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Wastewater Treatment


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Title: Wastewater Treatment

Wastewater Treatment
  • Environmental Chemistry
  • TIP 2011

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

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

Image http//www.acumen.com.my/content/chemical-
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

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

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?

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
  • 9.1 x 10-9 mol/L
  • 9.1 x 10-9 M Pb

Why Treat it?
  • The environment is able to naturally dilute and
    degrade water contaminants, but only in small
  • Wastewater treatment reduces pollutants to levels
    that the environment can safely handle and

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

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
  • 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
  • Possible to purify into clean drinking water again

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

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,
  • 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
  • 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

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)

Separation of Oil and Grease
  • Oil and grease are typically long chains of
    hydrocarbons, making them nonpolar, hydrophobic
  • 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
  • Oils and grease rise to the top of water due to a
    difference in density

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
  • Mechanical skimmers or rakes remove oils and
    grease from the surface.
  • May be recovered to use in saponification

  • 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
  • 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
  • Increasing dissolved oxygen in water compensates
    for the increased BOD and helps with the sludge
    settling process

Example 2 Concentration
  • 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
  • 1000000g H2O 1 L H2O 32 g O2
  • 3 x 10-4 M O2

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

Secondary Treatment Aerobic Biological
  • performed in the presence of oxygen by aerobic
  • Aerobic in presence of oxygen
  • principally bacteria and protozoa
  • metabolize the organic matter in the wastewater,
    including sugars, fats, and short-chain
  • Results in production of several inorganic
    products, including CO2, NH3, and H2O, as well as
    reproduction of more microorganisms

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

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.

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
  • Floc can either be filtered out of wastewater or
    left to settle out as sludge

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

Colloids and Flocculation
  • Many flocculating agents under appropriate
    conditions (such as pH, temperature and salinity)
    will react with water to form insoluble
  • These hydroxides will precipitate from solution
    and link together to form long chains or meshes
  • physically traps small particles into the larger
  • Polymers can also be used as flocculants

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)

Treatment Process Step 4 Tertiary Treatment
  • Tertiary treatment (also known as advanced
    treatment) includes the remaining processes
    necessary to remove the following from
  • 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
  • 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

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
  • 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

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

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

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

Biological Nutrient Removal Zones
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
  • Nitrogen gas is then released into the atmosphere

Nutrient Removal Nitrogen
  • Nitrification
  • Nitrification occurs in the 3rd zone (aerobic
  • 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

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

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

Example 4 Redox Reactions
  • Redox Reactions Balance the three redox half
    reactions associated with nitrogen removal
  • Nitrification Step 1
  • Nitrification Step 2
  • Denitrification

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

Nutrient Removal Phosphorous
  • Phosphorous may occur as organic or inorganic
  • 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
  • Typical forms include
  • Orthophosphates easily used in biological
  • Polyphosphates contain two or more phosphorous
    atoms in a complex molecule. Can slowly undergo
    hydrolysis to orthophosphates
  • Phosphorous may be removed biologically or

Nutrient Removal Phosphorous
  • Biological Removal
  • Biological phosphorous removal takes place in
    Zones 1 and 2 of the five zone system mentioned
  • 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
  • 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

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

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?

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

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
  • Aluminum may adversely affect some of the
    bacteria used in sludge and digestion and should
    be used carefully

Chemical Removal of PhosphorousUsing Iron
  • Iron (III) chloride or sulfate or iron (II)
    sulfate can be used to form iron phosphate
  • Fe3 HnPO4 3-n ? FePO4 nH
  • Lime is usually added to raise the pH to enhance
    the reaction

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

Example 6 Solution
  • Balanced Reaction
  • Fe2(SO4)3(aq) 2Na3PO4(aq) ? 2FePO4(s)
  • Net
  • Fe3(aq) PO43-(aq) ? FePO4 (s)

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

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

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
  • HClO ? H ClO-
  • In acidic solution, the major species are Cl2 and
    HOCl while in basic solution only ClO- is
  • Very small concentrations of ClO2-, ClO3-, ClO4-
    are also found

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

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

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?

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

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
  • Damage to the constituents of the nucleic acids
    (purines and pyrimidines)
  • Breakage of carbon-nitrogen bonds leading to

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

Treatment Process Step 5 Sludge Treatment
  • Sludge consists of all the solid material removed
    from wastewater during the water treatment
  • While the water in treatment is ready for release
    into streams and groundwater, sludge requires
    further treatment before it can be disposed or
  • Must reduce the amount of organic matter
  • Must reduce the number of disease causing
  • Remove as much remaining liquid as possible
  • Sludge treatment options include
  • Aerobic digestion
  • Anearobic digestion
  • Composting
  • Incineration

Sludge Treatment
  • Sludge is most often processed by biological
    anaerobic digestion
  • Bacteria metabolize the organic material in the
  • Occurs over a period of 10 to 60 days, depending
    on the capabilities of the digesting tanks
  • Reduces the volume of sludge that requires
  • Makes the sludge more stable
  • Improves the dewatering characteristics of the
  • 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

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
  • Energy cost associated with adding oxygen to
    process and blowers to remove CO2

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
  • 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
  • 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
  • Dried sludge can be converted into fertilizer
    pellets which are usually rich in phosphorous

Water Treatment
  • View the entire process in action

(No Transcript)
  • Severn Trent Water. The Water Treatment
    Process Online. 9 July 2011.
  • http//www.youtube.com/watch?v9z14l51ISwg
  • United States Geological Survey. Wastewater
    Treatment Water Use Online. 9 July 2011.
  • South Carolina Office of Regulatory Staff.
    Overview of Basic Wastewater Treatment Process
    Online. 9 July 2011. http//www.regulatorystaff.s
  • Author Unknown. Sewage Treatment Online. 8
    July 2011. http//en.wikipedia.org/wiki/Sewage_tre
  • United States Geological Survey. A visit to a
    wastewater-treatment plant Primary treatment of
    wastewater Online. 9 July 2011
  • Natural Resources Management and Environment
    Department. Water Treatment Online. 10 July
    2011. http//www.fao.org/docrep/t0551e/t0551e05.h
  • Environmental Protection Agency. Water
    Treatment Process Online. 8 July 2011.
  • Environmental Protection Agency. Wastewater
    Technology Fact Sheet Ozone Disinfection.
    (1999) Online. 11 July 2011 http//water.epa.go
  • Author Unknown. Chlorination Online 11 July
    2011. http//water.me.vccs.edu/courses/ENV149/chl
  • Lenntech Water Treatment Solutions. Phosphorous
    removal from wastewater. Online 10 July 2011.
  • Author Unknown. Flocculation Online 9 July
    2011. http//en.wikipedia.org/wiki/Flocculation
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