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
1
Engineering, Prentice Hall
2. 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
2
3. Conventional drinking water treatment
Design or primary objectives are removal of
(coliforms
coliforms)
Microbial pathogens (coliforms) – health concerns
Particles (color and turbidity) – health and aesthetic concerns
Total dissolved solids removal (hard waters) - health and aesthetic
concerns
Secondary objectives are removal of dissolved pollutants – health
concerns (based on IS:10500)
General: Odor, taste, pH
Inorganic
Cl,
Mn,
Hardness, Alkalinity, Fe, Cl, F, Ca, Mg, Cu, Mn, SO42-, NO3- Hg,
Cd, Se, As, CN-, Pb, Zn, Cr(VI), Al, B, radioactive materials,
Cd,
Pb,
residual free chlorine, TDS
Organic
Pesticides,
Natural Organic Matter (NOM), Pesticides, Oils, PAHs, Anionic
detergents, Phenols
3
4. Conventional drinking water treatment
Groundwater (GW): In comparison to surface waters
GW tends to have lower dissolved oxygen compared to surface
waters
Can have very little microbial contamination especially if GW is from
a deep aquifer
Much higher concentrations of inorganic compounds (or ions)
sulfides),
Anions: chloride, carbonates, sulfates (or sulfides), bromide,
nitrates, fluorides, arsenite and arsenate
Cations:
Mn,
…..(Hardness
Cations: Ca, Mg, Fe, Mn, Al, As, …..(Hardness is the conc of Ca
and Mg in GW)
Surface waters (SW)
High turbidity and microbial concentrations
Dissolved oxygen concentrations vary depending on organic matter
concentrations
4
5. Water intake or infiltration well
preScreening or pre-sedimentation tank: Turbidity, TSS
removal
Coagulation and flocculation: Turbidity, colloid
removal
TURBID
SURFACE
WATER
Settling tank: floc removal
Filtration: Turbidity, TSS, floc removal
Disinfection and storage: Pathogen removal
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6. HARD GROUNDWATER
Aeration
Low DO levels, presence of other gases, precipitation of
reduced minerals like Fe, As, Mn due to oxidation
Softening
Removal of calcium and magnesium hardness
Filtration, with or without pre-chlorination
Turbidity, TSS, colloid removal
Chlorine to prevent biological growth on filter media
Disinfection and storage: pathogens are destroyed; provides
contact time for disinfection apart from water storage
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7. Conventional drinking water treatment processes
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
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12. 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
12
14. Particle sizes
Stable particles that
must be chemically and
physically conditioned
for removal
Discrete particles
can be removed by
settling
QMZ, 2000
14
15. 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
15
16. Conventional drinking water treatment
processes: coagulation
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!
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17. Dilute solution in nature – low ionic strength
Particles with negative
surface charges
After addition of coagulants to solution – high ionic strength
Particles with negative surface charges
17
18. Conventional drinking water treatment
processes: coagulation
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
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20. Residual turbidity results
Procedure for coagulation and flocculation in the laboratory flocculator.
0 mg/L
1 mg/L
2 mg/L
5 mg/L
10 mg/L
20 mg/L
Samples of the coagulated and settled supernatant from the jar tests (after step 3)
Narayan and Goel - 2011
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21. Conventional drinking water treatment
processes: flocculation
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!
21
24. 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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29. 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
29
30. 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
30
31. Chlorine chemistry: reactions in water
Addition of chlorine to water, results in the formation of hypochlorous
HOCl]
[HCl
HCl]:
[HOCl] and hydrochloric acids [HCl]:
Cl2 + H2O → HOCl + HCl
pK = 3.39
Depending on the pH, hypochlorous acid partly dissociates to hydrogen
and hypochlorite ions:
HOCl → H+ + OClpK = 7.57
The hypochlorite ion then most often degrades to a mixture of chloride
and chlorate ions:
3 OCl- → 2 Cl- + ClO3-
31
32. 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-
32
33. Example of inactivation assays or disinfection
experiments
dN
= − kN
dt
N
ln
= − kt
N0
− kt
N = N 0e
Harriette Chick’s law of
disinfection (1908)
Inactivation rate k is a f(time,
cell conc, disinfectant conc,
temperature, pH)
TFC-8ed
33
34. 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, 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
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
34
37. Alkalinity
Alkalinity is the measure of a water’s ability to
absorb hydrogen ions without significant pH
change
Buffering capacity of water
37
38. 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
38
40. Water classes based on salinity
CLASS
SOURCE
TDS, mg/L
Fresh
Rivers, lakes, GW
<500
Slightly saline
Ground, river, lake
500 - 1000
Estuaries
1000 - 2000
Inland and brackish mix
2000 - 10,000
Inland and coastal
10,000 - 30,000
Offshore seas and oceans
30,000 - 36,000
Mildly saline
Moderately saline
Severely saline
Sea water
TDS = A*C where
A = conversion factor, 0.55 to 0.75
C = electrical conductivity, microS or micromhos
TDS = total dissolved solids, mg/L
40
41. Demineralization or TDS removal
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
41
42. Demineralization
Processes for removing TDS from water
Membrane processes
Electric current driven: electrodialysis or electrodialysis
reversal
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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43. 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
43
44. Membrane Processes
Treated water or
effluent
Qp, Cp
Raw water or influent,
Q0, C0
Concentrate or rejectate,
Qr, Cr
Mass balance around system or process:
Flow: Q0 = Qp + Qr
Mass of contaminant: Q0C0 = QpCp + QrCr
44
![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
1
Engineering, Prentice Hall](https://image.slidesharecdn.com/ev20001watertreat2013-131105062416-phpapp01/85/water-treatment-slides-1-320.jpg)
