Transcript Wastewater Collection Systems

ENV H 440/ENV H 541
Wastewater treatment
processes (I)
John Scott Meschke
Gwy-Am Shin
Office: Suite 2249,
Office: Suite 2339,
4225 Roosevelt
4225 Roosevelt
Phone: 206-221-5470
Phone: 206-543-9026
Email:
[email protected]
Email:
[email protected]
Key points
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Purpose of the individual unit processes
The typical operating conditions
The outcome of the processes
Microbial reduction of the processes
How much wastewater do we
produce each day?
Source
Domestic sewage
Shopping centers
Average Daily Flow
60-120 gal/capita
60-120 gal/1000 ft2 total floor
area
240-480 gal/bed
18-36 gal/student
Hospitals
Schools
Travel trailer parks
Without individual
90 gal/site
hookups
With individual
210 gal/site
hookups
Campgrounds
60-150 gal/campsite
Mobile home parks
265 gal/unit
Motels
40-53 gal/bed
Hotels
60 gal/bed
Industrial areas
Light industrial area 3750 gal/acre
Heavy industrial
5350 gal/acre
Source: Droste, R.L., 1997. Theory and Practice of
Water and Wastewater Treatment
These values are
rough estimates only
and vary greatly by
locale.
Wastewater Characteristics
Wastewater treatment systems
• Decentralized
– Septic tank
– Waste stabilization ponds
• Facultative lagoon
• Maturation lagoon
– Land treatment
– Constructed wetland
• Centralized
Sewer systems
Typical composition of untreated domestic wastewater
Microorganism concentrations in untreated wastewater
(Minimum) Goals of wastewater
treatment plants
• <30 mg/L of BOD5
• <30 mg/L of suspended solids
• <200 CFU/100mL of fecal coliforms
Conventional Community (Centralized) Sewage
Treatment
Secondary Treatment Using Activated Sludge Process
Pathogen Reductions Vary from:
low (<90%) to Very High
(>99.99+%)
Sludge drying bed or
mechanical dewatering
process
Typical Municipal Wastewater Treatment System
Preliminary or Pre- Primary
Treatment
Treatment
Sludge Treatment
& Disposal
Secondary
Treatment
Disinfection
Preliminary Wastewater Treatment System
Preliminary or PreTreatment
Solids to Landfill
Preliminary Treatment - Bar Racks
Bar Racks: are used to remove large objects that
could potentially damage downstream
treatment/pumping facilities.
Ref: Metcalf & Eddy, 1991
Preliminary Treatment Facilities
Preliminary Treatment - Grit chamber
Grit chamber: used to remove small to medium sized,
dense objects such as sand, broken glass, bone
fragments, pebbles, etc.
Primary Wastewater Treatment
Primary
Treatment
Primary sedimentation
• To remove settleable solids from wastewater
Primary Clarification
Scum: Oil, Grease,
Floatable Solids
Primary
Effluent
Primary
Sludge
Influent from Preliminary
Treatment
Section through a Circular Primary Clarifier
Primary Treatment
Primary sedimentation
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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
Secondary Wastewater Treatment
Secondary
Treatment
Secondary treatment processes
• To remove suspended solids, nitrogen,
and phosphate
• 90 % removal of SS and BOD5
• Various technologies
– Activated sludge process
– Tricking filter
– Stabilization ponds
Secondary Treatment Using Activated Sludge Process
Sludge drying bed or
mechanical dewatering
process
Secondary
Treatment
Secondary Treatment
Simplified Activated Sludge Description
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
General Microbial Growth
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Carbon Source:
Dissolved organic matter
Energy Source:
Dissolved organic matter
Terminal Electron Acceptor: Oxygen
Nutrients:
Nitrogen, Phosphorus, Trace
Metals
• Microorganisms:
Indigenous in
wastewater, recycled
from secondary clarifier
Secondary Treatment
Activated Sludge Aeration Basins
Empty basin, air
diffusers on bottom
Same basin,
in operation
Secondary Treatment
The Oxidation Ditch
Ref: Reynolds & Richards,1996, Unit Operations and Processes in
Environmental Engineering
Secondary Treatment
The Oxidation Ditch
Ref: Reynolds & Richards,1996, Unit Operations and Processes in
Environmental Engineering
Secondary Treatment
Circular Secondary Clarifier
Secondary
Effluent
Return (Secondary)
Sludge Line
Influent from Activated
Sludge Aeration Basin
or Trickling Filter
Section through a Circular Secondary Clarifier
Secondary Treatment
Activated sludge process
• To remove suspended solids, nitrogen, and phosphate
• Food to microorganism ratio (F:M ratio): 0.25 kg BOD5
per kg MLSS (mixed liquor suspended solids) per day at
10 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 or
more 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
Secondary Treatment Using Trickling Filter Process
Trickling
Filter
Secondary
Treatment
Secondary Treatment
Trickling Filter
Primary effluent
drips onto rock or
man-made media
Rotating arm to
distribute water
evenly over filter
Rock-bed with slimy
(biofilm) bacterial growth
Treated waste to
secondary clarifier
Primary effluent pumped in
http://www.rpi.edu/dept/chem-eng/Biotech-Environ/FUNDAMNT/streem/trickfil.jpg
Trickling Filter
http://www.eng.uc.edu/friendsalumni/research/labsresearch/biofilmreslab/Tricklingfilter_big.jpg
Tricking filter process
• To remove suspended solids, nitrogen, and
phosphate
• Organic loading (BOD5 X flow/volume of filter):
0.1 kg BOD5 per m3 per day
• Hydraulic loading: 0.4 m3 per day per m3 of plan
area
• 90 % removal of SS and BOD5
• ~20 % removal of phosphate
• Variable removal levels of viruses, 20-80 %
removal of bacteria and > 90 % removal of
protozoa
Stabilization Ponds
• The oldest wastewater treatment systems
– Requires a minimum of technology
– Relatively low in cost
– Popular in developing countries and small
communities in the US (90 % communities with
population <10,000)
• Used for raw sewage as well as primary- or
secondary-treated effluents.
• Facultative ponds and aerated lagoons
Ponds and Lagoons
Facultative Pond
Ponds and Lagoons
Facultative ponds
• 3 zones: upper photic (aerobic) zone, facultative (aerobic
and 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 organic
matter to carbon dioxide, inorganic nutrients, water and microbial
biomass.
– Lower anaerobic zone: anaerobic bacteria degrade the biomass
from upper zones
• Influence by many factors
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Sunlight
Temperature
pH
Biological activities
Characteristics of wastewater
Ponds and Lagoons
Facultative ponds
• To remove suspended solids, nitrogen, phosphate, and
pathogens
• 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 depending
upon pond design, operating conditions and climate.
– 90-99% removal of indicator and pathogenic bacteria
– 99 % removal of PV1
– 99.9 reduction of Giardia and Cryptosporidium
Aerated Lagoons
Stabilization Lagoon
Aerated Lagoons
http://www.lagoonsonline.com/marshill.htm
Ponds and Lagoons
Aerated lagoons
• Biological activity is provided by mainly
aerobic bacteria
• Influence by many factors
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Aeration time
Temperature
pH
Biological activity
Characteristics of wastewater
Aerated lagoons
• To remove suspended solids, nitrogen, phosphate, and
pathogens
• Operating water depth: 1-2 meters
• Retention time: <10 days
• 85% BOD removal (at 20oC and an aeration period of
5 days)
• 65% BOD removal (at 10oC and an aeration period of
5 days)
• Microbe removal may be quite variable depending
upon pond design, operating conditions and climate
Wastewater Disinfection
Disinfection
Typical Municipal Wastewater Treatment System
Preliminary or Pre- Primary
Treatment
Treatment
Sludge Treatment
& Disposal
Secondary
Treatment
Disinfection
Sludge processing
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Thickening
Digestion
Dewatering
Disposal
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
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Mechanical thickening
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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
Gravity belt thickener
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)
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< 1000 MPN/g of fecal coliforms
< 3 MPN/4g of Salmonella sp.
< 1 PFU/4g of enteric viruses
< 1/4g of Helminth ova
~ 5 log removal
Processes to significantly reduce pathogens
(PSRP) for a Class B biosolids
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Aerobic digestion
Anaerobic digestion
Air drying
Composting
Lime stabilization
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 40
days or 58 oF for 60 days
– Solids and BOD reduction: 30-50 %
Anaerobic digestion
• Sludge is treated in the absence of air
• Operation conditions (optimum)
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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
Anaerobic digester
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 periodically
mixed 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
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Lime stabilization
– Sufficient lime should be added to raise the pH 12 after 2 hour
contact
– 4 log inactivation of enteric viruses, 2-7 log inactivation of
indicator bacteria, no inactivation of Acaris ova
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 oF for 30 minutes
• Beta- or gamma ray irradiation
– Sludge is irradiated with either beta- or gamma ray
– > 1.0 Mrad at room temperature
Dewatering
• To concentrate sludge by removing water
• Pressure filtration, centrifugation, and screw
press
• Pressure filtration (belt filter press and plateand-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 %
Belt filter press
Disposal
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Land application
Landfill
Incineration
Ocean dumping (no longer allowed in US)