Sunday, February 3, 2013

BASIC PRINCIPLES OF WATER TREATMENT




WATER & ITS IMPURITIES :

Water is never found in a pure state in nature because it is an extemely good solvent.  As it falls through the atmosphere in the form of rain, it dissolves gases, in particular oxygen, carbon dioxide, sulphur dioxide and other acid gases.  This means that when the rain reaches the ground, it is in fact a mixture of dilute acids.

At ground level the acids can have a dissolving action on the rocks, thus taking mineral matter into solution.  The amount and type of minerals dissolved depends upon the variety of the rock encountered.  In limestone or chalk areas the principal minerals are calcium and magnesium carbonates.  These are acted upon by the acids in the rain to produce principally calcium and magnesium bicarbonates and calcium and magnesium sulphates.  Lesser amounts of the chlorides and nitrates are also found.  Other minerals taken into solution include iron, aluminium, sodium and become alkaline.  In areas where the principal rock type is granite, the solvent action of the rain is much less.  The acids in the rain are only partially neutralised and thus the water remains on the acid side of neutrality.  The amount of mineral matter is then usually small.  Silica levels, however, can be similar to those found in waters from limestone areas.

Thus essentially the raw water essentially contains salts consisting of following ions :

            CATIONS                               ANIONS

            Calcium                                   Sulphates
            Magnesium                             Carbonates
            Sodium                                    Bicarbonates
            Iron                                          Chlorides
            Potassium                               Nitrates                                               
           Barium                                     Flourides
            Strontium                                Phosphates
            Manganese                             Carbondioxide
            Ammonia                                 Silica





Depending on the various application and use the water has to be essentially treated :

Various treatment process involve :

1)         Filtration
2)         Activated Carbon Filtration
3)         Chlorination
4)         Softening
5)         De-mineralisation
6)         Reverse Osmosis
7)         Ultraviolet Disinfection

FILTRATION

Water filtration essentially  consists of :
1)    Coagulation
2)    Clarification
3)    Filtration

Coagulation :

The suspended solids in water acquire a negative charge which prohibits these particles from coming together to form a large mass for easier settleability.  Coagulants such as Alum, Ferrous Sulphate, Ferric Chloride which have positive charged ions like Aluminum, Iron, etc. are then dosed into the water to hasten the settling of the suspended particles.  Suitable dosing systems  are employed for this purpose.  Coagulated particles are known as “Flocs”.

Clarification :

The purpose of clarification is to bring the flocs together to form a larger mass.  The larger the mass of flocs, the heavier the particles and easier settleability.

Operation :

Raw water is pumped to a chamber outside the flash-mixer tank.  Coagulant (Alum) is dosed into this chamber through a dosing pump.  Water then enters the flash mixing chamber.  Mixing of coagulant with water takes place with the help of motorised mixer and then it enters the flocculation chamber.  Slow speed agitator mounted on this chamber rotates at gentle speed, which coalesces the floc.  The overflow of flocculator tank passes into the clarifier chamber. Clear water moves up and is collected in the outlet launder.


PRESSURE QUARTZ FILTER :

The Pressure Quartz Filter is a rapid flow filter using very Fine Silex Quartz as Filter Media.  It is ideal for filtration of water having very fine suspended matters like mud, rust, particles, dirt, etc.

A Filter is a bed of granular material which physically removes suspended matter from the water passing through it. The only change in water quality resulting from filtration is the reduction of suspended solids. Raw Water flows downwards through the filter bed and the turbidity and suspended matter is retained on the quartz surface.  Filtered water is evenly collected by an under drain system in the bottom of the vessel and flows through the outlet to service.

When the pressure drop increases to a given level, the filter is clogged and requires cleaning by backwashing.The fine suspended impurities are removed from the filter bed by backwash at very high velocity..

CHLORINATION :

The use of chlorine is the oldest and most common disinfection method for private water supplies.  Chlorine is inexpensive and readily available, reliable, easy to use and monitor, and effective against most pathogenic bacteria, virus and cyst organisms.  It also kills non-pathogenic iron, manganese and sulfur bacteria.

Chlorine is also a strong oxidizing agent which causes a problem mineral such as soluble iron and manganese to change to an insoluble precipitate so it can be filtered from the water.

Chlorination may be done in many ways.  Chlorine may beused continously in the dry or liquid form that is dropped or injected into the well water using a chemical feed pump.  For periodic or shock water treatment, chlorine can also be poured in or fed in solution using a hose.

ACTIVATED CABON FILTRATION :

The  Activated Carbon Filter is used for effective removal of colour, odour and organic contamination in the water.  It also removes dissolved chlorine in the water.  It is a rapid flow filter using water treatment grade of activated carbon granules supported by very fine quartz filter media.  It is ideal for filteration of water from underground sources having either colour, odour or organic contamination which is not accepted for potable applications.


Regular backwashing is adequate to loosen up the bed and expose fresh surfaces of activated carbon granules to trap the dissolved impurities in the water.  When the carbon granules are exhausted, it is either replaced with new activated carbon granules or is thermally reactivated.

The water flows downwards through the carbon filter bed, the colour, odour and organic contamination is trapped by adsorption on the surface of activated carbon granules. Water is evenly collected by an under drain system in the bottom of the vessel and flows through the outlet to service.  It is highly recommended to feed filtered water to Activated Carbon filter to avoid fouling of activated carbon.

SOFTENER:

Hardness & Scale Formation :

Water containing susbtantial quantities of caclium and magnesium compounds is called hard water.

It is hardness which is largely responsible for scale and deposit formation in boilers and cooling water systems.  Hardness is often classified for convenience into temporary or alkaline hardness and permanent or non-alkaline hardness.  Temporary hardness arises from the presence of calcium and magnesium bicarbonates.  When water containing these substances is boiled, the soluble bicarbonates decompose to the insoluble carbonates which form scale.  This can be represented chemically as follows :

            Ca(HCO3)2                              CaCO3                               
                                                                                    +     CO2     +    H2O
            Mg(HCO3)2                              MgCO3

Softening offers a simple means of removing the undesirable calcium and magnesium scale forming salts from the water.  A diagramatic form of the plant is reproduced below :

The softening process may be simplified as indicated below :-

2NaR   +          CaSO4                                       Na2SO4           +          CaR2
2NaR   +          MgSO4                                        Na2SO4           +          MgR2

R = Resin


The calcium and magnesium cations have a strong affinity for the active sites incorporated into the resin structure and hence become associated to the resin at the expense of the sodium ions, which then pass freely into the flowing water at a rate in proportion to the removal rate of the calcium and magnesium cations.

When all the sodium sites on the resin have become occupied by hardness cations (a situation which in practice should not arise), the resin is termed “exhausted” and requires “regeneration”.  The regeneration cycle involves exposing the resin to a strong solution of brine whereby the sodium ions, due to the grossly excessive numbers, liberate the hardness cations off the resins and to waste.

Regeneration process :

CaR2                                                               CaCl2
            + 2 NaCl                       2 NaR   +
MgR2                                                               MgCl2

The base exchange softening process should be capable of producing a water quality containing 0 - 5 ppm of residual hardness.

DEMINERALISATION PLANT  :

The process of producing demineralised or deionised water by ion-exchange is basically a two stage process.  The raw water to be deionised passes through a two stage deioniser comprising of Cation Exchanger followed by an Anion Exchanger.

In the cation exchanger the water is passed through a column of cation exchange resin charged with mobile replaceable hydrogen ions.  The cations in the raw water essentially consisting of Ca, Mg and Na get adsorbed on the resin surface which in turn releases hydrogen ions in the water.  Hence, the water coming out of cation exchanger will contain acid salts such as Hydrochloric acid, Sulphuric acid, Carbonic acid.  The composition is as follows :          

                        CATIONS                                ANIONS
                                                                        Carbonates
                                                                        Bicarboantes
                        Hydrogen                                 Chlorides
                                                                        Sulphates
                                                                        Silica

Hence, the pH of Decationised water will be about 3 to 3.5.  This water will be fed to Anion exchanger, in which it will pass through a column of anion resins charged with mobile replaceable hydroxyl ions.  The anions in decationised water essentially consist of carbonates, bicarbonates, chlorides, sulphates and silica and get adsorbed on the anion resin surface, which in turn release hydroxylions in the water.  The composition of water after anion exchanger is as follows :

                        H + OH                         H2O (Pure Water)

The absence of cations and anions in the water at the outlet of anion exchanger will be indicated by low electrical conductivity of DM water. 

After the hydrogen charge on the cation resin gets exhausted, a solution of hydrochloric acid is passed through the cation resin bed.  This process will regenerate the cation resin and restore the hydrogen charge and the cation exchanger will be ready for the next cycle of operation.

Similarly after the hydroxyl charge on the anion resins gets exhausted, a solution of Sodium Hydroxide (Caustic Soda) is passed through the anion resin bed.  This process will regenerate the anion resins and restore the hydroxyl charge and the anion exchanger will be ready for next cycle of operation.

Normally, the cation and anion exchanger columns will allow a small slippage of both cations and anions through the respective beds. Hence, the conductivity of D. M. Water at the outlet of anion exchanger will be upto 10 ms/cm equivalent to approximately upto 10 ppm of dissolved solids in D. M. Water.

This D. M. Water is further polished by passing through a Mixed Bed Polisher Unit, in which the remaining cations and anions are removed to get D. M. Water confirming to IP Standards for deionised water.

The Mixed Bed Unit consists of a mixture of both cation and anion exchange resins which remove the traces of remaining cations and anions from the D. M. Water.

REVERSE OSMOSIS:

Osmosis is a natural process involving fluid flow across a semipermeable membrane barrier.  It is selective in the sense that the solvent passes through the membrane at a faster rate than the dissolve solids.  The difference of passage rate results in solvent solids sparation.  The direction of solvent flow is determined by its chemical potential which is a function of pressure, temperature and concentration of dissolved solids.

Pure water in contact with both sides of an ideal semipermeable membrane at equal pressure and temperature has no net flow across the membrane because the chemical potential is equal on both sides.  If a soluble salt is added on one side, the chemical potential of this salt solution is reduced.  Osmotic flow from the pure water side across the membrane to the salt solution side will occur until the equilibrium of chemical potential is restored (Figure 1a).  Equilibrium occurs when the hydrostatic pressure differential resulting from the volume changes on both sides is equal to the osmotic pressure.  This is a solution property independent of the membrane.

Application of an external pressure to the salt solution side equal to the osmotic pressure will also cause equilibrium.  Additional pressure will raise the chemical potential of the water in the salt solution and cause a solvent flow to the pure water side, because it now has a lower chemical potential.  This phenomenon is called reverse Osmosis (Figure 1b).

A semi-permeable membrane is selective in that certain component of a solution, usually the solvent, can pass through it, while others, usually the solute (dissolved solids) cannot.  Osmotic flow from the pure water side to salt solution side will occur across the membrane until equilibrium is reached, at which chemical potential on both sides of membrane is equal (fig 1a & 1b).

  
 

Applied Pressure
 
         Dilute      Concentrate                                         
         Solution    Solution     










 

            Osmosis                                                                        Reverse
                                                                                                Osmosis
          Fig. 1a                                                               Fig. 1b

Reverse Osmosis is a membrane separation process in which the solvent (water) molecules from a pressurised solution flow through appropriate semi-permeable membrane.  The mebrane acts as a barrier to the flow of solute (dissolved solids) molecules, thereby separating solvent from solute. RO process is generally used for desalination of water.  The permeate (the liquid flowing through the membrane or purified water) which generally emerges at near atmospheric pressure, is reduced in salt content, while the feed solution which is pressurised on the other side of the membrane increases in salt content.  The only other side of the membane increases in slat content.  The only energy input required is that for pressurising the feed.  Unlike thermal desalination, membrane desalination operates at ambient temperature and without phase change, and hence energy consumption for membrane desalination is lower than that for thermal desalination.  This process is at times also also referred as Hyperfiltration.

ULTRAVIOLET LIGHT DISINFECTION :

Ultraviolet light is a method of disinfecting private water systems.  Ultraviolet radiation adds nothing to the water and does not produce any taste or odour.  The UV light is produced by a low pressure mercury vapor lamp which produces a disinfecting dose rated in microwatt-seconds per square centimeter (Mws/cm²).  Values of 30,000 Mws/cm² will kill most types of pathogenic bacteria.  However, viruses are more resistant and variable and may need upto 45,000 Mws/cm².

An Ultraviolet water treatment device is quite simple.  The most common design consists of a stainless cylindrical chamber with a cylindrical mercury are lamp located in it.     Water enters one end of the chamber, flows through the chamber around the lamp and exits the other end within a few seconds.

To be effective as a disinfection treatment, ultraviolet radiation must pass through every particle of water.  The thinner the water film and the slower the water flow, the more effective the system will be.  Also, the water cannot have any turbidity, suspended soil particles, or organic matter.  As a consequence, ultraviolet light treatment should only be attempted on clear water.  A prefilter is recommended on ultraviolet systems as is periodic inspection and lamp cleaning.

DRINKING WATER - SPECIFICATION

AS PER IS 10500:91

Sr.No.
Particulars
Desirable Limit

Essential Characterstics

1
Colour, Pt-Co
5
2
Odour
Unobjectionable
3
Taste
Agreeable
4
Turbidity, NTU
5
5
pH Value
6.5 to 8.5
6
Total Hardness (as CaCO3) mg/l
300
7
Iron (as Fe), mg/l
0.3
8
Chlorides (as Cl), mg/l
250
9
Residual free chlorine
0.20




Desirable Characterstics

10
Dissolved solids, mg/l
500
11
Sulphates (as SO4), mg/l
200
12
Nitrate (as NO3), mg/l
45
13
Alkalinity, mg/l
200





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