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Water Treatment
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Water tretment process
Basic steps
Raw Water
Storage
Mixing Flocculation
Sedimentation
Filtration
Clear Well
Distribution
AerationCoagulant, pH Adjustment
Disinfectant (Cl2, NaOCl)
Screening
Raw water
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Coagulation
Find the requirement of alum and lime to
treat water (107 L/day) at alum dosage (30mg/L) when original alkalinity present is 8.5
mg/L.
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Alum required =7
= 300 /
4.5 mg/L alkalinity (CaCO3) is required for 10mg/L dosage of alum.
Alkalinity required = (4.5/10)* 30 8.5= 5mg/ L
56 mg of CaO is required for obtaining 100 mg/L
of CaCO3.Lime required = 5*(100/56)*(107 / 106)
= 90 kg/day
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Sedimentation
Factors affect for size of settling basin
Detention time
Overflow rate Settling velocity of particle
Horizontal velocity (for rectangular tanks)
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Detention time =
Detention time (days)
Basin volume(m3)
Volumetric flow rate (m3/day)
Horizontal velocity =
Flow area
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Settling velocity of particle
=
Total surface area of the basin
Overflow rate surface loading) =
Over flow rate (m3
/m2
day)
Length of the tank =
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Strokes law
=
18
2
=
18 1 2
, Density of particle and water respectively
Particle diameter
Viscosity of water
Specific gravity of particle
Acceleration due to gravity
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Filtration
The required filtration rate is calculated using the formulabelow
( 2) = ( )
2
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Filter backwash
The amount of water required for backwash depends on,
Design of the filter
Quality of the water being filtered
( 2) = ( ) 2
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Chlorination
Chlorine usage in the treatment of 18.9
million litres of water is 7.71 kg/day. The
residual after 10 min contact is 0.2 mg/L.
Compute the dosage in milligrams per liter
and chlorine demand of the water.
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Dosage= 7.71 *1000/ 18.9*106
= 0.407 mg/L
Chlorine demand = DosageResidual
= 0.407- 0.2
= 0.207 mg/L
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Activated Carbon Filters
Activated carbon filtration can effectively
reduce,
certain organic compounds such as volatile
organic compounds, pesticides and benzene
and chlorine in drinking water.
the quantity of lead and harmless taste- and
odor-causing compounds
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Treatment Principles
An adsorptive process in which thecontaminant is attracted to and held
(adsorbed) onto the surface of the carbon
particles.
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Medium for an activated carbon filter
petroleum coke
bituminous coal
lignite wood products
coconut shell or peanut shells
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Preparation of activated carbon
Subject carbon medium to steam and hightemperature (2300F) without oxygen to activatethe product
the carbon can process by an acid wash or coat
with a compound to enhance the removal ofspecific contaminants
activation produces carbon with many smallpores and, therefore, a very high surface area
Activated carbon is then crushed to produce agranular or pulverized carbon product
This creates small particles with more outsidesurface area available for adsorption
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The efficiency of the adsorption process is
influenced by carbon characteristics (particle and pore size,
surface area, density and hardness)
the contaminant characteristics (concentration,
tendency of chemical to leave the water, solubility
of the contaminant, and contaminant attraction to
the carbon surface)
contact time between the water and the carbon(the rate of water flow)
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Breakthrough point
When the activated carbon becomes
saturated (all adsorption sites filled),
contaminants can flow from the carbon back
into solution. This is called breakthrough.
In order to prevent breakthrough, some AC
filtration units will shut off the water supply
after a specified number of gallons have beentreated
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Advanced Water
Treatment
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Ion exchange
In the ion exchange process an insoluble resin
removes ions of either positive charge or
negative charge from solution and releases
other ions of equivalent charge into solutionwith no structural changes in the resin
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Purpose of using ion exchanger in
water treatment
Remove
Anions- nitrate, fluoride, arsenic and other
contaminants
CationsCalcium, Magnesium
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Types of ion exchangers
Natural: Proteins, Soils, Lignin, Coal, Metal oxides,
Aluminosilicates (zeolites) (NaOAl2O3.4SiO2).
Synthetic zeolite gels and most common -
polymeric resins (macroreticular, large pores).
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Ion exchange resin
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Plastic beads made of cross linked polystyrene
with functional groups (sulphonates) that actas ion exchange sites.
The sulphonate group has a negative charge
allowing it to attract and hold (exchange)
positive ions or cations such as H+, Ca+2, Mg+2,
Fe+2, Na+.
Those ions remain on the bead until the bead
encounters other ions for which it has a
greater affinity
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Classification of ion exchange resins
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Resin classification
Resins are classified based on the type of
functional group they contain and their % of
cross-linkages
Cationic Exchangers:- Strongly acidicfunctional groups derived from strong
acids e.g., R-SO3H (sulfonic).
- Weakly acidicfunctional groups derived from weak
acids, e.g., R-COOH (carboxylic).
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Cation exchange Softening
Use to reduce hardness
Cation exchange reaction
++
++ 2
+
represent the anionic component of the resin(Ca and Mg cations are absorbed and an equivalentamoun of Na ions is released to the solution)
Reaction during regeneration
2
++
++
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Regeneration cycle consist of three stages Fill the resin bed with brine
Slow rinse
Back wash
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The sodium concentration after regeneration
should not exceed recommended maximum
value
The concentration of sodium in the softened
water increases in proportion to the hardness
ions removed
Ex: Hardness of 43 mg/l produces water
containing 20 mg/l of sodium
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Anionic Exchangers
Strongly basicfunctional groups derived
from quaternary ammonia compounds, R-N-
OH.
Weakly basic - functional groups derived from
primary and secondary amines, R-NH3OH or R-
R-NH2OH.
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Disadvantages
High operating cost
The problem of brine water disposal
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Factors influencing resin life
Type of resin
Chemical characteristics of the water beingtreated
Operating temperature Regeneration temperature
Regeneration level ( salt applied per unit bedvolume)
Water feed rate
Bed depth
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Membrane Processes
Microfiltrtion
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A membrane is a selective barrier that permits
the separation of certain species in a fluid bycombination of sieving and diffusion
mechanisms
Membranes can separate particles andmolecules and over a wide particle size range
and molecular weights
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Membranes commonly consist of a porous
support layer with a thin dense layer on top
that forms the actual membrane
Active layer
Porous support layer
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Pressure-Driven Membrane Processes
Separate by size and chemistry
Concentration, Porosity Effects
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MF
10-300 kPa
RO
0.5-1.5 MPa
NF
0.5-1.5 MPa
UF
50-500 kPaP=
Bacteria, parasites, particles
High molecular substances, viruses
Mid-size organic substances,multiple charged ions
Low molecular substances, single charge
ions
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Membrane filtration configuration
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The growing use of MF
1. More attention paid to environmental
problems linked to drinking and non-drinking
water
2. Increased demand for water (using
currently available sources more effectively)
3. Market power
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Pore size of MF membranes
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Pores and pore geometries
Porous MF membranes consist of polymeric matrix in which pores
are present.
The existence of different pore geometries implies that different
mathematical models have been developed to describe transport
phenomena.
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MF membranes preparation
Stretched PTFE memb rane
Stretching
Semycristalline polymers (PTFE, PE, PP)
if stretched perpendicular to the axis of
crystallite orientation, may fracture in
such a way as to make reproduciblemicrochannels.
The porosity of these membranes is very
high, and values up to 90% can be
obtained.
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Phase inversion (PI)
A polymer is transformed in a controlled
manner from liquid to solid phase. The
process of solidification is initiated by the
transition from one liquid state into two
liquids (liquid-liquid demixing) at a certain
stage during demixing. The high polymer
concentration phase will solidify and a
solid matrix is formed.By contrtolling the initial stage of phase
transition the membrane morphology can
be controlled and porous as well as
nonporeous membranes can be prepered.
Chemical phase inversion
0.45m PVDF membrane
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4. Sintering
This method involves compressing a powder consisting of
particles of a given size and sintering at high temperatures.
The required temperature depends on the material used.
HEAT
pore
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Materials used in MF
Synthetic polymeric membranes:
a) Hydrophobic
b) Hydrophilic
Ceramic membranes
PTFE, teflon
PVDF
PP
PE
Cellulose esters
PC
PSf/PES
PI/PEI
PAPEEK
Alumina, Al2O3
Zirconia, ZrO2
Titania, TiO2
Silicium Carbide, SiC
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1. Polymeric MF membranes
Phase inversion Stretching
Track-etching
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2. Ceramic MF membranes
Anodec, anodic oxidation (surface) US Filter, sintering (cross section, upper part)
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Retentate: how will it be used?
1.Sent to a treatment plant
2.Discharged into a body of water
3.Sent to a storage facility
4.For ground applications5.Recycled back to water source
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Some other industrial applications
1. Waste-water treatment2. Clarification of fruit juice, wine and beer
3. Ultrapure water in the semiconductor industry
4. Metal recovery as colloidal oxides or hydroxides5. Cold sterilization of beverages andpharmaceuticals
6. Medical applications: transfusion filter set,
purification of surgical water7. Purification of condensed water at nuclear plants
8. Separation of oil-water emulsions
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Membrane Processes
Reverse Osmosis
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Pore size in reverse osmosis
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Reverse Osmosis
This process will allow the removal of particles as smallas ions from a solution.
Reverse osmosis is used to purify water and removesalts and other impurities in order to improve the color,taste or properties of the fluid.
The most common use for reverse osmosis is inpurifying water. It is used to produce water that meetsthe most demanding specifications that are currently inplace.
It can be used to purify fluids such as ethanol and
glycol, which will pass through the reverse osmosismembrane, while rejecting other ions andcontaminants from passing.
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Contd.
Reverse osmosis uses a membrane that is semi-permeable, allowing the fluid that is being purified topass through it, while rejecting the contaminants thatremain.
The process of reverse osmosis requires a driving force
to push the fluid through the membrane, and the mostcommon force is pressure from a pump. As the concentration of the fluid being rejected
increases, the driving force required to continueconcentrating the fluid increases.
Reverse osmosis is capable of rejecting bacteria, salts,sugars, proteins, particles, dyes, and other constituentsthat have a molecular weight of greater than 150-250daltons.
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What is osmosis?
If two solutions of differentconcentration are separated bya semi-permeable membranewhich is permeable to thesmaller solvent molecules butnot to the larger solutemolecules, then the solventwill tend to diffuse across themembrane from the lessconcentrated to the more
concentrated solution. Thisprocess is called osmosis.
The energy which drives theprocess is usually discussed in
terms of osmotic pressure.
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Pressure and flux range
Membrane processPressure range
(bar)Flux range(l/m2hbar)
Microfiltration 0,1 - 2,0 >50
Ultrafiltration 1,0 - 5,0 10 50
Nanofiltration 5,0 20 1,4 - 12
Reverse Osmosis 20 - 100 0,05 - 1,4
The pressures used in reverse osmosis range from 20 to 100 bar and the
flux from 0,05 to 1,4 l / m2h
Energy Requirements
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Energy Requirements
Pressure driven processes
Power devices applied in pressure driven
membrane process
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Energy requirements
The energy
consumption to
pressurize a liquid is
given by:
A turbine may be
utilized to recover the
energy:
pump
pump
PqE
PqE turbineturbine
fluxq
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Membrane selection
o Membrane accounts for 15 to 40 percent of the price in
reverse osmosis.
o Membranes must be replaced periodically
o
CAREFUL MEMBRANE SELECTION ISESSENTIAL
SELECTION CRITERIA:
Chemical tolerance
Mechanical suitability
Price
Cleanability
Separation performance
GOOD DESIGN:
Consistent performance
Needs less frequent membranecleaning
Reasonable consum of power
Little operational attention
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Membrane configurations in RO
Spiral-wound configuration
Next logical step from a flat
membrane but with higher
packing density300 1000 m2/m3
Permeate is collected in the
central tube
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Tubular conf iguration
Not self supporting in
contrast to hol low fiber
modulesPermeate crosses the
membrane layer to the
outside
Low sur face-volume ratio
Usual ly the active layer is
inside
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Capil lary/hollow fiber conf igurationFibers diameter:
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Applications
Production of drinking water
Treatment of urban waste water
Production of water for industrial uses
Treatment of different wastes
Concentration of fruit juices, white of an egg,
whey...
Fermentation