Wastewater treatment processes (I)
ENV H 440/ENV H 541
John Scott Meschke
Office: Suite 2249,
4225 Roosevelt
Phone: 206-221-5470
Email: [email protected]
Gwy-Am Shin
Office: Suite 2339,
4225 Roosevelt
Phone: 206-543-9026
Email: [email protected]
Key points
• 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?
Wastewater Characteristics
Source Average Daily FlowDomestic sewage 60-120 gal/capitaShopping centers 60-120 gal/1000 ft2 total floor
areaHospitals 240-480 gal/bedSchools 18-36 gal/studentTravel trailer parks
Without individualhookups
90 gal/site
With individualhookups
210 gal/site
Campgrounds 60-150 gal/campsiteMobile home parks 265 gal/unitMotels 40-53 gal/bedHotels 60 gal/bedIndustrial areas
Light industrial area 3750 gal/acreHeavy 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.
Wastewater treatment systems
• Decentralized– Septic tank– Waste stabilization ponds
• Facultative lagoon• Maturation lagoon
– Land treatment– Constructed wetland
• Centralized
(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
Pathogen Reductions Vary from: low (<90%) to Very High (>99.99+%)
Secondary Treatment Using Activated Sludge Process
Sludge drying bed or mechanical dewatering process
Typical Municipal Wastewater Treatment System
Preliminary or Pre-Preliminary or Pre-TreatmentTreatment
PrimaryTreatment
SecondaryTreatment
Disinfection
Sludge Treatment& Disposal
Preliminary Wastewater Treatment System
Preliminary or Pre-Preliminary or Pre-TreatmentTreatment
Solids to Landfill
Preliminary Treatment Facilities
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 - Grit chamber
Grit chamber: used to remove small to medium sized, dense objects such as sand, broken glass, bone fragments, pebbles, etc.
Primary sedimentation • To remove settleable solids from wastewater
Primary Clarification
PrimarySludge
PrimaryEffluent
Influent from Preliminary Treatment
Section through a Circular Primary Clarifier
Primary Treatment
Scum: Oil, Grease, Floatable Solids
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
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
SecondaryTreatment
Secondary Treatment
Sludge drying bed or mechanical dewatering process
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
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, recycled from secondary clarifier
Secondary Treatment
Activated Sludge Aeration Basins
Empty basin, airdiffusers 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
SecondaryEffluent
Influent from Activated Sludge Aeration Basin
or Trickling Filter
Section through a Circular Secondary Clarifier
Return (Secondary) Sludge Line
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
SecondaryTreatment
Secondary Treatment
TricklingFilter
Trickling Filter
http://www.rpi.edu/dept/chem-eng/Biotech-Environ/FUNDAMNT/streem/trickfil.jpg
Primary effluent drips onto rock orman-made media
Rotating arm todistribute water evenly over filter
Rock-bed with slimy (biofilm) bacterial growth
Primary effluent pumped inTreated waste to secondary clarifier
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 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– 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
http://www.lagoonsonline.com/marshill.htmPonds and Lagoons
Stabilization Lagoon
Aerated Lagoons
Aerated lagoons
• Biological activity is provided by mainly aerobic bacteria
• Influence by many factors– 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
Typical Municipal Wastewater Treatment System
Preliminary or Pre-Preliminary or Pre-TreatmentTreatment
PrimaryTreatment
SecondaryTreatment
Disinfection
Sludge Treatment& 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
• 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
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 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
• 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)
– 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
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• 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 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 %