1 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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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)
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)
� 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)
� 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
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!
Conventional drinking water treatment
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
Conventional drinking water treatment
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!
Conventional drinking water treatment
processes: flocculation
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Clariflocculator
http://www.environengg.com/clariflocculators.html
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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 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
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:
� 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:
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
Slightly salineSlightly salineSlightly salineSlightly saline Ground, river, lakeGround, river, lakeGround, river, lakeGround, river, lake 500 500 500 500 ---- 1000100010001000
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