Transcript Wastewater Treatment

Wastewater
Treatment
Municipal WW Management Systems
Sources of
Wastewater
Transmission
and Pumping
Processing at
the Source
Treatment
Wastewater
Collection
Reuse/Disposal
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
Typical WW Characteristics
Parameter
Conc.
BOD
TSS
COD
Ammonia
TOC
Chloride
250 mg/L
250 mg/L
500 mg/L
30 mg/L
100 mg/L
+ 50 mg/L
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
Conventional WW Treatment
Preliminary
Treatment
Secondary
Sedimentation
Primary
Sedimentation
Biological Process
Sludge
Disinfection
Sludge
Preliminary Treatment
 To
remove materials that will interfere with
subsequent treatment
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Coarse Screening (bar racks)
Medium Screening
Comminution
Flow measuring
Pumping
Grit removal
Pre-aeration
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
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
Grit Chamber
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
Primary Sedimentation
– Primary Treatment
– Removes ~50% of suspended solids
Parameter Design Range
Typical
Value
Overflow
Rate
35-45 m/d
800-1200 gal/ft2/d
40 m/d
1000 gal/ft2/d
Detention
Time
1.5-2.5 h
2h
2
275 m2/d
Weir loading 125-500 m /d
10,000-40,000 gal/ft/d 20,000 gal/ft/d
rate
Primary Sed. Tank
Primary Clarifier: Center Feed
Secondary Treatment
 Generally
includes some biological process
plus secondary clarification
 Required under PL92-500
 Converts soluble and colloidal organic
materials to biomass and CO2
Biological Treatment
 Suspended
Growth
– Activated Sludge
» Conventional, Extended Aeration, Contact
Stabilization
– Aerated lagoons
– Aerobic digestion
 Attached
Growth
– Trickling Filters
– Rotating Biological Contactors
Suspended Growth Systems
Air
Aeration Tank
Return Activated Sludge
Secondary
Sedimentation
Sludge
Waste
Activated Sludge
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
Rotating Biological Contactors
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Drum diameters are typically 10-12 ft.
Rotation speed: ~1.5 rpm
May be in several stages
No flow recycle
Requires piloting
Secondary Clarifier
Sludge Disposal
 Thickening
– gravity, flotation
 Digestion
– aerobic, anaerobic
 Mechanical
Dewatering
– Vacuum filtration, centrifugation, pressure filtr.
 Disposal
– land application, burial, incineration
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
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
Belt Filter Press (Komline-Sanderson)