Download - Lecture 15 2009 Revised
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Wastewater treatment
processes (I)
ENV H 440/ENV H 541
John Scott Meschke
Office: Suite 2249,
4225 Roosevelt
Phone: 206-221-5470
Email:
Gwy-Am Shin
Office: Suite 2339,
4225 Roosevelt
Phone: 206-543-9026
Email:
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Key points
Purpose of the individual unit processes
The typical operating conditions
The outcome of the processes Microbial reduction of the processes
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How much wastewater do we
produce each day?
Wastewater Characteristics
Source Average Daily Flow
Domestic sewage 60-120 gal/capita
Shopping centers 60-120 gal/1000 ft2 total floorarea
Hospitals 240-480 gal/bed
Schools 18-36 gal/student
Travel trailer parks
Without individualhookups
90 gal/site
With individualhookups
210 gal/site
Campgrounds 60-150 gal/campsite
Mobile home parks 265 gal/unit
Motels 40-53 gal/bedHotels 60 gal/bed
Industrial areas
Light industrial area 3750 gal/acre
Heavy industrial 5350 gal/acre
Source: Droste, R.L., 1997. Theory and Practice ofWater and Wastewater Treatment
These values are
rough estimates only
and vary greatly by
locale.
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Wastewater treatment systems
Decentralized
Septic tank
Waste stabilization ponds Facultative lagoon
Maturation lagoon
Land treatment
Constructed wetland
Centralized
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Sewer systems
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Typical composition of untreated domestic wastewater
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Microorganism concentrations in untreated wastewater
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(Minimum) Goals of wastewater
treatment plants
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Conventional Community (Centralized) Sewage
Treatment
Pathogen Reductions Vary from:
low (99.99+%)
Secondary Treatment Using Activated Sludge Process
Sludge drying bed ormechanical dewatering
process
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Typical Municipal Wastewater Treatment System
Preliminary or PrePreliminary or Pre--
TreatmentTreatmentPrimary
Treatment
Secondary
TreatmentDisinfection
Sludge Treatment& Disposal
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Preliminary Wastewater Treatment System
Preliminary or PrePreliminary or Pre--
TreatmentTreatment
Solids to Landfill
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Preliminary Treatment Facilities
Preliminary Treatment - Bar Racks
Bar Racks: are used to remove large objects that
could potentially damage downstreamtreatment/pumping facilities.
Ref: Metcalf & Eddy, 1991
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Preliminary Treatment - Grit chamber
Grit chamber: used to remove small to medium sized,dense objects such as sand, broken glass, bone
fragments, pebbles, etc.
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Primary Wastewater Treatment
Primary
Treatment
Primary
Treatment
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Primary sedimentation
To remove settleable solids from wastewater
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Primary Clarification
PrimarySludge
Primary
Effluent
Influent from Preliminary
Treatment
Section through a Circular Primary Clarifier
Primary Treatment
Scum: Oil, Grease,
Floatable Solids
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Primary sedimentation
To remove settleable solids from wastewater
Average flow: 800-1200 gpd/ft2
Retention time: 1.5 - 2.0 hours (at maximum flow)
50 - 70 % removal of suspended solids 25 - 35 % removal of BOD5 ~20 % removal of phosphate
~50 % removal of viruses, bacteria, and protozoa
90 % removal of helminth ova
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Secondary Wastewater Treatment
Secondary
Treatment
Secondary
Treatment
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Secondary treatment processes
To remove suspended solids, nitrogen,
and phosphate
90 % removal of SS and BOD
5
Various technologies
Activated sludge process
Tricking filter
Stabilization ponds
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Secondary Treatment Using Activated Sludge Process
Secondary
Treatment
Secondary Treatment
Sludge drying bed ormechanical dewatering
process
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Aerobic microbes utilities carbon and
other nutrients to form a healthy
activated sludge (AS) biomass
(floc)
The biomass floc is allowed to
settle out in the next reactor;
some of the AS is recycled
Secondary Treatment
Simplified Activated Sludge Description
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General Microbial Growth
Carbon Source: Dissolved organic matter
Energy Source: Dissolved organic matter
Terminal Electron Acceptor: Oxygen
Nutrients: Nitrogen, Phosphorus, Trace
Metals
Microorganisms: Indigenous in
wastewater, recycledfrom secondary clarifier
Secondary Treatment
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Activated Sludge Aeration Basins
Empty basin, air
diffusers on bottom
Same basin,in operation
Secondary Treatment
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The Oxidation Ditch
Ref: Reynolds & Richards,1996, Unit Operations and Processes inEnvironmental Engineering Secondary Treatment
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The Oxidation Ditch
Ref: Reynolds & Richards,1996, Unit Operations and Processes inEnvironmental Engineering Secondary Treatment
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Circular Secondary Clarifier
Secondary
Effluent
Influent from ActivatedSludge Aeration Basin
or Trickling Filter
Section through a Circular Secondary Clarifier
Return (Secondary)
Sludge Line
Secondary Treatment
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Activated sludge process
To remove suspended solids, nitrogen, and phosphate
Food to microorganism ratio (F:M ratio): 0.25 kg BOD5per kg MLSS (mixed liquor suspended solids) per day at10 oC or 0.4 kg BOD5 per kg MLSS per day at 20
oC
Residence time: 2 days for high F:M ratio, 10 days ormore for low F:M ratio
Optimum nutrient ratio: BOD5:N:P =>100:5:1
90 % removal of SS and BOD5 ~20 % removal of phosphate
> 90 % removal of viruses and protozoa and 45 - 95 %removal of bacteria
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Secondary Treatment Using Trickling Filter Process
Secondary
Treatment
Secondary Treatment
Trickling
Filter
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Trickling Filter
http://www.rpi.edu/dept/chem-eng/Biotech-Environ/FUN DAMNT/streem/trickfil.jpg
Primary effluent
drips onto rock or
man-made media
Rotating arm to
distribute water
evenly over filter
Rock-bed with slimy(biofilm) bacterial growth
Primary effluent pumped in
Treated waste to
secondary clarifier
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Trickling Filter
http://www.eng.uc.edu/friendsalumni/research/labsresearch/biofilmreslab/Tricklingfilter_big.jpg
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Tricking filter process
To remove suspended solids, nitrogen, andphosphate
Organic loading (BOD5 X flow/volume of filter):0.1 kg BOD
5
per m3 per day
Hydraulic loading: 0.4 m3 per day per m3 of planarea
90 % removal of SS and BOD5 ~20 % removal of phosphate
Variable removal levels of viruses, 20-80 %removal of bacteria and > 90 % removal ofprotozoa
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Stabilization Ponds
The oldest wastewater treatment systems Requires a minimum of technology
Relatively low in cost
Popular in developing countries and smallcommunities in the US (90 % communities with
population
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Facultative Pond
Ponds and Lagoons
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Facultative ponds
3 zones: upper photic (aerobic) zone, facultative (aerobicand anaerobic) zone and lower anaerobic zone. Upper aerobic zone: algae use CO2, sunlight and inorganic
nutrients (photosynthesis) to produce oxygen and algal biomass.
Facultative zone: bacteria and other heterotrophs convert organicmatter to carbon dioxide, inorganic nutrients, water and microbial
biomass. Lower anaerobic zone: anaerobic bacteria degrade the biomass
from upper zones
Influence by many factors Sunlight
Temperature pH
Biological activities
Characteristics of wastewater
Ponds and Lagoons
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Facultative ponds
To remove suspended solids, nitrogen, phosphate, andpathogens
Operating water depth: 1-2.5 meters
(maximum) BOD loading: 2.2-5.6 g/m3 /day
Retention time: 3-6 months
>90 % SS and BOD removal (warm and sunny climates)
Microbe removal may be quite variable dependingupon pond design, operating conditions and climate. 90-99% removal of indicator and pathogenic bacteria
99 % removal of PV1
99.9 reduction ofGiardia and Cryptosporidium
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Aerated Lagoons
http://www.lagoonsonline.com/marshill.htm
Ponds and Lagoons
Stabilization Lagoon
Aerated Lagoons
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Aerated lagoons
Biological activity is provided by mainly
aerobic bacteria
Influence by many factors Aeration time
Temperature
pH
Biological activity
Characteristics of wastewater
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Aerated lagoons
To remove suspended solids, nitrogen, phosphate,and pathogens
Operating water depth: 1-2 meters
Retention time:
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Wastewater Disinfection
Disinfection
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Typical Municipal Wastewater Treatment System
Preliminary or PrePreliminary or Pre--
TreatmentTreatmentPrimary
Treatment
Secondary
TreatmentDisinfection
Sludge Treatment& Disposal
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Sludge processing
Thickening
Digestion
Dewatering Disposal
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Sludge thickening
To reduce the volume of sludge to increase sludge solids at least 4 %
Gravity thickening and mechanical thickening
Gravity thickening Used for primary and tricking filter solids
Without chemical flocculants loading rate: 30-60 kg/m2 per day
Mechanical thickening Used for activated sludge solids
With chemical flocculants
dissolved air flotation, gravity belt thickeners, and centrifuge thickening
loading rate: 10-20 kg/m2 per day (dissolved air flotation), 400-1000 L/m(gravity belt thickeners), 1500-2300 L/m (centrifuge thickening)
The concentration of pathogens increased during this process
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Gravity belt thickener
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Regulatory requirement for
disposal of sewage sludge
Class B biosolids (agriculture land) < 2 million MPN/g of fecal coliforms
Seven samples over 2-weeks period
~2 log removal Class A biosolids (home lawn and garden)
< 1000 MPN/g of fecal coliforms
< 3 MPN/4g ofSalmonella sp.
< 1 PFU/4g of enteric viruses < 1/4g of Helminth ova
~ 5 log removal
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Processes to significantly reduce pathogens
(PSRP) for a Class B biosolids
Aerobic digestion
Anaerobic digestion
Air drying
Composting
Lime stabilization
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Digestion
To stabilize organic matter, control orders,and destroy pathogens
Aerobic digestion and anaerobic digestion
Aerobic digestion
Sludge is agitated with air/oxygen
loading rate (maximum): 640 g/m2 per day
Temperature and retention time: 68 oF for 40days or 58 oF for 60 days
Solids and BOD reduction: 30-50 %
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Anaerobic digestion
Sludge is treated in the absence of air
Operation conditions (optimum) Temperature: 85-99 oF (98 oF)
pH: 6.7-7.4 (7.0-7.1)
Alkalinity: 2000-3500 mg/L
Solid loading: 0.02-0.05 lb/ft3/day
Retention time: 30-90 days
Treatment outcome
Solid reduction: 50-70 % Significant reduction of most pathogens
Gas production: methane and carbon dioxide
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Anaerobic digester
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Air drying, composting, and lime
stabilization Air drying
Sludge is dried on sand beds/(un)paved basins
Retention time: minimum of 3 months
Composting
Various methods: in-vessel, static aerated file, and periodicallymixed windrows
File temperature should be raised > 40 oC for 5 days
For 4 hours during the 5 days, the file temperature should be >55 oC
Lime stabilization
Sufficient lime should be added to raise the pH 12 after 2 hourcontact
4 log inactivation of enteric viruses, 2-7 log inactivation ofindicator bacteria, no inactivation of Acaris ova
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Processes to further reduce pathogens
(PFRP) for a Class A biosolids
Heat drying Sludge is dried by contact with hot gases
The temperature of gas is >80 oC
Thermophilic aerobic digestion Sludge is agitated with air/oxygen
132-149 oF for 4-20 hours
Pasteurization
158
o
F for 30 minutes Beta- or gamma ray irradiation Sludge is irradiated with either beta- or gamma ray
> 1.0 Mrad at room temperature
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Dewatering
To concentrate sludge by removing water
Pressure filtration, centrifugation, and screw
press
Pressure filtration (belt filter press and plate-and-frame filter)
Usually with polymer flocculation
Loading rate: 40-60 gpm/m (hydraulic) and 500-1000
lb/m/h (solid)
Feed solid: 1-6 %
Cake solids: 15-30 %
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Belt filter press
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Disposal
Land application
Landfill
Incineration Ocean dumping (no longer allowed in US)