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Phyto-Remediation for Landfill Leachate Treatment John R. Buchanan Associate Professor, University of Tennessee, Walter H. Eifert Principal Hydrologist, ELM Site Solutions, Inc.
Phyto-Remediation for Landfill
Leachate Treatment
John R. Buchanan Associate Professor, University of Tennessee,
Walter H. Eifert Principal Hydrologist, ELM Site Solutions, Inc.
Phyto-Remediation for Landfill
Leachate Treatment • Issue
– at the end of their useful life
• modern landfills are capped
• covered with a low permeability material to
minimize infiltration of precipitation
– before the advent of modern regulations
• completed landfills may have only been covered
with soil
Infiltration
• Without a cap
– more infiltration occurs
– more water comes in contact with solid
wastes
• becomes leachate
– increases hydraulic pressure on sidewalls
• seepage and slumping
– increases the potential for deep percolation
• groundwater contamination
Leachate
• At legacy landfills
– there are several conventional options for
dealing with leachate production
• replace “cover” with a “cap”
• install a geosynthetic clay liner over landfill
• collect leachate & haul to WWTP
• install & operate a WWTP
Leachate
• Making a decision about which solution to
implement depends on
– leachate chemistry
– evaluation of the hazard
• to groundwater
• to surface water
– availability of a WWTP
– cost
Potential Alternative Solution
• Create an Evapotranspiration Cover
– use the existing soil cover to store
precipitation
– use plants to remove the moisture from the
cover
– apply leachate to the cover during high ET
conditions
A Water Balance Approach
http://regclim.coas.oregonstate.edu/wp-content/uploads/BATS_highres.png
Water Balance Approach
• Moisture holding capacity
– volume of water that can be held by the soil
before deep percolation out of the root zone
• depends on texture (sand, silt, & clay content)
– Units
• inches of water per foot of soil
• typical silty clay → about 2” of water per foot of soil
Water Balance Approach
• Soil matrix potential
– often called soil
moisture tension
– soil matrix forces pull
on the moisture
• can hold moisture
above the force of
gravity
Water Balance Approach
• Infiltration and runoff
– if precipitation rate is greater than the
infiltration rate
• then the difference becomes runoff
– when precipitation rate is less than or equal to
the infiltration rate
• then precipitation infiltrates into the soil
– infiltration rate is dynamic
• rate decreases as water moves into soil
Water Balance Approach
• Evapotranspiration rates
– governed by how much moisture the air can
hold and move away
– function of
• temperature
• relative humidity
• wind speed
• plant characteristics
Trend for ET in Tennessee
0
1
2
3
4
5
6
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Inches
Month
Precipitiation Evapotranspiration
Remember
• Only about 75% of
rainfall infiltrates into
the soil
– the remainder is runoff
• There will be wetter
years (such as 2018)
• There will be drier
years (such as 2016)
As Compared to a Cap
• A compacted clay cap still has some
infiltration
– 1x10-9 to 1x10-5 cm/s
– 0.032 to 315 cm of water per year
– 0.0124 to 124 inches of water per year
• ET cover is considered a success if
infiltration is equivalent to compacted clay
cap
Lowland, Tennessee
• Former 764-ac BASF manufacturing area
– previously known as Liberty Fibers
• rayon plant, originally American Enka Company
• opened in 1948
– now a very complex site in terms of
• ownership
• stormwater management
• environmental regulations
• local politics
North Landfill
• BASF owns two landfills on this site
– the focus is on the North Landfill
– used to store
• fly ash
– approximately 90% of volume
• production wastes
• wastewater treatment sludge
• garbage generated at plant
Closure
• Disposal operations ended in 1983
– covered with 30 inches of soil
– additional 30 inches added later to limit
infiltration
– considered “closed” in 1985
• non-RCRA closure
Leachate Management
• Historically, leachate has been collected
– by a subsurface toe drain system installed
around landfill base to intercept leachate
– this drainage system connected to a gravity
line that conveyed leachate to a lift station
– then,
• gravity flow, lift station, and gravity flow to WWTP
WWTP is Nearby But Pipes are Leaky
• With all the I&I
– the flow from former
manufacturing site has
been restricted
– little of the water was
leachate from BASF
– forced BASF into a
pump & haul system
for leachate disposal
A Potential Solution
• Is to minimize leachate production
– reduce percolation through cover by storing
moisture in the cover soil
– pull moisture out of the soil using ET to
reestablish storage
• use plant material that encourage transpiration
• high leaf surface area
• open canopy
Water Balance – Morristown, TN
0
1
2
3
4
5
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Inches
Month
Precipitiation Grass Reference ET
Accounting for Runoff Assuming 85% Infiltration
0
1
2
3
4
5
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Inches
Month
Infiltration Grass Reference ET
Soil Moisture Balance w/Grass Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
I 3.4 3.5 3.5 3.3 3.5 3.1 4.0 3.0 2.6 1.9 2.9 3.4
ET 0.2 0.3 1.1 2.1 3.4 4.3 4.9 4.8 3.6 2.3 1.0 0.3
Net -3.2 -3.2 -2.4 -1.2 -0.2 1.1 1.0 1.8 1.0 0.4 -1.9 -3.1
S 8.2 10.0 10.0 10.0 10.0 8.9 7.9 6.1 5.1 4.8 6.7 9.8
D 0.0 1.4 2.4 1.2 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0
I = Infiltration
ET = Evapotranspiration
Net = ET – I
S = Moisture Storage in Soil
D = Deep Percolation
Grass Reference ET
• began year with 5” of soil moisture
• ended year with 9.8” of soil moisture
• deep percolation: 5.1”
Hybrid Poplars
• Cross between
Eastern Cottonwood
and Black Poplar
– utilizes a prolific
volume of water
– tremendous leaf
surface area available
for transpiration
– NM6 and DN34
genotypes https://www.leachate.us/poplars/
Development of the Phytoplot
• Top surface of North Landfill
– 5 acres planted with 2,400 hybrid poplar trees
• November 2017
• 10-ft by 10-ft spacing
• 435 trees per acre
– Anticipated ET rates
• Year 1 – 1.2 gpd per tree
• Year 3 – 9.8 gpd per tree
• Year 5 – >20 gpd per tree
At Maturity
• Assuming 20 gpd per tree
– 8,700 gallons per acre
– 0.32 inch per day
– Greater than the Grass Reference ET
• Estimated 217 growing-days per year
– for trees in the Ridge and Valley of East
Tennessee
– essentially March through September
Water Balance with Trees Assuming 25% Greater ET
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Inches
Month
Infiltration Tree Potential ET
Water Balance w/Trees (25%) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
I 3.4 3.5 3.5 3.3 3.5 3.1 4.0 3.0 2.6 1.9 2.9 3.4
ET 0.2 0.3 1.3 2.7 4.2 5.4 6.1 6.0 4.5 2.3 1.0 0.3
Net -3.2 -3.2 -2.1 -0.7 0.7 2.2 2.2 3.0 1.9 0.4 -1.9 -3.1
S 8.2 10.0 10.0 10.0 9.3 7.1 4.9 2.0 0.1 -0.3 1.6 4.7
D 0.0 1.4 2.1 0.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
I = Infiltration
ET = Evapotranspiration
Net = ET – I
S = Moisture Storage in Soil
D = Deep Percolation
Assuming Tree ET is 25% greater than grass ET
• began year with 5” of soil moisture
• ended year with 5” of soil moisture
• deep percolation: 4.1”
Water Balance with Trees Assuming 35% Greater ET
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Inches
Month
Infiltration Tree Potential ET
Water Balance w/Trees (35%) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
I 3.4 3.5 3.5 3.3 3.5 3.1 4.0 3.0 2.6 1.9 2.9 3.4
ET 0.2 0.3 1.4 2.9 4.5 5.8 6.6 6.4 4.8 2.3 1.0 0.3
Net -3.2 -3.2 -2.0 -0.5 1.0 2.6 2.7 3.4 2.3 0.4 -1.9 -3.1
S 8.2 10.0 10.0 10.0 9.0 6.3 3.7 0.2 -2.0 -2.4 -0.5 2.6
D 0.0 1.4 2.0 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
I = Infiltration
ET = Evapotranspiration
Net = ET – I
S = Moisture Storage in Soil
D = Deep Percolation
Assuming Tree ET is 35% greater than grass ET
• began year with 5” of soil moisture
• ended year with 2.6” of soil moisture
• deep percolation: 3.8”
With a 35% ET Increase
• Soil moisture storage is maximized
– deep percolation is minimized
• will always have some deep percolation
• Now have excess ET capacity
– could use it to ET leachate
• irrigate phytoplot to ensure moisture is available to
be evapotranspired
• could apply 2.4 inches per year
Leachate Production
• Leachate flows were monitored
– October 2016 to February 2017
• peak flow, 25 gpm, heavy rain event
• wet weather flow, 6.6 gpm
• dry weather flows, 1.5 gpm
• For the phyto-remediation project
– the design flow became 3.5 gpm
• or 5,040 gallons per day
• 1.8 million gallons per year
Proposed Irrigation System
• Subsurface drip
irrigation
– one row of tubing per
row of trees
– effective irrigation area
is 1.5’ to each side of
tubing
– 1.4 acres of effective
irrigation area within 5-
ac phytoplot
10-ft row spacing
wetted soil volume
Irrigation Scheduling
• Proposed irrigation system is designed so
that no leachate is recirculated through
the solid wastes
– must have sufficient soil moisture storage to
hold additional moisture until evapotranspired
– application rate is equivalent to 0.04 inch per
hour
Leachate Application
• 5,040 gallons over 1.4 acres
– is 0.13 inch of moisture
• this is the design daily leachate production
• At anticipated ET of hybrid poplar trees
– there would be days when 0.32 inch of
moisture could be applied
• which is 2.4 times the daily design leachate
production
Performance Monitoring
• Sensors
– evapotranspiration
• sap flow meters, soil moisture sensors, Rh,
temperature, rain gauge, evap pan
– leachate flow and quality
• doppler meter in collection system
• automatic sampler
– Irrigation system
• flow meters and pressure gauges
Leachate Storage
• Three 21,000-gallon
tanks
– AKA, frac tanks
– plumbed in series
– surge tanks for pump
& haul
• Pump station for
proposed irrigation
system
Leachate Quality Most Results below Reporting Limit
• nitrate 0.02 mg/L
• nitrite
• ammonia 5.42 mg/L
• TSS 9.8 mg/L
• settleable solids 0.10 mg/L
• TKN 6.21 mg/L
• BOD5
• pH 7.70
• phenolics
• arsenic
• cadmium
• cyanide 0.0077 mg/L
• chromium
• copper
• lead
• molybdenum
• nickel
• phosphorus 0.147 mg/L
• selenium
• silver
• zinc
• mercury
Dissolved Constituents
• Precipitate will occur with evaporation
– selected hybrid poplars are salt tolerant
• Wet season rainfall will flush salts through
soil profile
• Recall
– that primary goal is to minimize leachate
production
– leachate treatment is a secondary benefit
Reduced Percolation means
Reduced Leachate Production
• With grass cover
– percolation estimated to be 5.1 inch per year
• equivalent to 4x10-7 cm/s
• With trees consuming 35% more than
grass
– percolation estimated to be 3.3 inch per year
• equivalent to 3.3x10-7 cm/s
For Legacy Landfills
• An evapotranspiration cover with leachate
irrigation is a potentially less expensive
solution
– for leachate management
– hybrid poplar trees have a luxurious
consumption of moisture
• maintains the moisture holding capacity of the
cover soil
• promotes evapotranspiration instead of percolation
QUESTIONS?
Thanks for your time!!
