Download Wastewater Collection Systems

ENV H 452/ENV H 542
Water and wastewater
treatment processes
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 processes
• <30 mg/L BOD5
• <30 mg/L of suspended solids
• <200 CFU/100ml 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
Clarification Theory - Circular Basins
Center of Clarifier
Basin
Outside Wall of
Clarifier Basin
Horizontal velocity decreases
with increasing distance from
center of clarifier basin
Particles that are removed have a settling trajectory such
that the particle settles before reaching the
outside wall or end of clarifier.
Primary Treatment
Primary sedimentation
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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
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
– Aerated lagoons
– Rotating biological contractors
Secondary Treatment Using Activated Sludge Process
Sludge drying bed or
mechanical dewatering
process
Secondary
Treatment
Secondary Treatment
Simplified Activated Sludge Description
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
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
• ~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 BOD5
• ~20 % removal of phosphate
• Variable removal levels of viruses, 20-80 %
removal of bacteria and > 90 % removal of
protozoa
Wastewater Disinfection
Disinfection
Wastewater disinfection
• To inactivate pathogens in wastewater
• Several choices
– Free chlorine and combined chlorine
– UV
– Ozone
– Chlorine dioxide
Process Chemistry of Free Chlorine
Chlorine does not inactivate microorganisms
directly. Microorganisms are inactivated by the
hypochlorous acid (HOCl) and the hypochlorite
ion (OCl-).


Cl2  H2O  HOCl  H  Cl
HOCl  H  OCl
pK a  7.5 @ 25 C
Disinfection
Breakpoint Reaction for Chlorine
• Reaction of ammonium with free chlorine:
NH4  HOCl  NH2Cl (monochlor amine)  H2O  H
NH2Cl  HOCl  NHCl 2 (dichloram ine)  H2O
NHCl 2  HOCl  NCl3 (nitrogen trichlorid e)  H2O
• Sum of chloramine residual concentrations
called combined residual
• Process is very dependent on pH,
temperature, contact time, and initial ratio of
chlorine to ammonia
• Organic nitrogen compounds react rapidly
with chlorine to form organochloramines
Disinfection
Breakpoint Reaction for Chlorine
Monochloramine,
organochloramines
Cl2:N < 5:1 mass basis
Dichloramine,
nitrogen trichloride,
and
organochloramines
Ref: Metcalf & Eddy, Inc., 1979. Wastewater Engineering, Treatment and Disposal. McGraw-Hill, New York.
Wastewater chlorination
• To inactivate pathogens in wastewater
• Dynamic chloramination and breakpoint
chlorination
• 5 - 20 mg/L for 30 minutes
• > 99.99 % reduction of total and fecal
coliforms, ~90 % reduction of enteric
viruses, ~50% reduction of Giardia, but <
10 % reduction of Cryptosporidium
Ultraviolet (UV) Disinfection
Closed-channel, horizontal, parallel to flow
Medium pressure, high-intensity lamps
Automatic cleaning
Disinfection
Ultraviolet (UV) Disinfection
Closed-channel, horizontal,
parallel to flow (Trojan)
Raised UV Lamp Unit
Disinfection
UV disinfection in wastewater
• To inactivate pathogens in wastewater
• Low pressure, low pressure high-output, or
medium pressure lamp
• 40 mJ/cm2
• Similar level of reduction for total and fecal
coliforms, and enteric viruses, but a lot
higher level of reduction for Giardia and
Cryptosporidium
Overall pathogen reduction in wastewater treatment
Water treatment processes
Water contaminants
• Chemicals
– Inorganics
– Organics
• Synthetic organic compounds
• Volatile organic compounds
• Microbes
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Viruses
Bacteria
Protozoa parasites
Algae
Helminths
Water contaminants (I)
Water contaminants (II)
Water contaminants (III)
Water contaminants (IV)
Water contaminants (V)
Multiple barrier concept
for public health protection
Barrier Approach to Protect Public
Health in Drinking Water
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Source Water Protection
Treatment
Disinfection
Disinfectant residual in distribution system
Water treatment processes
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)
Physico-chemical processes
• To remove particles in water
• Coagulation/flocculation/sedimentation
• Filtration
Rapid Mix
• Intense mixing of
coagulant and other
chemicals with the
water
• Generally performed
with mechanical
mixers
Chemical Coagulant
Major Coagulants
• Hydrolyzing metal salts
– Alum (Al2(SO4)3)
– Ferric chloride (FeCl3)
• Organic polymers (polyelectrolytes)
Coagulation with Metal Salts
Soluble Hydrolysis Species
+
+
Al(OH)3
(Low Alum Dose)
(High Alum Dose)
Colloid
Colloid
Alx(OH)y
Colloid
Al(OH)
Al(OH)3
Al(OH)3
Floc
Colloid
Al(OH)3
Colloid
Al(OH)3
Al(OH)3
Al(OH)3
Colloid
Al(OH)3
Charge Neutralization
Al(OH)3
Sweep Coagulation
Horizontal Paddle Flocculator
Flocculation Example
Water coming from
rapid mix.
Water goes to sedimentation
basin.
Sedimentation Basin
Sedimentation Basin Example
Water coming from
flocculation basin.
Sludge to solids
treatment
Water goes to
filter.
Floc (sludge) collected
in hopper
Coagulation/flocculation/and
sedimentation
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To remove particulates, natural organic materials in water
Coagulation
– 20 -50 mg/L of Alum at pH 5.5-6.5 (sweep coagulation)
– rapid mixing: G values = 300-8000/second
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Flocculation:
– Slow mixing: G values = 30-70/second
– Residence time:10 -30 minutes
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Sedimentation
– Surface loading: 0.3 -1.0 gpm/ft2
– Residence time: 1 – 2 hours
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Removal of suspended solids and turbidity: 60-80 %
Reduction of microbes
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74-97 % Total coliform
76-83 % of fecal coliform
88-95 % of Enteric viruses
58-99 % of Giardia
90 % of Cryptosporidium
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
Filter Example
Water coming from
sedimentation
basin.
Anthracite
Sand
Gravel (support
media)
Water going to disinfection
Mechanisms Involved in Filtration
Floc particles
Interception: hits
& sticks
Flocculation:
Floc gets
larger within
filter
Entrapment:
large floc gets
trapped in
space between
particles
Sedimentation:
quiescent, settles,
& attaches
Granular media,
e.g., grain of sand
Removal of bacteria, viruses and protozoa by a
granular media filter requires water to be coagulated
Rapid filtration
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To remove particulates in water
Flow rate 2-4 gpm/ft2
Turbidity: < 0.5 NTU (often times < 0.1 NTU)
Reduction of microbes
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50-98 % Total coliform
50-98 % of fecal coliform
10-99 % of Enteric viruses
97-99.9 % of Giardia
99 % of Cryptosporidium
Disinfection in water
• To inactivate pathogens in water
• Various types
– Free chlorine
– Chloramines
– Chlorine dioxide
– Ozone
– UV
Trend in disinfectant use (USA, %
values)
Disinfectant
1978
1989
1999
Chlorine gas
91
87
83.8
NaClO2 (bulk)
6
7.1
18.3
NaClO2 (onsite)
0
0
2
Chlorine
dioxide
0
4.5
8.1
Ozone
0
0.4
6.6
Chloramines
0
20
28.4
Comparison of major disinfectants
Consideration
Disinfect ants
Cl2
Oxidation
potential
Residuals
Mode of
action
Disinfecting
efficacy
By-products
Strong
ClO2
O3
Stronger? Strongest
Yes
No
No
Proteins/ Proteins/ Proteins/
NA
NA
NA
Good
Very good Excellent
Yes
Yes
Yes?
NH2Cl
Weak
Yes
Proteins
Moderate
No