water treatment - "let's water passes ,not the trashes"

8/13/2019 Water Treatment - "Let's Water Passes ,Not the trashes"

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Water Treatment Plant - Let's WaterPasses,Not Trash

The various source of Water are :

1. Above the surface : Rain2. Surface water Reservoirs, rivers, streams,ponds, tanks and lakes.3. Ground Water Shallow wells, deep wells and springs.

Types Of Impurities are:

Constituent Effect Of Impurities Way of TreatmentTurbidity Deposits in water lines, process

equipment,Coagulation, settling,and filtration

Hardness Calcium and magnesiumsalts, expressed as CaCO3Scaling in heat exchangeequipment, boilers,pipe lines Demineralization;internal boiler watertreatment

AlkalinityBicarbonate (HCO3

-),

carbonate (CO32-

), and

hydroxide(OH-),

expressed as CaCO3

Carryover of solids with

steam; embrittlementof boiler

steel, a source of corrosion in

condensate linesAcid treatment
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Carbon

Dioxide CO2corrosion in water lines,particularly steam and

condensate lines

aeration, deaeration,neutralization with

alkalies

PHHydrogen ion

concentration defined as:pH = log

1

[H ]

pH varies according to acidic

or alkaline solids in water;most natural waters have a pH

of 6.0-8.0

pH can be increased by

alkalies and decreased

by acids

Chloride Cl - Increases corrosive character owater

Demineralization,

Reverse osmosis,electrodialysis,

evaporation

Sodium Na+ Causes corrosion in boilersunder certain conditions

demineralization, reverse

osmosis, electrodialysis,

evaporation

Silica SiO2scaling in boilers and cooling

water systems, turbine blade

deposits

Absorption by highlybasic anion exchange

resins reverse osmosis

Iron Fe2+(ferrous)Fe3+(ferric) Source of deposits in water

lines, boilers.

aeration; coagulation and

filtration

cation exchange

Oxygen O2corrosion of water lines, heatexchange equipment, boilers,

return lines,Deaeration , corrosion

inhibitorsDissolved

Solids noneSuspended

Solids noneDeposits in heat exchange

equipment, boilers, water lines,

etc.Filtration preceded by

coagulation and settling

Types Of Water Treatment Plant :

1. Pre Treatment Plant2. Post Treatment Plant

Pre Treatment PlantThe Various Steps Involved are: Screening Raw Water Storage


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Aeration Chlorination Coagulation & Flocculation Sedimentation Clarification FiltarationScreening Water from the river is first allowed to pass through Coarse Screen (D = 25mm). The steel bar forming the screen are normally quite substantial (about 25 mm dia.) and are spacedabout 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 aregenerally 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 atthe same time not long enough to encourage other organism to develop.

Chlorination

Chlorine is injected into the raw water soon after it enters the plant. There is a Chlorine dosage of 2.5 mg/l while feeding raw water to plant. 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.

Chlorine hydrolyses in water to form hypochlorous acid and the hypochlorite ion.Cl2 + H2O HOCl + HCl

This reaction occurs readily at pH >3.0Further,

HOCl H++ OCl

-

This reaction is favoured by high pH.

The prime oxidising agent is HOCl and to a much lower degree OCl-

ion. Hence as pH increases, the

effectiveness of chlorine decreases.
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Chlorine Demandis the difference between the amount of Chlorine applied to a water and the free residual

chlorine that remains after a certain contact time.

Aeration

It is a cheap and valuable means of controlling taste, odor and corrosion. Aeration is the process by which compressed air is forced through a liquid or substance, normallyfrom the bottom of a tank, in order to reach a certain level of dissolved oxygen concentration.

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

Coagulation Chemical such asalumis added which produces positive charges to neutralize the negative charges onthe particles. Then the particles can stick together, forming larger particles which are more easily removed.

Coagulation is the process of destabilisation by charge neutralisation. Finely divided particles suspended in surface water repel each other because their surfaces arepredominantly negatively charged. Once neutralised, particles no longer repel each other and can be
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brought together. This is achieved by adding inorganic salts of aluminium or iron which neutralise the

charge on the turbidity particle.

e.g. Alum, Al2(SO4)3+ 6 H2O= Coagulation

Flocculation

This is the process whereby smaller particles (inorganic and organic), water-stable soilaggregates, or flocsaggregate to form larger particles (flocs) in a flowing medium. The formationof flocs is a complicated process that is driven by a combination of mechanisms, physical (e.g.,turbulence), chemical (e.g., ionic concentration), and biological (bacterial populations and

extracellular polymeric material).

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

Al(pH 6-7) and Iron Hydroxide (pH 5-6) acting as good floc. Coagulation and flocculation are aided and enhanced by Coagulant / Flocculant aids likepolyelectrolytes.These are long chain polymer molecules with higher charge density. They help

form largerfloc which settles faster. Unlike inorganic coagulants they do not decrease the pH of thetreated water nor increase the TDS.

Al2(SO4)3+ 6 H2O Al(OH)3 + 3 H2SO4Sedimentation

This refers to the physical removal from suspension or settling that occurs once the particlesare coagulated and flocculated.

Clarification

The Three processes happening in a clarification equipment are :- Rapid mix for coagulation

- Moderate mixing for flocculation and

- Floc and water separation.The most common design is the upflow clarifier where the water flows up while the floc settles.

The two types are :- Sludge blanket type and- Solids contact type

Both utilise part of the settled floc to enhance sedimentation.

FiltarationGravity and pressure filters

Multimedia filters Gravel as support media and sand / anthracite as filter media. Underdrain System For the support of gravel bed. Assures the uniform collection of filteredwater and the uniform distribution of the backwash water.

Control devices The Rate of Flow (ROF) and the Loss of Head (LOH) meters ensures the efficientoperation of the filters.

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

Ion Exchange Resin - ION Exchange resins are Synthetic polymers.


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

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

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

Cation Exchange Resin - Cation exchange resins are nothing but acid and can be representedas: RH+,

where R is resin matrix completely insoluble in water and only H+is mobile in water.

Types of Cation Exchange Resin

Strong acid cation exchange resins (SAC) : When the functional group attached to resin matrix is strong acid group.SAC can split allsalts 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 resins (WAC) : When the attached functional group is of weak acid, it is called WAC resin. WAC can onlysplit weak electrolyte (carbonate and bicarbonate) only.

It performs better with high pH water and with lower pH water its performancedecreases.

When pH < 4, actually regeneration takes place.

Degassifier After the cation exchanger the effluent is acid and all the bicarbonate present in water isconverted to CO2. This CO2can be removed in Degasser very cheaply.
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Anion Exchange Resin - Anion exchange resins can be simply represented as R+OHandis

nothing but an alkali / base. OHis only mobile in water.

Types of Anion Exchange Resin

Strong Base Anion exchange resins (SBA) : Functional group is strong base, it's performance is not influenced by water pH & itexchanges with both strong & weak acid.

Weak Base Anion exchange resins (WBA) : Functional group is weak base , it Perform better at low pH. As pH > 11 regeneration takes place.

If water after pre treatment passes through a cation exchanger, all the cations are exchangedwith H+of cation exchange resins.pH drops around 3.5 and water becomes soft.

The above water is then passed through anion exchanger, all anions are exchanged with OHof SBA resins and equivalent amount of water is produced. 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. For cation: Ca++> Mg++> Na+> H+ For anions: HSO4> NO3> Cl> SiO32

Sodium Slip When water containing Ca, Mg, Na ions is passed through cation exchanger bed, Ca ions areretained in the 1stlayer 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 withsodium salt ( say NaCl)

Effluent coming out from cation exchanger is not 100% acid but contain a little amount ofSodium salt.Increased bed depth reduces this amount of slip but never be reduced to zero.

Mixed Bed Units
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The water passing through cation and then through anion exchanger, it is passed throughmixed bed unit , resulting final effluent of very good quality water.

Regeneration of Cation Exchanger: Regeneration of the cation exchanger is done when the sodium leakage increases to certainpre set value. The following steps involved in regeneration :

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

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

Regeneration of Anion Exchanger:\

The following steps involved in regeneration :1. Backwash This 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.

2.

4% caustic at 4-8 Ibs/cubic ft. resin is injected for a contact time of about 1hr.and regenerant temperature is (50C).

Regeneration of Mixed Bed : MB is normally regenerated when the effluent conductivity is more than present value ofsilica is more than 0.02ppm.

The following steps involved in regeneration :1. Air Scrubbing Water is drained to top of the resin bed and air scrubbing is done for10 min.

2. 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 cationresin.

Mixed Bed Outlet Water Quality

Conductivity: 0.2

0.3 micro S/cmpH : 6.8 7.2SiO2 :

water treatment - "let's water passes ,not the trashes"

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