water tech be notes

8/10/2019 Water Tech BE Notes

1/14

1

U.R.BAGWANM.Sc

Water Technology and Nanomaterials :

Water Technology:

Introduction, sources and impurities of water;

boiler feed water, boiler troubles with disadvantages -scale and sludge formation, priming

and foaming, boiler corrosion(due to dissolved O2, CO2 and MgCl2).

Determination of DO, BOD and COD, numerical problems.

Sewage treatment:

Primary, secondary (activated sludge method) and tertiary methods.

Softening of water by ion exchange process.

Desalination of sea water by reverseosmosis & electrodialysis (ion selective).

Nano Materials :

Introduction,

properties (size dependent).

Synthesis- bottomup approach (sol-gel, precipitation, gas condensation, chemical vapour

condensation, hydro thermal & thermolysis processes).

Nano scale materials

nano crystals & clusters, nano crystalline, fullerenes, carbon nano tubes,

nano wires, nano rods, dendrimers & nano composites.


2/14


3/14

3

U.R.BAGWANM.Sc

Boiler feed water: A boiler is closed vessel in which water is converted into steam by

heating at high pressure.The water is used in boilers is called boiler feed water.Boiler requies

pure water ,if impurities are present in water following problems are observed. a) Scale and

sludge formation b)Priming and foaming c) Boiler corrosion.

a) Scale and sludge formation

When water is converted into steam , impurities concentrate and finally precipitate out. The

precipitation adheres to inner walls of boiler. If the precipitation is in the form of hard ,dense

and sticky coating it is called scale.If precipitation is inform greasy and loosely held mud , it is

called sludge.

SCALES:these are hard deposits which stick to inner surface of boiler and are difficult to

remove. The impurities like bicarbontes , hydroxides , sulphates ,phosphates , oxides of Ca ,Mg

and silica are responsible for scale formation .Formation of scales takes place due to

i) Due to increase in concentration of salt by evaporation.

Ii)Decrease in solubility of salt with increase in temperature For ex: CaSO4

iii)Precipitation of substance by chemical reaction.

For example

a) Calcium sulphate scale: The solubility 0 f CaSO4 decreases with increase in temperature.

Temperature will be higher at surface than the interior of the boiler. CaSO4 get saturated at the

surface and get deposited in the form of thin film. Due to continuous process a thick, hard,

multilayer scales are formed.

b)Calcium carbonate scale :Calcium bicarbonate decomposes into calcium carbonate in

presence of heat. It has low solubility and deposits in the form of scales.

SLUDGE :It is loose accumulation of suspended solids, found in the colder and less turbulent

sections of the boilers.It is formed by impurities which have greter solubility in hot water than

in cold water. The impurities like MgCl2, MgCO3,MgSO4, CaCl2are soluble in hot water but

precipitate out in cold water. Iron and silica are also commonly found in sludge. If sludge is not

removed periodically , it may settle down to form scale.

Harmful effects due to scale and sludge formation

1) Efficiency of boiler decrease

2)Scales and sludge do not conduct heat , hence wastage of fuel.

3)Over heating may lead to expansion of tubes called as bagging.

4) Bagging leads to cracking and seepage of water from boilers


4/14

4

U.R.BAGWANM.Sc

4) Colder parts of the boiler and distribution lines (pipes) may get blocked due to sludge .

2)PRIMING AND FOAMING

Priming: The steam produced in boiler , some time carry water droplets along with impurities.

This is called priming. The steam with water droplets is called wet steam. The impurities carriedby wet steam may adhere to blades of turbine , hence efficiency of turbine decreases. Primimg

occurs due to impurities present in water, improper construction of boilers, high level of water

in boiler and highsteam velocity. Primming can be prevented by slow discharge of steam, proper

construction of boiler and by maintaining low water level.

FOAMING :It is the formation of persistent bubbles on the surface of boiler water. It fills up the

steam space of boiler.Foaming is caused by alkalis, oils, fats and other suspended oranic matter.

Foaming can be avoided by using antifoaming agents . Oil and grease can be removed by sodium

aluminates. Clay and suspended matter can be removed using coagulating agent.

BOILER COROSION

It is process in which boiler surface deteriorates (undergo destruction) due to attack by

impurities present in boiler feed water and environment .Corrosion in boiler occurs due to

a)Presence of dissolved oxygen

b) Presence of dissolved CO2

c) Presence of Magnesium chloride.

a)Presence of dissolved oxygen : Oxygen in feed water is main reason for boiler corrosion. It

reacts with iron at 350 -4500C to form ferrous hydroxide. It is further oxidized to ferric

hydroxide by dissolved oxygen. This process continuous until all dissolved oxygen is exhausted.

Fe +2H2O + O2 2Fe(OH)2

Fe(OH)2 + O2 Fe2O3.3H2O

b) Presence of dissolved CO2: The dissolved CO2 reacts with water to form carbonic acid.

Hence acidic condition leads to corrosion.The bicarbonate ions furnished by carbonic acid

combines with Fe to form iron bicarbonate . The Fe(HCO3)2 decomposes to Fe(OH)2 and

CO2.Thus the process continues

Fe(HCO3)2 Fe(OH)2+ CO2

c) Presence of Magnesium chloride.The salts like MgCl2 and FeCl2 undergo hydrolysis

producing mineral acids which leads to corrosion.

MgCl2+2H2O Mg(OH)2+2HCl


5/14

5

U.R.BAGWANM.Sc

FeCl2+2H2O Fe(OH)2+2HCl

DISSOLVEDOXYGEN

Oxygen in water is measured as dissolved oxygen(DO).It is expressed in mg/l or ppm. It

maintains the balance in water ecosystem. Oxygen is required for aquatic plants and animals

and also to prevent odors. Oxygen enters from atmosphere into water surface and also

produced by aquatic plants, algae, phytoplanktons during photosynthesis. Dissolved oxygen

can range from 0 to 18mg/l. Most natural water systems require 5 to 6 mg/l of O2to support

life in water. A high percentage of dissolved oxygen supports aquatic life. If dissolved oxygen

declines, aquatic plants and animals will die .There will be increase in worms and fly larvae and

also anaerobic respiration increases. DO analysis is key test to check on downstream water. If

organic waste such as animal waste is not properly treated and allowed to enter water body. It

decreases amount of dissolved oxygen.

Factors affecting dissolved oxygen 1)Water temperature 2)Aquatic plants 3)Stream Flow

4)Altitude/ atmospheric pressure 5) Human activities 6)Organic wastes.

DETERMINATION OF DISSOLVED OXYGEN (DO)

. BY WINKLER METHOD. (5MARKS)

Principle: In this method ,the given water sample is treated manganous sulphate and potassium

iodide .In alkaline medium O2 oxidises Mn+2

to Mn+4

.On acidification Mn+4

oxidises iodide and

liberates equivalent amount of iodine. The liberated iodine is titrated against standard sodiumthiosulphate solution using starch as indicator.

MnSO4+ 2KOH Mn(OH)2+K2SO4

2Mn(OH)2+O2 2MnO(OH)2 (BROWN) (Manganic oxide)

2MnO(OH)2 + 2H2SO4 2Mn(SO4)2+6H2O

2Mn(SO4)2+ 4 KI 2MnSO4+2 K2SO4+ I2

2I2

+ 4Na2S

2O

3 2Na

2S

4O

6+ 4NaI

Procedure : 250ml of water sample taken in a bottle. Add 2ml of MnSO4 solution and 3ml of

alkaline iodine solution.The bottle is stoppered and shaken well for 10-15minutes and allowed

to stand for 2 minutes for precipitate to settle. Then 1ml of conc H2SO4is added, stoppered and

shaken well to dissolve the MnO2precipitate.100ml of the solution are is pipetted out from the


6/14

6

U.R.BAGWANM.Sc

bottle into conical flask and titrated with standard Na2S2O3solution using starch as indicator till

the violet color disappears.

Normality of the dissolved oxygen =N1

Volume to water sample =V1 ml

Normality of Na2S2O3 soution = N2

Volume of Na2S2O3 solution consumed =V2 ml

N1V1=N2V2

Normality of dissolved oxygen = N2 V2

. V1

Mass of oxygen dissolved per /dm3=Normality Equivalent massof oxygen

= N1 8 = A g/l

= A 1000 = A mg/l or ppm

SEWAGE

Sewage is the waste material which comes from different sources. Sewage may be broadly

classified into

1)Domestic sewage

2)industrial Sewage

1)Domestic sewage : Domestic sewage is the liquid wastes from residences ,institutions ,

business buildings , kitchens, baths and laboratories . They contain organic wastes, pathogenic

bacteria ,plant mineral, pesticides ,detergents and other waste material.

2) Industrial Sewage: Industrial sewage is from industrial establishments such as chemical

plants,fertilser factories ,tanneries ,refineries ,textiles etc. They contain acids, oils,toxic

chemicals ,metals non biodegradable compounds.etc


7/14

7

U.R.BAGWANM.Sc

In recent years use of chemicals in high scale and disposing in improper

way by industries has created many environmental problems. Sewage treatment is done to

avoid its harmful effects on environment. Sewage treatment involves breaking down of

complex molecules of organic wastes into simple molecules by bacteria

Bacterial degradation of organic matter is of two types.

a)Aerobic degradation involves degradation of organic waste by bacteria in presence of

oxygen and producing harmless productssuch as carbon dioxide and water.

b)Anaerobic degradation involves degradation of organic waste by bacteria in absence of

oxygen and producing ahrmful end products such as methane ,hydrogen sulphide and ammonia

BIOLOGICAL OXYGEN DEMAND (BOD)

Defination :Biological oxygen demand (BOD) is defined as the amount of oxygen required by

microorganisms to oxidise the organic wastes present in one litre of waste water under

aerobic conditions at 200C over a period of 5 days

BOD is expressed in mg/l or ppm.The amount of organic matter present in the water is

measured interms of amount of dissolved oxygen required by micro organisms to oxidise the

organic matter.This is called as BOD.Higher value of BOD of particular water sample indicates

that water is having more amount of organic waste.Increase in temperature also leads to high

BOD value.

Measurement of BOD: The BOD value of water sample determined by measuring dissolved

oxygen (DO) using winkler method.The DO of water sample placed in BOD bottle is

determined.The bottle is sealed and incubated in dark for 5 days at 200C .After the end of

incubation again the DO is measured. The difference in DO value gives the oxygen consumed by

microrganisms to oxidise the organic waste.

Significance of BOD: Its important to know the BOD of sewage before disposing into rivers or

lakes because ,dissolved oxygen content in water will be decreased by the sewage.If the BOD of

sewage is high ,fishes and other aquatic plants will die.In such cases sewage is diluted with

required amount of clean water and then left in water bodies. Bod gives following information.1)Polluting power of sewage or its nuisance value.

2)The load of organic matter on the sewage treatment plants .

3)The amount of clean diluting water required for disposal of sewage.


8/14

8

U.R.BAGWANM.Sc

CHEMICAL OXYGEN DEMAND (COD)

Defination: COD is defined as the amount of oxygen consumed to oxidise organic and inorganic

waste present in 1 litre of water by using strong oxidisng agent likeK2Cr2O7

It is expressed in mg/l or ppm.COD is always greater than BOD, because COD involves oxidationof both biodegradable and non-biodegradable matter.

EXPERIMENTAL METHOD

Principle:Water sample is made to react with acidified K2Cr2O7 in presence of Ag2SO4and

HgSO4. The un reacted K2Cr2O7 is made to react with ferrous ammonium sulphate solution.The

amount of K2Cr2O7consumed to oxidize the water sample corresponds to COD of water sample.

3CH2O + 16H+ + Cr2O7

24Cr

+3+CO2 + 11 H2O

Procedure : Take water sample in conical flask .Add 10cm3of 0.25N K2Cr2O7 and 30cm

3of of

6N H2SO4.Now add 1g Ag2SO4 as a catalyst and 1g of HgSO4to avoid interference of chloride

ions.Attach a reflux condenser and gently reflux the contents for two hours .Cool and titrate the

excess K2Cr2O7against ferrous ammonium sulphate solution using frroin as indicator till the

bluish green color turns to reddish brown.Let the volume FAS consumed be Aml .Now carry out

the blank titration by taking distilled water in place water sample. Let the volume of FAS

consumed be B mlCalculation:

Volume ofK2Cr2O7 required to react with water sample in terms of FAS V2 = (BA) ml

Normality of FAS = N2

COD of water sample = N1

Volume of water sample = V1

N1 V1 = N2V2= N2 (BA)

. V1

mass/dm3

= N1 Equivalent mass of O2

= N1 8 g/l


9/14

9

U.R.BAGWANM.Sc

= N1 8 1000 mg/l or ppm

SEWAGE TREATMENT

Sewage contains pathogenic bacteria ,organic, inorganic and suspended impurities. If left

untreated in water bodies .It causes death of aquatic plants and animals.Sewage treatment is

done to reduce BOD level and to remove other impurities. Sewage treatment is done in three

steps.

1)Primary treatment.

2) Secondary treatment.

3) Tertiary treatment.

1) Primary treatment.

It involves removal of floating and suspended impurities.These impurities are removed by

passing sewage water through bar screens and mesh screens in which impurities get filtered.

Sand ,broken glass piecesare removed by passing sewage water slowly through grit chamber.

Here heavy material settels down due to gravity.Oil and greaseare removed by passing

compressed air to form froth which floats on water and skimmed off.Suspended particlesare

removed my coagulation method (sedimentation process) by addiing coagulating agents like

alum , FeSO4.

2) Secondary treatment (Biological treatment )

Waste water after sceondary treatment is mixed with activated sludgein aeration tank

.Activated sludge is mixture containg microrganism and bacteria. Airis blown for several hours

from the bottom of the tank ,to bring good contact between bacteria and organic wastes.

Under sunlight and oxygen bacteria oxidises (aerobic oxidation) organic matter.The purified

water and sludged is sent to a tank in which sludge settles down.A part of sludge is used to

purify fresh sewageand remaining is used as fertilisers.Purified water is collected separately

and chlorinated to remove bacteriaand finally discharged into running water or used for

watering plants.This method operates at 90-95% efficiency of BOD treatment. If the treated

water contains a high concentration of phosphates ,heavy metals , colloidal impurities and non

degradeble organic compounds , the water is sent to tertiary treatment.


10/14

10

U.R.BAGWANM.Sc

3) Tertiary Treatment

Phosphates are removed by adding lime to form calcium phosphate.Heavy metals are removed

by treating with S-2

ions to form insoluble sulphides. Water is also treated with activated

charcoal where impurities get adsorbed. Colloidal particals if not removed in sceondary methodis now removed by adding alum to reduce BOD level

WATER POLLUTION

Water pollution is contamination of water with foreign material. With rapid globalization and

industrialization water bodies are unlawfully used to dump waste material.Water bodies are

contaminated with, pathogenic bacteria, organic wastes , pesticides, heavy metals ,radioactive

wastes, industrial wastes and heat. Water suitable for drinking purpose is called as potable

(safe) water .Water for drinking purpose should be soft, clear and odorless , free from

pathogenic organisms ,free from any harmful foreign material and should be of good taste.

DESLAINATION OF WATER

The process of removal of dissolved salts from sea (or brackish) water to make it usable is

called desalination of water. Desalination is expensive process usually applied in countries

where there are no sources of fresh water. Salt water is desalinated to produce fresh water

suitable forhuman consumption or irrigation. There are three grades of water based on the

level of dissolved salts.

a) Fresh water contains less than 1000mg/l of dissolved salts.

b) Brackish water is one whose salinity lies between fresh water and sea water. It contains

100035,000mg/l of dissolved salts

c) Sea water contains more than 35,000mg/l of salts.

Desalination of water is done by

a)Reverse Osmosis b) Electro dialysis
http://en.wikipedia.org/wiki/Drinking_waterhttp://en.wikipedia.org/wiki/Drinking_water


11/14

11

U.R.BAGWANM.Sc

REVERSE-OSMOSIS (5 MARKS)

Defination: The process of demineralization of water by passing water under pressure

greater than osmotic pressure through semi permeable membrane is called as Reverse

Osmosis .It is commonly referred to as RO.

Pressure greater than OP

Osmosis is the movement of solvent (water) molecules from higher concentration

to lower concentration through semi permeable membrane. When salt water and pure water

are separated by semi permeable membrane a difference in concentration of salt arises. This is

called chemical potential. Salt water shows greater chemical potential than pure water. To

equalize the chemical potential water molecules from pure water diffuse through semi

permeable membrane into salt water , this is called as osmosis and the pressure with which

molecules are diffusing is called osmotic pressure (OP).

Osmosis can be reversed by applying pressure on saline water.

The applied pressure should be greater than osmotic pressure. The amount of pressure

required depends on concentration of the salt water. Concentrated salt water requires more

pressure to overcome osmotic pressure(OP). Sea water exerts an osmotic pressure of about

240psi and brackish water exerts less OP value. Reverse osmosis is done applying pressure in

the range of 410 psi to 540 psi. A reverse osmosis unit consists of a membrane, a vessel and

high pressure pump. The semi permeable membranes are generally made up of cellulose

acetate or nylon which allows on water molecules to pass through.


12/14

12

U.R.BAGWANM.Sc

Softening of water by Ion exchange process (6 Marks )

Ion exchange process is one in which there is a reversible exchange of ions between solid

phase and solution phase.If an aqueous solution containing B+cations is placed in contact with

solid exchanger A+X

--. Following reaction takes place.

B+ + A+X-- XB+ + A+

Ionexchange resins are polymers of high molecular mass and with porous cross linked

structure.Ion exchange property arises due to presence of acidic or basic group in resins. The

acidic group (sulphonic or carbonyl) exchange H+ions for cations (Ca

+2 ,Mg

+2). The basic group

(amino) exchange OH

ions for anions (SO4-2

,Cl

)

In ion-exchange process:Hard water is allowed to pass through cation exchange resins, whichremove Ca

+2and Mg

+2ions and exchange equivalent amount of H

+ions.

Cation Exchange Resins2RH

++ Ca

+2 2RCa

+2+ 2H

+

2RH+

+ Mg+2

2RMg+2

+ 2H+

(RH+

= cation exchange resin)

Now water is passed through anion exchange resins which

remove bicarbonates, chlorides and sulphates ,fluoride and nitrate from water and exchange

equivalent amount of OH

ions.

.Anion Exchange Resins

ROH + Cl R Cl

+ OH-

2ROH-+ SO4-2 2RSO4 -2+ 2OH- (ROH = anion exchange resin)


13/14

13

U.R.BAGWANM.Sc

Regeneration of resin

When cation exchanger looses capacity of producing H+ ions and anion exchanger

Looses capacity of producing OH

--

ions, they are said to be exhausted. The exhausted cationexchanger is regenerated by passing it through dilute HCl or H2SO4The exhausted anion

exchanger is regenerated by passing a dilute solution of NaOH.

Advantages of ion exchange process

1)The process is completed in a few seconds

2)Ion exchange resin can be easily regenerated.

3)Water of 1 to 2 ppm purity could be obtained.

ELECTRODIALYSIS (5MARKS)

Principle: Electrodialysis process is based on the principle that ions migrate towards oppositely

charged electrodes when an emf is applied.

The direction of ions are made specific by using ion selective membranes.

Cation membrane(C)allows only cations to pass through it and anion selective membrane (A)

allows only anions to pass through it.The process of decreasing the concentration of salt in

saline water using ion selective membranes under the influence of an applied field is called

electrodialysis.

An electrodialysis cell consists of series of alternative cation and anion membranes

( C & A)as shown in figure. The anode is placed near the anion permeable membrane. The

cathode is placed near cation permeable membrane.A suitable emf is applied and it depends

upon concentration of salt in water.Under the influence of the applied emf, Na+ions move

through the cation permeable membrane (C) and anion Cl

ions move through anion

permeable membrane. The net result is decrease in concentration of salt content in CA

compartment and increase in salt concentration in AC compartment .The fresh water formed

at CAcompartment is collected. The concentrated salt solution (brine) is discharged through

outlet.


14/14

14

U.R.BAGWANM.Sc

+ C A C A C A

Na+

Na+ Na

+

Cl

Cl

Cl

water tech be notes

Documents