Outline Water/Wastewater Treatment (Chapter 8)

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OutlineWater/Wastewater Treatment (Chapter 8)

• Overview
• Removal of solids (Chapter 8.6)
• ? Removal of hardness in water (Chapter 8.7)
• Kill of pathogenic bacteria (Chapter 8.11)
• Removal of dissolved inorganics (Chapter 8.8)
• Removal of nutrients (N P) (Chapter 8.8)
• Removal of dissolved organics (Chapter 8.9)
• Treatment disposal of sludge (Chapter 8.10)

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An Overview Water / Wastewater Treatment

• Similarity between Water Treatment and Wastewater
  Treatment
• Both require solids removal and disinfection
• Difference between Water Treatment and Wastewater
  Treatment
• Purpose Drinking water vs. sewage
• Unit process and operation w/ and w/o biological
  process
• Regulation SDWA vs. NPDES

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Types of Water / Wastewater Treatment
MethodsCommon classification (for wastewater)
(p. 232 - 237)

• Primary treatment
• Purpose To remove settable and floatable solids
• Method screening, grit removal, primary
  sedimentation
• Secondary treatment
• Purpose To aerobically and biologically remove
  biodegradable organic matter
• Method activated sludge, trickling filter,
  rotating biological contactors
• Tertiary (advanced) treatment
• Purpose To further remove SS, dissolved organics
  (refractory compounds) and inorganics (N, P
  nutrients)
• Method activated carbon, nitrification /
  denitrification

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Types of Water / Wastewater Treatment Methods

• Based on contaminants to be removed (detailed in
  handout)
• Suspended solids sedimentation,
  coagulation/flocculation, filter
• Biodegradable organics activated sludge,
  trickling filter
• Volatile organics air stripping, carbon
  adsorption
• Pathogens disinfection (Cl2, O3, UV)
• Nutrients (N) nitrification/denitrification, NH3
  stripping (p.252)
• Nutrients (P) precipitation, biological removal,
  adsorption (Al2O3)
• Refractory organics carbon adsorption, O3/UV
  radiation
• Hardness (Ca2, Mg2), heavy metals
  precipitation, ion exchange, membrane processes,
  chelation (sequestion)
• Grease flotation, biological processes
• Dissolved solids ion exchange, membrane processes

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Water / Wastewater Treatment Selected Topics

• Removal of suspended solids
• Coagulation / Flocculation
• Removal of hardness
• Water Softening
• Removal of pathogenic bacteria
• Disinfection

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Typical Water Treatment Plant (Chapter 8.2)
Primary Settling
Coagulation/ Flocculation
Secondary Settling
Surface Water
Filtration
Disinfection
Clean water to consumer
Sludge
Sludge
Primary Settling
Coagulation/ Flocculation
Secondary Settling
Filtration
Disinfection
Aeration
Ground Water
Clean water to consumer
Sludge
Sludge
Well
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Typical Wastewater Treatment Plant (Chapter 8.4)
Bar Screen Grit Removal
Primary Settling
Aeration (Biological)
Secondary Settling
Disinfection
Effluent discharged to receiving water
Wastewater
Return Sludge (Bacteria)
Sludge Thickening
Sludge Stabilization
Waste Sludge
Sludge Dewatering
Dewatered sludge to landfill
Supernatant
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Topic I. Removal of Solids (Chapter 8.6)

• Solids in water and wastewater
• Dead animals, plant biomass, food debris
• Soil particles (clay, sand, etc.)
• Colloidal particles (humic substances)
• Bacterial cells, algal cells, virus
• Sludge
• Primary methods for the removal of solids
• Screening physical process
• Filtration physical process
• Settling physicochemical processes including
  coagulation and flocculation

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Coagulation for the Removal of SolidsWhat? Why?
How? (p. 132 p.238-239)

• What? Coagulation involves the reduction of
  electrostatic repulsion such that colloidal
  particles of identical materials may aggregate.
• Why? Colloidal particles are prevented from
  aggregating by electrostatic repulsion of the
  electrical double layers. They are small in size
  and very stable in water.
• How? By the addition of coagulants followed by
  flocculation (p. 132)

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Common Coagulants

• Alum
• Hydrated aluminum sulfate Al2(SO4)318H2O
• Alum, when added to water, will be hydrolyzed to
  form gelatinous hydroxide Al(OH)3 precipitate.
  This will carry suspended solids as it settles by
  gravity.
• Anhydrous Fe3
• Forms Fe(OH)3 (s) in a wide range of pH 4-11
• Anhydrous Fe2 (copperas, FeSO47H2O)
• Must be oxidized to Fe3 first at pH higher than
  8.5
• Natural and synthetic polyelectrolytes
• Starch, cellulose derivatives, proteinaceous
  materials, and gums composed of polysaccharides
• Synthetic polymers

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ExampleWater Treatment for Solids Removal

• Suspended solids are to be removed through
  coagulation / flocculation followed by gravity
  settling. Alum and lime are added at
  stoichiometric rates sufficient to form 25 mg/L
  of aluminum hydroxide sludge. For the following
  data, calculate the alum and lime dosage, the dry
  weight mass of solids generated per day, and the
  wet volume per day of sludge.
• Given data
• Flow 1.0 MGD
• TSS 5.0 mg/L
• Thickened sludge 20,000 mg/L
• Given reaction
• Al2(SO4)3 3Ca(OH)2 ? 2Al(OH)3 ? 3CaSO4

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Topic II. Removal of Hardness (Ca2, Mg2)Water
Softening

• Hardness is an important water quality parameter
  in determining the suitability of water for
  domestic and industrial uses
• Hard waters require considerable amounts of soap
  to produce foam
• Hard waters produce scale in hot-water pipers,
  heaters and boilers
• Ca2 2HCO3- ? CaCO3 (s) CO2 (g) H2O
• Groundwater is generally harder than surface
  water
• Principal cations causing hardness and the major
  anions associated with them (in decreasing order
  of abundance in natural waters)
• Cations Ca2, Mg2, Sr2, Fe2, Mn2
• Anions HCO3-, SO42-, Cl-, NO3-, SiO32-

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Water Hardness

• Hardness expressed in mg/L as CaCO3
• Methods of determination
• Calculation (see example)
• Hardness (mg/L) as CaCO3 M2 (mg/L) x 50 / EW
  of M2
• EDTA titrimetric method
• M2 Eriochrome Black T (blue) ? (M
  Eriochrome Black T)complex (wine red)
• Water softening is needed when hardness is above
  150-200 mg/L Hardness 50-80 is acceptable in
  treated water

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Carbonate and Noncarbonate Hardness

• Total hardness Carbonate hardness
  Noncarbonate hardness
• Carbonate hardness temporary hardness
  eliminated at elevated temperatures in boilers
• Ca2 2HCO3- ? CaCO3 CO2 H2O
• Ca2 2HCO3- Ca(OH)2 ? 2CaCO3 2H2O
• Noncarbonate hardness permanent hardness can
  not be removed or precipitated by boiling.
  Noncarbonate hardness cations are associated with
  SO42-, Cl- and NO3-.

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Example Harness Calculation

• Calculate the hardness of a water sample with the
  following analysis

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Water Softening Methods (p. 240-244)

• Ion exchange
• Reverse osmosis
• Coagulation /flocculation (most commonly used)
• Ca2, Mg2 ? CaCO3 (s), Mg(OH)2 (s)
• lime-only process when Ca2 is present primarily
  as bicarbonate hardness
• lime-soda Ca(OH)2-Na2CO3 process when
  bicarbonate is not present at substantial level

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Lime-Soda Ca(OH)2-Na2CO3 Process for Water
Softening Chemical Reactions Involved

• Lime to remove Ca2 in the form of natural
  alkalinity
• Ca(HCO3)2 Ca(OH)2 ? 2CaCO3? 2H2O
• Lime to remove Mg2 in the form of natural
  alkalinity
• Mg(HCO3)2 Ca(OH)2 ? MgCO3 (soluble) CaCO3?
  2H2O
• additional lime must be added to remove MgCO3
• MgCO3 Ca(OH)2 ? CaCO3? Mg(OH)2?
• Mg2 hardness in the form of a sulfate requires
  both lime and soda ash
• MgSO4 Ca(OH)2 ? CaSO4 Mg(OH)2?
• CaSO4 Na2CO3 ? CaCO3? Na2SO4?
• CO2 in the water will also consume lime
• CO2 Ca(OH)2 ? CaCO3 ? H2O

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Lime-Soda Ca(OH)2-Na2CO3 Process
Recarbonation by bubbling CO2 after softening

• Recarbonation is usually required after lime-soda
  process
• Why?
• To prevent super-saturated CaCO3 (s) and Mg(OH)2
  (s) from forming harmful deposits or undesirable
  cloudiness in water at a later time
• CaCO3 (s) CO2 H2O ? Ca2 2HCO3-
• MgCO3 (s) CO2 H2O ? Ca2 2HCO3-
• To neutralize excessively high pH caused by
  Na2CO3
• OH- CO2 ? HCO3-

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Topic III. Disinfection Kill of Pathogenic
Bacteria in Water

• Disinfection is typically the last step in a
  water / wastewater treatment system
• Residual chlorine is needed in distribution
  system after water / wastewater treatment
• In addition to disinfection, chlorine also has
  the following functions
• taste and odor control as an oxidizing agent
• oxidation of Fe2 and Mn2 in groundwater
• ammonium removal in domestic waste treatment
• slime, biofouling control control of sludge
  bulking

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Types of Disinfection

• Gaseous Cl2
• Most commonly used
• Advantage provide residual chlorine for the
  protection from bacterial growth in distribution
  system
• Disadvantage The formation of disinfection
  by-products (trihalomethanes) presents a health
  risk
• Chlorine dioxide (ClO2)
• No disinfection by-products such as
  trihalomethanes
• Ca(ClO)2
• Safer than Cl2
• Ozone (p. 256-257)
• UV

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Chemistry of Chlorine in Water

• Cl2 H2O ? H Cl- HOCl
• HOCl is a weak acid with Ka 4.5x10-4 (HOCl
  H OCl-)
• HOCl and OCl- are free available chlorine which
  are very effective in killing bacteria
• Small amount of ammonium (NH4) in water is
  desired
• Chloramine NH2Cl, NHCl2, NCl3 (p. 255)
• Chloramines (combined available chlorine) are
  weaker disinfectants than free available chlorine
  but are desired residual chlorine to be retained
  in water distribution system
• Excessive amount of ammonium (NH4) in water is
  undesirable because it consume excess demand of
  Cl2
• Extra chlorine may be removed by SO2 , a process
  called dechlorination
• SO2 HOCl H2O ? Cl- SO42- 3H

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Example Chlorine Requirement

• Determine the quantity of chlorine gas (lb/day)
  that must be supplied for breakpoint chlorination
  of municipal wastewater that contains 3 mg/L of
  ammonia-nitrogen. The wastewater flow rate is 1.0
  MGD (Mgal/day). The relevant reactions are
  provided below
• Cl2 H2O ? HCl HOCl
• 2NH3 3HOCl ? N2 3H2O 3HCl

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Other Methods Removal of Dissolved Inorganics
Organics

• Inorganics
• Distillation
• Membrane process
• Electrodialysis
• Ion exchange
• Reverse osmosis
• Filtration
• Nanofiltration
• Ultrafiltration
• Microfiltration

• Organics
• Adsorption
• Activated carbon (AC)
• PAC (powered)
• GAC (granulated)
• Synthetic polymer
• XAD
• Oxidation
• O3
• H2O2
• O2

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Other Methods Removal of Nutrients

• Nitrogen (p. 251-252)
• Air stripping NH3
• Nitrification followed by denitrification
• NH4 ion exchange
• Biosynthesis
• Chlorination

• Phosphorus (p. 249-251)
• Biological uptake
• activated sludge
• Precipitation (Table 8.1)
• Adsorption
• Activated alumina, Al2O3