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Water treatment

Sudha Goel, Ph.D.

Assistant Professor (Environmental Engineering)

Department of Civil Engineering, IIT Kharagpur

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

Engineering, Prentice Hall

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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 health health health concernsconcernsconcernsconcerns

� Particles (color and turbidity) Particles (color and turbidity) Particles (color and turbidity) Particles (color and turbidity) –––– health and health and health and health and aesthetic concernsaesthetic concernsaesthetic concernsaesthetic 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

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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

� tends to have lower dissolved oxygen compared to surface waterstends to have lower dissolved oxygen compared to surface waterstends to have lower dissolved oxygen compared to surface waterstends to have lower dissolved oxygen compared to surface waters

� 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 (sulfides), bromide, Anions: chloride, carbonates, sulfates (sulfides), bromide, Anions: chloride, carbonates, sulfates (sulfides), bromide, Anions: chloride, carbonates, sulfates (sulfides), 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, …..(Hardness is the , …..(Hardness is the , …..(Hardness is the , …..(Hardness is the concconcconcconc of all of all of all of all multivalent multivalent multivalent multivalent cationscationscationscations –––– mainly Ca and Mg in GW)mainly Ca and Mg in GW)mainly Ca and Mg in GW)mainly Ca 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, floc removalTurbidity, TSS, floc removalTurbidity, TSS, floc removalTurbidity, TSS, floc removal

Settling tank: floc removalSettling tank: floc removalSettling tank: floc removalSettling tank: floc removal

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

Screening or preScreening or preScreening or preScreening or pre----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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Cascade aerators

(Gangtok water treatment plant)

Source: RN Sharma

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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

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� Coagulation and flocculation: turbidity and 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!

Conventional drinking water treatment

processes: coagulation

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Clariflocculator

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

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Circular clariflocculator

Source: Internet(msu)

Conventional drinking water treatment

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 rapid sand filters is essential to regain head loss due to clogging

• Generally done with chlorinated water to disinfect filters

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Disinfection

� Destruction Destruction Destruction Destruction or removal of or removal of or removal of or removal of vegetative pathogensvegetative pathogensvegetative pathogensvegetative 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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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

• Arrhenius’ effect

•HOCl is a stronger

disinfectant than OCl-

20TFC-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 multivalent cations like Ca, Mg (mainly), and Fe, Mn, Sr, Al, etc.� 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, CaCO3 and 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; IS standard – 300 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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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 > 300 mg/L (IS 10500)

� 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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DemineralizationProcesses for removing TDS from water

� Ion exchange

� Membrane processes

� Electric current driven: electrodialysis or electrodialysisreversal

� Pressure driven: reverse osmosis (RO), nanofiltration, ultrafiltration, microfiltration

� Distillation

� Multi-stage flash distillation (MSF)

� Multiple effect evaporation (or distillation) - MED

� Vapor compression (VC)

� Solar distillation

� Freezing

Distillation and RO account for 87% of the desalination capacity in

the world

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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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