Bioretention Technology

Presented by:

The Low Impact Development Center, Inc. A non-profit water resources and sustainable design organizationwww.lowimpactdevelopment.org

Presented by:

The Low Impact Development Center, Inc. A non-profit water resources and sustainable design organizationwww.lowimpactdevelopment.org

The Low Impact Development Center, Inc. has met the standards and requirements of the Registered Continuing Education Program. Credit earned on completion of this program will be reported to RCEP at RCEP.net. A certificate of completion will be issued to each participant. As such, it does not include content that may be deemed or construed to be an approval or endorsement by RCEP.

COPYRIGHT MATERIALS

This educational activity is protected by U.S. and International copyright laws. Reproduction, distribution, display, and use of the educational activity without written permission of the

presenter is prohibited.

© Low Impact Development Center, 2012

The purpose of this presentation is to provide detailed information on bioretention pollutant removal, design variations, and sizing methods

At the end of this presentation, you will be able to:• Describe how bioretention works physically and

chemically • Design bioretention systems

Purpose and Learning Objectives

Overview

• Performance research

• State-of-the-art in bioretention design

• Design tools

What is Bioretention?

Filtering stormwater runoff through a terrestrial aerobic (upland) plant / soil / microbe complex to remove pollutants through a variety of physical, chemical and biological processes.

The word “bioretention” was derived from the fact that the biomass of the plant / microbe (flora and fauna) complex retains or uptakes many of the pollutants of concern such as N, P and heavy metals.

It is the optimization and combination of bioretention, biodegradation, physical and chemical that makes this system the most efficient of all BMP’s

Pollutant Removal MechanismsPhysical / Chemical / Biological

ProcessesSedimentationFiltration AdsorptionAbsorptionCation Exchange Capacity Polar / Non-polar SorptionMicrobial Action (aerobic / anaerobic)

decomposition / nitrification / denitrificationPlant UptakeCycling Nutrients / Carbon / MetalsBiomass Retention (Microbes / Plant)Evaporation / Volatilization

System Components

Mulch

Course Sand

Pore Space

Surface Area

Complex Organics

Microbes

Biofilm

Plants

“Ecological Structure”

Plant-and-Microbe-Mediated Pollutant Removal

• Phytoremediation o Translocateo Accumulate o Metabolizeo Volatilizeo Detoxifyo Degrade

• Exudates

• Bioremediation• Soils

o Capture / Immobilize Pollutants

Nitrogen Removal

• Step 1: Nitrificationo Ammonia/urea → nitrateo Aerobic processo Nitrate is highly mobile, and tends to be exported

• Step 2: Denitrificationo Nitrate → nitrogen gaso Anaerobic processo May occur in gravel storage layer beneath underdrain

Phosphorus Removal

• Dependent on the amount of phosphorus present in the BSM • Measured by the p-index of the topsoil used to mix BSM• High p-index soils export phosphorus

Other Pollutants

• Heavy metals o Adsorb to clay and humus in BSMo May be taken up by plants

• Organics (oil and grease, pathogens, PAHs, etc)o Filtered by mulch and BSMo Digested by microbeso Taken up by plants

• TSS o Filtered by mulch and BSMo Bioturbation by earthworms may prevent clogging

Bioretention Pollutant RemovalUniversity of Maryland

Cumulative Depth

(ft) Copper Lead ZincPhos-

phorus TKN Ammonia Nitrate

1 90 93 87 0 37 54 -972 93 99 98 73 60 86 -1943 93 99 99 81 68 79 23

Field 97 96 95 65 52 92 16

Removal Efficiency (%)

Box Experiments

Dr. Allen Davis, University of Maryland

Pollutant Mass RemovalUniversity of Maryland

Pollutant Mass removal

TSS 57 %

TP 78 %

Cu 80 %

Pb 86 %

Zn 62 %

NO3-N 93 %

• Field experiments• Small events produced zero

effluent, so comparing inflow/outflow EMC underestimates removal

• Mass removal is a better metric, but produces misleadingly low removal rates for pollutants occurring at low concentrations (e.g. Cu, Pb, and Zn)

Volume Reduction University of Maryland

• Allen Davis at the University of Maryland has found that even lined bioretention cells with underdrains reduced runoff volume by at least 33% for 55-62% of events

• 18% of storm events had no outflow

Louisburg BioretentionDr. Bill Hunt

North Carolina State Research

Load Reductions: Louisburg Removal vs. P-Index

Cell TN TP

L-1(unlined)

64% 66%

L-2(lined)

68% 22%

June 2004- February 2005

P-Index

1 to 2

85 to 100

Inflow vs. Outflow Rates

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

1/13/2005 12:00 1/14/2005 0:00 1/14/2005 12:00 1/15/2005 0:00 1/15/2005 12:00 1/16/2005 0:00

Dis

char

ge

(cfs

)

0

0.2

0.4

0.6

0.8

1

1.2

Dep

th (

in)

Inflow

Outflow

CumulativeRainfall

Design Considerations

• Design Objectives (Quality / Volume / Flow / Recharge) • Media Specifications / Consistency • Sizing • Offline / Flow–Through Systems• Pretreatment • Unique configurations / designs (costs)• Custom Application (Bacteria / Metals / Oil and Grease)

Bioretention Design Objectives

• Peak Discharge Control o 1-, 2-, 10-, 15-, 100-year stormso Bioretention may provide part or all of this control

• Water Quality Controlo ½”, 1” or 2” rainfall most frequently usedo Bioretention can provide 100% control

• Ground water rechargeo Many jurisdictions now require recharge

( e.g., MD, PA, NJ, VA)

2’

2” Mulch

Infiltration System

Highly Pervious Soils

Existing Ground

2’

2” Mulch

Drain Pipe

Combination Filtration / Infiltration

Gravel

Sandy Organic Soil

Existing Ground

Bioretention

Shallow Ponding - 4” to 6”

• Mulch 3”

• BSM Depth 2’ - 2.5’

• BSM• 50% Sand

• 30% Sandy Loam

• 20% Shredded Hardwood/ Compost

• Underdrain System

• Plants

X 2’

Under Drain

Bioretention Soil Medium Final proportion Component Properties

50% by volumeSand Conforms to ASTM C33 Fine

Aggregate

20% by volumeOrganic Material Compost or shredded

hardwood mulch

30% by volume

Topsoil Sand (2.0 – 0.050 mm) 50 – 85% by weight Silt (0.050 – 0.002 mm) 0 – 50% by weight Clay (less than 0.002 mm) 10 – 20% by weight * Organic Matter 1.5 – 10% by weight pH 5.5 – 7.5 (NOTE: pH can be

corrected with soil amendments if outside acceptable range)

Magnesium Minimum 32 ppm (NOTE: magnesium sulfate can be added to increase Mg)

Phosphorus (Phosphate - P2O5)

Not to exceed 69 ppm

P-index should be less than 25 Potassium (K2O) Minimum 78 ppm (NOTE:

potash can be added to increase K)

Soluble Salts Not to exceed 500 ppm* If the proposed topsoil is known to contain expansive clays, clay content should not exceed 10% by weight.

Other Media Considerations

• Homogenous Mixture• Peat / Clays / Silts slow flows • Test and standardize the media! • But performance varies with source! • Min 1.0’ depth of media • Max depth varies with vegetation.• Organic Component (Shredded Hardwood vs. Compost)

Underdrain System • Needed for subsoils with percolation rates less than ½” per

hour

• Filter fabric vs pea gravel diaphragm

• Minimum of 3" of gravel over pipes; not necessary underneath pipes

• Underdrain Piping ASTM D-1785 or AASHTO M-2786" rigid schedule 40 PVC 3/8" perf. @ 6" on center, 4 holes per row; or corrugated perforated HDPE

• Observation wells

Design Configuration Considerations

• Off line vs. Flow-through• Inlet • Surface Storage• Underdrain – Dewater media

Off-line

2005 Lake County, OH

Flow-through

2005 Lake County, OH

Plant Considerations

• Pollutant uptake • Evapotranspiration • Soil ecology / structure / function • Number & type of plantings may vary,

o Aestheticso Morphology (root structure trees, shrubs and herbaceous) o Native plants materialso Trees 2 in. caliper / shrubs 2 gal. size / herbaceous 1 gal size. o landscape plan will be required as part of the plan. o Sealed by a registered landscape architect.o Plants are an integral part no changes unless approved o Plant survival

• Irrigation – Typical / customary

Sizing

• Flow rate• Infiltration rate• Volume• Void space• Drainage area (Smaller the Better)

“Kerplunk” Method

• Bioretention cell is sized to store a target runoff volume within the ponded area and soil/gravel pore space.

• This method makes several simplifying assumptions, but works reasonably well (see Reese, Stormwater Magazine, September 2011)

“Kerplunk” Method

𝐴𝑟𝑒𝑎=𝑅𝑢𝑛𝑜𝑓𝑓 𝑣𝑜𝑙𝑢𝑚𝑒

𝐸𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒𝑠𝑡𝑜𝑟𝑎𝑔𝑒 h𝑑𝑒𝑝𝑡

𝐸𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒𝑠𝑡𝑜𝑟𝑎𝑔𝑒 h𝑑𝑒𝑝𝑡 =𝑝𝑜𝑛𝑑𝑖𝑛𝑔 h𝑑𝑒𝑝𝑡 +(𝐵𝑆𝑀 h𝑑𝑒𝑝𝑡 ∗𝐵𝑆𝑀 𝑝𝑜𝑟𝑜𝑠𝑖𝑡𝑦 )+(𝑔𝑟𝑎𝑣𝑒𝑙 h𝑑𝑒𝑝𝑡 ∗𝑔𝑟𝑎𝑣𝑒𝑙𝑝𝑜𝑟𝑜𝑠𝑖𝑡𝑦)BSM porosity ≈ 0.35Gravel porosity ≈ 0.4

RECARGA

• Developed by the University of Wisconsin-Madison Department of Civil Engineering

• Capable of single event or continuous simulation• Incorporates infiltration, evapotranspiration, overflow, and

underdrain flow

RECARGA

http://dnr.wi.gov/runoff/stormwater/technote.htm

Thank you for your time.

QUESTIONS?

Low Impact Development Center, Inc.www.lowimpactdevelopment.org

301.982.5559