water treatment slides

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Water treatment

Sudha Goel, Ph.D.

Associate Professor (Environmental Engineering)

Civil Engineering Department, IIT Kharagpur

Reference: Masters GM [1998] Water treatment systems in Introduction to Environmental Science and

Engineering, Prentice Hall

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Goal: safe and clean drinking water

REQUIREMENTS

� Identify source water in terms of

� Quantity and quality

� Location

� Cost and sustainability

� Protect source water from contamination

� Watershed management plans

� Appropriate treatment of raw water (source water)

� Safe distribution of treated water

Clean, safe drinking water at the tap

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Conventional drinking water treatment

� Design or primary objectives are removal ofDesign or primary objectives are removal ofDesign or primary objectives are removal ofDesign or primary objectives are removal of

� Microbial pathogens (Microbial pathogens (Microbial pathogens (Microbial pathogens (coliformscoliformscoliformscoliforms) ) ) ) –––– health concernshealth concernshealth concernshealth concerns� Particles (color and turbidity) Particles (color and turbidity) Particles (color and turbidity) Particles (color and turbidity) –––– health and aesthetic concernshealth and aesthetic concernshealth and aesthetic concernshealth and aesthetic concerns� Total dissolved solids removal (hard waters) Total dissolved solids removal (hard waters) Total dissolved solids removal (hard waters) Total dissolved solids removal (hard waters) ---- health and aesthetic health and aesthetic health and aesthetic health and aesthetic concernsconcernsconcernsconcerns

� Secondary objectives are removal of dissolved pollutants Secondary objectives are removal of dissolved pollutants Secondary objectives are removal of dissolved pollutants Secondary objectives are removal of dissolved pollutants –––– health health health health concerns (based on IS:10500)concerns (based on IS:10500)concerns (based on IS:10500)concerns (based on IS:10500)

� General: Odor, taste, pHGeneral: Odor, taste, pHGeneral: Odor, taste, pHGeneral: Odor, taste, pH

� Inorganic Inorganic Inorganic Inorganic

� Hardness, Alkalinity, Fe, Hardness, Alkalinity, Fe, Hardness, Alkalinity, Fe, Hardness, Alkalinity, Fe, ClClClCl, F, Ca, Mg, Cu, , F, Ca, Mg, Cu, , F, Ca, Mg, Cu, , F, Ca, Mg, Cu, MnMnMnMn, SO, SO, SO, SO44442222----, , , , NONONONO3333

---- Hg, Hg, Hg, Hg, CdCdCdCd, Se, As, CN, Se, As, CN, Se, As, CN, Se, As, CN----, , , , PbPbPbPb, Zn, Cr(VI), Al, B, radioactive materials, , Zn, Cr(VI), Al, B, radioactive materials, , Zn, Cr(VI), Al, B, radioactive materials, , Zn, Cr(VI), Al, B, radioactive materials, residual free chlorine, TDSresidual free chlorine, TDSresidual free chlorine, TDSresidual free chlorine, TDS

� OrganicOrganicOrganicOrganic

� Natural Organic Matter (NOM), PesticidesNatural Organic Matter (NOM), PesticidesNatural Organic Matter (NOM), PesticidesNatural Organic Matter (NOM), Pesticides, Oils, PAHs, Anionic , Oils, PAHs, Anionic , Oils, PAHs, Anionic , Oils, PAHs, Anionic detergents, Phenolsdetergents, Phenolsdetergents, Phenolsdetergents, Phenols

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Conventional drinking water treatmentConventional drinking water treatmentConventional drinking water treatmentConventional drinking water treatment

� Groundwater (GW): Groundwater (GW): Groundwater (GW): Groundwater (GW): In comparison to surface waters In comparison to surface waters In comparison to surface waters In comparison to surface waters

� GW tends GW tends GW tends GW tends to have lower dissolved oxygen compared to surface to have lower dissolved oxygen compared to surface to have lower dissolved oxygen compared to surface to have lower dissolved oxygen compared to surface waterswaterswaterswaters

� Can have very little microbial contamination especially if GW is from Can have very little microbial contamination especially if GW is from Can have very little microbial contamination especially if GW is from Can have very little microbial contamination especially if GW is from a deep aquifera deep aquifera deep aquifera deep aquifer

� Much higher concentrations of inorganic compounds (or ions)Much higher concentrations of inorganic compounds (or ions)Much higher concentrations of inorganic compounds (or ions)Much higher concentrations of inorganic compounds (or ions)

� Anions: chloride, carbonates, sulfates Anions: chloride, carbonates, sulfates Anions: chloride, carbonates, sulfates Anions: chloride, carbonates, sulfates (or sulfides(or sulfides(or sulfides(or sulfides), bromide, ), bromide, ), bromide, ), bromide, nitrates, nitrates, nitrates, nitrates, fluorides, fluorides, fluorides, fluorides, arsenitearsenitearsenitearsenite and arsenateand arsenateand arsenateand arsenate

� CationsCationsCationsCations: Ca, Mg, Fe, : Ca, Mg, Fe, : Ca, Mg, Fe, : Ca, Mg, Fe, MnMnMnMn, , , , Al, As, …..(Al, As, …..(Al, As, …..(Al, As, …..(Hardness is the Hardness is the Hardness is the Hardness is the concconcconcconc of of of of CaCaCaCaand Mg in GW)and Mg in GW)and Mg in GW)and Mg in GW)

� Surface Surface Surface Surface waters (SW)waters (SW)waters (SW)waters (SW)

� High turbidity and microbial concentrations High turbidity and microbial concentrations High turbidity and microbial concentrations High turbidity and microbial concentrations

� Dissolved oxygen concentrations vary depending on organic matter Dissolved oxygen concentrations vary depending on organic matter Dissolved oxygen concentrations vary depending on organic matter Dissolved oxygen concentrations vary depending on organic matter concentrationsconcentrationsconcentrationsconcentrations

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Disinfection and storage: Disinfection and storage: Disinfection and storage: Disinfection and storage: Pathogen removalPathogen removalPathogen removalPathogen removal

Filtration: Filtration: Filtration: Filtration: Turbidity, TSS, Turbidity, TSS, Turbidity, TSS, Turbidity, TSS, flocflocflocfloc removalremovalremovalremoval

Settling Settling Settling Settling tank: tank: tank: tank: flocflocflocfloc removalremovalremovalremoval

Coagulation and Coagulation and Coagulation and Coagulation and flocculation: flocculation: flocculation: flocculation: Turbidity, colloid Turbidity, colloid Turbidity, colloid Turbidity, colloid removalremovalremovalremoval

Screening or Screening or Screening or Screening or prepreprepre----sedimentation tank: sedimentation tank: sedimentation tank: sedimentation tank: Turbidity, TSS Turbidity, TSS Turbidity, TSS Turbidity, TSS removalremovalremovalremoval

Water intake or infiltration wellWater intake or infiltration wellWater intake or infiltration wellWater intake or infiltration well

TURBID

SURFACE

WATER

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Disinfection and storage: pathogens are destroyed; provides contact time for disinfection apart from water storage

Filtration, with or without pre-chlorination

Turbidity, TSS, colloid removal

Chlorine to prevent biological growth on filter media

Softening

Removal of calcium and magnesium hardness

Aeration

Low DO levels, presence of other gases, precipitation of reduced minerals like Fe, As, Mn due to oxidation

HARD GROUNDWATER

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� Aeration: necessary for GWs that are anoxic

� Oxidation of reduced forms of Fe(II) to Fe(III) and Mn(II) to Mn(IV)

� For As-contaminated water, it can result in substantial removal of As, too

� Types of aerators: cascade, fountain, tray, diffusers

� Screening: necessary for most surface waters, especially at intake points

� Removes large floating and suspended debris

Conventional drinking water treatment processes

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Cascade aerators (Gangtok water treatment plant)

Source: RN Sharma

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PLAIN SEDIMENTATION TANK (with fountain type aerators; Gangtok water treatment plant)

Source: RN Sharma

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PLAIN SEDIMENTATION TANK (with fountain type aerators; Gangtok water treatment plant)

Source: RN Sharma

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Cascade aerators (Gangtok water treatment plant)

Source: RN Sharma

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Solids and suspensions

� Discrete particles

� Particles do not change size, shape and specific gravity over time

� Flocculating particles

� Size, shape and specific gravity of particles changes over time as they aggregate or coalesce

� Dilute suspensions

� If conc of particles in suspension is insufficient to displace water as the particles settle

� Concentrated suspensions

� If conc of particles in suspension is sufficient to displace water as the particles settle

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Particle sizes

QMZ, 2000

Discrete particles can be removed by

settling

Stable particles that must be chemically and

physically conditioned

for removal

Solids separation: Sedimentation and clarification

� Sedimentation

� Removal of discrete particles (>1 micron) that are heavy enough to settle by gravity alone

� Sedimentation or settling tanks for floc removal as well

� Detention times range from 1 to 10 hours

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� Coagulation and flocculation: turbidity and suspended solids (SS)

removal

� Design objective is removal of colloidal particles (1 nm to 1

micron)

� Can remove bacteria, soil, sand and clay particles

� Concomitant removal of associated compounds or smaller

particles like NOM, heavy metals, pesticides, etc.

� Stable particles in natural systems

� Particles in natural waters (generally in pH range of 6 to 8) are

–vely charged

� Like charges repel each other and remain suspended in

solution (stable particles and no aggregation is possible)

� A turbid solution!


processes: coagulation

Particles with negative surface charges

Particles with negative

surface charges

Dilute solution in nature – low ionic strength

After addition of coagulants to solution – high ionic strength

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� Coagulation mechanisms

� Charge neutralization: Addition of Al or Fe salts and organic polymers provides high concentrations of counter ions that neutralize negative surface charges of particles

� Reduces electrostatic repulsive interaction forces, and net interaction energy becomes attractive (mainly Van der Waal’s forces)

� Net attractive forces lead to aggregation, and settling of aggregates or floc formation

� Sweep floc formation: precipitation of salts at high concentration

� In settling, the precipitate ‘sweeps’ colloidal particles along with itself

� Interparticle bridging: polymers attach to more than one particle leading to aggregation and floc formation


processes: coagulation

19PRT 1985PRT 1985PRT 1985PRT 1985

Adsorption

and

interparticle

bridging

Residual turbidity results

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0 mg/L 1 mg/L 2 mg/L 5 mg/L 10 mg/L 20 mg/L

Narayan and Goel - 2011

Procedure for coagulation and flocculation in the laboratory flocculator.

Samples of the coagulated and settled supernatant from the jar tests (after step 3)

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� Flocculation or mixing

� Rapid mixing: for mixing the coagulant

� Detention time is approx. 0.5 min

� Slow mixing: for floc formation

� Too fast will break floc; slow enough to maximize number of particle collisions

� Optimum speed has to be determined experimentally

Practical examples: milk and tea as colloidal suspensions!


processes: flocculation

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Clariflocculator

http://www.environengg.com/clariflocculators.html

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Clariflocculator

Source: Internet(msu)


processes: filtration

Filtration: removal of flocculated particles of smaller size (those

that cannot be removed by settling)

• Rapid sand filters: higher throughput

• Slow sand filters: lower throughput

• Adsorption is another important mechanism for particle

removal

• Backwashing of filters is essential to regain head loss due to

clogging

• Generally with chlorinated water to disinfect filters

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Slow sand filters

http://www.google.co.in/imgres?imgurl= 25

Rapid sand

filters

Peavy HS, Rowe DR and Tchobanoglous G (1985) Chapter 4, Environmental Engineering, McGraw Hill

International Ed., NY, US26

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Disinfection

� Destruction of vegetative pathogensDestruction of vegetative pathogensDestruction of vegetative pathogensDestruction of vegetative pathogens

� Not sterilization which implies destruction of all life forms Not sterilization which implies destruction of all life forms Not sterilization which implies destruction of all life forms Not sterilization which implies destruction of all life forms

(microbes, spores, cysts, viruses, etc.)(microbes, spores, cysts, viruses, etc.)(microbes, spores, cysts, viruses, etc.)(microbes, spores, cysts, viruses, etc.)

� Autoclaving, membrane filtrationAutoclaving, membrane filtrationAutoclaving, membrane filtrationAutoclaving, membrane filtration

� Physical methodsPhysical methodsPhysical methodsPhysical methods

� Membrane FiltrationMembrane FiltrationMembrane FiltrationMembrane Filtration

� Radiation: UV, XRadiation: UV, XRadiation: UV, XRadiation: UV, X----rays, gamma rays rays, gamma rays rays, gamma rays rays, gamma rays

� Chemical methods (disinfectants)Chemical methods (disinfectants)Chemical methods (disinfectants)Chemical methods (disinfectants)

� Chlorinated compounds Chlorinated compounds Chlorinated compounds Chlorinated compounds

� chlorine, chloramines, chlorine dioxidechlorine, chloramines, chlorine dioxidechlorine, chloramines, chlorine dioxidechlorine, chloramines, chlorine dioxide

� Ozone (hydroxyl radical mechanism)Ozone (hydroxyl radical mechanism)Ozone (hydroxyl radical mechanism)Ozone (hydroxyl radical mechanism)

� Potassium permanganatePotassium permanganatePotassium permanganatePotassium permanganate

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Oxidation potentials and disinfection power of disinfectants

DisinfectantOxidation potential

(Volts)

Fluorine -3.06

Hydroxyl free radical (OH-) -2.80

Oxygen (atomic) -2.42

Ozone (O3) -2.07

Hypobromous acid (HOBr) -1.59

Hypochlorous acid (HOCl) -1.49

Chlorine (Cl2) -1.36

Oxygen (molecular) -1.23

Bromine (Br2) -1.07

Chlorine dioxide (ClO2) -0.95

Monochloramine (NH2Cl) -0.75

Dichloramine (NHCl2) -0.74

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Chlorine remains the most popular, why?

� Potent germicide

� High oxidation potential

� Residual in distribution system� Chloramine can do the same but is a less powerful oxidant

� Taste and odor control

� Oxidation of NOM and removal of compounds causing taste and odor problems

� Biological growth control

� Growth of algae and bacteria in storage reservoirs and water supply systems

� Chemical control

� Iron and manganese removal

� Oxidation of SOCs

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Problems with chlorine!� Hazardous material

� Difficulty in transportation, handling and storage

� Pungent compound

� Disagreeable taste and odor

� Dermal and eye irritation

� Microbial resistance to chlorine

� More effective against bacteria rather than spores, cysts and viral particles

� Disinfection by-products (DBPs) formation

� Potential health hazard

� Carcinogenic, mutagenic, teratogenic

� Non-carcinogenic effects – little information or discussion in literature

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� Addition of chlorine to water, results in Addition of chlorine to water, results in Addition of chlorine to water, results in Addition of chlorine to water, results in the formation the formation the formation the formation of of of of hypochloroushypochloroushypochloroushypochlorous

[[[[HOClHOClHOClHOCl] and hydrochloric acids [] and hydrochloric acids [] and hydrochloric acids [] and hydrochloric acids [HClHClHClHCl]:]:]:]:

� ClClClCl2222 + H+ H+ H+ H2222O O O O → HOClHOClHOClHOCl + + + + HClHClHClHCl pKpKpKpK = 3.39= 3.39= 3.39= 3.39

� Depending on the pH, Depending on the pH, Depending on the pH, Depending on the pH, hypochloroushypochloroushypochloroushypochlorous acid partly dissociates to hydrogen acid partly dissociates to hydrogen acid partly dissociates to hydrogen acid partly dissociates to hydrogen

and hypochlorite ions:and hypochlorite ions:and hypochlorite ions:and hypochlorite ions:

� HOClHOClHOClHOCl→ HHHH++++ + + + + OClOClOClOCl---- pKpKpKpK = 7.57= 7.57= 7.57= 7.57

� The hypochlorite ion then most often degrades to a mixture of chloride The hypochlorite ion then most often degrades to a mixture of chloride The hypochlorite ion then most often degrades to a mixture of chloride The hypochlorite ion then most often degrades to a mixture of chloride

and chlorate ions:and chlorate ions:and chlorate ions:and chlorate ions:

� 3 3 3 3 OClOClOClOCl----→ 2 2 2 2 ClClClCl---- + ClO+ ClO+ ClO+ ClO3333----

Chlorine chemistry: reactions in water

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Effect of pH and temperature on chlorine speciation

• Temperature effect

on equilibrium

constants

• Arrhenius’ effect of

temperature on

reaction kinetics

•HOCl is a stronger

disinfectant than OCl-

33TFC-8ed

Example of inactivation assays or disinfection experiments

kteNN

ktN

N

kNdt

dN

−

=

−=

−=

0

0

ln

Harriette Chick’s law of disinfection (1908)

Inactivation rate k is a f(time, cell conc, disinfectant conc, temperature, pH)

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Hardness� Hardness: due to presence of cations like Ca and Mg

� Other cations like Fe, Mn, Sr, Al, etc. may be present

� Formation of soap curd (lack of frothing or foaming that is essential for bringing dirt particles into solution), increased soap requirement and subsequent difficulty in all cleaning activities

� On heating, scale formation or precipitation of these ions, CaCO3and Mg(OH)2, leads to reduced efficiency of heating elements, and failure

� Synthetic detergents can reduce the problem but not eliminate it

� General level of acceptance is ≤ 150 mg/L

� Carbonate hardness� Due to anions like carbonates and bicarbonates

� Also called temporary hardness, since it can be precipitated by boiling

� Non-carbonate hardness� Amount of hardness in excess of carbonate hardness

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Hardness

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Hardness classification

DescriptionDescriptionDescriptionDescription Hardness, Hardness, Hardness, Hardness, meqmeqmeqmeq/L/L/L/L Hardness, mg/LHardness, mg/LHardness, mg/LHardness, mg/LSoftSoftSoftSoft < 1 <50 to 75

Moderately hardModerately hardModerately hardModerately hard 1 to 3 50 or 75 – 150

HardHardHardHard 3 to 6 150 - 300

Very hardVery hardVery hardVery hard > 6 > 300

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Alkalinity

� Alkalinity is the measure of a water’s ability to

absorb hydrogen ions without significant pH

change

� Buffering capacity of water

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Softening� Surface waters are generally softer than GWs

� For hardness levels < 200 mg/L as CaCO3, no softening is required

� Softening is often required for GW

� Especially when hardness is > 1000 mg/L

� Processes

� Lime-soda (gives crude levels of removal, cheap)

� Quick lime (CaO) or hydrated lime (Ca(OH)2) is added to water

� Carbonates of Ca precipitate out of solution

� Mg(OH)2 precipitates at pH >11, excess lime has to be added

� Can bring hardness down to 30-40 mg/L of CaCO3� Ion exchange (for finer applications, expensive, for <30 to 40 mg/L of CaCO3)

� Zeolites: can be natural or synthetic

� Ion exchange resins: cationic or anionic

� Na+ or H+ is exchanged for Ca 2+and Mg2+, does not contribute to hardness

� Regeneration required; much higher removal efficiencies can be achieved

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Zeolites

Wikipedia 2007

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Water classes based on salinity

CLASSCLASSCLASSCLASS SOURCESOURCESOURCESOURCE TDS, mg/LTDS, mg/LTDS, mg/LTDS, mg/L

FreshFreshFreshFresh Rivers, lakes, GWRivers, lakes, GWRivers, lakes, GWRivers, lakes, GW <500<500<500<500

Slightly salineSlightly salineSlightly salineSlightly saline Ground, river, lakeGround, river, lakeGround, river, lakeGround, river, lake 500 500 500 500 ---- 1000100010001000

Mildly salineMildly salineMildly salineMildly saline EstuariesEstuariesEstuariesEstuaries 1000 1000 1000 1000 ---- 2000200020002000

Moderately salineModerately salineModerately salineModerately saline Inland and brackish mixInland and brackish mixInland and brackish mixInland and brackish mix 2000 2000 2000 2000 ---- 10,00010,00010,00010,000

Severely salineSeverely salineSeverely salineSeverely saline Inland and coastalInland and coastalInland and coastalInland and coastal 10,000 10,000 10,000 10,000 ---- 30,00030,00030,00030,000

Sea waterSea waterSea waterSea water Offshore seas and oceansOffshore seas and oceansOffshore seas and oceansOffshore seas and oceans 30,000 30,000 30,000 30,000 ---- 36,00036,00036,00036,000

�TDS = A*C where

� A = conversion factor, 0.55 to 0.75

� C = electrical conductivity, microS or micromhos

� TDS = total dissolved solids, mg/L

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Processes for removing TDS from water

� Membrane processes

� Electrodialysis (ED) and Electrodialysis reversal (EDR)

� Reverse Osmosis (RO)

� Distillation

� Freezing

� Distillation and RO account for 87% of the desalination

capacity in the world

Demineralization or TDS removal

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DemineralizationProcesses for removing TDS from water

� Membrane processes

� Electric current driven: electrodialysis or electrodialysisreversal

� Pressure driven: reverse osmosis, nanofiltration, ultrafiltration, microfiltration

� Distillation

� Multi-stage flash distillation (MSF)

� Multiple effect evaporation (or distillation) - MED

� Vapor compression (VC)

� Solar distillation

� Freezing

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Membrane Processes

� Defined as processes in which a membrane is used to permeate high-quality water while rejecting passage of dissolved and suspended solids

� Used for demineralization (or desalination) and removal of dissolved and suspended particles

� Major applications in water treatment are NOM removal, and desalting (demineralization)

� Analytical instruments and methods

� Industrial applications:

�Medical applications include separation of various components of body fluids

� Purification processes

QMZ (2000) Ch-18; Sincero (1996) Ch-9

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Raw water or influent,

Q0, C0

Concentrate or rejectate,

Qr, Cr

Treated water or

effluent

Qp, Cp

Membrane Processes

Mass balance around system or process:

Flow: Q0 = Qp + Qr

Mass of contaminant: Q0C0 = QpCp + QrCr

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