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Water and wastewater treatment processes
ENV H 452/ENV H 542
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]
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Key points
• Purpose of the individual unit processes
• The typical operating conditions
• The outcome of the processes
• Microbial reduction in the processes
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Wastewater treatment processes
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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.
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Wastewater treatment systems
• Decentralized– Septic tank– Waste stabilization ponds
• Facultative lagoon• Maturation lagoon
– Land treatment
• Centralized
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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 processes
• <30 mg/L BOD5
• <30 mg/L of suspended solids
• <200 CFU/100ml fecal coliforms
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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
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Typical Municipal Wastewater Treatment System
Preliminary or Pre-Preliminary or Pre-TreatmentTreatment
PrimaryTreatment
SecondaryTreatment
Disinfection
Sludge Treatment& Disposal
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Preliminary Wastewater Treatment System
Preliminary or Pre-Preliminary 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 downstream treatment/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
PrimaryTreatmentPrimary
Treatment
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Primary sedimentation • To remove settleable solids from wastewater
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Primary Clarification
PrimarySludge
PrimaryEffluent
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• Maximum flow: 30 - 40 m3 per day• Retention period: 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
SecondaryTreatmentSecondaryTreatment
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Secondary treatment processes
• To remove suspended solids, nitrogen, and phosphate
• 90 % removal of SS and BOD5
• Various technologies– Activated sludge process– Tricking filter– Aerated lagoons– Rotating biological contractors
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Secondary Treatment Using Activated Sludge Process
SecondaryTreatment
Secondary Treatment
Sludge drying bed or mechanical dewatering process
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The Activated Sludge 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
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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 BOD5 and SS• ~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
SecondaryTreatment
Secondary Treatment
TricklingFilter
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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
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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, 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 BOD5 and SS• ~20 % removal of phosphate• Variable removal levels of viruses, 20-80 %
removal of bacteria and >90 % removal of protozoa
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Wastewater Disinfection
Disinfection
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Wastewater disinfection
• To inactivate pathogens in wastewater
• Several choices– Free chlorine and combined chlorine– UV– Ozone– Chlorine dioxide
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Overall pathogen reduction in wastewater treatment
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Water treatment processes
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Water contaminants
• Chemicals– Inorganics– Organics
• Synthetic organic compounds• Volatile organic compounds
• Microbes– Viruses– Bacteria– Protozoa parasites– Algae– Helminths
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Water contaminants (I)
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Water contaminants (II)
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Water contaminants (III)
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Water contaminants (IV)
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Water contaminants (V)
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Multiple barrier concept for public health protection
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Barrier Approach to Protect Public Health in Drinking Water
• Source Water Protection
• Treatment Technology
• Disinfection
• Disinfectant residual in distribution system
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Water treatment processes
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Oxidation
• To remove inorganics (Fe++, Mn++) and some synthetic organics– Cause unaesthetic conditions (brown color)– Promote the growth of autotrophic bacteria (iron bacteria): taste
and order problem• Free chlorine, chlorine dioxide, ozone, potassium
permanganate– Fe++ + Mn ++ + oxygen + free chlorine → FeOx ↓ (ferric oxides) +
MnO2 ↓ (manganese dioxide)– Fe (HCO3)2 (Ferrous bicarbonate) + KMnO4 (Potassium
permanganase) → Fe (OH)3 ↓ (Ferric hydroxide) + MnO2 ↓ (manganese dioxide)
– Mn (HCO3)2 (Manganese bicarbonate) + KMnO4 (Potassuim permanganase) → MnO2 ↓ (manganese dioxide)
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Physico-chemical processes
• To remove particles in water
• Coagulation/flocculation/sedimentation
• Filtration
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Rapid Mix
• Intense mixing of coagulant and other chemicals with the water
• Generally performed with mechanical mixers
Chemical Coagulant
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Major Coagulants
• Hydrolyzing metal salts– Alum (Al2(SO4)3)
– Ferric chloride (FeCl3)
• Organic polymers (polyelectrolytes)
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Coagulation with Metal Salts
Al(OH)
Alx(OH)y
Colloid
Al(OH)3
Al(OH)3 Colloid
Al(OH)3
Al(OH)3
Colloid
+ +Soluble Hydrolysis Species
(Low Alum Dose)
Colloid
Colloid
Colloid
Al(OH)3Al(OH)3
Al(OH)3
Al(OH)3
Al(OH)3
(High Alum Dose)
Floc
Sweep CoagulationCharge Neutralization
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Horizontal Paddle Flocculator
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Flocculation ExampleFlocculation Example
Water coming from Water coming from rapid mix.rapid mix. Water goes to sedimentationWater goes to sedimentation
basin.basin.
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Sedimentation Basin
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Sedimentation Basin ExampleSedimentation Basin ExampleWater coming from Water coming from flocculation basin.flocculation basin.
Water goes to Water goes to filter.filter.
Floc (sludge) collectedin hopperSludge to solids
treatment
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Coagulation/flocculation/and sedimentation
• To remove particulates and natural organic materials in water• Coagulation
– 20 -50 mg/L of Alum at pH 5.5-6.5 (sweep coagulation)– rapid mixing: G values = 300-800/second
• Flocculation: – Slow mixing: G values = 30-70/second– Residence time:10 -30 minutes
• Sedimentation– Surface loading: 0.3 -1.0 gpm/ft2
– Residence time: 1 – 2 hours• Removal of suspended solids and turbidity: 60-80 %• Reduction of microbes
– 74-97 % Total coliform – 76-83 % of fecal coliform – 88-95 % of Enteric viruses– 58-99 % of Giardia– 90 % of Cryptosporidium
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Filtration
• To remove particles and floc that do not settle by gravity in sedimentation process
• Types of granular media– Sand– Sand + anthracite– Granular activated carbon
• Media depth ranges from 24 to 72 inches
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Filter ExampleFilter Example
Water coming from Water coming from sedimentation sedimentation basin.basin.
AnthraciteAnthracite
SandSand
Gravel (supportGravel (support
media)media)
Water going to disinfectionWater going to disinfection
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Mechanisms Involved in Filtration
Interception: hits & sticks
Sedimentation: quiescent, settles, & attaches
Flocculation: Floc gets larger within filter
Entrapment: large floc gets trapped in space between particles
Floc particles
Granular media, e.g., grain of sand
Removal of bacteria, viruses and protozoa by a granular media filter requires water to be coagulated
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Rapid filtration
• To remove particulates in water• Flow rate: 2-4 gpm/ft2
• Turbidity: < 0.5 NTU (often times < 0.1 NTU)• Reduction of microbes
– 50-98 % Total coliform – 50-98 % of fecal coliform – 10-99 % of enteric viruses– 97-99.9 % of Giardia– 99 % of Cryptosporidium
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Disinfection in water
• To inactivate pathogens in water
• Various types– Free chlorine– Chloramines– Chlorine dioxide– Ozone– UV
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Trend in disinfectant use (USA, % values)
Disinfectant 1978 1989 1999
Chlorine gas 91 87 83.8
NaClO2 (bulk) 6 7.1 18.3
NaClO2 (on-site)
0 0 2
Chlorine dioxide
0 4.5 8.1
Ozone 0 0.4 6.6
Chloramines 0 20 28.4
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Comparison of major disinfectants
Consideration Disinfect ants
Cl2 ClO2 O3 NH2Cl
Oxidation potential
Strong Stronger? Strongest Weak
Residuals Yes No No Yes
Mode of action
Proteins/NA
Proteins/NA
Proteins/NA
Proteins
Disinfecting efficacy
Good Very good Excellent Moderate
By-products Yes Yes Yes? No