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Wastewater Treatment
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Municipal WW Management Systems
Sources of Wastewater
Processing at the Source
WastewaterCollection
Transmissionand Pumping
Treatment Reuse/Disposal
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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
Parameter Conc.BOD 250 mg/LTSS 250 mg/LCOD 500 mg/LAmmonia 30 mg/LTOC 100 mg/LChloride + 50 mg/L
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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 BOD30 mg/L BOD55
– 30 mg/L TSS30 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 TreatmentBiological ProcessPreliminary
Treatment
SecondarySedimentation
SludgeSludge
Disinfection
PrimarySedimentation
SludgeSludge
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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 Stoke’s 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 Primary effluent is largely composed of largely composed of soluble and colloidal soluble and colloidal organics which can be organics which can be converted to settleable converted to settleable microbial solids and microbial solids and COCO22 by biological by biological
treatmenttreatment
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Primary Sedimentation– Primary Treatment– Removes ~50% of suspended solids
Parameter Design Range TypicalValue
OverflowRate
35-45 m/d800-1200 gal/ft2/d
40 m/d1000 gal/ft2/d
DetentionTime
1.5-2.5 h 2 h
Weir loadingrate
125-500 m2/d10,000-40,000 gal/ft/d
275 m2/d20,000 gal/ft/d
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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
SludgeSludge
SecondarySedimentation
Return Activated SludgeReturn Activated Sludge
WasteWasteActivated SludgeActivated Sludge
Aeration TankAirAir
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Attached Growth: Trickling Filters
Rocks or Plastic Media
Underdrain
Rotary Distributor
With rocks, depth is limited to 2-3 m because of oxygen needsWith rocks, depth is limited to 2-3 m because of oxygen needs
Air
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Rotating Biological Contactors
• Drum diameters are typically 10-12 ft.Drum diameters are typically 10-12 ft.• Rotation speed: ~1.5 rpmRotation speed: ~1.5 rpm• May be in several stagesMay be in several stages• No flow recycleNo flow recycle• Requires pilotingRequires 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 daysSludge held without aeration for 10-90 days• Process can be accelerated by heating to 35-40Process can be accelerated by heating to 35-40ooCC• These are called High Rate Digestors (10-20 days)These are called High Rate Digestors (10-20 days)• AdvantagesAdvantages
•low solids productionlow solids production•useable methane gas produceduseable methane gas produced
• DisadvantagesDisadvantages•high capital costshigh capital costs•susceptibility to shocks and overloadssusceptibility 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)