Water treatmentLecture #0317-10-2011

By Kareem Bakhsh Ansari

Sequence for water conditioning at WTPi)Removal of suspended solids• Coagulation & flocculation(carried in settling tank)• Filteration(carried in sand filters)ii)Removal of dissolved solids• Demineralization(carried ion exchangers).iii) Removal of dissolved gasesa)Physical method• Dearteation(carried in deareator)b)Chemical method• Chemical scavenging(carried in boiler)

Water treatment plant

WTP flowchart visualization

Canal water :• Due to less dissolved impurities in it is preferred

over well water.Clarification of Canal Water:• Normally, turbidity is removed by adding a

coagulant prior to the sedimentation process.• Alum is added at moga pit and in second screen

pit for thorough and uniform mixing of coagulant with water.

• Canal water from a nearby canal flows through a moga and concrete pipes to a 73x40x17 feet settling tank(dimensions for DHCL Plant).

• Mud settles effectively in a two hours residence time settling tank. Settled mud is removed by special pumps.

Coagulation• Remove of colloidal impurities from water by

conglomeration of small colloidal particles into bigger particles having enough mass to settle by the action of gravity.

• process used to de stablize the colloidal systems.• Due to like charges , colloidal particles repel each

other and can not spontaneously conglomerates into larger particles.

• Coagulation is affected by doing of stimulants that neutralize the charge of the colloidal particles.

• These neutralized particles coagulates on collision with similar other neutralized particles.

• These coagulated particles grow in size and become too large to outweigh gravity by buoyant force exerted upon them by water.

• They settle on bottom of the settling tank as gelatinous and porous flakes.

CHEMICALS as coagulants/flocculants :• Alum• Aluminum sulphate.• Ferrous/ferric sulphate

• Ferric chloride• Polyelectrolytes :polyacrylamide are mostly used

as flocculants.• The process of coagulation is very sensitive to

pH,a)For Al-Coagulant it should be 5-7b)For Fe-Coagulant it should be 8-10It is necessary to maintain some degree ofalkalinity in the water being treated with coagulantbecause the (H ions) are liberated due to hydrolysisof metal ions that reduce the alkalinityof water.

FlocculationBridging of a large ,and more efficient ,agglomerate

via settling.• Suspended particles tend to have a surface charge,

the charge on the polymer is quite important in flocculation.

• If the fouling material has a negative charge as in the case with mud, silt and biological matter, a cationic polyelectrolyte is used to neutralize the charge resulting in the formation of a floc.

• This floc, which is of low density, will remain in solution and can be removed by blow down or filtration

• Flocculants will bridge smaller colloids together using charge and molecular weight.Polyacrylamides are used as flocculating agents.

How flocculants work????• STOKE’S LAW predicts that spherical particles

suspended in a fluid medium settle at a rate proportional to the fourth power of the radius of the particle.

• Thus large particles settle much faster than smaller ones

Filteration

• It is the process of clarification of water by passing it through a filter bed composed of a porous material that retain coarse suspended solids on its surface and in the pores.

• Raw water with turbidity less than approximately 30 NTU (Nephlometric Turbidity units) is pumped to raw water sand filters.

• Different numbers of filters are used depends upon the quantity of water to be filtered at the plant.

• Raw water, well or canal is stored in a STORAGE TANK after passing through these sand filters, turbidity is decreased upto 5 NTU.

• The driving force of filtration is the pressure drop across the filter bed.

• Pressure drop through the filtering bed depends upon the following factors;

1)Height of the filtering bed2)Rate of filtration.3)Particle size ( i.e. grain diameter of the filtering

material)4)Degree of contamination of the filter bed by

impurities trapped.• Each filter to have to filter certain GPMs of water.

• Each filter has filtering bed composed of charcoal, sand, granite and gravel.

When the filter bed backwashed???• Backwashing is reversal in flow of wash water.• When pressure drop across the filter increases or

turbidity at outlet increases the filtering bed is backwashed.

• Before backwash is surface washed to loosen the packed mud layer.

Removal of Dissolved Gasses• Dissolved gasses in water can cause corrosion in boiler

condensate and feed water system.• Dissolved impurities are removed by two methods i)Physical(deaeration) ii)Chemical(chemical scavenger)

Deareation• A deaerator is a device widely used to expel the dissolved

gasses, in particular oxygen , from the water, prior to its use in steam generating boilers.

• This also reduces the need for oxygen scavengers.• Deaeration of boiler feed water is usually accomplished by

steam heating.

• In particular, dissolved oxygen in boiler feed waters will cause serious corrosion damage in steam systems by attacking to the walls of metal piping and other metallic equipment and forming oxides (rust).

• Water also combines with any dissolved carbon dioxide to form carbonic acid that causes further corrosion.

• The typical horizontal tray-type deaerator has a vertical domed deaeration section mounted above a horizontal boiler feedwater storage vessel.

• Boiler feedwater enters the vertical deareation section above the perforated trays and flows downward through the perforations.

• Low-pressure deareation steam enters below the perforated trays and flows upward through the perforations.

• The steam strips the dissolved gas from the boiler feedwater and exits via the vent at the top of the domed section.

• The deaerated water flows down into the horizontal storage vessel from where it is pumped to the steam generating boiler system.

• Low-pressure heating steam, which enters the horizontal vessel through a sparger pipe in the bottom of the vessel, is provided to keep the stored boiler feedwater warm.

• External insulation of the vessel is typically provided to minimize heat loss.

• Close deareating heaters of such type operate at 105 C & GENERALLY lower the oxygen content to below 0.01 PPM.

Chemical Scavenging• Any of remaining traces of the oxygen can then be

chemically combined by using an oxygen scavenger such a;

Sodium sulfite or hydrazine hydrate.• Such complete deoxygenation is desireable to

minimise corrosion in the modern high temperature high pressure boilers

Removal of Dissolved solids• Several method are available for the removal

of unwanted dissolved salts in the boiler feed water.

selection of the method depends upon the nature of source and post-treatment conditions required.

Dissolved salts are generally removed from water by.

Water softening methods.Lime softeningCold lime-soda softeningHot-lime soda processSodium zeolite softeningdemineralization.Cation exchange resinsAnion Exchange Resins

Cold soda lime process

• Now a days ,organic polyelectrolytes are used instead of these tradititional inorganic coagulants,because the latter can result in carry over of aluminium and iron which in turn may cause problems in downstream equipments. Most objectionable feature of this process is large volume of sludge formed,disposal of which is troublesome & expensive

Hot lime-sada process• This process is carried out near the boiling point

of water.• The reaction involved are same as in cold lime-

soda process.• Partial removal of dissolved oxygen also take place.• High temperature : 80-150 C• Process completion time: 15 mint.• No coagulants used.

• Water with 15-30 PPM hardness is removed.• Substantial amount of silica is removed as

magnesium silicate. • Hardness is further reduced due to low

solubility of calcium carbonate and nagnesium hydroxide in hot water.

• Old technique ,Used for conditioning of BFW.• It operates at BP of water,so,reactions proceed

faster,coagulation & precipitation are facilitated

• STEPS • Analysis of raw water.• Heating of raw water with steam.• Mixing & proportioning of lime & soda ash in

conformance with the raw water analysis.• Pumping of lime slurry & soda.

• Reaction of lime and soda, facilitated by mixing with or with or without previous heating coagulation or release of the “super saturation “by variation method , such as slow agitation or contact with “seeds” by sludge recirculation

• Setting or removal of the precipitate with or without filtration pumping away of the softened water periodic washing away of the sludge from the cone tank bottom (and from the clarifying filters)

Demineralization• Demineralization means the removal of cations of cal.

& mag.of salts dissolved in water.Ion EXCCHANGE method• Raw water dissolved salts are removed by ion

exchange method using cationic and anionic resins.• Each train has one cation, one primary anion and one

secondary anion vessel.• Cation resin removes cationic part of salt and anion

resin removes anionic part .• Raw water enters cation bed, cations are removed by

chemical reaction of salt with resin• Ion exchange is a chemical reaction in which mobile

hydrated ions are exchanged,equivalent for equivalent,for ions of like charge in solution.

• The solid has open fishnet like structure,and mobile ions electrically neutralize charged,or potentially charged,groups attached to the solid matrix,called the ion exchanger.

• Ion exchange resins are synthetic polymers having exchangeable positive or negative ions depending upon the type of resin.

CLASSIFICATIONS OF ION EXCHANGE RESINS• Ionizable groups attached to the resin bead

determine the functional capability of the resin. Industrial water treatment resins are classified into four basic categories:

• Strong Acid Cation (SAC)• Weak Acid Cation (WAC)• Strong Base Anion (SBA)• Weak Base Anion (WBA)

Cation exchange resins• They have fixed negatively charged sites and

exchangeable positive ions are associated with these sites.

• These sites take positive ions from water and give up their ion to water.

Anion exchange resinsThey have fixed positive charged sites and exchangeable

negative ions are associated with these sites.These sites take negative ions from water and give up

their negative ion to water.

• From cation the effluent water with certain FMA and nil hardness leads to Degasser where dissolved CO2 is degassified.

• It is the removal of carbon dioxide present in water.

• Before anion exchangers mechanical decarbonation is applied to reduced the size of anion installment.

• Counter current flow of air and water is achieved through a packed bed with rasching ring packing.

Mixed bed

• Combination of strong acid cation and strong base anion resins are applied to produced high purity water for 1500 psig steam generation.

For 1500 psig steam, boiler feed water should haveDissolved solids 0.1 ppm maximumSilica 0.005 ppm maximumConductivity 1.0 micro-mhos maximum

Out put of mixed bed is our final outlet product

Ion exchange resin beads

Microscopic view of cellular resin beads (20-50 mesh) of a sulfonated styrene-divinylbenzene strong acid

cation exhcanger. (Courtesy of Rohm and Haas Company.)

Close

Ion Exchange resin• An ion-exchange resin or ion-exchange polymer

is an insoluble matrix (or support structure) normally in the form of small (1–2 mm diameter) beads, usually white or yellowish, fabricated from an organic polymer substrate. The material has highly developed structure of pores on the surface of which are sites with easily trapped and released ions.

• Most typical ion-exchange resins are based on crosslinked polystyrene. The required active groups can be introduced after polymerization, or substituted monomers can be used.

• Ion-exchange devices consist of a bed of plastic (polymer) beads covalently bound to anion groups, such as -COO-. The negative charge of these anions is balanced by Na+ cations attached to them. When water containing Ca2+ and Mg2+ is passed through the ion exchanger, the Ca2+ and Mg2+ ions are more attracted to the anion groups than the Na+ ions. Hence, they replace the Na+ ions on the beads, and so the Na+ ions (which do not form scale) go into the water in their place.

Exchange Capacity :• The total capacity of an ion exchange resin is

defined as the total number of chemical equivalents available for exchange per some unit weight or unit volume of resin. The capacity may be expressed in terms of milliequivalents per dry gram of resin or in terms of milliequivalents per dry gram of resin or in terms of millequivalents per milliliter of wet resin.

How resins look like????