Effect of Perched Water Conditions in MSW Landfills ...
Transcript of Effect of Perched Water Conditions in MSW Landfills ...
Effect of Perched Water Conditions in MSW Landfills:
Considerations for Landfill Operators
Timothy Townsend Department of Environmental Engineering
University of Florida
ESD 17th Annual Solid Waste Technical ConferenceEast Lansing, Michigan
April 4, 2006
Motivation
• A common observation at solid waste landfills is the possible presence of saturated waste layers in the deeper parts of landfills.
• The presence and cause of these saturated layers can be interpreted differently.
• The question that we asked: “what should one expect?”
Implications of Perched Water
• Problems with gas recovery?
• Slope stability concerns?
• Leachate collection system problems?
• Future side slope seepage issues?
Let’s examinethe scenario where only
waste around the well is saturated
Some source of water is added to the well at a rate greater that it can drain out. Possible sources:
• Gas condensate• Perched zones of
leachate in the landfill• Short circuiting from
liquids addition
Modified Version of Richard’s Equation
( )t
St
C
kz
kz
Krr
kKr
kr
K
s
zr
r
∂∂
+∂
∂=
=
+
∂∂
∂∂
+∂∂
+
∂∂
∂∂
ψψψ
ψψψ
Richard’s equation was solvedusing a USGS program called
SUTRA
r (ft)0.0 1.0 2.0 3.0 4.0
z (ft
)
0
10
20
30
0 ft1 ft2 ft3 ft4 ft5 ft
Simulation Parameters
K = 10-5 cm/sec
Q = 17 gallons/day
Duration of Moisture Addition = 10 days
Head in the well ~ 8 ft
• If the liquids are added to the landfill at a rate greater than the hydraulic conductivity, saturated conditions will result
Let’s examinethe scenario where saturated conditions will develop in the
landfill even if barrier layers are not present
Consider a Liquids Infiltration Pond
• The waste underneath the pond will become saturated
• In the absence of cover soil layers, a saturated zone will extend to the leachate collection system
Can saturated conditions develop if the liquids are added at a rate less than the permeability of the waste?
• Yes, if the permeability of the waste is reduced with depth
DecreasingPermeability
Col 21 vs Col 22
Density (t/m3)
0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
Hyd
raul
ic C
ondu
ctiv
ity (m
/sec
)
10-8
10-7
10-6
10-5
10-4
10-3
At ~1400 pcyK = 8x10-5 cm/sec
Air permeability of waste at NRRL at different depths
Air Permeability, k (X10-12 m2)
<0.1
0.1-
2.5
2.5-
5.0
5.0-
7.5
7.5-
10.0
10.0
-12.
5
12.5
-15.
0
15.0
-17.
5
17.5
-20.
0
20.0
-22.
5
22.5
-25.
0
>25.
0
Num
ber o
f Loc
atio
ns
0
3
6
9
12
15
Air Permeability, k (X10-12 m2)
<0.1
0.1-
2.5
2.5-
5.0
5.0-
7.5
7.5-
10.0
10.0
-12.
5
12.5
-15.
0
15.0
-17.
5
17.5
-20.
0
20.0
-22.
5
22.5
-25.
0
>25.
0
Num
ber o
f Loc
atio
ns
0
3
6
9
12
15
Air permeability of waste at NRRL at different depths
Air Permeability, k (X10-12 m2)
<0.1
0.1-
2.5
2.5-
5.0
5.0-
7.5
7.5-
10.0
10.0
-12.
5
12.5
-15.
0
15.0
-17.
5
17.5
-20.
0
20.0
-22.
5
22.5
-25.
0
>25.
0
Num
ber o
f Loc
atio
ns
0
3
6
9
12
15
Air permeability of waste at NRRL at different depths
Simulation Parameters
Decreasing K = 10-5
cm/sec (top) to 5X10-6
cm/sec (bottom at 60 ft deep)
Q = 8.5 gallons/day
Duration of Moisture Addition = 10 days
Head in the well ~ 5 ft
r (ft)0.0 1.0 2.0 3.0 4.0
z (ft
)
0
10
20
30
0 ft1 ft2 ft3 ft4 ft
Review
• The existence of standing liquids in gas wells in landfills does not necessarily result from a phreatic liquid surface in the landfill.
• Liquids added to wells as a result of perched layers in the landfill, gas condensate or other sources can result in relatively large depths of water in the well.
Review
• The decreasing permeability of landfilled waste with depth should have impact.
• Saturated waste conditions may be present, but the pressure of this water may not be accurately reflected by the depth of water that would be measured if a well was installed.
• At large liquid addition rates, saturated conditions in deeper layers may develop.
Implications
• The presence of liquids in gas wells in “dry” landfills should not automatically assumed to represent a phreatic surface.
• In “wet” landfills, the liquid levels in wells may result from both situations.
• When evaluating slope stability, careful thought must be given to the pressures that truly occur.
• Leachate collection systems need to be designed and operated correctly.
MSW
10’
5’
20’
30’
40’
VW piezometers
Data Station
Injection Well #1
Cover Soil
10’
20’
35’
Injection Well #2
Piezometer
• Is an instrument to measure pressure– Specifically, the Water Pressures within the
Pores caused by Recirculating Leachate
Installed inCotton bagswith moist sand
Piezometer T212
14.45
14.5
14.55
14.6
14.65
14.7
14.75
14.8
14.85
0 5 10 15 20 25 30 35
Days of December
Abs
olut
e Pr
essu
re (p
si)
Barometer
T212
Bioreactor Research in Florida
• Leachate recirculation has been practiced in Florida since the 1980s
• Researchers are currently involved with bioreactor activities at multiple landfills
Fine concrete
3% Ca(OH)2
1% Ca(OH)2
Sand
Compost
10% CaCO3
Testing Plots
Sampling Tubes
Fine concrete
3% Ca(OH)2
1% Ca(OH)2
Sand
Compost
10% CaCO3
Testing Plots
Sampling Tubes
Contact Info
• Tim Townsend – [email protected]• www.ees.ufl.edu/homepp/townsend