ESM 214 05

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ESM 214 05

• Lecture 5 / 6 Physical Chemical Processes in
  Waste Treatment

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Physical Chemical Processes

• May precede Biological Treatment
• e.g. flow equalization
• e.g. preliminary and primary treatment
• May contribute to Biological Treatment
• e.g. precipitation in secondary clarifier
• e.g. aeration
• May follow Biological Treatment
• e.g. advanced treatment

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Concept of Flow Equalization

• Dampen out diurnal fluctuations
• Divert toxics
• Balance loads

High flow diversion
Low flow return
Qi
Qi
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Some physical and chemical processes used with
biological treatment are for combining

• -- mixing flocculation
• -- aeration

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Often the purpose of, physical and chemical
processes is to separate

• -- solids from liquid
• -- dissolved chemicals from liquid
• -- gases from liquid

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Separating Solids

• Settling
• Discrete particles
• Flocculants
• Precipitants
• Flotation
• Filtration
• Granular media
• Membrane
• Centrifugation

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Discrete Particle Settling

• Modeled by Stokes law (p 363 ME)
• Spherical, homogeneous particles
• No flocculation
• Vc depth? (Q / V) Q/A overflow rate
• Only particles with velocity gt Vc are removed
• Grit removal 30 sec to a few minutes
• Primary sedimentation (clarification) 1000
  g/sf-d

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Example Primary clarification

• Circular or rectangular basins
• 1.5 to 2.5 hour detention time
• Significant removal of BOD (up to 45)
• Removal of TSS (up to 70)
• Fractionation of VSS from TSS

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Flocculation

• Flocculation formation of aggregates that settle
• Can occur during settling
• May be helped by coagulants

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Flocs in activated sludge.
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Coagulants / Precipitants
Alum Al2(SO4)318H2O Aluminum chloride
AlCl3 Lime Ca(OH)2 Ferric chloride
FeCl3 Ferric sulfate Fe2(SO4)3 Ferrous
sulfate FeSO4 7H2O Sodium aluminate Na2Al2O4
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Dissolved Air Flotation Thickner (DAFT)
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Depth vs Surface Filtration
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Granular filter filtration cycle
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Granular filter backwash cycle
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Granular filter variables

• Influent and effluent quality
• Filter media (size, shape, etc.)
• Filter bed (depth, mixing, porosity,
  stratification)
• Filtration flow rate
• Chemicals
• Headloss allowed
• Backwashing

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Membrane filtration process
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Membrane processes by mechanism

• Hydrostatic pressure ?
• Microfiltration (MF) 0.08 2.0 mm
• Ultrafiltration (UF) 0.005 0.2 mm
• Nanofiltration (NF) 0.001- 0.01 mm
• Reverse Osmosis (RO) 0.0001 0.001 mm
• Diffusion across a concentration ?
• Dialysis (2 to 50 nm pore size)
• Electromotive force
• Electrodialysis ( less than 2 nm)

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Sieving (top) as in MF and UF Ion
rejection (bottom) as in NF and RO
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Osmosis
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Reverse Osmosis
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Separating Dissolved Chemicals from Liquids

• Membrane filtration (as before)
• Adsorption
• Ion Exchange

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Adsorption

• Definition accumulation of substances at an
  interface (e.g. solid / liquid)
• Adsorbate substance being transferred
• Adsorbent recipient interface
• Activated carbon may be used in WWT
• Char from organics
• Activation develops pores (1 nm to 25 nm)
• GAC and PAC

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Sorption onto activated carbon
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Activated carbon plug flow reactor.
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Ion Exchange

• See Fig. 11-77 in text. (pg. 1195)
• Ions in solution displace and replace ions on
  exchange substrate
• e.g. Ca and Mg replacing Na
• zeolites (aluminosilicates)
• NH4 and NO3- can be removed
• Cationic and anionic resins for TDS removal
• Heavy metal (e.g. Cd, Zn, Ni, Cu) removal

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ion exchange schematic
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Gas Stripping

• Moves gas from liquid phase into gas phase and
  away
• Contact liquid with clean air
• Ammonia, VOCs, odorous gases
• Continuous or intermittent flow
• Counter-, Co- or Cross-current flow regimes

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Counter current stripping tower for VOC removal.
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Ammonia stripping
NH4 ? NH3 H
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Advanced Treatmentmay include

• Filtration for solids removal
• RO for dissolved organics and inorganics
• Adsorption (e.g. GAC) for organics
• Air Stripping for volatile organics and ammonia
• Ion exchange for dissolved ions, TDS