Transcript PowerPoint

Environmental Engineering
CIV 257
Braum Barber
Wastewater Treatment
Unit 4
Wastewater Flow
Sources of Wastewater
• Residential & Commercial
– Treated in municipal wastewater plant
• Industrial
– Often pre-treated before discharged to sanitar
sewers.
• Surface drainage
– Often drained directly to natural watercourse.
Domestic Wastewater
• Volume varies from 50 - 250 usgpd/person
– 450 lpd/person on average
– Depends upon appliances.
• Organic matter is approximately:
– Suspended Solids of 0.24 lb/d/person
– BODs of 0.20 lb/d/person.
• BOD = amount of oxygen consumed by microorganisms
under aerobic conditions at 20oC for 5 days.
• Seasonal flow and BOD load can vary 20 to 30%
over the average yearly values.
Domestic Wastewater
• “Rule of thumb” design
– 80% of average daily water consumption
– or 100 % of winter water consumption.
– 450 lpd/person is “safe”
• QPDW = ( G )( P )( Pf )/86.4
where:
QPDW = Peak dry weather design flow (l/s)
G
P
Pf
= Average daily flow (lpd/person)
= Population in thousands
= Peaking factor which is the higher of 2.5 or
Harmon’s factor = ( 1+ 14/( 4 + P0.5 )
Domestic Wastewater
Parameter
Raw
[mg/l]
After Settling
[mg/l]
Biologically
Treated
[mg/l]
Total Solids
800
680
530
Total Volatile Solids
440
340
220
Suspended Solids
240
120
30
Volatile Suspended Solids
180
100
20
Biochemical Oxygen
Demand (BOD)
200
130
30
Inorganic Nitrogen
22
22
24
Total Nitrogen
35
30
26
Soluble Phosphorus
4
4
4
Total Phosphorus
7
6
5
Based on 450 l/d/person
Biological Treatment
• The minimum BOD/N/P ratio required for
biological treatment is 100/5/1.
Effluent Standards
• BOD
• Suspended Solids
Where required:
• Ammonia
• Phosphorus
• Fecal Coliforms
• Total Coliforms
25 mg/l
25 mg/l
5 - 10 mg/l
1 mg/l
200/100 ml
1000/100 ml
Industrial Wastewater
• Typically discharge into City wastewater
systems after pretreatment.
• Uncontaminated cooling water is disposed
directly into storm sewer.
• Requires stringent control since many
manufacturing wastes are only partially
removed through conventional treatment.
Infiltration & Inflow
• Groundwater entering the sanitary sewers
through defective joints, cracked pipe, or
manholes.
– 500 usgpd / mile of pipe / inch diameter.
– 46 l/d / kilometer of pipe / mm diameter
– 3 to 5% of peak hourly domestic flowrate or
10% of average flow.
Municipal Wastewater
• New systems are usually designed on the
basis of an average daily / capita flow rate
of 450 litres (including infiltration).
• Design peak flows:
– 1500 lpd for laterals and submains
– 950 lpd for mains and trunks.
• Fluctuates based on season and day of
week.
– Summer flows exceed winter flows by 20%
Wastewater Flow
BOD Concentration
Composite Sampling
• Testing required to appraise plant
performance and loadings.
• Regular samples are taken (each hour)
with flow meter readings.
• Composite samples are portioned based
on the flow rate.
Multiplier = volume of composite desired
flow rate x samples / d
Evaluation of Wastewater
• Typically based on BOD and Suspended
Solids.
• Data should consider:
– wastewater flow,
– day of the week,
– weather/rainfall,
– abnormal wastewater discharges
– equipment performance
Wastewater Collection
Storm Sewer Design
• Typically designed for:
– Minimum flow of 0.9 m/s
• Self-cleansing
– Maximum flow of 3.0 m/s
• Minimize erosion
• Concrete pipe (designed for rainfall
frequency) is buried as shallow as
possible to minimize excavation.
Sanitary Sewer System
• Typically designed for:
– 1500 l/d/person for laterals and submains.
• Minimum 8” diameter
– 950 l/d/person for mains and trunks.
• Include normal infiltration, and exclude industrial
wastewater.
• Slopes designed to maintain velocity of
0.90 m/s (for self-cleansing).
– Use Manning’s Formula with n=0.013
Sanitary Sewer System
Sewer Diameter
Minimum Slope
[%]
Flowing Full Discharge
[usgpm]
8
0.33
310
10
0.25
490
12
0.19
700
18
0.14
1080
15
0.11
1570
21
0.092
2160
24
0.077
2820
27
0.066
3570
30
0.057
4410
36
0.045
6330
Sanitary Sewer System
• Buried to prevent freezing and allow
drainage from house basements.
• Laterals are placed not less that 3.3m
below the top of the house foundation.
• Sewers are placed at least 3 m
horizontally from the water main.
– or 18” below the bottom of the water main and
in a separate trench.
Sanitary Sewer System
• Inverted Siphons used to drop below the
hydraulic gradient to avoid an obstacle.
– Flow is split and a velocity > 0.9 m/s is
maintained to prevent deposition of solids.
Wastewater Processing
Wastewater Plant
Sanitary Wastewater
• 450 liters per person per day.
• Solids content is approximately 0.1%
– 240 mg/l suspended solids
– 200 mg/l BOD
• BODs are converted to settable biological
solids during treatment.
Sanitary Sludge Removal
• Liquid sludge from settling amounts to 2
l/person (solids content about 5%)
• Cake amounts to about 0.25 l/person
(solids content about 30%)
Parameter
Raw
[mg/l]
After Settling
[mg/l]
Biologically
Treated
[mg/l]
Total Solids
800
680
530
Total Volatile Solids
440
340
220
Suspended Solids
240
120
30
Volatile Suspended Solids
180
100
20
Biochemical Oxygen
Demand (BOD)
200
130
30
Inorganic Nitrogen
22
22
24
Total Nitrogen
35
30
26
Soluble Phosphorus
4
4
4
Total Phosphorus
7
6
5
Based on 450 l/d/person
Effluent Quality
• Maximum monthly average of:
– 25 mg/l BOD
– 25 mg/l suspended solids
– 10 mg/l oil and grease
– pH between 6.0 and 9.0
– Fecal coliform bacteria < 200/100 ml
Process
Design Criteria - Loading
Flow Measurement
Peak Hourly Flow
Bar Screens
Peak Hourly Flow
Pumps
Peak Hourly Flow
Minimum Hourly Flow
Grit Chamber
Maximum Monthly Flow
Peak Hourly Flow
Primary Settling
Maximum Monthly Flow
Biological Treatment
Maximum Monthly BOD loading
Check Peak Hourly BOD loading
Final Settling
Maximum Monthly Flow
Disinfection
Peak Hourly Flow
Thickening
Maximum Daily Sludge Flow
Check Maximum Solids Loading
Digestion
Maximum Monthly Volatile Solids Load
Check Maximum Monthly Sludge Flow
Dewatering
Maximum Sludge Flow
Check Maximum Solids Loading
Land Application
Maximum Nutrient Loading (sludge)
Maximum Hydraulic Loading (water)
Preliminary Treatment
Preliminary Treatment
• Parshall Flume
– Low had loss and smooth flow.
• Screens & Shredders
– Clear bar openings between ½” and 1”
– Mechanically cleaned by a travelling rake.
– Shredders (smaller plants) grinds solids to
about ¼”
http://www.elmhurst.edu/~chm/vchembook/310wastewater.html
Preliminary Treatment
• Grit Chamber
– Removes large & heavy particles
• 0.2 mm diameter, specific gravity of 2.7
– Grit clarifiers that allow settling
– Forced vortex grit units create a vortex which
induce solids towards the center.
http://www.elmhurst.edu/~chm/vchembook/310wastewater.html
Preliminary Treatment
• Pumping Station
– Wet well to maintain pump requirements and
some surge capacity.
– Pumps must be designed to:
• Reduce stop/start cycles
• Optimize operating efficiency over flow range.
– Number of pumps required to meet total pump
capacity with the largest pump out of service.
– Centrifugal (with constant- or variable speed
drives) or Screw Pumps.
http://www.elmhurst.edu/~chm/vchembook/310wastewater.html
Primary Clarifier
Primary Clarifier
• Length to width ratios between 3:1 to 5:1
• Widths range from 3m to 6 m
• Depths are typically 2 to 2.5 m
Overflow Rate
Vo = Q / A
Where
Vo = Overflow rate or surface
loading [m³/m²/day]
Q = Average daily flow [m³/day]
A = Total surface of the basin [m²]
Detention Time
t=V/Q
Where
t = detention time [hours]
V = basin volume [m³]
Q = Average hourly flow [m³/hr]
(or mean daily flow / 24 hours)
Weir Loading
Weir Loading = Q / weir length
Where
Weir Loading [m³/d/m length]
Q = Average daily flow [m³/day]
Weir Length [m]
Primary Clarifier
Design Standards
• Overflow Rates
– 600 usgpd/ft² to 800 usgpd/ft²
• For average monthly flow
– 32.5 to 49 m³/d/m²
• Weir Loading
– 10,000 usgpd/lineal foot
• For plants < 1 mgpd
– 125 to 500 m³/d/m
Biological Treatment
• Biological systems contact wastewater
with microbial growths attached to the
surfaces of supporting media.
• Trickling Filter uses a media of crushed
rock.
• Biological Tower uses a synthetic media.
• Biological Contactor uses a series of
rotating plates
Biological Process
Trickling Filters
http://www.engin.swarthmore.edu/~lgoodfr1/trickling.jpg
Trickling Filters
http://carroll1.cc.edu/~jclausz/microbiology/filter.jpg
Design Loadings
BOD Loading = settled wastewater BOD
Volume of filter media
Hydraulic Loading
= wastewater flow (inlet + recirculation)
surface area of filters
Trickling Filter Loadings
• BOD Loading
– 30 to 90 lb/ 1000 ft³ / day
– 0.15 to 1.5 kg/d/m³
• Hydraulic Loading
– 0.16 to 0.48 gpm / ft²
– 9.4 to 28 m³/d/m²
• Recirculation Ratio
– 0.5 to 3.0
• Conversions
– 1 lb/ 1000 ft³ / day = 16 g/m³ d
– 1 mgpd/acre = 0.935 m³/m²/d
Biological Towers
•
•
•
•
Better liquid distribution
Light weight
Chemical resistance
Ability to treat high-strength wastewaters.
http://www.rauschertus.com/process_technologies/stpt.html
Rotating Biological Contactor
http://www.wscllc.com/proucts_bp.htm
Biological Aeration
• Bacteria metabolize waste organics
(BODs) and dissolved oxygen, producing
new growth and carbon dioxide.
• Effluent quality is effected by temperature
(10°C to 20°C, with better quality at higher
temperatures).
• Biological floc is separated in the clarifier
and “activated sludge” is returned to the
aeration basin.
http://www.elmhurst.edu/~chm/vchembook/310wastewater.html
Aeration Loadings
F/M = Q x BOD / (V x MLSS)
Where:
F/M = food-to-microorganism ratio
Q
= wastewater flow [m³/d]
BOD = wastewater BOD [g/m³]
V
= liquid volume of aeration tank [m³]
MLSS = ‘mixed liquor suspended solids’ in
the aeration basin [g/m³]
Conventional Loadings
• BOD Loading
– 20 to 40 lb
BOD/d/1000ft³
• MLSS
– 1000 to 3000 mg/l
• F/M ratio
– 0.2 to 0.5 lb BOD/d
per lb MLSS
• Sludge age
– 5 to 15 days
• Aeration period
– 4.0 to 7.5 hours
• Return sludge rates
– 20% to 40%
• BOD removal efficiency
– 80% to 90%
Sludge Settleability
Extended Aeration
(endogenous growth)
Conventional
Aeration
(declining growth)
High Rate
(accelerated growth)
Poor settleability
Fair settleability
Good settleability
0.05
0.2
F/M
0.5
1.0
Final Clarifier
• Similar to primary clarifier design
– More buoyant floc with greater variability in settling
velocities.
• Overflow rate:
– Typically 600 gpd/ft² (24 m³/d/m²)
– Should not exceed 800 gpd/ft² (33 m³/d/m²)
• Minimum side-wall depth of 10 ft.
• Detention time of 2 to 3 hours.
• Weir loading between 10,000 to 20,000 gpd/ft (125 to
250 m³/d/m)
http://www.elmhurst.edu/~chm/vchembook/310wastewater.html
Effluent Disinfection
• To inactivate pathogenic organisms,
including enteric bacteria, viruses, and
protozoans.
• Mean concentrations test for coliform
bacteria is 200 per 100 ml after 30
consecutive days and 400 per 100ml after
7 consecutive days.
– From an inlet concentration of 100,000 to
10,000,000 per 100 ml.
Chlorine Contacting Basin
• Designed for rapid mixing (8 to 15 mg/l of
chlorine) with a minimum contact time of
20 to 30 minutes at peak flow.
http://www.elmhurst.edu/~chm/vchembook/310wastewater.html
http://www.elmhurst.edu/~chm/vchembook/310wastewater.html
Sludge Treatment
Quantity of Waste Sludge
• At 0.6 usg/person
– the total sludge (Wp + Ws) is about 5 m³ per
1000 m³
• Primary Sludge:
Wp = f x SS
where f = fraction of SS removed (use 50%)
SS = suspended solids in unsettled
wastewater [g/day]
Quantity of Waste Sludge
• Biological Sludge
Ws = K x BOD
where K = fraction of BOD removed
BOD = BOD in wastewater after primary
sedimentation [g/day]
• For F/M of 0.2, K = 0.42
• For F/M of 0.5, K = 0.51
Quantity of Waste Sludge
• Wet Volume (from total dry sludge)
V = W / (s/100)
where V = volume of wet sludge [l]
W = total weight of dry sludge [kg]
s = solids content [%]
Sludge
Processing
Thickening
• For waste-activated sludge:
– Dissolved air flotation
• Release air bubbles that attached to sludge particles and
cause them to float.
• Sludge is removed from the surface of the tank.
– Gravity belt
• Requires the addition of a polymer in a flocculation tank.
• The floc is spread over a permeable fabric, allowing the
water to pass through.
– Centrifuge
• Uses centrifugal force to remove the water.
Anaerobic Sludge Digestion
• Used to convert odorous sludge to inert material.
• Acid-forming bacteria is used to convert organic
matter to CO2, CH4, trace H2S, and organic
acids.
• Acid-splitting, methane-forming bacteria convert
the organic acids to CO2 and CH4
• Gas emissions are:
– 450 to 500 liters / lb Volatile Solids digested
– CO2 (35%) & CH4 (65%)
http://www.elmhurst.edu/~chm/vchembook/310wastewater.html
Anaerobic Sludge Digestion
• Optimum pH of 7.0 to 7.1
• Volatile Solids Loading
– Conventional: 0.02 to 0.05 lb VS / day / ft³
– First-stage: 0.05 to 0.15 lb VS / day / ft³
• Retention time:
– Conventional: 30 to 90 days
– First-stage: 12 to 20 days
• Volatile Solids reduction of 50% to 70%
Two-Stage Digestion
Digester Sizing
• Trickling Filter Plant
– 0.11 m³/capita of design load
• Activated Sludge Plant
– 0.17 m³/capita of design load
Dewatering
• Belt Filter Press
– Sludge is compressed on a porous belt that
winds through a series of rollers.
• Water is recycled to the beginning of the process.
• Cake is sent to disposal
•
•
•
•
Centrifugation
Composting
Lagoon
Sand Drying Beds
Belt Filter Press
http://www.elmhurst.edu/~chm/vchembook/310wastewater.html
Disposal
• Application on agricultural land
– Federal limits on pollutants and pathogens.
• Landfill
• Incineration
– For complete drying, and ash is separated
and disposed.
Video: http://www.lethbridge.ca/living-here/water-wastewater/Pages/Wastewater.aspx
Stabilization Ponds
• Typical of secondary treatment for small
treatment facilities.
• Facultative Ponds
– Bacteria breaks down waste organics,
releasing nitrogen and phosphorous (&CO2)
– Algae use the nitrogen and phosphorous, and
sunlight to grow, releasing oxygen (and H2S).
– The bacteria use the oxygen, closing the
cycle.
• Oxygen can also come from wave action.
Facultative Stabilization Ponds
Facultative
Stabilization
Ponds
Facultative Pond Loading
• Maximum allowable BOD loading
– 2.2 g/m²d in northern climates to reduce
odours in the spring.
– 5.6 g/m²d in the south.
• Retention time is 2 months
– Depending upon applied load, depth of
wastewater, evaporation rate, and seepage
losses.
Other Ponds
• Aerated Ponds for continuous flow
• Evaporation Ponds where there is no
place to discharge
– Evaporation and precipitation rates to be
considered.
Tertiary (Polishing) Ponds
• Used after activated sludge or trickling
filter secondary treatment.
• Retention and surface aeration reduces
suspended solids.
• 1.7 g/m²d BOD loading
• 10 to 15 day retention
• 2 to 3 ft depth for mixing & sunlight
penetration.
Summary
 Sanitary flow may be estimated based on
population and BOD & SS concentrations.
 Collection systems are designed for maximum
and minimum flowrates.
 Wastewater treatment systems include
preliminary treatment including primary settling,
secondary biological treatment with final
settling, and disinfection.
 Sludge is treated through digestion and
dewatering.
 Facultative ponds may be designed for small
wastewater treatment systems.