waste water treatment
DESCRIPTION
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Wastewater
Treatment
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Municipal WW Management Systems
Sources of
WastewaterProcessing at
the SourceWastewater
Collection
Transmission
and PumpingTreatment 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/L
TSS 250 mg/L
COD 500 mg/L
Ammonia 30 mg/L
TOC 100 mg/L
Chloride + 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 BOD5
30 mg/L TSS
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Conventional WW Treatment
Biological ProcessPreliminary
Treatment
Secondary
Sedimentation
Sludge
Disinfection
Primary
Sedimentation
Sludge
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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
Parameter Design Range Typical
ValueOverflow
Rate
35-45 m/d
800-1200 gal/ft2/d
40 m/d
1000 gal/ft2/d
Detention
Time
1.5-2.5 h 2 h
Weir loading
rate
125-500 m2/d
10,000-40,000 gal/ft/d
275 m2/d
20,000 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
Sludge
Secondary
Sedimentation
Return Activated Sludge
Waste
Activated Sludge
Aeration Tank
Air
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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 needs
Air
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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)