chemical treatment processes
TRANSCRIPT
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Topic 5.1: Chemical Treatment Processes
Dr. Lee Khia Min
Department of Civil Engineering
UEMX3653 Water and Wastewater Treatment
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ObjectivesObjectives
Review some basic concepts of the chemistry involve in chemical treatment processes.
To discuss types of chemical processes and their roles within treatment systems.
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Inorganic Chemicals and Compounds Valence – the combining power of an element relative to that of
the hydrogen atom which has an assigned value of 1. An element with a valence of 2+ can replace two hydrogen atoms in a
compound. An element with a valence of 2- can react with two hydrogen atoms.
Equivalent or combining weight of an element is equal to its atomic weight divided by the valence. For example, the EW of calcium equals 40 g divided by 2, or 20 g.
Equivalent weight of a molecule is equal to its molecular weight divided by the number of positive or negative electrical charges. For example, the EW of sulfuric acid equals 98.1 g divided by 2, or 49 g.
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Units of Expression mg/g ppm %1 1 mg/L = 8.34 lb/million gal
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Units of Expression (mg/L as CaCO3)The concentration given in milligrams of
weight may not relate to specific element whose concentration is being expressed. Hardness: Ca2+ and Mg2+
Alkalinity: OH-, CO32-, and HCO3
-
The common units is given in mg/L as CaCO3.
Ammonia, nitrate, and organic nitrogen expressed as mg/L as N.
Phosphates expressed as mg/L as P.
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Units of Expression (meq/L) The term milliequivalents per liter (meq/L) expresses
the concentration of a dissolved substance in terms of its electrical charge or its combination in reaction.
Milliequivalents are calculated from milligrams per liter for elemental ions by the following equation:
and for radicals or compounds by the following equation:
EW
mg/L
weightatomic
valencemg/Lmeq/L
EW
mg/L
weightmolecular
charge electricalmg/Lmeq/L
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Milliequivalents-per-Liter Bar Graph For better visualization of the chemical composition, meq/L bar graph is
used. The top row consists of major cations arranged in the order of Ca2+, Mg2+,
Na+, K+. Anions in the bottom row are aligned in the sequence of carbonate (if present), bicarbonate, sulfate, and chloride.
The sum of the positive meq/L must equal the sum of the negative meq/L. Hypothetical combinations of “+” and “–” ions can be written from a bar
graph, and are useful in evaluating a water for lime-soda ash softening.
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Example 11.2 The results of a water analysis are Ca 40 mg/L, Mg 10
mg/L, Na 11.7 mg/L, K 7 mg/:, HCO3- 110 mg/L, SO4
2- 6.2 mg/L, and Cl- 11 mg/L. Draw a meq/L bar graph and express the hardness and alkalinity in units of mg/L as CaCO3.
Answer:
Hardness = 141 mg/L
Alkalinity = 90 mg/L
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Alkalinity and pH Relationships
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Coagulation
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Definitions Coagulation is
The addition and rapid mixing of coagulantsThe destabilization of colloidal and fine particlesThe initial aggregation of destabilized particles
Flocculation isThe gentle agitation to aggregate destabilized
particles to form rapid-settling floc
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Colloidal Characteristics What are colloidal particles
Colloids
Have particle size between 0.001 to 1.0 micron
Colloids do not settle by the force of gravity
Are stable in suspensions because Extremely small size
State of hydration (chemical combination with water)
Surface electrical charge
10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 cmMoleculesions
colloids silt sand
1nm 1µm
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Colloidal Characteristics
Have large surface area per unit volume – surface phenomena such as electrostatic repulsion and hydration become important.
Adsorb substances from surrounding water. Have electrostatic charge (mostly negatively
charged). Can be hydrophilic (organic colloids) or hydrophobic
(inorganic colloids). Attract ions of opposite charge to its surface (fixed
layer and diffused layer).
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Coagulation Theory
Colloid particles are in constant motion in water and tend to collide each other.
Forces on the colloid Electrostatic repulsive forces
Attractive force: Van der Waals force
1 2 1 212 2 2
1 2( )
q q q qF k k
R r r
When the two particles collide
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Coagulation Theory – Force between particles
Electrostatic force ~ 1/r2,Repulsive
Net force
Van der Waals force ~ 1/r
Energy barrier
Distance
Energ
y/
Forc
e
Repuls
ive
Att
ract
ive
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Stabilization/Destabilization Destablization of hydrophobic colloids can be
accomplished by double layer compression, charge neutralization, enmeshment, and interparticle bridging.
1. Adding electrolytes into the solution (Figure 11.10b) destablizes colloids by double layer compression. Counterions of the electrolyte suppress the double-layer charge of the colloids to permit particles to contact due to excession van der Waals forces.
2. Charged species adsorb to (attach to the surface of) of the colloid and reduce the surface charge (Figure 11.11a). Trivalent metals hydrolyze in water to produce hydroxo complexes that carry positive charge and have an affinity to attach to negatively charged colloids, thereby reducing overall colloidal charge and resulting in aggregation.
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Stabilization/Destabilization A colloidal suspension is stable when the dispersion shows little
or no tendency to aggregate (Figure 11.10a). The repulsive force disperses particle and prevents aggregation.
Particles with a high zeta potential produce a stable sol. Factors tending to destabilize a sol are van der Waals forces of
attraction and Brownian movement. Van der Waals are the molecular cohesive forces of attraction
that increase in intensity as particle approach each other. Brownian motion is the random motion of colloids caused by their
bombardment by molecules of the dispersion medium. This motion has a destabilizing effect on a sol because aggregation may result.
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Figure 11. 10 Schematic representations of coagulation and bridging of colloids. (a) A stable suspension of particle where forces of repulsion exceeds forces of attraction. (b) Destablization and coagulation caused by counterions of a coagulant suppressing the double-layer charges.
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Stabilization/Destabilization3. At particular pH conditions and high enough Al or Fe
concentrations, the metal will precipitate. Colloids tend to adsorb to these solids and become enmeshed in the resulting solid (Figure 11.11b). This mechanism is called sweep floc coagulation.
4. Interparticle bridging is the adsorption of colloids to a relatively large molecule, such as organic polymer, to destablize and aggregate the colloidal suspension. The long polymer molecule attaches to absorbent surfaces of colloidal particles by chemical or physical interactions, resulting in aggregation.
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Figure 11.11 Coagulation mechanisms: (a) charge neutralization, (b) sweep floc, (c) agglomeration of destablized particle by attachment of coagulant ions and bridging of polymers.
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Destabilization Two basic destabilization mechanisms:
Coagulation reduces the net electrical repulsive forces at particle surfaces by adding coagulant chemicals.
Flocculation is a agglomeration of the destabilized particles by chemical joining and bridging.
In water treatment, both are used to destabilize turbidity, color, odor-producing compounds, pathogens, etc.
In wastewater treatment, coagulation precedes tertiary filtration necessary to clarify a biologically treated effluent for effective chemical disinfection.
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Coagulation process The common unit operations and chemical additions in
the treatment of surface waters for a potable supply (Figure 11.12).
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Coagulants Mainly aluminum and iron salts
Aluminum sulfate (most common) Ferric sulfate (second most common) Ferric chloride
Lime [Ca(OH)2]
Aluminum salts are cheaper, but iron salts are more effective over wider pH range (pH 4-9), and are more effective at removing NOM.
For some waters, cationic polymers are effective as a primary coagulant (the higher the charge on the cation, the more effective is the coagulant), but polymers are more commonly applied as coagulant aids.
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Aluminum Sulfate (Filter Alum) Aluminum sulfate reacts with natural alkalinity in water to form
soluble Al hydroxo complexes or Al hydroxide floc:
The production of H+ will tend to depress the pH. So, the types of hydrolysis products formed depend on the pH. Addition of base to control the pH shift may be required.
If sufficient alum is added, and pH = 6-8, aluminum hydroxide precipitate is formed for the sweep floc coagulation mechanism:
Each mg/L of alum decreases water alkalinity by 0.5 mg/L as CaCO3.
O14.3H3SO2H2Al(OH)OH2O14.3H)(SOAl 2-2
42
22342
O14.3H3SO6H2Al(OH)OH6O14.3H)(SOAl 2-2
4322342
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Aluminum Sulfate (Filter Alum) Lime or soda ash is added to provide necessary alkalinity
and control the pH of the coagulation process:
Soda ash does not increase hardness, only corrosiveness, but lime is more popular and less expensive.
Dosage range : 5-50 mg/L; optimum pH: 5.5-8.0.
O14.3H3CaSO2Al(OH)3Ca(OH)O.14.3H)(SOAl 24322342
O14.3H3COSO3Na2Al(OH)
O3HCO3NaO.14.3H)(SOAl
22423
2322342
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Ferric Sulfate
It reacts with alkalinity presents in water:
Fe(OH)3 is dense and settle fast. If alkalinity is not enough, hydrated lime is used. Optimum pH is between 4 and 12.
24323342 6CO3CaSO2Fe(OH))3Ca(HCO)(SOFe
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Ferric Chloride It reacts with natural alkalinity
If alkalinity is insufficient, lime is added
Each of the above equation has an optimum pH range.
223233 6CO3CaCl2Fe(OH))3Ca(HCO2FeCl
2323 3CaCl2Fe(OH)3Ca(OH)2FeCl
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Advantages of Ferric Coagulation
Coagulation is possible over a wider pH range, generally 4-9 for most waters.
The floc settes better than alum floc. more effective in the removal of NOM, taste,
and odor compounds.
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Synthetic Polymers Are water-soluble high-MW organic compounds that have
multiple electrical charges along a molecular chain of C atoms.
Three types: Cationic polymer: the ionization groups have a positive charge Anionic polymer: the ionization groups have a negative charge Nonionic polymer: no charges are exhibited or if the net charge is
zero
Are extensively used as coagulant aids with aluminum and iron coagulants in treatment of turbid waters to build larger floc by bridging mechanism.
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Synthetic Polymers Cationic polymers can be effective for coagulation, without
hydrolyzing metals, by producing destablization through charge neutralization and interparticle bridging. Common dosage: 0.5-1.5 mg/L
Anionic and nonionic polymers are effective coagulant aids. After destablizing the colloidal suspension by hydrolyzing metals such as alum, polymers promote larger floc by a bridging mechanism (Figure 11.11c). Common dosage: 0.1-0.5 mg/L
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pH Adjustment Used if pH of water to be treated is not within
the optimum pH of the coagulant. pH is increased using lime, sodium
hydroxide, and soda ash. pH is reduced using sulfuric acid,
phosphoric acid.
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Factors Affecting Coagulation
1. Coagulant
2. Coagulant aids
3. pH
4. Alkalinity
5. Temperature
6. Time
7. Velocity
8. Zeta potential
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Water Softening
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Causes of Hardness Water hardness is principally caused by:
calcium ionsmagnesium ions
Source of calcium and magnesium ionsgeological formations
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Types of Hardness
Carbonate hardness - carbonates and bicarbonates of calcium, magnesium and sodium, which can be removed and settled by boiling of water.
CO32-, HCO3
-
Noncarbonate hardness - metallic cations of iron, manganese and strontium
SO42-, Cl-, NO3
-
Total hardness = carbonate + noncarbonate
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Water Hardness The maximum level of hardness considered for public
supply is 300 to 500 mg/l, though many customers object to water harder than 150 mg/l.
Disadvantages of hardnessexcessive soap consumption during launderingscale-formation in hot water heaters and pipes.
The use of synthetic detergents and pipe linings can overcome those problems.
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Hardness Ranges
Degree of HardnessHardness Concentration (mg/L as CaCO3)
US International
Soft 0 – 60 0 – 50
Moderate Soft 51 – 100
Slightly Hard 101 – 150
Moderately Hard 61 – 120 151 – 200
Hard 121 – 180 201 – 300
Very Hard > 180 > 300
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Softening In precipitation softening, lime (CaO) and soda ash
(Na2CO3) are used to precipitate calcium and magnesium form water. Lime treatment can also:
kill bacteriaremove ironhelp in clarification of surface water (coagulant)
Lime treatment will raise the pH value, so recarbonation by carbon dioxide is used to lower the pH by converting the hydroxide and carbonate ions to bicarbonate ion.
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Lime Lime is commercially available in the forms of:
quicklimehydrated lime
Quicklimeavailable in granular formcontains minimum of 90% CaOmagnesium oxide is the primary impurity
Hydrated Limecontains about 68% CaO
Slurry lime is written as Ca(OH)2.
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CO2
Carbon dioxide is:gascolorlessclearproduced by burning fuel such as coal, oil, or gasused to recarbonate lime-softened waterapplied through diffusers immersed in the treatment
tank
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Removal of hardness with soda ash and lime
.....(8)CaCONaCl2CaClCONa
.....(7)CaCOSONaCaSOCONa
.....(6)Mg(OH)CaClMgClCa(OH)
.....(5)Mg(OH)CaSOMgSOCa(OH)
.....(4)Mg(OH)CaCOMgCOCa(OH)
O.....(3)2HCaCO2Mg(OH))Mg(HCO22Ca(OH)
O.....(2)2H2CaCO)Ca(HCOCa(OH)
O....(1)HCaCOCOCa(OH)
3232
342432
2222
2442
2332
232232
23232
2322
Free of hardness?
Minimum practical limits of precipitation softening are 30 mg/L of Ca2+ and 10 mg/L Mg2+ expressed as CaCO3.
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Reactions From the reaction equations, it can be seen that:
Lime reacts first with free carbon dioxide (eq. 1)Next, lime reacts with calcium bicarbonate (eq. 2)Lime also reacts with magnesium carbonate and
bicarbonate (eqs. 3 and 4)Noncarbonate hardness (magnesium sulphate and chloride)
requires the addition of soda ash for precipitation (eqs. 5 and 6)
Noncarbonate hardness (calcium sulphate and chloride) requires the addition of soda ash only for precipitation (eqs. 7 and 8)
Hardness?
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Pros and Cons of Softening Advantage of precipitation softening:
the lime added is removed along with the hardness taken out of solution.
TDS of the water are reduced.the chemical reactions can be used to estimate the quantity
of sludge produced.
Disadvantage of precipitation softening:sodium ions, from the addition of soda ash, remain in the
finished water.
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Recarbonation
Recrabonation is used to stabilize lime-treated water, and thus reducing its scale-forming potential.
Carbon dioxide is used for the recarbonation process. It converts lime to calcium carbonate. Further recarbonation will convert carbonate to bicarbonate.
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Recarbonation Reactions
23232
2322
2322
)Ca(HCOOHCaCOCO
8.5)(pHcontrolpHforionRecarbonat
OHMgCOMg(OH)CO
OHCaCOCa(OH)CO
9.5) (pH
control pH and lime excess of removalfor ionRecarbonat
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Stoichiometric Requirement Based on the above equations, the stoichiometric
requirement for lime and soda ash expressed in equivalent per unit volumes are
Approximately 1 eq/m3 of lime in excess of the stoichiometric requirement must be added to bring the pH to above 11 to ensure Mg(OH)2 complete precipitation. After precipitation, recarbonation is needed to bring pH down to a range of 9.2 to 9.7.
alkalinityMgCa)(eq/m requiredash Soda
excessMgHCOCO)(eq/m required Lime223
2-32
3
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Example Water has the following composition: calcium = 82 mg/L,
magnesium = 33 mg/L, sodium = 14 mg/L, bicarbonate = 280 mg/L, sulfate = 82 mg/L, and chloride = 36 mg/L. Determine carbonate hardness, noncarbonate hardness, and total hardness, all in terms of mg/L of CaCO3.
Solution:
The species concentration in meg/L is computed as
The species concentration in mg/L of CaCO3 is calculated as
3CaCOfor502
100
weightequivalent
mg/Lmeq/L
species ofweight equivalent
50species ofmg/L
50speciesofmeq/LCaCO asmg/L 3
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Solution Construct a table for ions in mg/L as CaCO3:
Ion species MW EWConcentration
mg/L meq/L mg/L as CaCO3
Ca2+ 40 20 82 4 200
Mg2+ 24.3 12.2 33 2.7 135
Na+ 23 23 14 0.6 30
Total: 7.3 365
HCO3- 61 61 280 4.6 230
Cl- 35.5 35.5 36 1 50
SO42- 96.1 48 82 1.7 85
Total: 7.3 65
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Solution Construct an equivalent bar diagram for the cationic and anionic
species of the water: The diagram shows the relative proportions of the chemical species
important to the water softening process. Cations are placed above anions on the graph. The calcium equivalent should be placed first on the cationic scale and
be followed by magnesium and other divalent species and then by the monovalent species sodium equivalent.
The bicarbonate equivalent should be placed first on the anionic scale and immediately be followed by the chloride equivalent and then by the sulfate equivalent.
0 200 335 365
0 230 280 365
Ca2+ Mg2+ Na+
HCO3- Cl- SO4
2-
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Solution Compute the hardness distribution:
Total hardness = 200 + 135 = 335 (mg/L as CaCO3)
Alkalinity = bicarbonate = 230 mg/L as CaCO3
Carbonate hardness = bicarbonate = 230 mg/L as CaCO3
Noncarbonate hardness = 335 – 230 = 105 mg/L as CaCO3
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Process Variations in Lime-Soda Ash Softening Depend on the degree of hardness and types and amount of
chemical added. Include
Excess-lime treatment Selective calcium removal Split treatment
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Excess-Lime Softening Carbonate hardness associated with Ca ion can be
effectively removed to the practical limit of CaCO3 solubility by stoichiometric additions of lime.
Precipitation of Mg ion needs additional 35 mg/L of CaO (1.25 meq/L) above stoichiometric requirements.
The practice of excess-lime treatment reduces the TH to about 40 mg/L (i.e., 30 mg/L of CaCO3 and 10 mg/L of Mg hardness).
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Excess-Lime Softening In excess-lime softening:
After excess lime addition, the water is flocculated and settled to remove CaCO3 and Mg(OH)2 precipitates.
After that, recarbonation is carried out in two stages.
In the first stage, CO2 is added to lower the pH to 10.3 and converts excess lime to CaCO3.
Water is then flocculated and settled. If needed, soda ash is added at this stage to remove noncarbonate
hardness.
In the second stage, CO2 is added to further lower the pH to the range of 8.5 to 9.5 to convert most of the remaining carbonate ion to bicarbonate ion in order to stabilize the water against scale formation.
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Figure 11.13 Schematic flow diagram for a two-stage excess-lime softening plant.
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Example 11.6 Water defined by the following analysis is to be softened by excess-lime
treatment in a two-stage system (Figure 11.13):
CO2 = 8.8 mg/L as CO2 Alk(HCO3-) = 115 mg/L as CaCO3
Ca2+ = 70 mg/L SO42- = 96 mg/L
Mg2+ = 9.7 mg/L Cl- = 10.6 mg/L
Na+ = 6.9 mg/L
The practical limits of removal can be assumed to be 30 mg/L of CaCO3 and 10 mg/L of Mg(OH)2, expressed as CaCO3.
a) Sketch a meq/L bar graph and list the hypothetical combinations of chemical compounds in the raw water.
b) Calculate the quantity of softening chemicals required in lb/Mgal of water treated and the theoretical quantity of CO2 needed to provide a finished water with one-half of the alkalinity converted to bicarbonate ion.
c) Draw a bar graph for the softened water after recarbonation and filtration.
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SolutionFigure 11.14 Meq/L bar
graph.
(a) Bar graph and hypothetical chemical combinations in the raw water.
(b) Bar graph of the water after lime and soda ash additions and settling but before recarbonation.
(c) Bar graph of the water after two-stage recarbonation and final filtration.
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Selective Calcium Removal If the water to be treated contains low concentration of magnesium
(< 40 mg/L as CaCO3), selective calcium removal can be used.
Magnesium hardness of more than 40 mg/L as caCO3 is not recommended due to the possible formation of hard magnesium silicate in high temperature waters (180F).
Enough lime is added to precipitate calcium hardness without providing any excess for Mg removal.
Soda ash may be used depending on the extent of noncarbonate hardness.
If precipitation of CaCO3 is not satisfactory, alum or a polymer can be used to aid flocculation.
Recarbonation is used to reduce scale formation on the filter and to produce stable water.
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Figure 11.15 Schematic diagram for a single-stage calcium-carbonate softening plant.
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Example 11.7 Determine the chemical dosages needed for selective calcium softening
of the water described in Example 11.6. Draw a bar graph of the processed water.
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Solution
Figure 11.16 Bar graph of the softened water after selective calcium removal.
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Iron and Manganese Removal
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Iron (Fe) and Manganese (Mn) Fe and Mn in concentrations greater than 0.3 mg/L of
Fe and 0.05 mg/L of Mn stain plumbing fixtures and laundered clothes.
Foul tastes and odors can be produced by growth of Fe bacteria in water distribution mains. These filamentous bacteria, using reduced Fe as an energy source, precipitate it, causing pipe encrustations. Decay of the accumulated bacterial slimes creates offensive tastes and odors.
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Chemistry of Fe and Mn Fe and Mn are abundant elements in the earth’s crust. They get into
natural water from dissolution of rocks and soil, from acid mine drainage, and from corrosion of metals.
Under reducing conditions (absence of DO and low pH), ferrous iron Fe2+ and manganous manganese Mn2+ are
chemically reduced soluble complexed with NOM
When exposed to air, they are oxidized to ferric iron Fe3+ and manganic manganese Mn4+, which are:
oxidized stable insoluble
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Chemistry of Fe and Mn Oxygen, chlorine, and potassium permanganate
(KMnO4) are the most frequent oxidizing agents.
Oxidation reactions using KMnO4 are
3Fe+ + MnO4- 3Fe3+ + MnO2
3Mn2+ + 2MnO4- 5MnO2
MnO2 acts as catalysts that increase the rate of Mn oxidation.
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Fe and Mn Removal Process The techniques for removing Fe and Mn are based on the
oxidation of Fe2+ and Mn2+ to the Fe3+, Mn3+ and Mn4+, and the oxidation of any organic-complex compounds. This is followed by filtration to remove insoluble compounds.
Four major techniques for Fe and Mn removal from water are: Aeration-Filtration Aeration-Chemical Oxidation-Sedimentation-Filtration Water Softening Greensand Filtration
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Aeration-Filtration Aeration (air oxidation) is the simplest oxidation
treatment in removing iron. The reaction that takes place is in the form of:
2Fe(HCO3)2 + 0.5O2 +H2O 2Fe(OH)3 + 4CO2
Aeration alone cannot remove manganese effectively. Increasing the pH to 8.5 can enhance the oxidation process. If manganese is not effectively removed from the water, it can cause problems with post-chlorination. When oxidized:
it can clog the solution-feed chlorinatorit cause a staining water
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Aeration-Chemical Oxidation- Sedimentation-Filtration This is a common method for removing iron and manganese
from well water without softening treatment. Contact tray aeration is designed to displace dissolved gases
(i.e., CO2) and initiate oxidation of the reduced Fe and Mn.
Chlorine, potassium permanganate, ozone, chlorine dioxide can oxidize Fe and Mn.
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Aeration-Chemical Oxidation- Sedimentation-Filtration When Cl2 is used, a free available chlorine residual is
maintained throughout the treatment process.
The KMnO4 oxidation is many times faster than Cl2 for the oxidation of Mn. Potassium permanganate oxidizes iron and manganese at rates faster than dissolved oxygen and its reaction is relatively pH independent.
Ozone oxidizes Mn faster than other oxidants, but if the dosage is too high, it may convert the manganese to permanganate and thereby cause the water to turn pink (the color of permanganate solution).
Since iron and manganese can not be completely removed by sedimentation, effective filtration is required.
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4H2ClOMnOO2H2ClOMn
2Cl4HMnOO2HClMn
4H2MnOO2HO2Mn
3HClOFe(OH)O3HClOFe
2Cl6H2Fe(OH)O6HCl2Fe
8H4Fe(OH)O10HO4Fe
22222
2222
2222
23222
3222
3222
Oxidation of Iron and Manganese with Oxygen, Chlorine, and Chlorine Dioxide
75
Water Softening Lime-soda ash softening will also remove Fe and Mn. Lime treatment has been used to remove organically bound Fe
and Mn from surface water. The process scheme aeration-coagulation-lime treatment-sedimentation-filtration can treat surface waters containing color, turbidity, and organically bound Fe and Mn.
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Greensand Filtration The mineral glauconite, commonly known as greensand, can be used
in pressure filters with either continuous or periodic addition of permanganate to remove Fe and Mn.
Oxides on the greensand surface can oxidize Fe and Mn. Permanganate oxidizes Fe and Mn and regenerates the greensand.
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Greensand Filtration KMnO4 is applied prior to filtration.
The filter is a dual-media filter with anthracite filter medium is placed on top of the manganese zeolite.
Iron and manganese are oxidized by KMnO4.
The upper layer will remove the insoluble metal ions. Any iron and manganese ions not oxidized, it will be captured
by the lower layer of manganese zeolite.
Any surplus KMnO4 will regenerate the greensand.
When the bed becomes saturated, it is backwashed by KMnO4 to remove particles from the upper layer and regenerate the greensand.
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2432
332
22
332
22
MnOZKMnOOMnZ
onRegenerati
MnOMnZMnMnOZ
FeOMnZFeMnOZ
Oxidation
Reaction Equations
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Disinfection
80
Disinfection Definition: the process of destruction or inactivation of waterborne
pathogens or living pathogenic microorganisms. Post-disinfection – to disinfect filtered water and leave an adequate
disinfectant residual in the treated water to ensure its safety in the distribution system until its use by the farthest consumer
Disinfection depends on: The physico-chemistry of the disinfectant The cyto-chemical nature and physical state of the pathogens The interaction of the above Temperature pH Electrolytes Interfering substances
81
Disinfection Most chemical disinfectants form harmful DBPs:
Chlorine: THMs and HAAs Chloramines: cyanogen chloride and chloropicrin Chlorine dioxide: chlorites and chlorates Ozone: bromates and aldehydes
The balance between proper disinfection and disinfection byproducts is the goal of each water utility.
82
Disinfection Methods
Chlorine Alternative disinfectants and oxidants:
Ozone Chlorine dioxide UV irradiation High pH Other halogens (iodine and bromine)
Reasons: DBPs, greater disinfection efficiency Disadvantages: Typically more costly, etc.
83
(1) Chlorine and Chloramines Chlorine (Cl2) is widely used
Effective at low concentration Cheap Helpful in controlling taste and odor, Fe, Mn, cyanides, and
phenols Forms residual if applied in sufficient dosages
Chlorine is applied as: Gas (most common) hypochlorite
Chlorine is a strong oxidizing agent It reacts with various organic substances, ammonia, and metals. It inactivates microorganisms by reacting with their enzymes.
84
Chlorination Reaction Chlorine gas reacts with water (hydrolyzes) to form (lower the
pH) Hypochlorous acid (HOCl) – the principal disinfecting form of
chlorine. H+ and Cl- (no disinfection potential)
At pH above 7.5, HOCl dissociates to hypochlorite ion (OCl–) ( a function of pH); thus, it becomes less and less effective. The higher the pH, the higher the ionization, the less effective the
Cl2.
ClHHOClOHCl 22
OClHHOCl So, ACID!!!
85
Figure 11.21
86
Chloramines Reaction with ammonia (a function of pH, [Cl2]/[NH3],
temperature)
At pH 4.5-8.5, monochloramine and dichloramine are formed. At pH > 8.5, monochloramine predominates. At pH about 4.5, dichloramine predominates. At pH 4.4, trichloramine predominates Chloramines are effective against bacteria but not viruses.
hloramine tric
OH3NCl3HOClNHCl
nedichlorami
O2HNHCl2HOClClNH
minemonochlora
OHClNHHOClNH
232
222
223
pH
87
Chlorination Reaction
Reaction with organicsReaction with phenol produces chlorophenolsReaction with humic substances (or NOM) produces
trihalomethanes (THMs) Chloroform (CHCl3)
Bromodichloromethane (CHCl2Br)
Dibromochloromethane (CHClBr2)
88
Dosages, Demand and Residuals Dosage: the amount of chlorine added. Demand: the amount of chlorine needed to oxidize materials,
or the difference between the amount added and the quantity of free and combined available chlorine remaining at the end of a specified contact period.
Residual: the amount of chlorine remaining after oxidation. Assume we used 3 mg/L chlorine dose in a sample, and after 30 min
there was 2 mg/L residual.
Chlorine demand is therefore 3 mg/L – 2mg/L = 1 mg/L.
89
90
Combined and Free Available Residual Chlorine Combined available residual chlorine
– residual chlorine existing in chemical combination with ammonia (chloramines) or organic nitrogen compounds.
Free available residual chlorine
– residual chlorine existing in water as hypochlorous acid or hypochlorite ion. (Note: some say +Cl2)
91
Chlorine Residual Curve When chlorine is added to water containing reducing agents
and ammonia, residual develop that yield a curve as shown in the next slide.
A-B When chlorine is added to water, chlorine first reacts with reducing agents, such as nitrites, ferrous ions, and H2S.
B-C The excessive addition of chlorine results in the formation of chloramines (combined available residual chlorine) which are much less effective than the free chlorine (faster and higher disinfection capacity).
C-D The previously produced chloramines are oxidized to produce nitrogen compounds, such N2, N2O, NO3
-.D Once most of the chloramines are oxidized, additional
chlorine creates an equal residual. Point D is referred to as the breakpoint.
Beyond point D, all added residual is free available chlorine.
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No residual
Hump
Breakpoint or dip
Free residual chlorine / breakpoint residual
HOCl
Completion of chlorination reactions
N2, N2O, NO3-
Reducing agents
A B
C
D
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Reactions of Chlorine with Reactants in Water Reactions in sequence: Reducing agents
Fe, Mn, H2S and nitrites neutralize Cl2 into chlorides.
Red water, black water, tastes and odors are removed. No residual chlorine at this stage.
Ammonia Choramines are formed until hump. Combined available residual chlorine
Destruction of combined residual chlorine Chlorine reacts with combine chlorine, resulting in a drop of residual
chlorine.
Breakpoint chlorination therefore ensures a proper disinfection after control of Fe, Mn, bacteria, tastes and odors.
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Breakpoint Chlorination The purpose of breakpoint chlorination is to
produce and maintain free-residual chlorine in the water after complete oxidation of substances that react with chlorine.
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Chloramination Often practiced in water treatment, particularly to
maintain a residual in the distribution system after some other disinfectant (chlorine, ozone, or chlorine dioxide) is used as the primary disinfectant.
Effective for bacteria, but not for viruses.
[Cl2]:[NH3] = 3-6.
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Manual-Control Chlorinator
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Automatic Proportional-Control Chlorinator
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Factors Affecting Chlorination pH
The lower the pH, the more effective the free-residual chlorine. Why?
The amount of HOCl decreases at above pH 7.5 as it ionizes into OCl-.
Type of residual chlorine HOCl is more effective than the OCl-.
Temperature The higher the temperature, the quicker the
disinfection and the shorter the required contact time is.
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Factors Affecting Chlorination Contact time period
Chlorine requires a certain amount of contact time at different temperatures to react with microorganisms.
The longer the contact time, the more effective the disinfection is. So, the concept of CT is used to ensure a proper disinfection.
CT stands for concentration of a disinfectant as mg/L and its contact time in minutes.
Required CT is constant for each disinfection at different temperatures.
In chlorination, it also takes into consideration the pH of the water. The lower the pH, the better the disinfection, and the lower the CT value is.
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Factors Affecting Chlorination Concentration
The higher the concentration, the more effective the disinfectant is.
Normally 0.5 to 1 ppm free-residual chlorine will effectively disinfect the water.
According to the SDWA, the maximum allowable chlorine residual in the distribution system is 4 mg/L and the minimum required is 0.2 mg/L.
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CT and CT Ratio C stands for the concentration of a disinfectant as mg/L, which is the
lowest residual value of the disinfectant at the highest flow at the effluent site.
T stands for the contact time in minutes, which is the detention time of a pipe or a basin at the highest flow during 24 hours.
CT value is CT as mg/L-minutes. CT value takes into consideration the lowest C and its shortest T, and
vice versa. CT calculation needs the disinfectant residual at the effluent end of
each pipe and basin and their detention times at the highest flow to satisfy CT requirements.
The calculated CT for each part of the disinfection train divided by he required CT (from the CT table provided by the EPA) for each disinfectant is called available CT.
The higher the available CT value, the better the disinfection is.
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Dechlorination Chlorinated effluents can have negative impacts
on receiving environment Chemicals used for dechlorination include:
Sulfur dioxide (most common)Sodium sulfiteSodium bisulfiteSodium thiosulfateHydrogen peroxideAmmonia
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Sulfur Dioxide (SO2) General characteristics
Has solubility in water of 18.6% at 0CWhen reacts with water, it forms a weak solution of sulfurous
acid (H2SO3)Sulfurous acid dissociates as follows:
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332
SOHHSO
HSOHSOH
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Sulfur Dioxide (SO2)
Free and combined chlorine forms react readily with sulfite ion (SO3
2-) as follows:
Required mass ratio of SO2 to Cl2 is 1.1:1
42422
23
24
23
NHClSOOHClNHSO
HClSOHOClSO
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(2) Chlorine Dioxide (ClO2)
ClO2 was originally used to remove taste and odor from water
General characteristics of ClO2
More powerful oxidant than chlorineIts effectiveness is not affected by ammonia and pHIt is very effective when followed by chlorine or
chloraminesDoes not react with water, so can be easily removed from
water by aerationReadily decomposed by exposure to UV radiationMaintains a stable residual
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Preparation of ClO2
Produced onsite by mixing sodium chlorite and (excessive) chlorine:
Acid and sodium chlorite
Sodium hypochlorite and sodium chlorite
HNaClClOHClNaClO 22
OH2NaCl2ClO2NaClOHClHOCl
HClHOClOHCl
222
22
222 H3NaCl2ClO2HClNaOCl2NaClO
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Advantages of ClO2 Disinfection
ClO2 is strong bactericide and viricide over a wide pH range.
ClO2 has longer lasting residual than HOCl.
In wastewater, ClO2 use is limited to phenolic wastes and the control of sulfide in wastewater collection systems.
ClO2 does not produce measurable amounts of THMs (trihalomethanes) or TOXs (total organic halogens).
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Disadvantages of ClO2 Disinfection
ClO2 is unstable compound that shortly reverts to chlorine. Being short lived, it is generated onsite and applied immediately.
It is explosive at a concentration above 10% in the air. It is relatively expensive to generate. It forms chlorites (can be reduced by ferrous ions) and
chlorates, which are toxic. Chlorite is regulated as a DBP in the primary drinking water standards at 1 mg/L, and ClO2’s maximum residual disinfectant level leaving the treatment plant is 0.8 mg/L. Chlorate is unregulated yet.
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(3) Ozone General characteristics of ozone (O3)
More reactive than Cl2 and ClO2.
The reactions are rapid in inactivating microorganisms, oxidizing Fe, Mn, sulfide, and nitrite, and slower in oxidizing organic compounds like humic and fulvic substances, pesticides, and VOCs.
Does not react with water to produce DBPs, but decomposes in water to produce O2 and OH·.
Unstable in aqueous solutions, has a half-life of 10 to 30 minutes in distilled water; therefore, it is produced on-site and cannot be stored.
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Ozone Production1. Air is refrigerated to remove moisture (dew point = -
40 to -60 C).
2. Air is dried through desiccants (silica gel and activated alumina).
3. Air is passed between oppositely charged plates .
4. This process converts a small % of O2 to O3 (1-3% from air, and 2-6 % from the O2 feed).
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Ozonation Processes Ozonation treatment of water can be divided into
3 parts: Preparation of feed gas is mainly filtration of air to
remove dust particles and moisture. Pure O2 feed is more efficient.
Production of O3
Contaction is contacting the microbes by the bubbling of an O3 and O2/air mixture through the water by dispersing it mostly by diffusers at the bottom of a contact chamber for the maximum O3 transfer.
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Ozonation System
Figure 11.25 Two-compartment ozone contactor with porous diffusers.
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Ozone Reactions A need to balance pathogen inactivation and DBP control. Ozone reacts rapidly with NOM, producing small organic
compounds. The biodegradability of the resulting organic carbon is increased. This biodegradable organic matter (BOM), measured as
biodegradable dissolved organic carbon (BDOC) or assimilable organic carbon (AOC), can cause problems of bacterial regrowth in the distribution system, production of tastes and odors, and increased residual chlorine demand.
Granular media filters can be operated in a biologically active state (i.e., microbial growth is encouraged in the filter to biodegrade the organics so they are not released to the distribution system.
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Alternatives of Ozonation
Various combinations of O3 and other disinfectants: Peroxone is the use of H2O2 and O3 to accelerate the
oxidation of some organics by 2-6 times when compared to O3 itself.
Soozone is a combination of ultrasonic waves and O3, where ultrasonic waves break organics particles, and O3 oxidizes them.
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Advantages of Ozonation
When compared to chlorination: Ozone degrades to O2, so no toxic residues.
Ozone has been used to remove tastes and odors, color, algae, THMs precursors, phenols, cyanides, sulfides, sulfites and heavy metals.
The required CT is very low.
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Disadvantages of Ozonation High cost of production. Short-lived with almost no residual effect. On-site production. Oxidizes bromide ion (Br-) to bromate (BrO3
-) (difficult to remove), a DBP regulated at 10 g/L.
Therefore, ozonation is less common than chlorination. Generally, ozonation is followed by chlorination or chloramination for the residual effect.
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(4) Ultraviolet (UV) Radiation
UV rays have wavelengths from 100 to 400 nm (Figure 11.26) and are produced by a variety of lamps (Table 11.6).
UV radiation is absorbed by and damages the nucleic acids in DNA and RNA of microorganisms which prevents their replication.
Although many microorganisms have enzymes that enable them to repair the damaged nuclei acids, higher UV doses and residual chemical disinfectants are expected to inhibit the repair and maintain inactivation.
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Figure 11.26 UV light in the electromagnetic spectrum
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U-V Tube
U-V Sterilizer
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Advantages and Disadvantages of UV In wastewaters, SS can shield bacteria and viruses from the UV
radiation. pH, temperature, alkalinity, and total organic carbon do not influence
the UV radiation. Potable water systems have low SS and turbidity. In potable water systems, chemical disinfection is used after UV
radiation to maintain a disinfectant residual. (LACK OF DISINFECANT RESIDUAL)
For wastewaters where residual disinfectants are not allowed, UV radiation has this particular advantage for disinfecting wastewaters. (DISINFECANT RESIDUAL IS NOT REQUIRED)
Chemical disinfection may be used together with UV radiation to inactivate viruses.
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Factors Affecting UV Treatment Turbidity
Turbidity shield microorganisms from radiation. The higher the turbidity, the less effective the UV treatment is
TDS Solids deposit on the lamp and foul it. The less the TDS, the better the treatment is.
Dissolved organic matter DOM absorbs the UV light and shields microbes from radiation. The less the DOM, the better the treatment is.
Depth The shallow the water, the more effective the UV treatment is
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Relative Strength of Disinfectants
O3 > ClO2 > HOCl > OCl- > NH2Cl
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Taste and Odor Control
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Causes of Taste and Odor
Taste and odor can be caused by: Decaying of natural organic compounds Microbes: algae and bacteria Dissolved gases (e.g., H2S) in GW Inorganic salts Metal ions (e.g., Fe, Mn, Pb, Cu, Zn) Disinfectants Organic compounds resulting from industrial activities Sewage
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Causes of Taste and Odor
In water systems, taste and odor problems are unique in each system.
in groundwater, odor is caused by dissolved gases that can be stripped from water by aeration.
Aeration
in surface water, odor is caused by nonvolatile organic compounds which can not be removed by aeration.
Breakpoint chlorination and AC treatment
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Prevention
Preventing taste and odor-producing substances from reaching surface water should be given priority.
if the problem is caused by industrial waste discharge, the source may be removed.
if algal growth is causing the problem, copper sulfate (one of the algaecides) or powered activated carbon can be used to stop the growth of algae.
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Treatment
Aeration
Oxidation
Activated Carbon
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(1) Aeration/Air Stripping
It is effective for removing dissolved gases and highly volatile odorous compounds.
Aeration as the first step in processing well water may acgieve any of the following: Removal of H2S, CO2 and CH4
Addition of DO for oxidation of Fe and Mn
Rarely effective in processing surface waters since the odor-causing substances are nonvolatile.
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(2) Oxidation Oxidation can be done by the use of:
chlorine (formation of THMs must be considered)chlorine dioxide (has the stronger oxidative power as
chlorine without forming THMs)potassium permanganate (strong oxidizing agent but
forms MnO2, so filtration must be applied after treatment with KMNO4)
Ozone
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Adsorption What is adsorption?
Attachment to interfaces between phases water and solid Air and solid
Effects removal of species
Uses removal of contaminants
activated carbon – removal of trace organic compounds ion exchange – resins for water softening metal hydroxides – coagulation in water treatment
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Adsorption on AC Adsorption on AC is the most effective treatment for the removal of taste
and odor from water. Activated carbon (AC) adsorption
Main treatment for the removal of tastes and odors, volatile organic compounds, and synthetic (pesticides) organic chemicals.
AC is a wood product made by burning plant matter to form charcoal through carbonation and activation.
Carbonation is the burning of wood material at 550 to 700C in the absence of air to form charcoal.
Activation is the further burning of charcoal at 800 to 900C in the presence of steam and CO2 to produce pores and crevices in and on the surface of the particles.
Two forms of AC in the water treatment: Powdered Activated Carbon (PAC) Granulate/Granular Activated Carbon (GAC) The choice depends on the water quality and design of the plant.
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Powdered Activated Carbon (PAC) Power is mixed with water to form slurry. Slurry is added to the water at
several points, starting from intake to the filter influent. AC is also a reducing agent because it reacts with disinfectants such as
chlorine, chloramines, chlorine dioxide and ozone. It adsorbs them and causes a higher disinfectant demand. Therefore, both should not be applied at the same time or in sequence without proper time interval.
AC is mostly applied into the influents of presedimentation basin and the final sedimentation basin. Other application points are rapid mix, flocculation basin and influent of the primary sedimentation basin. It is also applied to the influent of microfiltration and ultrafiltration processes. It is also applied after the recarbonation of the softened water to prevent the blocking of the adsorption sites by calcium carbonate.
A typical dose of PAC varies from 3 to 50 mg/L. Low capital cost and dose, flexibility of application point, no required
regeneration, coagulation aid by providing particulate matter for flocculation, but produces extra sludge.
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Granulate/Granular Activated Carbon (GAC) Used for a continuous treatment when there is a need for
continuous removal of volatile and synthetic organics compounds in addition to occasional taste and odor control.
It does filtration and adsorption. GAC needs regeneration when GAC is exhausted and then
releases adsorbed substances. More expensive than PAC due to regeneration and initial cost,
no flexibility of application at different points. So, PAC is more popular.
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Fluoridation
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Fluoridation Water fluoridation is the precise adjustment of the natural fluoride
concentration in a public water supply up to the level recommended for preventing tooth decay.
Low levels of fluoride result in increasing incidence of caries, while excessive fluoride results in mottled tooth enamel.
Recommended limits (Table 11.15) are based on air temperature, since this influences the amount of water ingested by people.
The US EPA has set the maximum contaminant level for fluoride in the drinking water at 4 mg/L.
To prevent dental decay, the optimum fluoride concentration has been established at 1 mg/L (American Dental Association, 1980).
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Table 11.15
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Fluoridation Most common fluoride compounds (Table 11.16):
Sodium fluoride (NaF) (45% F)
Sodium silicofluoride (Na2SiF6) (61% F)Most common, economical, convenient, and safe.
Fluosilicic acid (H2SiF6) (79% F)
Application of fluoride is best in a channel or water main coming from the filters, or directly to the clear well. If it is applied prior to filtration, some of the fluoride
may be lost due to reactions with other chemicals.
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Water Fluoridation Chemicals Sodium Fluoride (NaF)
Crystalline powder Relatively constant (low) solubility pH 7.6 Saturator Systems
Sodium Fluorosilicate (Na2SiF6) Crystalline powder Solubility varies with water temperature pH 3.5 Dry-feed Systems
Fluorosilicic Acid (H2SiF6) Liquid Infinite solubility pH 1.2 Venturi Systems
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Venturi Chemical InjectorBypass venturi Bypass venturi injection device injection device for injection of for injection of liquid chlorine, liquid chlorine, liquid fertilizer or liquid fertilizer or acid.acid.
Cutaway of a Cutaway of a venturi injector venturi injector cross-section.cross-section.
throttling valvethrottling valve
chemical suction portchemical suction port
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Example 11.14 The fluoride ion concentration in a water supply is
increased from 0.3 mg/L to 1 mg/L by applying 98% pure sodium silicofluoride. How many milligrams of chemical are required per liter of water?
Solution:
From Table 11.16, Na2SiF6 is 61% F.
Dosage = (1 – 0.3)/(0.98 0.61) = 1.17 mg/L
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Synthetic Organic Chemical Removal
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Removal of Synthetic Organic Chemicals
Synthetic organic compounds (SOCs) include a large number of chemicals used in industrial, agricultural, and household activities, e.g., pesticides, VOCs, and THMs.
Some of the SOCs are toxic substances and can cause cancer or damage to vital organs, others can impair the nervous system. The maximum allowable level of SOCs in water range from 0.0002 to 0.1 mg/l.
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Sources of SOCs
Groundwaters can be polluted by:leaching of agricultural pesticidesseepage from improper disposal of industrial volatile
organic chemicals (VOCs)
Surface waters can be polluted by SOCs through runoff from:
agricultural landsdischarge of industrial wastewaterspillage of chemicals
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Treatment Conventional water treatment (coagulation -
sedimentation - filtration) is not effective in removing SOCs.
Other alternatives that can improve the treatment include:
changing the coagulantschanging the polymersadjustment of pHuse of activated carbon adsorption – poor adsorption due
to pore structure of PAC, short contact time between carbon particles and dissolved organic chemicals, and interference by adsorption of other organic compounds.
use of filtration through a bed of granular activated carbon
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Treatment
Granular activated carbon (GAC) can be reactivated by heating in a furnace.
Pilot plant studies need to be conducted in order to:
select the best type and dosage of carbondetermine the contact timedetermine the effect of water quality variationdetermine the effectiveness of reactivation
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Involves the incorporation of PO43- into TSS and the
subsequent removal of those solids. Phosphorus can be incorporated into either biological
solid (microorganism) or chemical precipitates. Phosphate precipitation can be brought about by the
addition of the salts of multivalent metal ions that form precipitates of separingly soluble phosphates.
eg: Ca2+, Al3+, Fe3+.
Chemical Precipitation for Phosphorus Removal
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Phosphate Precipitation with Calcium
Ca is added in the form of lime, Ca(OH)2. When lime is added to water, it reacts with
the natural bicarbonate alkalinity to CaCO3.
Ca(OH)2 + Ca(HCO3)2 2CaCO3 + 2H2O
When pH > 10, excess Ca2+ will precipitate out PO4
3- as hydroxylapatite.
10Ca2+ + 6PO43- + 2OH- Ca10(PO4) 6(OH) 2
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The quantity of lime required depends on the alkalinity of the water but not on the amount of phosphate present.depends on the alkalinity of the water but not on the amount of phosphate present.
Quantity required to precipitate P is typically ~1.4 to 1.5 times the total alkalinity expressed as CaCO3.
Phosphate Precipitation with Calcium
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Phosphate precipitation with Al:Al3+ + HnPO4
3-n AlPO4 + nH+
Phosphate precipitation with Fe:Fe3+ + HnPO4
3-n FePO4 + nH+
Many competing reaction occur at the same time when Al or Fe precipitate out the PO4
3-. This causes the requires precipitant dosage cannot be estimated.
Phosphate Precipitation with Aluminium and Iron
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Al & Fe salts are added to the untreated water, in the activated sludge aeration tank or the final clarifier influent channel.
P is removed from the liquid phase through precipitation, adsorption, agglomeration and removed as 1’ or 2’ sludges.
Phosphate Precipitation with Aluminium and Iron