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 6 5 4 3 Inches 2 1 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 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 5 4 3 Inches 2 1 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month Precipitiation Grass Reference ET Accounting for Runoff Assuming 85% Infiltration 5 4 3 Inches 2 1 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 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 Grass Reference ET ET = Evapotranspiration • began year with 5” of soil moisture Net = ET – I • ended year with 9.8” of soil moisture S = Moisture Storage in Soil • deep percolation: 5.1” D = Deep Percolation 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 7.0 6.0 5.0 4.0 Inches 3.0 2.0 1.0 0.0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 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 Assuming Tree ET is 25% greater than grass ET ET = Evapotranspiration • began year with 5” of soil moisture Net = ET – I • ended year with 5” of soil moisture S = Moisture Storage in Soil • deep percolation: 4.1” D = Deep Percolation Water Balance with Trees Assuming 35% Greater ET 7.0 6.0 5.0 4.0 Inches 3.0 2.0 1.0 0.0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 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 Assuming Tree ET is 35% greater than grass ET ET = Evapotranspiration • began year with 5” of soil moisture Net = ET – I • ended year with 2.6” of soil moisture S = Moisture Storage in Soil • deep percolation: 3.8” D = Deep Percolation 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 10-ft row spacing tubing – 1.4 acres of effective irrigation area within 5- ac phytoplot 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