Transcript Wastewater Treatment - WaterTreatment-TIP3-2011

Wastewater Treatment
Environmental Chemistry
TIP 2011
What is Wastewater Treatment?
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Wastewater treatment is also referred to as
sewage treatment
Process of removing physical, chemical, and
biological contaminants from wastewater and
household sewage
Goal is to separate wastewater into:
Environmentally-safe fluid waste stream
 Solid waste to be disposed or reused
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Wastewater Sources
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Wastewater comes from:
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Homes
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Sinks, showers, toilets,
washing machines,
dishwashers
Businesses
 Industrial facilities
 Storm runoff
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From roads, parking lots,
roofs
Image: http://www.acumen.com.my/content/chemical-testing?q=node/6
What Needs to be Removed?
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Wastewater may contain a variety of substances:
trash and debris
 human waste
 food scraps
 oils
 grease
 soaps
 chemicals (cleaning, pesticides, industrial)
 pharmaceuticals and personal care products
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http://www.westfield.ma.edu/personalpages/draker/edcom/final/webprojects/sp11/triparoundworld/Marine.html
What Needs to be Removed?
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Wastewater from both domestic and industrial sources
may contain a variety of potentially harmful
contaminants, including:
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Bacteria
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Viruses
Nitrates
Metals
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E. coli (right), Giardia, Hepatatis A
mercury, lead, cadmium, chromium, arsenic
Toxic materials
Salts
http://www.anh-usa.org/the-european-e-coli-outbreak-the-real-story/
Example 1: Concentration
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A 4.2 mL wastewater sample was tested and
found to contain 7.6 ng of lead (II) ions. What
is the molarity of lead (II) ions in this solution?
Example 1 Solution
7.6 ng Pb x 10-9 g x 1 mol Pb x 1 mL soln
4.2 mL
1 ng
207.2 g Pb
10-3 L
= 9.1 x 10-9 mol/L
= 9.1 x 10-9 M Pb
Why Treat it?
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The environment is able to naturally dilute and degrade
water contaminants, but only in small amounts
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Wastewater
treatment reduces
pollutants to levels
that the
environment can
safely handle and
process
Why Treat it?
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Decaying solid matter left in water consumes
dissolved oxygen from the water
Known as Biochemical Oxygen Demand (BOD):
the amount of dissolved oxygen needed by aerobic
organisms to break down organic matter
 Lack of oxygen can kill plants and aquatic life
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Excessive nutrients (nitrogen and phosphorous)
can also lead to deoxygenation
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Increased plant and algae growth, which eventually
die and decompose, lead to an increased BOD
Process of Water Treatment
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Water which enters a water treatment facility
undergoes a series of steps to cleanse the water
using physical, chemical, and biological
processes
Upon exiting the water treatment facility, the
decontaminated water is released into rivers or
streams, entering again into the environment
Sometimes used specifically for agriculture and
irrigation
 Possible to purify into clean drinking water again
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Steps of the Wastewater Treatment
Process
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1. Pretreatment
2. Primary Treatment
3. Secondary Treatment
4. Tertiary Treatment
5. Sludge Processing
Water Treatment Process
Image: http://www.cityofdunbarwv.com/node/22
Treatment Process
Step 1: Pretreatment
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Prepares waste water for entering
the treatment plant
Removal of larger debris by
screening (shown right)
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Trash
Tree limbs
Removal of grit and gravel by
screening and settling
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Gravel must be removed early as it
can damage machinery and
equipment in the treatment plant
http://www.alard-equipment.com/wastewater/index.htm
Treatment Process
Step 2: Primary Treatment
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In Primary Treatment, as much solid material is
removed as possible by relying on gravity
Removes most of the sludge and scum
Sludge: Organic and inorganic materials which will
naturally settle
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Scum: Materials which will float (oil, grease, soap)
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removed by sedimentation
removed by skimming
This step successfully removes 50 to 70% of suspended
solids and up to 65% of oil and grease
Colloidal and dissolved materials are not affected by
this step
Separation of Oil and Grease
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Oil and grease will naturally separate from water due to
differences in polarity
This is also known as the hydrophobic effect
Water is considered a polar substance, while oils and grease are
considered nonpolar substances
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A polar molecule is one in which
electrons are unevenly
distributed within the molecule
due to differing
electronegativities
Nonpolar molecules generally
have evenly distributed electrons
andhave no areas of partial
charges
http://novocreamseparators.com/blog/clean-separation/
Separation of Oil and Grease
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Water molecules have regions of differing electron
density, making one end of the molecule have a
partially negative side, while the other is partially
positive
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Water molecules are attracted to one another due to
attractions between these positive and negative regions
http://bioweb.wku.edu/courses/biol115/Wyatt/Bonds.htm
(hydrogen bonding)
http://marineodyssey.co.uk/abioticoceans.html
Separation of Oil and Grease
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Oil and grease are typically long chains of
hydrocarbons, making them nonpolar,
hydrophobic substances
Mixing a hydrophobic substance such as oil
into water disturbs the attractions between
polar water molecules
Hydrophobic substances tend to aggregate
together in water in order to minimize the
surface area that contacts the water which
minimizes the disturbance
Oils and grease rise to the top of water due
to a difference in density
http://en.wikibooks.org/wiki/Structural_Biochemistry/Chemical_Bonding/Hydrophobic_interaction
Primary Treatment:
Physical Separation
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Sewage flows through large tanks known as primary
clarifiers or primary sedimentation tanks
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Round or rectangular basins, 3 to 5 meters deep
Water retained here for 2 to 3 hours
Sludge will settle toward the bottom of tanks, while
scum will rise to the top. Both are removed and
pumped to sludge treatment tanks
Mechanical scrapers continuously drive sludge into a
well at the bottom of the tanks to be removed
Mechanical skimmers or rakes remove oils and grease
from the surface.
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May be recovered to use in saponification
Saponification
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Saponification is the base hydrolysis of fats and
oils to produce glycerol and a crude soap
triglyceride (fat)
glycerol
Primary Treatment: Aeration
Another process during primary treatment is
aeration
Water is agitated and exposed to air, which
serves two purposes:
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Allows some dissolved gases to escape, such as foul
smelling hydrogen sulfide gas
Allows more oxygen to be dissolved into the water.
Oxygen may be bubbled into water at this point.
Increasing dissolved oxygen in water
compensates for the increased BOD and helps
with the sludge settling process
Example 2: Concentration
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9 ppm is considered a healthy dissolved oxygen
concentration in water. What is this
concentration expressed in molarity?
Example 2 Solution
9 g O2
x 1000 g H2O x 1 mol O2
1000000g H2O
1 L H2O
32 g O2
= 3 x 10-4 M O2
Treatment Process Step 3:
Secondary Treatment
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Secondary treatment is designed to remove
residual organic materials and suspended solids
that were not removed during primary treatment
Works to degrade the biological content of the
sewage that comes from human waste, food
waste, soaps and detergent.
Removal of biodegradable dissolved and
colloidal organic matter using aerobic
biological treatment and flocculation
Secondary Treatment:
Aerobic Biological Treatment
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performed in the presence of oxygen by aerobic
microorganisms
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Aerobic = in presence of oxygen
principally bacteria and protozoa
metabolize the organic matter in the wastewater,
including sugars, fats, and short-chain
hydrocarbons
Results in production of several inorganic
products, including CO2, NH3, and H2O, as well
as reproduction of more microorganisms
Secondary Treatment:
Flocculation
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Process in which colloids
come out of suspension
to form flakes, or floc
Differs from
precipitation!
Precipitation involves particles which are dissolved
in a solution
 Flocculation involves particles that are suspended
within a liquid, not dissolved
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http://www.tech-faq.com/flocculation.html
Colloid Properties
Colloids contain microscopic particles
dissolved evenly throughout a substance
 Particles finer than 0.1 µm in water remain
in constant motion because they often
carry an electrostatic charge which causes
them to repel each other.
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Colloids and Flocculation
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If the electrostatic charge of colloid
particles is neutralized, the finer particles
start to collide and combine together into
larger groups of particles
 Due
to the influence of Van der Waals forces:
These larger and heavier particles are called
flocs
 Floc can either be filtered out of
wastewater or left to settle out as sludge
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Colloids and Flocculation
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Flocculants, or flocculating agents are
chemicals that promote flocculation by
causing colloids and other suspended
particles in liquids to combine, forming a
floc.
Many flocculants are multivalent cations
such as aluminum, iron, calcium, and
magnesium
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Often, colloid particles carry a negative charge
These positively charged flocculant
molecules interact with negatively charged
colloid particles and molecules to reduce
the barriers to aggregation.
http://water.me.vccs.edu/courses/env110/lesson4.htm
Colloids and Flocculation
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Many flocculating agents
under appropriate
conditions (such as pH,
temperature and salinity)
will react with water to
form insoluble hydroxides
These hydroxides will
precipitate from solution
and link together to form
long chains or meshes
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physically traps small
particles into the larger floc
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Polymers can also be
used as flocculants
http://www.kolonls.co.kr/eng/product/pop02_16.asp
Common Flocculants
Chemical Flocculating
Agents:
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Alum
Aluminum chlorohydrate
Aluminum sulfate
Calcium oxide
Calcium hydroxide
Iron (II) sulfate
Iron (III) chloride
Polyacrylamide
Sodium silicate
Natural Products Used as
Flocculants:
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Chitosan
Isinglass
Horseradish tree seeds
Gelatin
Guar Gum
Alginates (from brown
seaweed)
Treatment Process
Step 4: Tertiary Treatment
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Tertiary treatment (also known as advanced
treatment) includes the remaining processes necessary
to remove the following from wastewater:
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Nitrogen
Phosphorus
additional suspended solids
remaining organics
heavy metals
dissolved solids
Final treatment stage before water is released into
rivers, lakes, or groundwater
Example 3: Dilution and Concentration
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Nitrogen is usually present in wastewater as
ammonia. 3.5 million L of wastewater entering
a treatment plant have an initial ammonia
concentration of 0.75 mM. By the time the
wastewater reaches the tertiary treatment phase,
the volume has been reduced to 2.9 million L.
What is the concentration of ammonia at this
point?
Example 3 Solution
(M1)(V1) = (M2)(V2)
(0.75 mM NH3)(3.5 million L)=(M2)(2.9 million L)
M2 = 0.91 mM
Tertiary Treatment
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Depending on the types of contamination and
the desired end use, one or more processes may
be used in tertiary treatment:
Sand filtration
 Nutrient removal (nitrogen and phosphorous)
 Odor removal
 Disinfection (via chlorination, ozone, or UV
radiation)
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Tertiary Treatment
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Sand filtration
Removes any remaining suspended solids not
removed by sedimentation and flocculation
 May be combined with filtering over activated
carbon to remove toxins and odors
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http://water.me.vccs.edu/concepts/filters.html
Tertiary Treatment
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Nutrient Removal
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Excessive release of nitrogen and phosphorous leads
to a condition known as eutrophication (presence of
excessive nutrients)
Eutrophication
encourages excessive
algae and weed growth
 Leads to
deoxygenation of
water
 Some algae can
release toxins into
water
http://05lovesgeography.blogspot.com/2011/02/eutrophication.html
Tertiary Treatment
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1.
Nutrient removal may be accomplished through
biological processes by passing wastewater through 5
different chambers:
Anaerobic fermentation zone
very low dissolved oxygen levels and the absence of
nitrates
2.
Anoxic zone
low dissolved oxygen levels but nitrates present
3.
4.
5.
Aerobic zone
Secondary anoxic zone
Final aeration zone
Biological Nutrient Removal Zones
http://www.wedotanks.com/anaerobic-aerobic-wastewater-treatment-plant.asp
Nutrient Removal: Nitrogen
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The majority of nitrogen in wastewater is in the
form of ammonia, NH3
Nitrogen removal takes place in two parts:
Nitrification: oxidation of ammonia to nitrate
 Denitirication: reduction of nitrate to nitrogen gas
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Nitrogen gas is then released into the
atmosphere
Nutrient Removal: Nitrogen
Nitrification
 Nitrification occurs in the 3rd zone (aerobic zone)
 Two step process
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Each step carried out by a unique bacteria
Step 1: oxidation of ammonia to nitrite (NO2 -1)
NH3  NO2 -1
Step 2: oxidation of nitrite to nitrate (NO3 -1 )
NO2 -1  NO3 -1
Nutrient Removal: Nitrogen
Denitrification
 After nitirification in the 3rd zone, wastewater rich in
nitrates is recycled back to the 2nd zone (first anoxic
zone)
 The recycled nitrates, in the absence of dissolved
oxygen, are reduced by bacteria to nitrogen gas
NO3 -1  N2
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Incoming organic carbon compounds present in this zone
act as hydrogen donors
Nutrient Removal: Nitrogen
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Denitrification
In zone 4, the second anoxic zone, any nitrates
not reduced in zone 2 are reduced by the
respiration of bacteria present
In zone 5, the re-aeration zone, oxygen levels are
increased to stop the denitirication process
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Stopping denitirification prevents problems with
settling
Example 4: Redox Reactions
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Redox Reactions: Balance the three redox half
reactions associated with nitrogen removal:
Nitrification Step 1
 Nitrification Step 2
 Denitrification
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Example 4 Solution
Nitrification Step 1
NH3  NO2-1
NH3 + 2H2O  NO2-1 + 7 H+
NH3 + 2H2O  NO2-1 + 7 H+ + 6 eNitrification Step 2
Denitrification
NO2-1  NO3-1
NO2-1 + H2O NO3-1 + 2 H+
NO2-1 + H2O NO3-1 +2H+ + 2e-
NO3-1  N2
2 NO3-1  N2
2 NO3-1 + 12 H+  N2 + 6H2O
2 NO3-1 + 12H+ + 11e-  N2 + 6H2O
Nutrient Removal: Phosphorous
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Phosphorous may occur as organic or inorganic forms
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Phosphorous is typically present in the form of
phosphates
Typical forms include:
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Of the 5 to 20 mg/L total phosphorous content in
wastewater, 1 to 5 mg/L is organic
Orthophosphates: easily used in biological metabolism
Polyphosphates: contain two or more phosphorous atoms
in a complex molecule. Can slowly undergo hydrolysis to
orthophosphates
Phosphorous may be removed biologically or
chemically
Nutrient Removal: Phosphorous
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Biological Removal:
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Biological phosphorous removal takes place in Zones 1 and 2
of the five zone system mentioned earlier
In Zones 1 and 2, the anaerobic fermentation zone and first
anoxic zone, bacteria are stressed by the low oxygen
conditions and release phosphorous to maintain cell
equilibrium
When these bacteria reach later zones with higher oxygen
supplies, they rapidly accumulate phosphorous in excess of
what they normally would
Removed along with sludge
Nutrient Removal: Phosphorous
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Chemical Removal
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Phosphorous can be precipitated out of the wastewater
mixture using salts of iron, aluminum, or calcium
Some of this is accomplished during flocculation
Produces more sludge due to precipitate formation
More expensive than biological removal (added cost of
chemicals)
Usually more reliable and more effective than biological
removal
Chemical Removal of Phosphorous
Using Calcium
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Usually added in the form of lime, Ca(OH)2.
Reacts with the natural alkalinity in the wastewater to
produce calcium carbonate
Ca(HCO3)2 + Ca(OH)2  2CaCO3 + 2H2O
As the pH value of the wastewater increases past 10,
excess calcium ions will then react with the phosphate, to
precipitate in hydroxylapatite:
10 Ca2+ + 6 PO43- + 2 OH- ↔ Ca10(PO4)*6(OH)2 (s)
Amount of lime required depends on pH of water rather
than amount of phosphate present
Neutralization may be required to lower the pH before
further treatment or disposal, typically by recarbonation
with carbon dioxide
Example 5: Acids, Bases, and pH
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The pH of domestic wastewater is about 7.2.
What are the concentrations of hydronium and
hydroxide ions in water at this point?
 Lime is only effective in removing phosphorous at a
pH higher than 10. What is the pH of 9800 L of
water treated with 50 g Ca(OH)2?
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Example 5 Solution
Part 1:
[H3O+] = 10 -pH
= 10 -7.2
= 6.3 x 10-8 M
Part 2:
50.0 g x 1 mol Ca(OH)2 x 2 mol OH
9800 L
74.1 g
1 mol Ca(OH)2
= 1.38 x 10 -4 M OH-
[H3O+] [OH-] = 1 x 10-14
[OH-] = 1 x 10-14
6.3 x 10-8
[H3O+] = 1 x 10-14
1.38 x 10 -4
= 7.25 x 10 -11 M H3O+]
= 1.6 x 10-7M
pH = -log[H3O+]
= -log (7.25 x 10 -11 M)
= 10.1
Chemical Removal of Phosphorous
Using Aluminum
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Typically use alum or hydrated aluminum sulfate
to precipitate aluminum phosphates (AlPO4).
Al3+ + HnPO43-n ↔ AlPO4 + nH+
Reaction affected by pH, equilibrium of
competing reactions, and presence of trace
elements in wastewater
Aluminum may adversely affect some of the
bacteria used in sludge and digestion and should
be used carefully
Chemical Removal of Phosphorous
Using Iron
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Iron (III) chloride or sulfate or iron (II) sulfate
can be used to form iron phosphate precipitates
Fe3+ + HnPO4 3-n ↔ FePO4 + nH+
Lime is usually added to raise the pH to enhance
the reaction
Example 6: Solubility and Net Ionic
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Write the full balanced equation (including
states) and the net ionic equation for the
reaction of iron (III) sulfate with sodium
phosphate.
Example 6 Solution
Balanced Reaction:
Fe2(SO4)3(aq) + 2Na3PO4(aq)  2FePO4(s) + 3Na2SO4(aq)
Net:
Fe3+(aq) + PO43-(aq)  FePO4 (s)
Tertiary Treatment: Disinfection
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Disinfection of wastewater reduces the number of
microorganisms in water that may lead to disease
before discharging back into the environment
Usually the very last step before discharge
Effectiveness depends upon conditions of treated
water at this point, including cloudiness and pH
Three major strategies: chlorination, ozone, and UV
radiation
http://www.purewater2000.com/Ultraviolet.html
Disinfection: Chlorination
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Most commonly used form
of disinfection due to low
cost and high effectiveness
The exact mechanism by
which chlorine disinfects is
not fully understood. It
likely involves oxidative
damage to microbial cell
membranes and vital
protein systems
Chlorination also helps to
reduce any odors in the
water
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Drawbacks:
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may create chlorinated organic
compounds that may be carcinogenic
Residual chlorine is toxic to aquatic life
May be necessary to dechlorinate
water before release
http://chlorination.us/chlorination/chlorination/
Disinfection: Chlorination
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When chlorine (Cl2) is injected into water, it forms
hypochlorous acid and hydrochloric acid in a pH
dependent equilibrium
Cl2 + H2O → HOCl + HCl
Depending on the pH, the hypochlorous acid will partly
dissociate to hydrogen and hypochlorite ions:
HClO → H+ + ClOIn acidic solution, the major species are Cl2 and HOCl
while in basic solution only ClO- is present.
Very small concentrations of ClO2-, ClO3-, ClO4- are
also found
Disinfection: UV Radiation
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Ultraviolet radiation damages the genetic structure of bacteria
and viruses which makes them incapable of reproduction
Since no chemicals are used, UV disinfection poses no risk to
organisms which will later encounter the treated water
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Requires highly treated water
with little cloudiness.
Suspended solids in the water
may block out the UV rays
Maintaining UV lamps can be
costly
http://www.blog.waterfilters.net/uv-technology-explained/1207
Disinfection: Ozone
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Ozone (O3) is generated by passing oxygen gas
(O2) through a high voltage potential. Voltage
breaks O2 into oxygen atoms which will
recombine as O3 gas
O2 + electricity  O3
Ozone is very unstable. Generated as needed
rather than stored
Produces fewer by-products than chlorination,
but much more costly
Example 7: Reaction Stoichiometry
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Write the balanced equation for the synthesis of
ozone from oxygen
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If 56.8 g of ozone must be synthesized, how
many moles of oxygen gas are required?
Example 7 Solution
3 O 2  2 O3
56.8 g O3 x 1 mol O3 x 3 mol O2
48 g O3
2 mol O3
= 1.78 mol O2
Disinfection: Ozone
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Ozone is very effective in destroying viruses and
bacteria and may act by several mechanisms:
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Direct oxidation and destruction of the cell wall with
leakage of cellular components
Reactions with radical by-products of ozone
decomposition
Damage to the constituents of the nucleic acids (purines
and pyrimidines)
Breakage of carbon-nitrogen bonds leading to
depolymerization
Tertiary Treatment: Odor Removal
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Odor in waste water typically form as a result of
anaerobic conditions
Most common odor is hydrogen sulfide gas
Odor is eliminated along the way by aeration,
chlorination, biological degradation, and circulation of
fluids
Other methods to eliminate hydrogen sulfide are by
adding iron salts, hydrogen peroxide, or calcium nitrate
Treatment Process
Step 5: Sludge Treatment
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Sludge consists of all the solid material removed from
wastewater during the water treatment process
While the water in treatment is ready for release into
streams and groundwater, sludge requires further
treatment before it can be disposed or used
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Must reduce the amount of organic matter
Must reduce the number of disease causing microbes
Remove as much remaining liquid as possible
Sludge treatment options include:
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Aerobic digestion
Anearobic digestion
Composting
Incineration
Sludge Treatment
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Sludge is most often processed by biological
anaerobic digestion
Bacteria metabolize the organic material in the
sludge
Occurs over a period of 10 to 60 days, depending on
the capabilities of the digesting tanks
 Reduces the volume of sludge that requires disposal
 Makes the sludge more stable
 Improves the dewatering characteristics of the sludge
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Shorter retention time and smaller tanks required
Requires higher temperatures, resulting in a higher
energy cost
Sludge Treatment
One byproduct of
anaerobic sludge digestion
is the production of
biogas
 Biogas contains about 60
to 65% methane (CH4)
and can be recovered as
an energy source.
 Methane is a combustible,
renewable fuel
CH4 + O2  CO2 + H2O
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http://home.comcast.net/~hollywastewater/Process.htm
Sludge Treatment
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In small sewage treatment plants, sludge is
processed using aerobic digestion
Under aerobic conditions, bacteria will consume
organic material and convert it into carbon
dioxide
Energy cost associated with adding oxygen to
process and blowers to remove CO2
Sludge Treatment
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Composting of sludge is similar to aerobic digestion,
except other organic materials such as sawdust are
mixed in with the sludge
Incineration is the least used method of sludge
treatment.
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Sludge burns poorly due to low calorific value, so extra
fuels must be added
Worries of emissions associated with sludge
High energy cost to vaporize residual water present in
sludge
Sludge Treatment
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Sludge that does not originate from
highly industrialized areas and is
for the most part free of toxic
chemicals can be used as fertilizer
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Water is removed
from sludge by
centrifugation and
addition of chemicals
that aid in polymer
formation
Dried sludge can be
converted into
fertilizer pellets
which are usually rich
in phosphorous
http://www.thewatertreatmentplant.com/sludge-treatment-equipment.html
Water Treatment
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View the entire process in action
http://photosmynthesis.wordpress.com/2010/09/16/wetland-services/water-treatment-plant/
Sources
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Severn Trent Water. “The Water Treatment Process” Online. 9 July 2011.
http://www.youtube.com/watch?v=9z14l51ISwg
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