SOURCES OF WATER

Sources of water

Surface water Rain water Underground water Sea water

Flowing water

Still water

Lakes Reservoirs Ponds

Springs Tube wells Wells

In India most of the power plants use River Water.

Hardness of Water

‘Prevents the lathering of soap’

Reasons: Presence of certain salts of Ca, Mg and other heavy metal ions like Al3+, Fe3+ and Mn2+ dissolved in water.

2C17H35COONa + CaCl2 (C17H35COO)2Ca + 2NaCl

2C17H35COONa + MgSO4 (C17H35COO)2Mg + Na2SO4

Types of Hardness:

Temporary Hardness:

Salts responsible: Ca(HCO3)2, Mg(HCO3)2 and carbonate of Iron.

Largely removed by boiling of water.

Ca(HCO3)2

∆

CaCO3 + H2O + CO2

Mg(HCO3)2

∆

Mg(OH)2 + 2CO2

Permanent Hardness:

Salts responsible: CaCl2, MgCl2, CaSO4, MgSO4, FeSO4, Al2(SO4)3

BOILER FEED WATER

A boiler – feed water should correspond with the following composition:

(i) Its hardness should be below 0.2 ppm

(ii) Its caustic alkalinity (due to OH–) should lie in between 0.15 and 0.45 ppm.

(iii) Its soda alkalinity (due to Na2CO3) should be 0.45 – 1 ppm.

Excess of impurities, in boiler feed water cause following problems.

1. Scale & sludge 2. Priming 3. Foaming

4. Carryover 5. Boiler Corrosion 6. Caustic embrittlerment

WATER TREATMENT PLANT

The impurities present in river water can be broadly classified into following groups :

● Floating solids

● Suspended solids

● Colloids

● Dissolved solids

● Dissolved liquids, gases etc.

For thermal power station the most objectionable impurities are dissolved solids

PRE-TREATMENT PLANT

Depending upon the impurities present in source water pre-treatment plants are designed.

Normally following processes are carried out in pretreatment plant

■ Screening

■ Storage of raw water

■ Aeration

■ Chlorination

■ Coagulation and flocculation

■ Filtration

Raw water to Clarified water

4/11/23 PMI Revision 00 7

Raw water bayRW make up pumps

Cl2 dosing

Lime dosing

Alum dosingFlash mixer

Clarified water

Clariflocculator

SCREENING

Coarse screens are provided at river intake to prevent floating material of fairly large size entering the works.

The steel bar forming the screen are normally quite substantial (about 25 mm dia.) and are spaced about 100 mm apart.

The velocity of water through the screen openings should not exceed 0.5 m/s

Fine screens are also normally fitted immediately after the coarse screens, the orifices of which are generally of the order of 6 mm.

RAW WATER STORAGE

The storage provided is for 7- 15 days of the average water demand.

This is sufficient of to reduce pathogenic bacteria, suspended solids and river algae, while at the same time not long enough to encourage other organism to develop.

PRE CHLORINATION

Chlorine is injected into the raw water soon after it enters the plant.

Dosing rate : 2- 5 mg/L

This destroys the high bacterial count, but also oxidises and precipitates iron and manganise, kills algae, reduce color and slime formation are generally assists settlement.

Chlorination

AERATION

It is a cheap and valuable means of controlling taste, odor and corrosion

Gases are absorbed or liberated from water until equilibrium is reached between the natural content of each gas in the atmosphere and its content in the water.

* If water is lacking in dissolved oxygen, it will pickup oxygen from air.

* Looses excess CO2 or H2S

* Iron and manganese in solution are oxidized and precipitated out

* Certain volatile substance is liberated by algae growths or decomposition of organic matter, can be released from water

Aeration

COAGULATION

Coagulation is a process by which small particles in suspension join together to form large agglomerate.

Fine suspended particles and colloids present in water carry charges on their surface and most of the common materials assume negative charge.

Obvious way of promoting coagulation is to neutralize or reverse the electrical repulsion effect. In that case, the particles will touch each other or pulled together and agglomerate. This can be done by two ways.

(i) By adding traces of surface- active materials called coagulation aids.

(ii) By introducing fresh particles with positive charge.

COAGULATION AIDS

Long chain molecule carrying electrically active groups all along the length of the chain.

For- example: long chain poly - acryl amides, which carries positive charge.

Most of the very effective aids (poly- acryl amides) are toxic and can not be used in potable water.

A common nontoxic aid is activated silica.

It is prepared on the site because it is unstable & requirement may be more than ten times than that of poly acryl amides.

FLOCCULATION

Ferric and aluminium salts when added in water forms insoluble hydroxides, which come out as floppy flocs, whose surface carry positive charge. This positive charge flocs attaches itself with the negative charge particles in water.

More effective; because

■ Large electropositive surface area

■ Greatly increases particle concentration in water, which increases, the probability of collision, proper pH, good mixing time and presence of nuclei on which precipitate can form, favors good floc formation.

Aluminium and iron hydroxide gives good flocs normally in slight acidic pH

Iron : 5 – 6 pH

Aluminium: 6 – 7 pH

When alum (Aluminium sulphate) is added in water aluminium hydroxide floc and sulphuric acid is formed.

Al2(SO4)3 + 6 H2O → Al(OH)3 ↓ + 3 H2SO4

If turbidity is high alum dosing is also high

Here pH may fall below the coagulation pH

Lime [Ca(OH)2] is generally used to increase pH and it is also have coagulation property.

Coagulation and flocculation

CLARIFIERS

Clarifiers are structure/system in which water is cleared from most of the suspended solids with or without addition of coagulant.

There are different types of clarifiers:

● Conventional Type Clarifiers.

● Solid Contact Unit Clarifier

(i) Slurry re-circulation clarifier

(ii) Sludge blanket clarifier

● Hopper Bottom Clarifier

Latest concept. The tank is usually square in plane with upper portion having vertical sides and the lower portion in the form of an inverted pyramid with sides at an angle of 60°to the horizontal size of the tank is governed by the turbidity and volume of the feed water.

CONTROL OF FLOCCULATION

A variable velocity valve adjustable from the top of the tank is fitted with inlet pipe to take care exceptional changes of water characteristics or wide variation of flow.

Inlet pipe has a fixed diameter outlet to give required discharge velocity.

The effect of increases flow through the tank is the expansion of the sludge blanket until the equilibrium is re-established. The surface of the sludge blanket will rise but carry over will not occur until the increase in flow is excessive.

COLLECTION OF CLARIFIED WATER

The water coming out of the sludge blanket passes up through straight position of the tank and is drawn off by a series of collecting troughs.

They have notched side and are provided with adjustable support to enable them to be set for uniform draw off. This eliminates chances of short- circuiting.

SLUDGE DISCHARGE

It is necessary to bleed off a small flow of sludge through concentrating pocket. It may be constructed in the concrete or it may be suspended from the top of the tank.

The main discharge pipe is periodically used to remove heavier sludge from the bottom of the tank.

Clarified water to Filtered water

PMI Revision 00 22

Clarified water

Clari-flocculator

DM PLANT

Filtered water

FW sump

FW pumps

Sand filters

Cl2Post-chlorination

Cl2 House

DW sump

DW pumps

To Township for drinking water

DM water for Plant purpose

SPECIFICATION OF CLARIFIED WATER

The clarified water should be seen by naked eye without any turbidity.

Almost 98% turbidity should be removed.

FILTRATION

Filtration means taking suspended solids out of fluids

There are some problems :

(i) Suspended matter and water is relatively viscous. This means that very small holes must be used and any reasonable rate of flow, through them will raise a high- pressure loss.

(ii) As the filtration process is in progress, the suspended matter slowly blinds the pores of the Screen resulting greater pressure loss and slowly stoppage of the process. Deposition of suspended matter on the pores of the screen is known as formation of filter cake.

There are two kinds of simple screen filter

(a) True screens, such as meshes, sieves, sinters, felts, fibrewound candles or cloth held in some device which forces water to flow through.

(b) Disposable screens which are created a fresh for every filter cycle. This class includes paper filters and cartridge filter which are used at place where there is used little suspended matter to remove, else filter replacement becomes too costly.

MICRO STRAINERS

Cylindrical drum covered with a very fine wire mesh.

Raw water flows into the middle of the drum and out through the mesh by gravity. The drum rotates continuously and bring the mesh under a high pressure jet.

DEEP BED FILTER

This is a vessel in which filter bed is created from a fine granular medium such as sand etc. through which water is made to flow.

The suspended matter gets caught in the discontinuities which occur within such a bed. When the bed accumulates lot of solids that is practicable, the flow of water practically stops and the bed is back washed for reviving the filtering capacity.

The deep bed filter differ from another filters:

(a) They are capable of filtering very fine solids including colloids.

(b) Fine sand gives finer filtration because the size of the channel is decreased and a more area of filtration is achieved.

(c) Particles tend to accumulate at the same place, promotes coagulation.

(d) Depending the bed increases the availability of the filter medium and improves the quality of filtrate.

(e) At the end of a filter cycle, the rising pressure loss can force accumulated dirt right through the filter.

RAPID SAND FILTER

It consists of a bed of grinded sand typically +30 mesh to 15 mesh about 0.6 meter in depth. This lies in a bed of grinded gravel in several layers.

The gravel sizes vary from 2.5 mm to 25 mm. This filter operates between 5 – 10 m3/m2/hr. and capable of removing particles up to 10 micron size from water containing 100 ppm of suspended load.

ACTIVATED CARBON FILTER: (AC FILTER)

Granular carbon is used as a filter.

The water, as it progressively looses its pollutants encounters zones of activated carbon which are less and less saturated and therefore more and more active.

LIMITATIONS

(a) Filtration:

This must often be reduced to minimum in order to avoid clogging of the bed. Carbon tend to exert absorbable products from the flow with which it is contact, causing premature saturation.

(b) Biological media:

The surface of carbon offers ideal conditions bacterial growth. This phenomenon does assist purification but can also be very dangerous if not properly controlled.

Anaerobic fermentation giving off odor, clogging of bed etc.

(c) Catalytic action:

The main function of activated carbon is oxidation of water by free chlorine.

Cl2 + H2O → 2HCl + ½ O2

The pH has considerable role to play. The de-chlorination properly is affected by any factor that interfaces between carbon and chlorine such as filterable impurities like flock, algae etc.

Thus a clean water perform better for activated carbon filter.

Absorption:

This is the principle role of the activated carbon. A grate care is taken to keep the bed active and unaffected.

MEMBRANE FILTERS

Organic membranes have very fine pores.

‘Ultra filtration’ membranes have pores small enough to remove most kind of non reactive silica with a pressure across the membrane less than 5 bars.

● Installed after DM plant to absorb colloidal silica.

● Reverse osmosis plant followed by DM plant can produce water of almost ultimate purity and is very economical in handling brackish salts or more than 500 ppm.

D.M. PLANT

Dissolved solids present in water are removed in DM plant by ION exchange process and for this ION exchange resins are used.

ION EXCHANGE RESINS

ION Exchange resins are Synthetic polymers

Most commonly used resins are gel type polystyrene resin, manufactured by polymerization of Vinyl Benzene (Styrene) and 8- 10% Divinyl Benzene.

DM Plant

Ion exchange resin are manufactured in bead form (0.3 mm or 1.2 mm size) optimum size 0.6 mm. Smaller size would restrict the flow and higher size, retention time would not be sufficient for proper exchange.

Resin Beads are insoluble in water. Functional groups are then attached to each of the benzene rings to make it chemically active.

Depending upon the functional group attached to resin Matrix, it behaves as cation exchange resin or anion exchange resin.

CATION EXCHANGE RESIN

Cation exchange resins are nothing but acid and can be represented as: R– H+, where R is resin matrix completely insoluble in water and only H+ is mobile in water.

Cation exchange resins are of two types.

Strong acid cation exchange resins (SAC) :

When the functional group attached to resin matrix is strong acid group.

SAC can split all salts and its performance is not influenced by pH of water.

Operational exchange capacity and regeneration efficiency of SAC is less than WAC.

Weak Acid Cation Exchange Resin

When the attached functional group is of weak acid, it is called WAC resin.

WAC can only split weak electrolyte (carbonate and bicarbonate) only.

It performs better with high pH water and with lower pH water its performance decreases.

When pH < 4, actually regeneration takes place.

Na2CO3

NaCl WAC H2CO3

NaClSAC H2CO3

HCl

Na2CO3

NaClH2CO3SAC

HCl

ANION EXCHANGE RESINS

Anion exchange resins can be simply represented as R+– OH– and is nothing but an alkali / base. OH– is only mobile in water.

Anion Exchange Resins are of two types:

Strong Base Anion Resins (SBA):

Functional group is strong base; performance is not influenced by water pH & it exchanges with both strong & weak acid.

Weak Base Anion Resins (SBA):

●Functional group is weak base ●Perform better at low pH

●pH > 11 regeneration takes place

Operational capacity and regeneration efficiency of WBA is higher than SBA

HCl

H2CO3

WBAH2O

H2CO3

SBAH2OH2O

Cation effluentH2SiO3

HClSBA

H2OH2O

Principle of Deionisation:

In normal river water most common salts are present:

Ca++

Mg++

Na+

&HCO3

– , CO32–

SO42–

Cl–

SiO32–

Ions

If this water passes through a cation exchanger, all the cations are exchanged with H+ of cation exchange resins.

R––H+ + CaCl2 R–– Ca++ + HCl

R––H+ + CaCO3 R–– Ca++ + H2CO3

pH drops around 3.5 and water becomes soft.

The above water is then passed through anion exchanger, all anions are exchanged with OH– of SBA resins and equivalent amount of water is produced.

H+–Cl– + R+–OH– R– Cl + H+OH– (H2O)

H2+CO3

2– + R+–OH– R– CO3 + H+OH– (H2O)

All the acids are converted to H2O

Selectivity of Ions

Resins have a preferences for exchange and it depends on charge and size of the ion.

Triple charge is preferred to double and double is preferred to single charge.

Charge being same preference is given to bigger size ions.

Thus for cation: Ca++ > Mg++ > Na+ > H+

Similarly for anions: HSO4– > NO3

– > Cl– > SiO32–

Sodium Slip

When water containing Ca, Mg, Na ions is passed through cation exchanger bed, Ca ions are retained in the 1st layer then Mg and in the last layer Na ions are retained. Ion exchange reactions are all reversible.

The reaction in the bottom part of the bed is with sodium salt ( say NaCl)

R–H + R–Na + HCl

Now even at very low concentration of R–Na some back reaction produces NaCl.

R–Na + HCl R–H +

NaCl

NaCl

Effluent coming out from cation exchanger is not 100% acid but contain a little amount of Sodium salt.

Increased bed depth reduces this amount of slip but never be reduced to zero.

The cation effluent containing some sodium when passes through anion exchanger, acids are converted to water but sodium salts are converted to NaOH.

R–OH + R–Cl + NaOHNaCl

R–OH + R–Cl + H2OHCl

Effluent coming out of anion bed contains NaOH that increases the pH and conductivity of the anion effluent.

Similarly silica-ship takes place.

Mixed Bed Units

After passing water through cation and then through anion exchanger, it is passed through mixed bed unit – resulting final effluent of very good quality water.

SAC SBA MB

Further, H2SO3 produced in SAC can be easily removed at low running cost in a Degasifier.

SAC Degasser SBA MB

D.M. PLANT

From filter water, chlorine is removed before allowing it to enter ion exchanger. It may be done by:

(a) Passing through activated carbon filter which absorbs chlorine.

(b) Dosing calculated amount of sodium sulphate, which reduces chlorine to chloride ion.

Depending upon the amount of water to be treated and quality of the filter water, different types of demineralization schemes are made.

(1) Simplest Arrangement:

Cation Unit Degasser Anion Unit M.B.Unit(SAC) (SBA)

(2) Where water requirement is more:

Cation Unit WBA Degasser SBA MB

(3) Where water contains more carbonate/ bicarbonate and requirement is more.

WAC SAC WBA Degasser SBA MB

Regeneration of Cation Exchanger:

Regeneration of the cation exchanger is done when the sodium leakage increases to certain pre set value.

The following steps are observed in regeneration.

(i) Backwash:

Backwash is done by up flow of water with air scouring. Purpose of backwash is to loose the bed and remove accumulated suspended solids, dirt, resin fines, fragments etc.

(ii) Acid Injection:

After backwash acid injection is required at specified concentration. Contact time is normally 30 min. Two types of acids are used (a) sulphuric acid (b) hydrochloric acid

After acid injection the bed is rinsed with water, first at slower rate and then at higher rate. Volume used is 30-40 gall. Cubic ft.

Weak Acid Cation Exchanger

For regeneration of weak acid cation exchanger acid strength may be 0.8 to 1%.

If WAC exists in the stream then normally acid coming out of SAC unit as the time of its regeneration is passed through the WAC to regenerate it. This is therefore called regeneration.

Anion Exchanger Resin’s Regeneration

The following steps are observed for regeneration of Anion Exchange Units:

(i) Backwash: Backwashing is done to loose the bed and remove resin fines and fragments. Normal backwash rate is

2-4 g pm/sq.ft. for 10 minutes.

(ii) Injection of Caustic: 4% caustic at 4-8 Ibs/cubic ft. resin is injested for a contact time of about 1 hr.

For better removal of silica particularly in WBA/SBA combination higher

regenerant level more contact time and higher temperature of regenerant (50°C) may be needed.

(iii) Rinse: Slow rinse at 1 g pm/sq.ft, then fast rinse at 5 to 8 g pm/sq.ft. for 1 to 1½ hour may be required to bring down the

conductivity and silica to acceptable level. Volume used is 40-100 gal/cubic ft.

WBA:

End point of WBA is detected by increase in the conductivity and lowering of pH.

The Regeneration of WBA need about 1%NaOH. Therefore regeneration is done for WBA where SBA outlet caustic is injected to WBA with some modification to avoid silica precipitation in the WBA unit.

Regeneration of Mixed Bed

MB is normally regenerated when the effluent conductivity is more than present value of silica is more than 0.02ppm.

The following steps are observed at the time of regeneration.

(i) Air Scrubbing: Water is drained to top of the resin bed and air scrubbing is done for 10 min.

(ii) Back Washing: Unit is filled with water and back washing is done at 4-9 g pm/sq. ft.; then the resin is allowed to settle for 10 minutes which separates the anion and cation resin.

(iii) Regeneration of Anion Resin Bed: Regeneration of anion resin bed is done with 4% caustic at 12 lbs/cubic ft resins.

(iv) Rinse of the Anion Resin Bed: For about an hour until effluent conductivity is below 10ms/cm.

(v) Regeneration of Cation Resin Bed: Regeneration of cation resin bed is done with 4% acid

at a regeneration level 8-12lbs/cubic ft.

(vi) Rinse of Cation Resin Bed: Till conductivity less than 10ms/cm (20 minutes)

(vii) Mixing of the Resins: The water is drained down to the surface of the resin bed and mixing is

done by air blowing for about 5 minutes then is allowed to settle.

(viii) Final Rinse: After mixing the unit is refilled with water and put to final rinse till the effluent comes to the

acceptable limit.

Mixed Bed Outlet Water Quality

Conductivity: 0.2 – 0.3 micro S/cm

pH : 6.8 – 7.2

SiO2 : < 0.02 ppm

DEGASSIFIER

After the cation exchanger the effluent is acid and all the bicarbonate present in water is converted to CO2. This CO2 can be removed in Degasser very cheaply.

DM Water Storage

DALTON’S LAW:

The total pressure exerted by a mixture of several gases is equal to the sum of the partial pressures of individual gas.

According to Charle’s law the partial pressure of each gas is determined by the amount of that gas in the mixture.

HENRY’S LAW:The solubility of the gas in water is directly proportional to the partial pressure of that gas in contact with water.

X = P/H

X = amount of gas dissolved in water.

P = partial pressure of the gas in contact with water

H = constant at that temperature

Solubility of the gases decreases as the temperature of the solvent is increased. When water reaches its saturation temperature all the uncombined gases are theoretically insoluble in it and may be removed.

Solubility of a gas may be decreased to effect more complete from water in several ways

(i) By lowering the partial pressure by inserting another gas in contact with water.

(ii) By decreasing the pressure.

(iii) By lowering the partial pressure by heating the water to boiling point corresponding to the pressure of the steam introduced.

In case of removing CO2 from cation effluent principal is normally adopted.

X = P/H P = Partial pressure of CO2 in atmosphere

X = concentration of CO2 dissolved in water.

COUNTER FLOW REGENERATION:

In the conventional ion exchanger during service run, water flows from top to bottom through the resin bed and the regeneration is done in the same direction.

Since like operation, Regeneration is not 100% the bottom part of the bed contains more ions. During service, when the final treated water leaves the bed it passes through less regenerated portion of the bed and thus picks up more ions.

If regenerant is injected from bottom to top then the bottom portion of the bed will be better regenerated.

During service when treated under leaves the bed it passes through the better regenerated bottom position and ion pick up is less. This gives a better quality of water and Regeneration level is low.

The requirements of CFR are as below:

(1) The exchanger should have normal arrangement for down flow service.

(2) Back washing is required time to time to regrade the bed and remove dirt & fines. For this 50 – 100% free expansion space is to be provided.

(3) Resin must be free to shrink and swell during the cycle.

(4) Arrangement is to be made to hold the bed packed and immobile during up flow regeneration.