enhancing o 2 transfer in subsurface-flow constructed wetlands

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


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


Basic Lagoon


Floating Mechanical Aerator

Aerated Lagoon


Impervious liner

Influent

Effluent

Emergent plants

Plant root systems

Level controldevice

Clean out pipe

Coarsegravel

Medium gravel(1cm diameter) media

Pea gravel at inlet

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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

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Subsurface Flow (SSF)Constructed Wetland

Subsurface-Flow CW


1

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ers

Shallow rooting cattail

(Typha latifolia)

Deeper rooting fox sedge

(Carex vulpinoidea)

Deeperrooting

river bulrush(Scirpus fluviatilis ) 1

0

0.5

1

2

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ers

Shallow rooting cattail

(Typha latifolia)

Deeper rooting fox sedge

(Carex vulpinoidea)

Deeperrooting

river bulrush(Scirpus fluviatilis )

Typical Wetland Plants


Media

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Plant Root

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O2


Extensive bacterialColonization of root

tip surface area!


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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


CleanWater

Waste

O2

Fill-and-Draw CW


CleanWater

Waste

Fill-and-Draw CW


Fill-and-Draw CW

CleanWater

Waste

O2


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


Project Location


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


Greenfield Wetland System


Septic Tanks


Wetland Cells 1 & 2


Wetland Cells 1 & 2

Vertical filter

Vegetated subsurface flow wetland


@ Outlet of Cells 1 & 2


Wetland Cell 3


Biofield


Automatic Sampler


Flow Meters


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


Daily Rainfall and Flow Totals

- Daily Cycle- Weekly Cycle- Seasonal Cycle

Tailing response to rainfall


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


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


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


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.


Days of Operation

% R

em

oval

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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


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%


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


The END …

Visit our website:https://engineering.purdue.edu/ResearchGroups/Wetland


WasteCleanWater?

Wastewater Treatment


o

o

o

- Very simple!

Basic Lagoon

WasteCleanWater


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


WasteCleanWatero

o

o

O2 O2

- Improved aeration- Better oxygen supply- Better biological kinetics- Smaller lagoon

Aerated Lagoon


CleanWater

Waste

Constructed Wetland (CW)


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


CleanWater

Waste

Enhanced?Biometabolism

How Does it Work?


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


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


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


Design Parameters

• Design flow rate: 10,000 gpd

• Wastewater characteristics:

– BOD5 ~ 450 mg/L

– NH3-N ~ 150 mg/L

– TSS ~ 180 mg/L

enhancing o 2 transfer in subsurface-flow constructed wetlands

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