Transcript Water Chemistry & Microbiology On completion of this

Module 5 Water Treatment
On completion of this module you should be
able to:
• Be aware of the objectives of water treatment
• Have an appreciation of the location, layout of a plant
• Describe the processes involved in water treatment
• Discuss the types of separation processes
• Design a simple sedimentation tank
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Basic Methods for Correcting Water
Quality Deficiencies
• The processes and extent of required treatment are
dependent on the nature and degree of quality
deficiencies to be corrected.
• There is virtually no water that cannot be treated to
potable standards. Cost effectiveness is one of the
guiding principles
• The basic methods are physical and chemical
processes and to a lesser extent, biological
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Water
Treatment
matrix
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Plant Layout and Headloss Through
the Plant
• Planning and environmental constraints
• Selected source
• Plant design factors
• Site factors
• Environmental factors
• Unit processes should lie on the system gravity
hydraulic grade line
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Preliminary Treatment
Depending on the source, the following unit processes
are likely
• Intake screens
• Aeration
• Preliminary settling tanks
• Pre-chlorination and algal control
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Intake screen
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Aeration
• Increase dissolved oxygen in ‘stale’ water
• Remove or reduce dissolved CO2 and other gases
• Precipitate out dissolved ferrous and manganese
compounds
• Reduce volatile impurities and odour
• Various methods of aeration e.g. spray, cascade, tray
and diffused air
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Preliminary settling tank
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Chemical treatment through coagulation
• Coagulants are chemicals that react with colloidal
matter to form absorbent bulky precipitates (flocs)
• Destabilisation of colloidal particles (10-3 - 1 m),
hydrophilic or hydrophobic in nature
• Salts of aluminium and iron form insoluble
hydroxides
• Reaction is pH dependent (6 - 7 optimum range)
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Aluminium salts are commonly used
• Aluminium sulfate; sodium aluminate
• Natural or added alkalinity is required
• Al2(SO4)3 + 3Ca(HCO3)2
2Al(OH)3 + 3CaSO4 + 6CO2
• Reaction is sensitive to pH
• May revert to soluble for if pH increases/decreases
• Some recent concerns relating to health issues
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Aluminium sulfate
In the absence of alkalinity
• Al2(SO4)3 + 6H2O
• H2SO4 + Ca(OH)2
2Al(OH)3 + 3H2SO4
CaSO4 + 2H2O
Natural alkalinity
• Al2(SO4)3 + 3Ca(HCO3)2
2Al(OH)3 + 3CaSO4 + 6CO2
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Iron salts are more difficult to control
• Ferric chloride/iron(III) chloride; ferric sulfate
•
2FeCl3 + 3Ca(HCO3)2
2Fe(OH)3 + 3CaCl2 + 6CO2
• Natural or added alkalinity is required
• Wider operating pH range
• Cheaper material and forms heavier floc
• Iron salts cake and are dirty to handle, difficult
sludge to dispose
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Coagulant aids
• They assist difficult coagulant processes and result in
dramatic improvement with increased floc formation
and faster settling
• Polyelectrolytes of organic synthetic high molecular
weight material with electrical charges
• Clays, lime, soda ash and activated silica are other
examples of coagulant aids
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Optimum coagulant dosage
• Use of laboratory jar test
• Determine least cost of chemicals that remove
turbidity, colour in an shortest possible time
• Comparison of first floc appearance, floc size,
dosage and settling time
• Optimum dosage also tested against pH
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Optimum
coagulant
dosage
using the jar test
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Flash/Rapid Mixing
To cause rapid dispersion at minimum power input
• Use of various devices e.g. bends, baffles, can result
in energy losses
• Energy for good mixing requires 3 - 15 kW.s/m3
• 30 - 60 sec detention time at maximum flow
• Rate of chemical diffusion is quantified by the shear
velocity gradient, G = [P/(V)]0.5
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Flash/Rapid Mixing (cont)
• G = 500 - 600 s-1 at 30 - 60 s residence time
• Mechanical power for head loss, P = Q  g h watt
• Head loss from hydraulic mixing varies 0.15 - 0.5 m
• Excessive G values can be harmful
• Increased contact time of 120 s or more achieve little
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Flash/Rapid Mixer
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Flocculation
Gentle stirring following rapid mixing so that floc
particles can coalesce and agglomerate
• Two phases are involved; initial perikinetic, orthokinetic
> 1 m
• Shear velocity gradient, G = 20 - 75 s-1
• Detention time, t = 20 - 60 minutes
• Camp No, G t of (12 to 270) x 103
• Mechanical flocculation power input
• Tapered flocculation using high G values and
progressively lower as floc size increase
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Relationship between Shear Gradient and time, t
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A Mechanical Flocculator
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Sedimentation
Removal of suspended particles in an aqueous medium
through gravity settling
• Class I Unhindered settling of discrete particles
• Class II Settling of dilute suspension of flocculent
particles
• Class III Hindered settling and zone settling
• Class IV Compressive settling (compaction)
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Class I settling
For discrete particles settling freely, the terminal velocity
is reached when gravitational force is balanced by
frictional drag force
• vs = g d2 (1 - )/(18 )
• As particle size increases, vs increases
• As CD increases vs decreases
• CD varies inversely as NR
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Class I settling (cont)
• Detention time, t = Volume/Q
• Depth of tank is not relevant, vs = Q/surface area
• Performance is influenced by overflow rate and
detention time
• High water temperature decreases CD and thus
increases vs
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Drag coefficient
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Shallow depth sedimentation
• Proposed as early as 1904 with initial failure
• Obvious inherent advantages
• Tube clarifiers with high surface loading rates
achieve 9 m/h
• Plated tanks in zig-zag pattern, vh = 44 m/h, HRT of
22 min
• Lamella separator with vs = 20 m/h
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Tube clarifier at Mt Kynoch settling tank
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Shallow depth sedimentation
Plate settler tank
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Shallow depth sedimentation
Lamella separator tank
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Difficult settling operation conditions
• Excessive suspended solids
• High colloidal content
• Coincidence of peak demand and high turbidity
• Low coefficient of fineness < 1
• Low temperature, overturn
• Persistent wind condition
• Streaming caused by density currents, temperature
gradients
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Settlement in horizontal flow tanks
• Overflow rates varies from 18 to 54 m/d
• Typically 28 m/d for a 3.5 m depth and 3 h HRT
• In tropical countries with more turbid water, 18 m/d
with 4 h HRT is appropriate with depths of 3 - 3.5 m
• In practice, particles are not wholly discrete and there
is merit in depth
• As a preliminary guide use HRT x (TSS/900)0.5 h to
adjust for varying TSS in water
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A typical horizontal flow sedimentation tank
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Settlement in upward flow tanks
• Area of tank to ensure vs > v = Q/A
• In practice, vs  2 v
•
•
•
vs
= 3 m/h for well formed floc
= 6 - 10 m/h with coagulant aids
= 8 m/h in water softening plants
• Types: hopper bottomed sludge blanket square
tanks, solid contact clarifiers, pulsator
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Vertical flow tank
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Pulsator
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Solids contact clarifier
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Flotation
An effective means of removal of particles of density
less than the liquid medium
• Use of air bubbles to separate solids/particulates from
a liquid phase
• Air bubbles (20 - 100 m) generated by dissolved air
flotation, diffused air flotation and vacuum filtration
• Attachment of solids to bubbles in a 3 phase system;
size of flocs less important
• Solids separation through a floating scum and
removed by a skimmer
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Flotation (cont)
• Advantage of high surface loading rates 5 - 12 m/h,
and the ability to remove oils, grease and algae
• Short HRT of 40 - 80 minutes; bubbles rise at 1 - 1.5
mm/s
• Flotation units are smaller in size than normal clarifiers
• Saving in chemical costs
• Optimum amount of air is determined from pilot
studies
• Disadvantage of additional equipment cost, high
operating cost and energy use
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Dissolved Air Flotation (DAF)
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Dissolved Air Flotation (DAF)
DissolvedModule
air5 flotation
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Filtration
A process of passing water through a sand bed or other
suitable medium at low speed to remove suspended
solids
• Removal of non-settleable flocs after coagulation and
sedimentation
• Properties of the medium (effective size, hardness
etc)
• More than a mechanism of straining
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Mechanisms of filtration
• Straining
• Sedimentation
• Interception
• Adhesion
• Flocculation
• Adsorption
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Rapid sand filter
A process of depth filtration as solids are removed
within the granular medium
• Sand bed 0.6 - 0.75 m deep of 0.4 - 0.7 mm effective
size and a uniformity coefficient  1.7
• Supporting gravel layer 0.3 - 0.5 m (graded 2 - 60
mm)
• Underdrain system to collect filtered water and to
discharge air scour and backwash water uniformly
• Filtration rate varies from 4 - 15 m/h
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Rapid sand filter (cont)
• Backwash when head loss  2 m
• Application of backwash water assumes practical
importance in the design of filters
• Some problems associated with rapid sand filters are
mud balls, air-binding, surface cracks and shrinkage
• Other forms are direct filtration, and up-flow filtration
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clogs up readily
ideal but unattainable
Arrangement of filter media
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Typical rapid sand filter
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Rapid sand filter isometric view (Droste 1997, p. 418)
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Types of filter underdrain system (McGhee, 1991, p.212)
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Flow control for rapid sand filters
Most systems include some means of automatic flow
control valves operated by signals from level-sensing or
flow-sensing elements
• Flow control systems are usually operated
hydraulically or pneumatically
• Avoid control conditions that lead to controller
instability e.g. 'hunting' caused by continual over
correction
• Downstream flow control
• Upstream flow control
• Control system with common head loss
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Rapid sand filter flow control systems
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Problems associated with rapid sand filters
• Negative head
• Dirty filter media (mud ball formation)
• Mineral deposits
• Gravel movement during backwashing
• Underdrain failure
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Negative head
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Slow sand filters
These are the oldest and effective method for removing
pathogenic microorganisms in water. Cake filtration
when solids are removed on entering the face of the
granular material
• No pre-treatment or chemicals are required
• Filter media 0.7 - 1.2 m layer of 0.2 - 0.4 mm effective
size with a uniformity coefficient  3
• Supported on gravel layer 0.1 m (graded 5 - 25 mm)
• Relies on surface straining and microbial action
(schmutzdecke)
• Slow filtration rates of 350 - 700 L/s.ha (3 - 6 m/d)
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Slow sand filters (cont)
• 1 - 3 months filter run or when head loss  1 m
• Surface renewal by removing 12 - 25 mm of surface
layer each time until 600 mm of sand layer is left
• Requires large land area
• Labour intensive to remove and clean the sand
• Suitable for reservoir-fed supply and small
communities requiring no technical supervision
• Does not remove colour but is able to deliver
bacteriologically superior water
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Slow sand filter
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Pressure filtration
• No different from rapid sand filters
• Filter lies on the HGL
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Chlorine disinfection
It is presently the most cost-effective disinfection
method but it has some adverse effects
• Properties of chlorine
• Reaction is highly pH dependent
• Cl2 + H2O
HOCl + HCL
• As pH increase the hypochlorous acid (HOCl) will
further dissociate to H+ and OCl- (hypochlorite ions)
• HOCL and OCl represent the free available chlorine
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Chlorine disinfection (cont)
At 20o C
pH
6
7
8
9
%HOCL
97
79
21
4
%Ocl
3
21
73
96
 Chlorine:ammonia reaction
 Breakpoint chlorination
 Superchlorination
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Chlorine - ammonia reaction
Formation of monochloramine (NH2Cl)
HOCl + NH3
H2O + NH2Cl
Cl2:NH3 < 5:1; pH  7
Formation of dichloramine (NHCl2)
NH2Cl + HOCl
H2O + NHCl2 Cl2:NH3 < 10:1
Formation of trichloramine (NCl3)
NHCl + HOCl
H O + NCl3
Cl2:NH3 < 20:1; pH < 4
Monochloramine and dichloramine represent the
combined available chlorine, with less disinfecting
power compared with the free available chlorine
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Breakpoint chlorination
• Oxidation of chloramines until appearance of free
available chlorine
• At this point the free available chlorine residual is
lowest
• Taste, odour are reduced through oxidation
• Some colour may also be removed
• Good control required to ascertain that breakpoint is
reached
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Breakpoint chlorination
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Superchlorination
• High concentration of Cl2 is used to completely
oxidise ammonia, organics, chlorophenols, colour,
taste & odour
• Short contact time and effective when contamination
is anticipated
• Dechlorination is necessary using SO2, sodium
bisulfate or activated carbon to remove the high
chlorine residue
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Factors affecting chlorine disinfecting
efficiency
• Turbidity and organic matter
• Metallic compounds
• Contact time
• Temperature
• pH value
• Type of microorganisms
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Other disfecting agents
• Ozone gas, O3
• Chlorine dioxide
• Iodine, bromine (halogens)
• Silver (metal ions)
• Simple retention time
• Heat
• Ultra-violet light
• Ultrasonic radiation
• Ultra-filtration
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Water softening
• Chemical precipitation and ion-exchange
• Carbonate hardness
Ca2+ requires lime to raise the pH to 9.5 – 10 when HCO3- is
changed to CO3
Mg2+ requires more lime to pH 10.5 – 11 when HCO3- is changed to
• Non-carbonate hardness
Ca2+ requires soda ash to precipitate CaSO4
Mg2+ requires soda ash and lime to precipitate to Mg(OH)2 and
CaCO3
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Ion-exchange
• Ca2+ and Mg2+ are exchanged for sodium ions from
zeolites or resin compounds (Na2R)
• Regeneration by washing with brine solution and
CaCl2 and MgCl2 are discharged
• Ion-exchange plants are easy to operate, in-line with
the hydraulic gradient
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Ion-exchange (cont)
• Flow rate 15 m/h and no solid sludge discharge but
not suitable for turbid water or iron > 5 mg/L
• For very hard water, precede with lime-soda ash
softening. Water should first be treated by
coagulation, sedimentation & filtration prior to ionexchange process
• Does not reduce total dissolved solids (TDS)
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Taste and odour
They are often the immediate and main sources of
consumer complaints
• Action by microorganisms, decomposition
• Reduction of sulfates to sulfides under anaerobic
condition
• Sewage and industrial discharges
• Reaction with phenols & organics by chlorine
• Urban runoff from asphaltic surfaces
• Leachates from landfills
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Taste and odour
Remedies
• Aeration may precipitate out iron and remove sulfuretted
hydrogen odours found in deep bores
• Superchlorination and dechlorination
• Chloramine of 1:2 – 1:4 of NH3:Cl2 to produce combined
available chlorine
• Chlorine dioxide has a stronger oxidising property than chlorine
• Ozonation is more powerful than chlorine and leaves no aftertaste, but is also more expensive
• Activated carbon in the form of powder, granular or filter beds,
which removes taste and odour by adsorption and also removes
a wide range of complex organics eg. pesticides
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Iron & manganese removal
Problem lies with the variety of reactions that can occur
with these element
 Oxidation
• Aeration followed by sedimentation and filtration
• Use of oxidising agents eg. Chlorine, chlorine dioxide,
potassium permanganate
 Lime softening
• Increasing pH after aeration to precipitate as Fe(OH)3
 Catalytic action
• Oxidation of manganese in zeolite or pyrolusite ore with
KMnO4
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Iron & manganese removal (cont)
 Ion-exchange
 Zeolite or ion-exchange resins to remove Fe3+ and Mn2+ if
associated with HCO3
 Suitable for groundwater devoid of O otherwise Fe3+ or
Mn2+ ions will clog the ion-exchange resin bed
 Sequestering
 Use of complex molecules to encase the ions of Fe3+ and
Mn2+ so that they no longer participate in future reactions
 Common sequestering agents are polyphospates or
organic compounds eg. ‘Calgon’ (sodium
hexametaphosphate) but subsequent heating may
destroy treatment
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Desalination
 Distillation
• Simple to multi-stage distillation
• Multi-stage flash distillation
• Vapour compression
• Solar stills
 Freezing
• H2O molecules form and attach to ice crystal, while salt
molecules remain in solution
• Latent heat of fusion is 333 kJ/kg
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Desalination
Multi-stage flash distillation (Barnes et al 1986, p.348)
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Vapour compression
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Desalination (cont)
 Ion-exchange
• Use of cation and anion exchange resins
• Simple recharge using acids for cation resin and
alkaline for anion resin
• Na+ + HR
H+ + NaR
• R(OH) + Cl-
OH- + RCl
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Desalination
Anion and cation exchange (Barnes et al 1986, p.352)
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Desalination (cont)
 Reverse osmosis
• Membranes permitting only water molecules but not
solute to pass
• Under normal osmosis water flows from low solute
concentration to high solute concentration
• Osmotic pressure is pressure to stop this flow
• When pressure applied > osmotic, then reverse osmosis
occurs
• Turbidity, iron, Mn2+, CaCO3 must first be removed
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Desalination
Reverse osmosis (Barnes et al 1986, p.354)
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Desalination
• DALBY commissioned a DESALINATION
PLANT (2004) to supplement water for its
10,000 residents whose regular supply from
bores is too brackish
• The $2.8m reverse osmosis plant will supply
a quarter of its annual needs at roughly the
same price - 32c/kL - as current price for
treated summer water drawn from the
Condamine River
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Desalination (cont)
 Electro-dialysis
• Use of electrodes to maintain an electric field in
which ions will move as in electrolysis
• 2 kinds of special membrane selective each to
cations and anions
• Conditions tend to be acid at anode and alkaline
at cathode. To prevent CaCO3 deposits at the
cathode it is continually washed with acid rinse
• Operating potential difference  1000 V
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Desalination
Electrodialysis process (Barnes et al 1986, p.357)
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End of Module 5 Water Treatment
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