enhancing o 2 transfer in subsurface-flow constructed wetlands
DESCRIPTION
Enhancing O 2 Transfer in Subsurface-flow Constructed Wetlands. T. P. Chan, N. R. Shah, T. J. Cooper, J. E. Alleman, R. S. Govindaraju. School of Civil Engineering Purdue University May 16, 2005. Project Involvement. INDOT: Financial support & technical guidance JTRP & Purdue: - PowerPoint PPT PresentationTRANSCRIPT
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World Water and Environmental Resources Congress 2005
Enhancing O2 Transfer in Subsurface-flow Constructed WetlandsT. P. Chan, N. R. Shah, T. J. Cooper, J. E. Alleman,R. S. Govindaraju
School of Civil EngineeringPurdue University
May 16, 2005
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World Water and Environmental Resources Congress 2005
Project Involvement
INDOT: Financial support & technical guidance
JTRP & Purdue: Research sponsorship‘Hydraulic’ project: to track and evaluate flow rates and fate‘Environmental’ project: to track wastewater treatment efficacy
J.F. New: Sub-contracted design engineering for wetlands
Indiana Department of Health: Wetland discharge permitting
RQAW: Design engineering
Heritage: Project construction
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World Water and Environmental Resources Congress 2005
Basic Lagoon
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World Water and Environmental Resources Congress 2005
Floating Mechanical Aerator
Aerated Lagoon
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World Water and Environmental Resources Congress 2005
Impervious liner
Influent
Effluent
Emergent plants
Plant root systems
Level controldevice
Clean out pipe
Coarsegravel
Medium gravel(1cm diameter) media
Pea gravel at inlet
o oo o
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Subsurface Flow (SSF)Constructed Wetland
Impervious liner
Influent
Effluent
Emergent plants
Plant root systems
Level controldevice
Clean out pipe
Coarsegravel
Medium gravel(1cm diameter) media
Pea gravel at inlet
o oo oo oo oo oo o
oooooooooooo
Subsurface Flow (SSF)Constructed Wetland
Subsurface-Flow CW
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World Water and Environmental Resources Congress 2005
1
0
0.5
1
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Met
ers
Shallow rooting cattail
(Typha latifolia)
Deeper rooting fox sedge
(Carex vulpinoidea)
Deeperrooting
river bulrush(Scirpus fluviatilis ) 1
0
0.5
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Met
ers
Shallow rooting cattail
(Typha latifolia)
Deeper rooting fox sedge
(Carex vulpinoidea)
Deeperrooting
river bulrush(Scirpus fluviatilis )
Typical Wetland Plants
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Media
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Plant Root
Plant Root
Plan
t Ro
ot
O2
O2
O2
O2
O2
O2
O2
O2O2
O2
O2O2
O2
O2
O2
O2
O2
O2
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Extensive bacterialColonization of root
tip surface area!
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World Water and Environmental Resources Congress 2005
Media
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Constructed Wetland- Plants release oxygen via photosynthetic activity- High microbial activity on, and adjacent, to plant roots- However, oxygen may still be a critical limiting factor
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CleanWater
Waste
O2
Fill-and-Draw CW
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CleanWater
Waste
Fill-and-Draw CW
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Fill-and-Draw CW
CleanWater
Waste
O2
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Challenges at Rest Areas
• Remote location– Rural locale
– Away from existing sewer and POTW
• High wastewater strength– Low-flush toilets
• High variability in wastewater flow– Large increase in traffic volume during rush hours
and holidays
• Limited personnel
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Project Location
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Project Overview
• Unique challenges– Long (3+ miles) sewer line to the city POTW
– Low flush toilets; flow restrictive faucets
– High strength wastes (BOD, Ammonia)
– Odor problem at city lift station
– Surcharges by the city
• Pretreatment using constructed subsurface wetlands– Biofield (and city sewer) for effluent disposal
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Greenfield Wetland System
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Septic Tanks
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Wetland Cells 1 & 2
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Wetland Cells 1 & 2
Vertical filter
Vegetated subsurface flow wetland
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@ Outlet of Cells 1 & 2
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Wetland Cell 3
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Biofield
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Automatic Sampler
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Flow Meters
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First Year in Operation
• Wetland cells planted on August 12, 2003
• October 2003 – June 2004– Startup period
– Overflow mode
– North side only
• Mid June 2004 – present– Draw-and-fill mode (time-based)
– Full Operation beginning in October
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Daily Rainfall and Flow Totals
- Daily Cycle- Weekly Cycle- Seasonal Cycle
Tailing response to rainfall
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D
Outflow from Cells 1 & 2
No overflow Overflow at filling cell
Draw-and-fill mode:12-hour cycle during Saturday – Monday24-hours cycle during Tuesday – Friday
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Hydraulic Retention Time
Flow rate HRT (days)
(gpd) W-1 & W-2 W-3Septic tanks
Total
Over-flow mode
Design 10,000 6.8 0.6 2 9.4
Summer 7,500 9.1 0.8 2.7 12.6
Winter 4,000 17 1.5 5 23.5
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Dynamic Modeling
• Complex flow scheme– Changing water levels
– Recirculation
– Overflows
• Simplifying assumptions– Treat wetland cells as giant buckets
– Instantaneous overflow
• Estimate HRT
• Basis for design of similar systems
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Hydraulic Retention Time
Flow rate HRT (days)
(gpd) W-1 & W-2 W-3Septic tanks
Total
Over-flow mode
Design 10,000 6.8 0.6 2 9.4
Summer 7,500 9.1 0.8 2.7 12.6
Winter 4,000 17 1.5 5 23.5
Draw-and-fill mode
10/20–22 5,280 8.7 (12.9)*
10/16–17 6,150 8.8 (11.1)*
*The value in parenthesis is the estimated HRT as if in the over-flow mode.
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Days of Operation
% R
em
oval
0
10
20
30
40
50
60
70
80
90
100
0 50 100 150 200 250 300 350 400 450 500
BOD TSS NH3 TSS BOD NH3
NH3
BOD
TSS
- Definite pattern of increased performance…- Obvious correlation with increased plant and root density- However, lagging ammonia removal problem!!- Strong suggestion that oxygen is limited!
Wetland Performance
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Wetland Performance
Sampling Date
3/16/05
After 1st septic tank
Outlet % Removal
BOD (mg/L) 260 82 68%
TSS (mg/L) 180 12 89%
NH3 (mg/L) 200 39 81%
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Continuing/Future Activities• Continuing data collection and analysis
• “Tweaking” the system for optimum treatment effectiveness
• Additional of a surge tank
• Development of hydraulic and treatment process model
• Modifying existing wetland design guidelines, tailored to rest area application
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The END …
Visit our website:https://engineering.purdue.edu/ResearchGroups/Wetland
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WasteCleanWater?
Wastewater Treatment
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o
o
o
- Very simple!
Basic Lagoon
WasteCleanWater
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WasteCleanWater
O2 O2 O2 O2 O2 O2 O2
- Very simple!- However, mixing depends on wind!- Therefore, sometimes poorly mixed- Poor mixing means poor aeration- Oxygen supply will then be poor- Lagoon may stink!!
o
o
o
Basic Lagoon
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WasteCleanWatero
o
o
O2 O2
- Improved aeration- Better oxygen supply- Better biological kinetics- Smaller lagoon
Aerated Lagoon
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CleanWater
Waste
Constructed Wetland (CW)
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CleanWater
Waste
Constructed Wetland- Wetland plants tolerate routine submergence- High plant density - High plant root mass- High rate plant root release of oxygen- High bacterial growth on plant root surfaces- High biochemical degradation of waste
Characteristics
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CleanWater
Waste
Enhanced?Biometabolism
How Does it Work?
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How to Increase DO?
• Direct aeration
• Rapid changes in water level– Expose thin water films and biofilms on the wetland
substrate and plant roots to air
– Large surface area rapid and substantial oxygenation of the rhizosphere.
• Greenfield rest area– Fill-and-Draw concept
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Flow Data
• Variability in flow– Daily Cycle
– Weekly Cycle
– Monthly Cycle
– Extreme events:• High traffic volume during holidays
• Rainfall
• > 2-fold increase in daily flow
• Effects of ET and rainfall– 1 mm ET / rainfall 300 gal decrease / increase in
flow volume
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Wetland Performance
• Ponding– Lack of air-filled layer between water table and peat
moss layer
• Limited root growth
• Invasive plant species
• Overflows during the fill cycle– Short-circuiting
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Design Parameters
• Design flow rate: 10,000 gpd
• Wastewater characteristics:
– BOD5 ~ 450 mg/L
– NH3-N ~ 150 mg/L
– TSS ~ 180 mg/L