/l. REMOVAL OF HARDNESS:

The hardness can be removed by two methods that are following: CLARK’S METHOD: In this method a measured amount of calcium hydroxide is added to hard water, insoluble carbonates are obtained due to the reaction and these are separated by filtration process. ION EXCHANGE METHOD: In iron exchange method, sodium zeolite is used as ion exchanger. In this e xchanger sodium zeolite is converted into calcium zeolite and magnesium zeolite that are insoluble in water and easily separated.


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WATER TREATMENT PROCESSES DISADVANTAGES OF HARDNESS: The disadvantages of hardness or hard water are f ollowing: IN DOMESTIC USE: Hardness affects the cleaning clea ning property of soap. IN INDUSTRIES: Hardness causing scaling inside the pipes. That’s That’ s why if we use hard water in turbines, boilers and heat exchangers then their pipes will be corroded. ON HEALTH: Due to hard water many stomach problems produced. COMPARISON OF NATIONAL AND INTERNATIONAL STANDARDS:

Physical parameters

Purposes

Domestic

COLOUR

TURBIDITY

PH

HARDNESS

ALKALINITY

National Standards

<15TSU

International Standards 5TSU

Boiler Feed Water

No objectionable

No objectionable

Cooling Tower

No objectionable

No objectionable

Domestic

5NTU

0.5NTU

Boiler Feed Water

0.2-1kg/cm2

0.03-0.1kg/cm2

Cooling Tower

<1kg/cm2

0.02-0.1kg/cm 2

Domestic

>8.5

6.5-8.5

Boiler Feed Water

9.6-12

10-11

Cooling Tower

8.3-9

9-12

Domestic

<3ooppm

0.05-0.5(French units

Boiler Feed Water

No detectable

0ppm

Cooling Tower

0.05ppm

0ppm

Domestic

0.05-0.3ml

0.01-0.03ml

Boiler Feed Water

100-700ppm

125-500ppm

Cooling Tower

100-350ppm

125-300ppm


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WATER TREATMENT PROCESSES Water from the source contain a number of desired and undesired impurities of various size and nature. Before going to any of the process equipment it has to be pre-treated. Following are some steps essential for water pre-treatment 

Screening



Sedimentation



Flocculation and coagulation



Boiling



Sludge separation



Filtration



Chemical addition

SEDIMENTATION

Sedimentation is also use in water treatment process. In sedimentation process, the velocity of waste water becomes lower than the suspension velocity and the particles settle down to the due to the action of gravity. The clear water is taken ta ken out from the tank. Slug is removed from the bottom of the tank.

PROCESS DIAGRAM:

DRAWBACK:

From sedimentation, small particles do not remove efficiently because they settle down to the bottom of water too slowly. COAGULATION


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WATER TREATMENT PROCESSES Coagulation is a technique which is also used for water treatment. In this process, some chemicals such as alum and sodium aluminate are added which collect the small particles in waste water to form large la rge aggregate. These aggregate then settle down to the bottom and collected. CHEMICALS:

Two type of chemicals are used in coagulant 

Primary coagulant



Coagulant acids

PRIMARY COAGULANTS:

Primary coagulants are always used in coagulant process. These coagulants neutralize the water so that small particles combine to each other to form crowed. Some primary coagulants are: 

Alum



Sodium aluminate



Ferric sulfate

COAGULANT ACIDS:

Coagulant acids are used to increase the density of low settling aggregates and add toughness to aggregates so that aggregate do not break out while settling. Some coagulant acids are: 

Lime

Coagulant process diagram

MEMBRANE FILTERATION PROCESS


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WATER TREATMENT PROCESSES These processes have been used for years for t he removal of bacteria, pathogens, organic matter causing color, taste and odor to water, for micro-organisms removal and disinfected products. The membrane processes are of many types some of which are Micro filtration Ultra- filtration Nano- filtration Reverse- osmosis Some important characteristics of these types are as follows MICRO-FILTRATION: It has a membrane of approximately 0.03-10 microns pore size(1 micron=0.0001milimeter) 

It successfully removes sand, silt, clay, some algae and bacteria 



Micro filters scarcely

remove organic matter in normal usage

ULTRA-FILTRATION: 

Membrane with pore size approximately 0.002-0.1 microns



It like MF removes bacteria, viruses and other micro-organisms



Water can easily pass at low pressure

NANO-FILTRATION:  





Pore size membrane of approximately 0.001 microns Requirement of high pressure so that water can pass through through this small pore size In addition to micro-organisms elimination it also purifies the water from alkalinity More energy is needed as compared to the all above

REVERSE OSMOSIS: This is the most common and the most reliable type, so we will discuss it in detail. PRINCIPLE OF OPERATION The membrane filtration process works on the principle of reverse osmosis. UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

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WATER TREATMENT PROCESSES OSMOSIS DEFINATION: Osmosis is the spontaneous movement of the solvent from a dilute to a more concentrated solution through a partially permeable membrane. This movement across the membrane is due to the pressure difference in both the regions. The two solutions comprises of different dissolved and suspended chemicals. In other words we can passage of solvent concentration region to region through a membrane.

say that osmosis is the from a high a low concentrated partially permeable

The pressure that the water applies on the membrane to enter in the concentrated region is known as the osmotic pressure. MEMBRANE STRUCTURE: The membrane used in the osmosis allows only the particles that are smaller in diameter than the human human hair’s dia, dia, to pass through not others because its structure is made up of a cluster of fibres that has the dia equal to the diameter of a human hair. Normally the material of which these membranes are made are either polymeric or ceramic material.

REVERSE OSMOSIS

DEFINITION: Reverse osmosis is the process whose basis lies in the process of osmosis. UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

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WATER TREATMENT PROCESSES Reverse osmosis is the reverse of osmosis. It is the opposite movement of water with the increase in pressure in the concentrated region. This overcomes the natural pressure also known as the osmotic pressure of the dilute solution.

EXPLANATION: Due to osmosis, the water enters the concentrated region by applying pressure on the membrane.with this the pressure keeps on increasing in the concentrated region and there comes a point where the osmotic pressure becomes equal to the pressure in the concentrated region.as a result the water moveme nt ceases and thus we say that the t he equilibrium has reached. now if the pressure is increased in the concentrated region the reverse process will occur. the water will move from the concentrated to the dilute solution or from high pressure to low pressure. this phenomenon is known as reverse osmosis. This process is carried out only in case of very high pressure that is established in the high pressure region.

TYPICAL REJECTION CHARACTERISTICS OF R.O. MEMBRANES

Given below are some of the components and there rejection percentages by the r everse osmosis membrane. nearly all the dissolved and the suspended solids ar e rejected by this membrane except the few quantities.

Chloride

85-92%

magnesium 94-98% zinc

95-98%

iron

94-98%

lead

95-98%

manganese

94-98%

barium

95-98%

selenium

94-96%

Nitrate

60-75%

sulfate

96-98%


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WATER TREATMENT PROCESSES mercury

95-98%

potassium

85-95%

cadmium

95-98%

calcium

94-98%

sodium

85-94%

nickel

96-98%

fluoride

85-92%

arsenic

92-96%

phosphate

96-98%

cyanide

84-92%

WORKING OF THE REVERSE OSMOSIS PROCESS The reverse osmosis process consists of the following components 

Pre filter



Large cylinders for storage



Reverse osmosis membrane



Booster pump



Automatic shut off valve



Check valve



Pressure gauges



Post filter

PROCEDURE; First of all the pre treatment of the raw water is done.pre treatment requires the removal of some of the solids before sending the water to the membrane because it can cause damade to it.this pre treatment involves the elimination of dust particles using sand filters,chlorine removal that can cause oxidation of membrane using carbon filters,adition of acid for minimum alkalinity otherwise can cause fouling of the membranoes structure and maintenance of ph by running water through 10 micron prefliter. the reverse osmosis membrane requires the ph of 6.0. UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

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WATER TREATMENT PROCESSES Now the water is free from many suspended solids but still it is high in dissolved solids.so now this treated water is ready to become a part of reverse osmosis ppocess and easily pass through the membrane with no sabotage. s abotage. The most common membrane structure used is the spiral wound membrane.this structure was specially designed to remove remove the dissolved solids.two membranes membranes sheets each also known as the leaf are wound on the perforated tube.three edges are glued to a leaf whereas the unglued portion is seled with the porous central tube. After the pre treatment water wate r is pumped and is given two paths.one with a valve and the other tube without valve.the valve is the check valve and is not completely closed.it is about 70-80%closed so that a little resistance is offered.the fluid first comes to this line but very little fluid is allowed to pass through so the pressure is increased the pressure at this point is also measured .this pressure must not exceed 400 psi psi otherwise it can mutilate the membranes.the water is pushed back to the membrane from where 95-98%of the dissolved solids are removes again the pressure is measured and then then the filterate is collected in the storage tank. The pump in this process is a booster pump and the pressure that it transfers to the fluid must be greater than 10 psi to avoid cavitation .this amount of pressure varies for different solutions for instance the amount of pressure required to move a viscous fluid is very high.THE water that is deionized is known as the permeate and the product water and that which is rejected by the membrane is known as the reject or the concentrate stream.the reject stream is drained or it can be fed back to the feed in order to save water loss.the product line is in the direction of the feed.automatic shut off valve is very essential because when the storage tank is completely filled with the product water this valve automatically shuts off and there is no removal of water from the drain side too. when the water level is lowered in the t he tank and the pressure drops this valve is opeed again all owing water to flow through the membrane and the drain pipe.


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WATER TREATMENT PROCESSES from the storage tank the water goes to the post filter for the elimination of any an y kind of odour or taste.or dissolved gases,this task is accomplished by using any filter.or any other removal source.now from the storage tanks the water reaches its final task.this water can now be used for drinking. purpose,boiler feed water and for cooling tower

REVERSE -OSMOSIS PERFORMANCE AND DIFFERENT

PARAMETERS: Some of the important parameters of reverse osmosis process are as follows 

Salt rejection%



Salt passage%



Recovery%



Concentration factor

SALT REJECTION: It is given by the formula Salt rejection%= rejection%=

         

×100


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WATER TREATMENT PROCESSES This relation shows that higher the percentage rejection of salt higher is its efficiency.

SALT PASSAGE: This is simply the reverse of salt rejection% Salt passage%=1-salt rejection%

The higher is the value of salt passage passa ge %, the lower is its effiency.this can mean that the membranes need to be cleaned or the pretreatment done was not accurate.

%RECOVERY: This is given by the formula %recovery=

     

×100

%recovery is the amount of fluid that is not sent s ent to the drain but has rather participated part icipated in the product stream. in other words we can say that it is the recovered water.

CONCENTRATION FACTOR: This can be calculated by making use of the expression given below Concentration factor=

 

The concept behind that the more water which is recovered as the permeate leads to the more contaminents collected in the concentrate stream.this is one of the major cause of membrane fouling and scaling.

Reverse osmosis and drinking water. The water after passig through the reverse osmosis process can readily be used for drinking purpose and to serve other domestic life.this water has the ph of about 7 that means it is neutral.before this treaeted water is packed into bottles,some industries add useful chemicals like fluoride for the reduction of cavities among children.flouride vis very useful component for teeth.this water has some dissolved solids as well but these are not harmful for the people.

Reverse osmosis and boiler The water that is fed to the boiler must have a very low or no amount of dissolved solids in it.


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WATER TREATMENT PROCESSES The temperature is really high in the boiler operating environment,this changes the alkalinity from carbonates to the bicarbonates then to hydroxides and then directly to carbon dioxide.if the alkalinity amount exceeds the acceptable amont,this results in the limitation of the boiler cycle. Thus the boiler fuel is wasted and the plant efficiency efficienc y is affected.carbon dioxide if present in the boiler feed fee d water has the ability to convert into carbonic acid.here the hardness removal procedures are of no use. The oil presence in the water that is being fed to the water cause pre heating of metal , can cut off heat transfer,can cause thickening of sludge,can produce foaming.this is a slight hitch that can be reduced using cartridge filters,floatation method and mnay more. Silica presence in the boiler feed water also limits its application.the more silica present in the water that is fed to the boiler the higher are the chances of boiler boil er fouling. this problem of bolier is solved by the internal water treatment program This program is essential and its purpose is 

To eliminate oxygen and other dissolved gases fron the feed



To prevent boiler corrosion



To stop the entry of dissolved and other solids



To prevent scaling of the boiler

INTERNAL TREATMENT OF WATER: FOR HARDNESS: The water that enters the boiler comprises of some dissolved solids which cause hardness.these dissolved solids are removed by blowdown.blowdown is the discharge of boiler water taking along many dissolved solids. FOR SILICA: If there is presence of silica in the feed water than due to high temperature it precipitates out and cause the scaling of boiler.in silica treatment,the boiler water alkalinity is kept very high so that the silica remains in solution and doesnot accumulate on the walls of the boiler. FOR SLUDGE DISPOSAL: Internal treatment results in the formation of insoluble precipitates that form sludge.this sludge can deposit on the wall of the boiler. conditioners keep these solids away from depositing.these sludge conditioners include 

Tannin is effective for hard feed water



Lignin for phosphate sludge



Starches for silica feed water



Synthetic polyners useful for almost all kinds of sludge removal UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

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WATER TREATMENT PROCESSES Advantages of the internal treatment in boiler: Some of the conspicuous advantages are



It is a simple procedure



Safe



Efficiency is improved



Prevents tube failures



Scales and corrosion is minimized



Reduced energy consumption



Reverse osmosis process for water provided to cooling tower: REVERSE OSMOSIS AND THE COOLING TOWER: The water in the cooling tower is continuously being fed to make up for the water being evaporated or drained.this make up water water comes from the natural source like rivers wells etc so this water w ater contains a higher percentage of dissolved dis solved and suspended solids.this makeup stream is fed into the tower after regular intervals. This water needs to be treated before entering the cooling tower otherwise the salts deposition can cause the fouling and scaling of the cooling tower. Like for drinking water and boiler feed water pre treatment is important.reverse osmosis is proved beneficial for the water treatment in the cooling tower.the ph of this water should be maintained between 8-8.5 otherwise alkalinity factor can disturb bthe performance The water unlike boiler boiler feed is not 100% desalined because here some impurity percentage is non objectionable and does not not cause any serious damage to the apparatus Industries using reverse osmosis water: 

Pharmaceutical



Boiler feed



Food and beverge



Semi conductor finishes



Metal finishes etc

Advantages of reverse osmosis phenomenon: 

Best technique for contaminants elimination



Improves taste,odor and appearance

 

energy consumption is low Low production cost UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

Page 23

WATER TREATMENT PROCESSES 

Easy to handle and keep clean



Long lasting if the membranes are carefully kept



No production of hazardious chemicals during during the process



Cross flow filtration



Very efficient even for viscous solutions Disadvantages: 

Build up of salts around the membrane hinders its functioning



Chlorine in the water can ruin the membrane



Pretreatment is essential



Time taking process

 

No removal of dissolved gases like CO2 etc Membrane cleaning is required

CHEMICAL PRECIPITATION PRECIPITATION

Chemical precipitation is a method to remove metals, suspended solids, gases, oil inorganic and organics substances from water. A chemical reaction, start by the reagent cause the dissolved matter to form solid precipitates. By changing the water composition, we form insoluble metal complex. Filtration mechanism can be used to remove precipitate from water. Performance of this process depends upon concentration of metal, type of metal and reagent. Solution with dissolved ion

clear water + insoluble complex

Metals are precipitated as carbonates, sulfites and carbonates precipitate. In some process, the specie which we want to be removed must be oxidized. Fluorine can be removed by using calcium chloride and phosphate can be removed by the use of aluminum salt. Advantage and disadvantage of chemical precipitation

ADVANTAGES: 



Chemical precipitation is well-founded technique with readily availability of many chemicals and equipment. Sometimes, the chemicals that are used for operations are inexpensive i.e. lime. UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

Page 24


Do not remove all the hardness.

DISADVANTAGES: 

Large amounts of chemicals are required to be conveyed to the required place.

Methods of chemical precipitation for water treatment

soda process lime process soda lime process 

Cold lime soda process



Hot lime soda process

LIME PROCESS

Lime softening method is considered to be an effective method to lower the magnesium and calcium hardness in water by adding calcium oxide or calcium hydroxide and sodium carbonate to form calcium carbonate and magnesium hydroxide which precipitate out. PRETREATMENT:

Pretreatment of water for lime softening consists of either pre sedimentation or aeration. Pre sedimentation is used for very muddy surface waters, in order to create a more consistent. Aeration is performed for those waters that have high CO2 concentrations, such as ground water. It will also remove carbon dioxide from water. Other impurities present in water such as manganese, iron, fluoride, heavy metals, total dissolved solids, silica and chloride are also removed by pouring limestone only or with the combination of other chemicals like sodium silicates, alum etc. The rise pH required in the process also inactivates many microorganisms. PH VALUE:

In order to produce magnesium hydroxide or calcium carbonate, the pH of the water must be elevated by the addition of lime. Magnesium compounds require a pH of 10.0 to 10.5 while calcium compound require a pH of 9.0 to 9.5.The pH adjustment is maintained by adding sulfuric or hydrochloric acid into the waste stream with a metering pump PROCESS DIAGRAM:

. UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

Page 25


DESCRIPTION:

When soda ash and lime are added, hardness producing salts form almost insoluble precipitates. Magnesium hardness is precipitated out as magnesium hydroxide (Mg (OH) 2). Calcium hardness is precipitated out as calcium carbonate (CaCO3). PRECIPITATE REMOVAL PROCESS:

These precipitates are then removed by liable processes of sedimentation, filtration, and coagulation. Because precipitates are present in very small quantities, some hardness remains in the water at about as CaCO3. This hardness level is compulsory to prevent corrosion problems linked with water being too soft and having little or no hardness. METHOD:

First of all, we add lime in the water to form calcium hydroxide, or Ca (OH) 2. The reaction between calcium and carbon dioxide take place as a result, calcium carbonate (CaCO3) precipitate out. The optimum pH for this process is a bout 10.3. REACTIONS:

Removal of calcium hardness CO2. + Ca (OH) 2

CaCO3 + H2O

Ca (HCO3)2 + Ca (OH) 2

2CaCO3 + 2H2O

Removal of magnesium hardness Magnesium precipitates out in the form of magnesium hydroxide, Mg (OH) 2. The optimum pH for this reaction in from 11.0 to 11.3. This pH can be obtained by using further lime. Mg (HCO3)2 + Ca (OH) 2

CaCO3 + MgCO3 + 2H2O


Page 26

WATER TREATMENT PROCESSES MgCO3 + Ca (OH) 2

CaCO3 + Mg (OH) 2

MgSO4 +

Ca(OH)2

Mg(OH)2

+

CaSO4

MgCl2 +

Ca(OH)2

Mg(OH)2

+

CaCl2

Carbon dioxide does not produce any type of hardness but it take some amount of lime to react with before the lime start to reduce the hardness of water. CO2. + Ca (OH) 2

CaCO3 + H2O

In this way, precipitate form which are collected collect ed and water is free from hardness. RE CARBONATION:

To remove carbonate and bicarbonate hardness of water, high pH is required and produces water having high concentration of dissolved calcium carbonate and lime. If we enter this water to supply system in this state, due to higher pH, corrosion will occur on boiler tubes. Excess calcium carbonate would precipitate out, producing scale. In order to avoid this type of difficulties, the pH of the water must be lowered. Re carbonation is the process to produce stable water by lowering the pH of water and excess calcium carbonate (CaCO3) and lime (CaO) precipitate out. After this process, water will be available in such a composition of calcium carbonate that would be neither dissolve (cause corrosion ) nor tend to precipitate out from water ( causing scaling). This is usually done by pumping carbon dioxide (CO2) into the water. Excess lime reacts with carbon dioxide, making calcium carbonate:

Ca (OH) 2 + CO2

CaCO3 + H2O

SLUG:

Lime process produces large amounts of sludge. In fact, for five pound of lime used, about ten pounds of sludge are formed. SLUG HANDLING:

Landfill disposal is the most common common process to dispose slug. Sludge may sometimes sometimes be sent to sanitary drains. Lime sludge has a high pH and has progressively been disposed of by applying it to farmed land to increase the pH of acidic soils. CHEMICALS USE IN LIME SOFTENING UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

Page 27

WATER TREATMENT PROCESSES Types of Lime The lime used for softening water available in two forms. 

Hydrated lime



Quick lime

Both types of lime are used to soften the water wate r in the same way, but the apparatus required requi red for the two types of lime is different. dif ferent. HYDRATED LIME

Hydrated lime (Ca (OH) 2) is known as calcium hydroxide or slaked lime. Hydrated lime can be added to water without needing any special equipment, so it is a general choice for small water treatment plants. QUICK LIME:

Quicklime (CaO) is also known as calcium oxide. It must be slaked before it is used. Slaking is the process of changing quicklime to hydrated lime by addition of water. CaO + H2O

Ca (OH)2

Advantage and disadvantage of lime process DISADVANTAGES: 

The temporary and permanent hardness of magnesium can be significantly removed but the permanent hardness of calcium cannot be removed.



This process cannot be used where permanent hardness is r equired to be removed.



Large quantity of slug is produced from this process.

ADVANTAGES: 

This process is inexpensive.

TREND OF HARDNESS: 





Concentration of calcium hardness decrease with the increase in lime dosage but concentration increases if we use dosage more than 90 percent. Hardness of magnesium decreases with the increase in lime dosage especially, when calcium hardness was increase if we use dosage more than 90 percent. PH increase with the increase in lime l ime due to the formation of hydroxyl ions (OH). SODA ASH PROCESS


Page 28

WATER TREATMENT PROCESSES Soda ash (Na2CO3) is available in only one form and does not need any treatment before it being added to the water. Safety issues are same as those for lime handling. Soda ash dust annoys the eyes and mucous membranes of the nose, so the operator should wear shielding dress, a dust mask and goggles. In addition, areas where soda soda ash is used should be prepared with a fresh air s ystem to deal with the dust.

Soda ash (Na2CO3) is used to improve hardness reduction . It reacts with non-carbonate calcium hardness to form precipitate. REACTIONS:

Removal of permanent calcium hardness

CaSO4

+

Na2CO3

CaCO3

+

Na2SO4

CaCl2

+

Na2CO3

CaCO3

+

2NaCl

Ca (NO3)2

+

Na2CO3

CaCO3

+

2NaNO3

REACTION OF MAGNESIUM:

In order to reduce magnesium hardness, it acquires some amount of calcium hydroxide MgSO4 + Na2CO3 + Ca (OH) 2

CaCO3 + Mg (OH) 2 + Na2SO4

MgCl2 + Na2CO3 + Ca (OH) 2

CaCO3 + Mg (OH) 2 + 2NaCl

Mg (NO3)2 + Na2CO3 + Ca (OH) 2

CaCO3 + Mg (OH) 2 + 2NaNO3

CAUSTIC SODA PROCESS

Caustic soda (NaOH) is known known as sodium hydroxide. hydroxide. It can also be used to remove the hardness of water. The treatment process using caustic soda uses the same steps as that of lime and ash softening. Firs of all, carbon dioxide reacts with the caustic soda to make water and sodium carbonate. CO2

+ 2NaOH

Na2CO3 + H2O

Then the remaining caustic soda can react with magnesium bicarbonate and calcium bicarbonate. Ca (HCO3) 2 + Mg (HCO3)2

2NaOH +

2NaOH

CaCO3 + Na2CO3 + 2H2O MgCO3 +Na2CO3 +2H2O

Permanent calcium and magnesium hardness is removed by using soda. UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

Page 29

WATER TREATMENT PROCESSES MgSO4

+

2NaOH

Mg (OH) 2 + Na2SO4

MgCl2

+

2NaOH

Mg (OH) 2 + 2NaCl

2NaOH

Ca (OH) 2 + Na2SO4

CaSO4

+

ADVANTAGES: 

It produces less amount of slug.



Permanent hardness of calcium is also removed by using soda lime.

DISADVANTAGE: 

This process is very expensive.



This process is uneconomical.



Caustic soda is very dangerous to operator and cause severe action to skin.



It may produce caustic embrillitement LIME SODA PROCESS

It is very popular and important process for softening of water. PRINCIPLE:

When lime and soda are added to the hard water, soluble calcium and magnesium are precipitate out as calcium carbonate (CaCO3) and magnesium hydroxide (Mg (OH) 2). Then we add some chemicals to collect these precipitates. In the last, we apply filtration technology to remove these precipitate. FUNCTION OF LIME:

Lime is used to remove permanent magnesium hardness, temporary hardness, dissolved iron and magnesium salt, dissolved carbon dioxide (CO2) and hydrogen sulfide (H2S) and mineral acids in water. Removal of permanent magnesium hardness: MgSO4

+

Ca (OH) 2

Mg (OH) 2

+

CaSO4

MgCl2

+

Ca (OH) 2

Mg (OH) 2

+

CaCl2

Removal of temporary hardness: Mg (HCO3)2 + Ca (OH) 2

CaCO3 + MgCO3 + 2H2O

Ca (HCO3)2 + Ca (OH) 2

2CaCO3 + 2H2O


Page 30

WATER TREATMENT PROCESSES Removal of dissolved iron: FeSO4

+ Ca (OH) 2

2Fe (OH) 2 + 2H2O + ½ O2

Fe (OH) 2 + CaSO4 2Fe (OH) 2

Removal of aluminum salts: Al2 (SO4)3 + 3Ca (OH) 2

2Al (OH) 3 +3CaSO4

Removal of dissolved carbon dioxide: CO2 +Ca (OH) 2

CaCO3 +H2O

Removal of hydrogen bisulfite: H2S +Ca (OH) 2

CaS + 2H2O

Removal of sulfuric acid: H2SO4 + Ca (OH) 2

CaSO4 +2 H2O

Removal of hydrochloric acid: 2HCl + Ca (OH) 2

CaCl2 + 2H2O

Function of Soda:

When lime is used to remove hardness, it can be seen that permanent calcium hardness like, calcium sulfate (CaSO4) and calcium chloride (CaCl2) is introduced in to water. Soda is very efficient to remove permanent calcium cal cium hardness of water. CaSO4

+

2NaOH

Ca (OH) 2 + Na2SO4

PROBLEM IN PROCESS:

At room temperature the precipitate like, calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2)are very small and do not settle down at the bottom of water of water container. So in this case, filtration becomes challenging. These small particles may take part in corrosion and clogging in case of boiler tubes and pump SOLUTION:

These drawbacks may be removed by adding some chemicals as a coagulant like sodium aluminate and alum. Advantages and disadvantages of lime soda processes


Page 31

WATER TREATMENT PROCESSES ADVANTAGES: 

Iron and sulphur are removed from this process.



Due to basic nature, pathogenic bacteria are removed.



It is economical processes.



It removes not only hardness but also minerals.

DISADVANTAGES : 

Operation expenses are higher.

Typical Softener water analysis table

Based CaCO3 ppm

on Raw water in

Removal of Lime soda calcium softening alkalinity cold cold lime

Lime soda softening

Lime softening

hot

cold

Magnesium hardness

100

60

46

5

5

Calcium hardness

150

85

35

15

115

Total hardness

250

145

81

20

120

PH

7.5

10.3

10.6

10.5

10.4

SIO2

20

19

18

1-2

1-2

“p”Alkalinity

0

27

37

23

18

44

55

40

28

“M”Alkalinity I50

Lime soda process is divided in two processes depending upon temperature. 1. Cold lime soda process 2. Hot lime soda process COLD LIME SODA PROCESS

Cold soda lime process provides water containing a remaining hardness of 50 to 60 ppm. The desired amounts of soda and lime are added to water at room temperature. Small quantities of coagulants like alum and sodium aluminate are also provided to foam aggregates of small precipitate like calcium carbonate and magnesium hydroxide. Sodium aluminate helps to remove oil and silica, if present in water . UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

Page 32


METHOD:

The apparatus used in cold lime soda is shown in figure. The calculated amount of soda and lime is added to the water to be treated with a small amount of coagulant. This mixture is added from the top of the inner circular cylinder. The inner cylinder is fitted with the rotating shaft having number of paddles. When the chemicals and raw water flow down, there is continues mixing and vigorous stirring take place. Here chemical reactions occur and hardness of the water is removed by changing hard water in to soft water. The insoluble precipitates settle at the bottom and form heavy slug which is then taken out from the outlet at the bottom of the apparatus from time to time. The treated water goes to the outlet chamber and rise up. It passes through the filtering media to remove remaining slug. The filtered water finally comes out constantly through the outlet at top. The soft water contains remaining hardness of 50 to 60 ppm . HOT LIME SODA PROCESS

It is a fast process to remove the hardness of water at higher temperature. The temperature is keep to be 90-1000C. This temperature is the boiling point of water therefore, reactions take place very fast. The precipitation and filtration technique also carried out easily and rapidly. APPARATUS:

The apparatus used in hot lime soda process consists of three parts UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

Page 33


Sand filter



Reaction tank



A conical sedimentation vessel

METHOD:

Calculated amount of lime and soda is dropped from the top of the reaction tank in to the water to be treated, where it is mixed with stirrer. This mixture is heated with super-heated steam . Reactions take place and hardness of water is removed. Treated water along with slug goes in to a conical sedimentation vessel where slug settle down and impurities are removed to the bottom of the tank. The treated water rises up and passes through the sand filter to remove remaining impurities. The soft water comes out at the outlet. Treated water contains remaining hardness of about 15-30 ppm. DIFFERENCE BETWEEN COLD AND HOT SODA LIME PROCESS

Hot soda lime process.

Cold soda lime process

Filtration is very rapid and easy.

Filtration is difficult.

It is done at high temperature.

It is carried out at room temperature.

It is fast process.

It is slow process.

Its softening capacity is high.

Its softening capacity is low.


Page 34

WATER TREATMENT PROCESSES Softer water has residual hardness about 5060 ppm.

Softer water has residual hardness about1530 ppm.

Advantages of hot soda lime process over cold lime soda process 

Viscosity of treated water is lower, so filtration becomes easy.



Slug forms rapidly so it does not require any type of coagulant.



Softer water has residual hardness about 15-30 ppm.



Reactions occur very fast due to temperature in hot lime soda process.

Precipitation removal techniques in chemical precipitation 

Coagulation



Sedimentation



filtration

BOILER FEED WATER FROM CHEMICAL PRECIPITATION:

Three elements are very dangerous to lower the boiler’s life. 

Oxygen



Carbon dioxide



Solids

ACID CORROSION:

Acid corrosion occurs in boiler due to the presence of carbon dioxide. As carbon dioxide react with water to form carbonic acid which is the reason of acid corrosion. In chemical precipitation, the carbon dioxide is removed successively. CO2 +H2O

H2CO3

SCALE:

Due to the presence of hardness, salts are present in water.


Page 35

WATER TREATMENT PROCESSES In boiler, when water evaporates it leaves behind a layer of solid which can be removed by proper water treatment and chemical precipitation. We cannot be use a lime process because it does not remove permanent calcium hardness, as a result it cause scale and poor the life of boiler. Re carbonation must be done in chemical precipitation otherwise high pH will cause corrosion on boiler tubes. Caustic embrittlement produces due to sodium carbonate and alkali nature of water which cause corrosion. It can be reduce by controlling the amount of sodium hydroxide in chemical precipitation. COOLING TOWER:

In cooling tower, when no. of cycles increases the concentration of salts increase and form scale. This scale then leads to corrosion. Due to the scale formation, the heat transfer decrease and system leads to complications So in order to remove the calcium and magnesium hardness, we use chemical precipitation process. From chemical precipitation, we cannot remove microorganisms which cause severe problems in cooling tower.


Page 36


ION EXCHANGE METHOD INTRODUCTION:

Water has a general formula H2O. Formula shows that it consist of two Hydrogen atoms and one Oxygen atom. The formula show very simple composition of water but actually it contain many other substances either in dissolve form or suspended form. These substances are present in various concentrations making water impure. Suspended solids can be removed by various ways e.g. by boiling, chemical precipitation, coagulation, reverse osmosis, etc depending upon the type and concentrations of impurities. These methods cannot remove dissolve impurities. Dissolve impurities can be removed by ion exchange method. SOLUBLE IMPURITIES: IMPURITIES:

We cannot see dissolve impurities in water from naked eye. These impurities are present in the form of electrically charged species called ions. If these ions carry positive charge they are called cations. And if they carry negative charge then they are called anions. Since water is a neutral compound so the number of cations must be equal to numbers of anions Ions can be single atom (monatomic) or may be in the form of group of atom like a molecule (polyatomic) and they can carry number of charges mostly 1 to 3. E.g. Na+, K +, Ca++, Mg++, Al+++, CrO4=, CO3=, NO3 – , Cl – , etc All cations and anions are free to move they are neither attached to anti-charged ion nor they are fixed; only they are in a fix fi x proportion to make water neutral. Salts in water are in the form of their dissociated ions that are free to move. Water molecule also exists in the form of OH _ and H+ ions. These ions entrap the ions of the salts and made them hydrated. E.g. Na 2CO3 has two monovalent Na + ions and one divalent CO3= ion to balance the specie. ION EXCHANGE RESINS:

These are polymeric substances mostly plastic beads that have pours in it usually about 0.6 mm in diameter. They contain a small amount of water that is the measure of moisture content in them. In their polymeric structure they have permanently fixed ions UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

Page 37

WATER TREATMENT PROCESSES which can neither move nor displaced. The appositively charged ions to that fixed ions are free to move in same ratio to maintain neutrality. They are of the following types: Cations Exchange Resins:(CER) Resins:(CER)

In this type the anions are fixed on the structure while cations are exchanging with other cations Strong Acidic Cation Exchange Resin (SAC)--- Inorganic    

They have functional group of Sulphonic Acid ( — SO SO3 – H+) Na+ replaces Ca++ and Mg++ ions to eliminate hardness. H+ replaces all the cations. Act as an acidic catalyst

Weak Acidic Cation Exchange Resin (WAC)---Organic  

They have functional group of Carboxylic Acid ( — COOH) COOH) The solutions that contain Alkalinity, divalent ions can be removed with the help of H+ ions.

Anions Exchange Resins:(AER) Resins:(AER)

These resins have cations fixed on their polymeric structure, while anions are free to move and are able to replace with wit h other anions. Strong Basic Anion Exchange Resin (SBA)---Inorganic   

They have functional group of Quaternary Ammonium ( — N(CH3)3+ OH – ) They can replace all anions from OH - in hydroxyl form In chloride form they can replace sulphates, nitrates and other anions from Cl-

Weak Basic Anion Exchange Resin (WBA)---Organic   

Functional group is amines ( — N(CH3)2) They cannot remove weak acids (CO 2, SiO2) They can eliminate anions like sulphates, chlorides, nitrates, etc after cation exchange

Cation

Anion Resin

Resin Bead

Bead


Page 38

WATER TREATMENT PROCESSES Selective And Chelating resins: 

      

Per chlorates, metals, Boric acid and other ions can be removed selectively from these resins Triethylammonium is used for the removal of NO 3 Thiol is for the removal of Hg, Cd, etc Aminophosphonic is used for the removal of Ca in brine Iminodiacetic is used for the removal of Ni, Cu, et c Methyl Glucamine is used for the removal of H 3BO3 Bis_Picolylamine is used for the removal of metals at low pH Thiourea is used for the removal of Cd, Hg, etc

Zeolite Ion Exchange Resin:      

Aluminosilicate framework Porous structure Crystalline solid Inorganic in nature Sodium zeolite (Na-Zeolite) is most common example Working principal is same as that of cation cat ion exchange resin

Ion Exchange Processes:

Ion exchange method includes various applications for water purification that are:       

Decationization---Removal of cations Dealkalization---Removal of Bicarbonates Softening---Removal of hardness Demineralization---Removal of all ions Nitrate removal Mixed bed---Polishing Selective removal---of various components

CATION REMOVAL:

In this process all the cations are eliminated from water using strong acidic cation resin in its H + ion form. This process is called Decationization CER-H + Na+ 2 CER-H + Ca++

CER-Na + H+ (CER)2-Ca + 2 H+

Where CER stand for Cation Exchange Resin By using strong acid the process can be used to regenerate resin CER-Na + H+

R-H + Na+


Page 39


Applications:

This process is used in:  

Start of demineralization Mixed bed process is proceeded by this process

BICARBONATE REMOVAL:

Alkalinity can be removed by using weak acidic cation exchange resin. CO 2 produced in the process can be removed by degasifier tower. This process is called Dealkalization 2 CER-H + Ca ++(HCO3 – )2 (CER)2-Ca + + – 2 H + 2 HCO3 H+ + HCO3 –

CO2 + H2O

Process can be reversed by adding acid to regenerate resin Applications:

It is used in:      

Initial stages of demineralization Breweries Boilers running at decreased pressure Water filers on small scale usually at home The removal of temporary hardness Lessen the alkalinity

HARDNESS REMOVAL:

Metal cations like Ca2+, Mg2+, Ba2+, Sr 2+, etc are the cause of hardness in water. The latter two are insoluble and can cause scale to form. These ions can be eliminated from water by passing strong acid cation exchange resin in its Na+ form. This process is called Softening. 2 CER-Na + Ca++

(CER)2-Ca + 2 Na+

When these metals cation are concentrated in resin NaCl is added to increase the concentration of Na to regenerate resin R 2-Ca + 2 Na+

2 R-Na + Ca ++


Page 40

WATER TREATMENT PROCESSES Applications:

This process is used in:     

Removal of hardness but not eliminate alkalinity Breweries Low pressure boilers Dish washers in foreign countries Soft drink factories

DEMINERALIZATION:

This process is also called Deionization because this method is used to remove all the ions. Many applications need water that is free from any type of ion. Concentration of ions in water is increased by heating, due to evaporation, which can be removed by this method. For this purpose two types of resins used in the same system. Cation exchange resin is used for removal of cation in hydrogen form (H +) and anion exchange resin used for the removal of anion in hydroxyl form (OH -). Firstly water is to pass through cation exchange resin where all types of cations are eliminated. CER-H + Na+ 2 CER-H + Ca ++

CER-Na + H+ (CER)2-Ca + 2 H+

In the next step anion exchange resin is used to eliminate anions AER-OH + Cl – 2 AER-OH + SO4=

AER-Cl + OH – (AER)2SO4 + 2 OH –

Where AER stand for anion exchange resin

Final step of this process is H+ + OH –

H2O

The above reactions are reversible and can be regenerated with a strong acid

CER-Na + H+

the resin bor alkali

R-H + Na+


Page 41

WATER TREATMENT PROCESSES AER-Cl + OH –

AER-OH + Cl –

Applications:

Demineralization is used in:         

Nuclear power stations Laboratories Chemical industries Paper industries High pressure boilers Batteries Fossil fuelled industries Removal of hardness and brininess both The production of computer chips and other electronic equipments

NITRATE REMOVAL:

Nitrates can be eliminated from water using strong st rong basic anion exchange resin in Cl form. Majorly it is used for nitrates but some other anions are also removed during the process. i.e. Sulphates, Bicarbonates, etc. resin can be regenerated by increasing Cl concentration using NaCl.

AER-Cl + NO3 –

AER-NO3 + Cl –

Application:

It is used in:  

Process where low cost is available Water treatment for metropolitan for metropolitan system

MIXED BED POLISHING

Mix bed polishing is usually used after deionization, i.e. when a very small amount of impurities left the product from the unit is extremely good with no impurities. In mixed bed unit strong acidic cation exchange resin and strong basic anion exchange resin are combined. But in this process regeneration is difficult because the resins need to apart first through backwash and then regenerate. Also the need of chemical is large that’s why it is used after demineralization Applications:

This process is used in:  

Condensate purification Semiconductor industries to produce ultra pure water UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

Page 42

WATER TREATMENT PROCESSES    

Polishing permeate obtained from reverse osmosis. Polishing of the fraction obtained after distillation of sea water Various power stations---as a condensate polisher Purification of demineralized water

SELECTIVE REMOVAL

This method is usually used to take away different heavy metals and other contaminants from drinking water using special chelating resins that produce stable complexes. This method depends upon the relative affinities of ions with the resin. The ions that have greater affinity will remove first than that which have less affinity. The sequences of selectivity or affinity are: For anions

SO4= > NO3 – > Cl – > HCO3 – > OH – > F –

For cations

Pb

++

++

> Ca

++

> Mg

+

+

> Na > H

Applications:

This process is used to make drinking water:    

Boron (Boric Acid) Nitrates Per chlorates Heavy metals like: Fe, Ni, Cr, Pb, Cd, Zn, Hg, etc


Page 43

WATER TREATMENT PROCESSES SUMMARY OF THE PROCESS:

PROCESS

IMPURITIES

ION EXCHANGE RESIN

PRODUCT MINERALS

Softening

Ca(HCO3)2 & CaSO4

Na2SO4 & NaHCO3

A

Dealkalization Of Product A Dealkalization

Na2SO4 & NaHCO3

NaCl & NaCl

B

H2CO3

C

Cationization

Ca(HCO3)2 & CaSO4

H2CO3 & H2SO4

D

Alkalization

H2CO3 & H2SO4

Strong Acid Cation Na+ Exchanger Weak Base Anion Cl Exchanger Weak Acid Cation H + Exchanger Strong Acid Cation H + Exchanger Strong Base Anion OH- Exchanger

H2O & H2O

E

Ca(HCO3)2

SUPPOSED PRODUCT NAM

Table For Basic processes   

Product D & E collectively called Demineralization Product B is also a Demineralized product Same method follow for nitrates and other anion as followed in process B & E

Basic Flow Sheet Diagram Of Ion Exchange Process


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WATER TREATMENT PROCESSES ION EXCHANGE METHOD AND DRINKING WATER:

For the purification of drinking water this process is not preferable. It has a small application for drinking water. It can only be used for the removal of some specific substances using specific resins. 

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Strong acidic cation exchange resin is used to takeaway Barium and Radium from drinking water Weak acidic cation exchange resin and some other softeners remove lead from drinking water Uranium present in uranyl ion (UO 3++). For its elimination strong basic anion exchange resin or weak basic anion exchange resin are used For the removal of chromates from drinking water weak basic anion exchange resin is applied Chelating resins take away heavy metals from drinking water Special type of selective resins take away nitrates, boron and per chlorates Hardness is removed by softening Other impurities cannot be removed by this method

ION EXCHANGE METHOD AND COOLING TOWER’S WATER:

Ion exchange method is used at little extent because it yields very soft water as required in cooling tower. Specifically ion exchange method is used for the removal of some special anions from cooling tower’s water using anion exchange resins. Fir stly Fir stly cation exchange resin allows water to pass through it. This will cause the unwanted cation to separate. Then water that is free from hazardous cation has to pass through the anion exchange resin. Targeted anions make ionic bond with resin by replacing less hazardous anion. This method is specifically used for the elimination of:     

ASO4-3 CrO4-2 SeO4-2 SeO3-2 ClO4-1

Some components removed automatically due to their selectivity like:   

PO4-3 NO3-1 F-1

Chelating ion exchange resin removes heavy metals by making covelant bond for stable complexes with them. They are highly selective for heavy metals mostly divalent like:     

Cu Ni Cd Cr 3+ Hg UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

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Etc

ION EXCHANGE METHOD AND BOILER FEED WATER:

Ion exchange water treatment method finds its way in the treatment of boiler feed water, where water with least impurities is required. Boiler feed water cannot be efficient if it contain large or moderate amount of impurities, a very small amount is tolerable as present in ion exchange treated water. The reason of small amount of impurities is that water have to converted into steam in the boiler. This process is accompanied by evaporation. As water evaporates, the residual water becomes more concentrated with impurities. If the impurities are in huge concentration, they can cause scaling in the boiler and eventually leads to the malfunctioning of equipment. Water from ion exchanger is best for boiler. Usually cation exchange resin is used which remove hardness causing cations i.e. Ca 2+ and Mg2+. Anion exchange resin can also be used to remove alkalinit y i.e. HCO3-1 A problem may occur if water contain large amount of sulphates (SO 42-). They along with Ca2+ ions form CaSO4 which is insoluble in water at high temperature, as boiler always operates at high temperature so concentration of this salt increases and cause scaling in the equipment. The best method for avoiding this problemis the removal of Ca 2+ and Mg2+ ions so that soluble salts of sodium formed instead of insoluble salts of calcium and magnesium. Some specifications for boilers are: 

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For high pressure boiler dematerialized water is used because impurity contents in demineralized water are least i.e. free from all types of ions, particles, gases and other contaminants. As high pressure tent to high temperature that leads to greater evaporation so if any impurity present it may cause damage ver y quickly Decataslized and dealkalized water is used in low pressure boiler, where temperature is low thus evaporation rate is also low and concentrating process is slow so a considerable amount of ions is acceptable, as these two processes not remove all ions but cation or anion

ADVANTAGES OF ION EXCHANGE METHOD:             

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Ion exchange method is environmentally a safe process Small assembly is required Take away toxic contaminant from drinking water Less power requirement Remaining impurities are less than 0.5 ppm or sometimes not measureable It can be used with varying flow rates. Economical process Post treatments are not required Very useful in nuclear and thermal power station Resin can be easily regenerated Processed water has high flow rate Better quality is achieved Production of semiconductor, display systems and computer chip is very useful application Initial investment of is low UNIVERSITY OF ENGINEERING AND TECHNOLOGY LAHORE

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WATER TREATMENT PROCESSES  

Product flow is consistent and independent of temperature Treated water quality is independent of small cha nge in the quality of raw water

DISADVANTAGES OF ION EXCHANGE METHOD: 

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The waste from the exchanger is highly concentrated and needs to be handled carefully Deionization consumes high cost especially for a raw water containing high concentration of solids Pretreatments are needed otherwise suspended impurities will cause problems: fouling of resin with colloidal matter can cause degradation, it also create problems in degeneration Processing or operating cost is very high CaSO4 fouling Microorganism adulteration This process cannot takeaway particle, bacteria and pyrogens Cationic resin in its Na form not eliminate silicates, salinity and undissolve solids If the concentration of TDS is very high then this process i s not suitable\ Organic matter adsorption Usually some initial treatments are required for surface water Organic and Cl adulteration There is a possibility of corrosion in cationic resin in its H form due to acidic pH


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Water Treatment Processes

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