Wastewater Treatment

1
Wastewater Treatment

• Environmental Chemistry
• TIP 2011

2
What is Wastewater Treatment?

• 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

3
Wastewater Sources

• Wastewater comes from
• Homes
• Sinks, showers, toilets, washing machines,
  dishwashers
• Businesses
• Industrial facilities
• Storm runoff
• From roads, parking lots, roofs

Image http//www.acumen.com.my/content/chemical-
testing?qnode/6
4
What Needs to be Removed?

• 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

http//www.westfield.ma.edu/personalpages/draker/e
dcom/final/webprojects/sp11/triparoundworld/Marine
.html
5
What Needs to be Removed?

• Wastewater from both domestic and industrial
  sources may contain a variety of potentially
  harmful contaminants, including
• Bacteria
• E. coli (right), Giardia, Hepatatis A
• Viruses
• Nitrates
• Metals
• mercury, lead, cadmium, chromium, arsenic
• Toxic materials
• Salts

http//www.anh-usa.org/the-european-e-coli-outbrea
k-the-real-story/
6
Example 1 Concentration

• 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?

7
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

8
Why Treat it?

• The environment is able to naturally dilute and
  degrade water contaminants, but only in small
  amounts

• Wastewater treatment reduces pollutants to levels
  that the environment can safely handle and
  process

9
Why Treat it?

• 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
• Excessive nutrients (nitrogen and phosphorous)
  can also lead to deoxygenation
• Increased plant and algae growth, which
  eventually die and decompose, lead to an
  increased BOD

10
Process of Water Treatment

• 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

11
Steps of the Wastewater Treatment Process

• 1. Pretreatment
• 2. Primary Treatment
• 3. Secondary Treatment
• 4. Tertiary Treatment
• 5. Sludge Processing

12
Water Treatment Process
Image http//www.cityofdunbarwv.com/node/22
13
Treatment Process Step 1 Pretreatment

• Prepares waste water for entering the treatment
  plant
• Removal of larger debris by screening (shown
  right)
• Trash
• Tree limbs
• Removal of grit and gravel by screening and
  settling
• Gravel must be removed early as it can damage
  machinery and equipment in the treatment plant

http//www.alard-equipment.com/wastewater/index.ht
m
14
Treatment ProcessStep 2 Primary Treatment

• 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
• removed by sedimentation
• Scum Materials which will float (oil, grease,
  soap)
• 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

15
Separation of Oil and Grease

• 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

• 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-separati
on/
16
Separation of Oil and Grease

• 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
• Water molecules are attracted to one another due
  to attractions between these positive and
  negative regions (hydrogen bonding)

http//bioweb.wku.edu/courses/biol115/Wyatt/Bonds.
htm
http//marineodyssey.co.uk/abioticoceans.html
17
Separation of Oil and Grease

• 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_Biochemist
ry/Chemical_Bonding/Hydrophobic_interaction
18
Primary Treatment Physical Separation

• Sewage flows through large tanks known as primary
  clarifiers or primary sedimentation tanks
• 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.
• May be recovered to use in saponification

19
Saponification

• Saponification is the base hydrolysis of fats and
  oils to produce glycerol and a crude soap
• triglyceride (fat) glycerol

20
Primary Treatment Aeration

• Another process during primary treatment is
  aeration
• Water is agitated and exposed to air, which
  serves two purposes
• 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

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Example 2 Concentration

• 9 ppm is considered a healthy dissolved oxygen
  concentration in water. What is this
  concentration expressed in molarity?

22
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

23
Treatment Process Step 3Secondary Treatment

• 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

24
Secondary Treatment Aerobic Biological
Treatment

• performed in the presence of oxygen by aerobic
  microorganisms
• 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

25
Secondary TreatmentFlocculation

• 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

http//www.tech-faq.com/flocculation.html
26
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.

27
Colloids and Flocculation

• 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

28
Colloids and Flocculation

• 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
• 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.ht
m
29
Colloids and Flocculation

• 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
• physically traps small particles into the larger
  floc

• Polymers can also be used as flocculants

http//www.kolonls.co.kr/eng/product/pop02_16.asp
30
Common Flocculants

• Chemical Flocculating Agents
• Alum
• Aluminum chlorohydrate
• Aluminum sulfate
• Calcium oxide
• Calcium hydroxide
• Iron (II) sulfate
• Iron (III) chloride
• Polyacrylamide
• Sodium silicate

• Natural Products Used as Flocculants
• Chitosan
• Isinglass
• Horseradish tree seeds
• Gelatin
• Guar Gum
• Alginates (from brown seaweed)

31
Treatment Process Step 4 Tertiary Treatment

• Tertiary treatment (also known as advanced
  treatment) includes the remaining processes
  necessary to remove the following from
  wastewater
• Nitrogen
• Phosphorus
• additional suspended solids
• remaining organics
• heavy metals
• dissolved solids
• Final treatment stage before water is released
  into rivers, lakes, or groundwater

32
Example 3 Dilution and Concentration

• 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?

33
Example 3 Solution

• (M1)(V1) (M2)(V2)
• (0.75 mM NH3)(3.5 million L)(M2)(2.9 million L)
• M2 0.91 mM

34
Tertiary Treatment

• 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)

35
Tertiary Treatment

• 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

http//water.me.vccs.edu/concepts/filters.html
36
Tertiary Treatment

• Nutrient Removal
• 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/eutro
phication.html
37
Tertiary Treatment

• 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
• Anoxic zone
• low dissolved oxygen levels but nitrates present
• Aerobic zone
• Secondary anoxic zone
• Final aeration zone

38
Biological Nutrient Removal Zones
http//www.wedotanks.com/anaerobic-aerobic-wastewa
ter-treatment-plant.asp
39
Nutrient Removal Nitrogen

• 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
• Nitrogen gas is then released into the atmosphere

40
Nutrient Removal Nitrogen

• Nitrification
• Nitrification occurs in the 3rd zone (aerobic
  zone)
• Two step process
• 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

41
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
• Incoming organic carbon compounds present in this
  zone act as hydrogen donors

42
Nutrient Removal Nitrogen

• 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
• Stopping denitirification prevents problems with
  settling

43
Example 4 Redox Reactions

• Redox Reactions Balance the three redox half
  reactions associated with nitrogen removal
• Nitrification Step 1
• Nitrification Step 2
• Denitrification

44
Example 4 Solution

• Nitrification Step 1
• NH3 ? NO2-1
• NH3 2H2O ? NO2-1 7 H
• NH3 2H2O ? NO2-1 7 H 6 e-

• Nitrification Step 2
• NO2-1 ? NO3-1
• NO2-1 H2O? NO3-1 2 H
• NO2-1 H2O? NO3-1 2H 2e-

• Denitrification
• NO3-1 ? N2
• 2 NO3-1 ? N2
• 2 NO3-1 12 H ? N2 6H2O
• 2 NO3-1 12H 11e- ? N2 6H2O

45
Nutrient Removal Phosphorous

• Phosphorous may occur as organic or inorganic
  forms
• Of the 5 to 20 mg/L total phosphorous content in
  wastewater, 1 to 5 mg/L is organic
• Phosphorous is typically present in the form of
  phosphates
• Typical forms include
• 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

46
Nutrient Removal Phosphorous

• Biological Removal
• 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

47
Nutrient Removal Phosphorous

• Chemical Removal
• 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

48
Chemical Removal of PhosphorousUsing Calcium

• 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

49
Example 5 Acids, Bases, and pH

• 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?

50
Example 5 Solution

• Part 1
• H3O 10 -pH
• 10 -7.2
• 6.3 x 10-8 M
• H3O OH- 1 x 10-14
• OH- 1 x 10-14
• 6.3 x 10-8
• 1.6 x 10-7M

• 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 1 x 10-14
• 1.38 x 10 -4
• 7.25 x 10 -11 M H3O
• pH -logH3O
• -log (7.25 x 10 -11 M)
• 10.1

51
Chemical Removal of PhosphorousUsing Aluminum

• 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

52
Chemical Removal of PhosphorousUsing Iron

• 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

53
Example 6 Solubility and Net Ionic

• Write the full balanced equation (including
  states) and the net ionic equation for the
  reaction of iron (III) sulfate with sodium
  phosphate.

54
Example 6 Solution

• Balanced Reaction
• Fe2(SO4)3(aq) 2Na3PO4(aq) ? 2FePO4(s)
  3Na2SO4(aq)
• Net
• Fe3(aq) PO43-(aq) ? FePO4 (s)

55
Tertiary Treatment Disinfection

• 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
56
Disinfection Chlorination

• 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

• Drawbacks
• 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/
57
Disinfection Chlorination

• 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 ClO-
• In 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

58
Disinfection UV Radiation

• 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

• 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-exp
lained/1207
59
Disinfection Ozone

• 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

60
Example 7 Reaction Stoichiometry

• Write the balanced equation for the synthesis of
  ozone from oxygen
• If 56.8 g of ozone must be synthesized, how many
  moles of oxygen gas are required?

61
Example 7 Solution

• 3 O2 ? 2 O3
• 56.8 g O3 x 1 mol O3 x 3 mol O2
• 48 g O3 2 mol O3
• 1.78 mol O2

62
Disinfection Ozone

• Ozone is very effective in destroying viruses and
  bacteria and may act by several mechanisms
• 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

63
Tertiary Treatment Odor Removal

• 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

64
Treatment Process Step 5 Sludge Treatment

• 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
• 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
• Aerobic digestion
• Anearobic digestion
• Composting
• Incineration

65
Sludge Treatment

• 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
• Shorter retention time and smaller tanks required
• Requires higher temperatures, resulting in a
  higher energy cost

66
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

http//home.comcast.net/hollywastewater/Process.h
tm
67
Sludge Treatment

• 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

68
Sludge Treatment

• 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.
• 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

69
Sludge Treatment

• Sludge that does not originate from highly
  industrialized areas and is for the most part
  free of toxic chemicals can be used as fertilizer

• 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-treat
ment-equipment.html
70
Water Treatment

• View the entire process in action

http//photosmynthesis.wordpress.com/2010/09/16/we
tland-services/water-treatment-plant/
71
(No Transcript)
72
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• Severn Trent Water. The Water Treatment
  Process Online. 9 July 2011.
• http//www.youtube.com/watch?v9z14l51ISwg
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  http//ga.water.usgs.gov/edu/wuww.html
• South Carolina Office of Regulatory Staff.
  Overview of Basic Wastewater Treatment Process
  Online. 9 July 2011. http//www.regulatorystaff.s
  c.gov/orscontent.asp?pageid654
• Author Unknown. Sewage Treatment Online. 8
  July 2011. http//en.wikipedia.org/wiki/Sewage_tre
  atment
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  wastewater-treatment plant Primary treatment of
  wastewater Online. 9 July 2011
  http//ga.water.usgs.gov/edu/wwvisit.html
• Natural Resources Management and Environment
  Department. Water Treatment Online. 10 July
  2011. http//www.fao.org/docrep/t0551e/t0551e05.h
  tm
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  Treatment Process Online. 8 July 2011.
  http//water.epa.gov/learn/kids/drinkingwater/wate
  rtreatmentplant_index.cfm
• Environmental Protection Agency. Wastewater
  Technology Fact Sheet Ozone Disinfection.
  (1999) Online. 11 July 2011 http//water.epa.go
  v/scitech/wastetech/upload/2002_06_28_mtb_ozon.pdf
  
• Author Unknown. Chlorination Online 11 July
  2011. http//water.me.vccs.edu/courses/ENV149/chl
  orinationb.htm
• Lenntech Water Treatment Solutions. Phosphorous
  removal from wastewater. Online 10 July 2011.
  http//www.lenntech.com/phosphorous-removal.htmix
  zz1RpIsY55O
• Author Unknown. Flocculation Online 9 July
  2011. http//en.wikipedia.org/wiki/Flocculation