Wastewater%20Treatment

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• Wastewater Treatment

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Municipal WW Management Systems
Sources of Wastewater
Processing at the Source
Wastewater Collection
Reuse/Disposal
Treatment
Transmission and Pumping
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Sources Types of WW

• Domestic Wastewater
• from residences, commercial institutional
• flows 70-90 of water supplied
• Industrial Wastewater
• highly dependent on industry
• Infliltration/Inflow
• enters through leaks, foundation drains, etc.
• Stormwater
• for combined sewers - largely in older cities

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Typical WW Characteristics
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1972 Federal Water Pollution Control Act

• PL 92-500 subsequently amended and now called the
  Clean Water Act
• established water quality goals fishable
  swimmable and timetable
• established National Pollution Discharge
  Elimination System (NPDES)
• construction grants for WW treatment
• required secondary treatment (30/30)
• 30 mg/L BOD5
• 30 mg/L TSS

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TREATMENT CLASSIFICATION

• PRIMARY
• Removal of solids
• SECONDARY
• Removal of organics
• TERTIARY
• Removal of nutrients (N and P)

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Conventional WW Treatment
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Preliminary Treatment

• To remove materials that will interfere with
  subsequent treatment
• Coarse Screening (bar racks)
• Medium Screening
• Comminution
• Flow measuring
• Pumping
• Grit removal
• Pre-aeration

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Bar Racks

• Metal bars spaced a few cm apart across water
  flow
• mechanical or manually cleaned
• size of unit set by approach velocity
• 0.6-1.0 m/s for mechanically cleaned
• 0.3-0.7 m/s for manually cleaned
• see Fig 10.4 and example 10.2 on pg. 311

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Grit Removal

• Grit chambers intended to remove sand, cinders,
  gravel that may enter system by cracks in pipes,
  inflow etc.
• Grit can cause excess wear in pipes and pumps
• small sedimentation tanks designed with the help
  of Stokes Law
• no organics removal

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Grit Chamber
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Primary Sedimentation

• Purpose to remove suspended solids (smaller than
  grit, and less harmful)
• Typical efficiency
• 67 TSS removal
• 33 BOD removal
• Design parameters
• overflow rate
• weir loading rate
• detention time

Primary effluent is largely composed of soluble
and colloidal organics which can be converted to
settleable microbial solids and CO2 by biological
treatment
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Primary Sedimentation

• Primary Treatment
• Removes 50 of suspended solids

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Primary Sed. Tank
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Primary Clarifier Center Feed
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Secondary Treatment

• Generally includes some biological process plus
  secondary clarification
• Required under PL92-500
• Converts soluble and colloidal organic materials
  to biomass and CO2

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Biological Treatment

• Suspended Growth
• Activated Sludge
• Conventional, Extended Aeration, Contact
  Stabilization
• Aerated lagoons
• Aerobic digestion
• Attached Growth
• Trickling Filters
• Rotating Biological Contactors

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Suspended Growth Systems
Activated Sludge!
Sludge
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Attached Growth Trickling Filters
Rotary Distributor
Rocks or Plastic Media
Air
Underdrain
With rocks, depth is limited to 2-3 m because of
oxygen needs
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Rotating Biological Contactors

• Drum diameters are typically 10-12 ft.
• Rotation speed 1.5 rpm
• May be in several stages
• No flow recycle
• Requires piloting

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Secondary Clarifier
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Sludge Disposal

• Thickening
• gravity, flotation
• Digestion
• aerobic, anaerobic
• Mechanical Dewatering
• Vacuum filtration, centrifugation, pressure
  filtr.
• Disposal
• land application, burial, incineration

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Anaerobic Digestion

• Sludge held without aeration for 10-90 days
• Process can be accelerated by heating to 35-40oC
• These are called High Rate Digestors (10-20 days)
• Advantages
• low solids production
• useable methane gas produced
• Disadvantages
• high capital costs
• susceptibility to shocks and overloads

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Sludge Dewatering

• Sludge drying beds
• historically the most common
• sand bed, 15-30 days, evaporation seepage
• Vacuum Filtration
• cylindrical rotating drum covered with fabric
• submerged with applied vacuum
• Continuous belt filter presses (follows)
• Plate pressure filters
• vertical plates mounted on a frame

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Belt Filter Press (Komline-Sanderson)