Geosynthetics 2013

April 1-4, Long Beach, California

Geomembrane Surficial Landfill Gas Collection Systems

C M. Richgels, P.E., Agru-America, Inc., USA, crichgels@agruamerica.com M. Ayers, P.E., Closure Turf, Inc., USA, mayers@closureturf.com D. Lewis, Closure Turf, Inc., USA dlewis@closureturf.com

ABSTRACT

Typical final cover systems can be damaged by differential settlement during postclosure, can suffer from slope veneer failures, and experience strain of the geomembrane beyond allowable yield with development of excessive landfill gas (LFG) pressure beneath geomembrane barrier layers. Given the risks with standard final cover systems, additional design elements have been recommended for use in final cover systems. For example, use of sub-barrier layer LFG collection has been recommended by Thiel (1998) and Richardson (2000) to control LFG pressure beneath geomembranes to improve stability. Exposed geomembranes have been suggested for consideration by Koerner (2010) as a temporary final cover system until the deleterious effects of landfill differential settlement, and LFG generation with the attendant slope stability risks, have reduced in the initial postclosure period. In addition, geosynthetic material can be used in substitution for typical LFG control devices being used today. This paper discusses a new alternate final cover system which can best be described as a geosynthetic final cover system. It consists of a sand ballast layer placed on a specialized HDPE artificial grass tufted into a double layered polypropylene woven geotextile. This assembly is deployed across a structured geomembrane that provides a sub drainage layer beneath the turf component. This hybrid Final Cover system (trade name “Closure Turf®”) can provide designers with an alternative LFG collection system that can significantly reduce requirements for more costly and easily damaged vertical LFG extraction wells for energy recovery projects. In combination with two other geosynthetic based LFG collection systems, an Integrated LFG Collection System can be deployed at modern sanitary landfills with geosynthetic materials. 1. INTRODUCTION This surficial LFG collection system can be combined with vacuum application to geosynthetic leachate collection and removal systems (LCRS), and placement of geosynthetic LFG strip collectors in the active landfill areas. These geosynthetic applications may be used in substitution for typical vertical LFG extraction well systems. The location of these applications is depicted in Figure 1. There are three general locations where geosynthetic materials could be applied as LFG collection layers:

1. A surficial collection layer beneath the geosynthetic final cover system. 2. Geosynthetic collection strips in active landfill areas. 3. Geosynthetic leachate collection and removal system (LCRS) layer at the landfill base.

Figure 1. Locations for LFG collection with geosynthetic materials

These applications are discussed below. 2. GEOSYNTHETIC FINAL COVER SYSTEM The geosynthetic final cover system can be placed in combination with geocomposite collection strips to control LFG beneath the final cover system. This practice has been recommended by Thiel (1998) and others to reduce the negative effects on LFG pressure build-up beneath final cover geomembranes on slope stability. The concept of this system is depicted in Figure 2.

Figure 2. Surficial LFG collection beneath geosynthetic final cover system

In general, a geocomposite strip of an engineered width is placed along the length of the landfill slope. At the top of the slope, the geocomposite is connected to an extraction point - a typical LFG wellhead tapped to a perforated pipe manifold. The geocomposite is wrapped around the perforated pipe manifold and can be secured in place using a gravel backfill. The geosynthetic final cover system is then deployed over the geocomposite collector strip(s) to complete the surficial gas collection system. The configuration shown is proposed for incremental closure construction along the landfill sideslopes. 2.1 Geocomposite Collection Strip Spacing and Flow Spacing of geocomposite collection strips can be determined based on allowable gas pressures beneath the final cover system. Using an interface shear strength accounting for gas uplift pressure (Equation 1), the allowable gas pressure (u(g-allow)) beneath the geosynthetic final cover system can be determined as 60 Pa (at FSallow = 1.5). !(!!!""#$) = hγcosβ −   !"!""#$!!"#$%!  !

!"#∅ [1]

From Thiel, LFG flow to geocomposite collector strips can be estimated using Equation 2:

Ug x = !!!

!!!!

γ! Lx  – !!

! [2]

Where,

Ug(x) = gas pressure, Pa (minimum applied vacuum in this scenario) Qi = LFG flow rate, M3/sec kg = gas permeability of flow media, m/sec

t = thickness of flow media, m γg = gas unit weight, N/m3 L = half distance between collection strips, m x = distance from collection strip, m The LFG flow rate, Qi, is related to the LFG generation rate and the volume of waste where gas is being generated. Qi is estimated below. Qi = rg(t x L x 1) γwaste [3]

Where rg is the LFG generation rate and γwaste is the unit weight of the in-place waste. Emcon (1980) estimated rg at 7 ml/day/kg. Thiel recommended a value of 17 ml/day/kg. The variable t represents the depth of waste generating LFG to be collected by the geocomposite strip. The unit weight of waste can be taken 700 and 800 kg/m3. The gas unit weight is essentially the combined unit weight of methane (6.54 N/m3) and carbon dioxide (17.9 N/m3). At a nominal mixture of 50/50 in a balanced extraction scenario, γg = 12.2 N/m3. The gas permeability, kg, of the flow media is derived from the refuse beneath the slope cover. Emcon reported an intrinsic permeability, ks, for refuse between 13 and 20 Darcies. Intrinsic permeability is a function of the solid material structure. Gas permeability of the waste can be found by Equation 4. kg = ks(γg)/µg [4] (Kou)

Converting to Darcies to SI units, and using a LFG dynamic viscosity, µg, as 1.15 x 10-5 Ns/m2 (Emcon), kg can vary from 1.2 x 10-5 m/sec to 1.8 x 10-5 m/sec. The slope cover will be too variable in thickness to act as a gas relief layer with any reliability. A refuse thickness, t, of 5 meters is assumed as immediately affected by an applied vacuum from the geocomposite beneath the final cover system. Using an average landfill depth of 30 meters in sideslope areas, it can be assumed that half that depth is generating landfill gas that will be collected by the geocomposite strip before vacuum influence from an active area horizontal collector strip (Section 3), or the LCRS (Section 4 below) compete for the flow. Substituting Equation 3 into Equation 2 and inserting the highest variable values, Ug(x) every 1 meter along the collector strip can vary according to L as shown in Table 1

Table 1. Surficial LFG Collection System Pressure and Collector Spacing

Collector Strip Half-Distance L (m)

Ug(x) (Pa)

5 3 10 28 15 93 20 220 25 430 30 743

Based on the estimated allowable gas pressure from Equation 1, geocomposite strips would be required every 20-30 meters for natural venting as was discussed by Theil. For active extraction as considered here, the unit flow (per meter of strip length) towards geocomposite strips placed along a nominal 50 meter long slope spaced every 60 meters, can be calculated using Equation 3 as 71 ml/sec/m (with rg = 17 ml/day/kg, t = 15 m) . Or 7,080 ml/sec across the length of the strip (both sides) applying vacuum to an area of 3,000 m2. 2.2 Actual Flow Conditions at an Active Landfill In March 2011, the LFG surficial collection system described above was applied to a partial closure at an active landfill in Missouri. The partial closure area was approximately 40,490 m2. The average slope (3H:1V) length in the closure area was 75 meters. A total of eight (8) 1.5-meter wide 5-mm thick single-sided geocomposite strips were placed on this slope at an average spacing of 60 meters. The strips were curled around a perforated pipe collector at the toe of the slope. The perforated pipe was connected to a typical LFG vertical well head as depicted in Figure 2. The well head delivered collected gas to a perimeter header pipe feeding an onsite flare. The lower end of LFG flow from this surficial LFG collection system was measure as of 4.1 ml/sec/m2. Applied over an area of 3,000 m2 as calculated above, this is

equivalent to 12,250 ml/sec, or 1.7 times the estimated flow rate calculated above. Adjusting the calculation method by the actual data suggests applied vacuum from the geocomposite strip may reach as deep as 25 meters into the waste mass. 2.3 Laminar Flow within the Geocomposite Strip As long as the Reynolds number for gas flow within the geocomposite strip is less than that for the equivalent of a pipe (Re = 2,000), laminar flow conditions are expected, therefore the equations above are expected to produce valid estimates. In the application at the Missouri site, the velocity for a 1.5 meter wide, 5-mm thick single-sided geocomposite (assuming very little geotextile intrusion due to the low overburden) is found from: V = Q/A = 12,250/(1.5 x .005) = 0.17 m/sec [5]

Reynolds number is then:

Re = =   �!"�

= (1.24 kg/m3)(0.17 m/sec)(.005 m)/(1.15 x 10-5 N⋅s/m2) = 92 [6]

Use of these equations in the above scenarios is acceptable with this low of a Reynolds number.

3. HORIZONTAL COLLECTION STRIPS IN ACTIVE LANDFILL AREAS The surficial collection system discussed above can also be expanded for use within the active landfill area to control surface emissions. Installation of horizontal LFG collection wells require landfill operations to move to another location in the landfill. The wells are excavated as trenches across the topdeck surface of the landfill. Excavation spoils and the trench itself release significant amounts of LFG exposing the construction workers and landfill operating staff to health risks that must be controlled with engineered controls and personal protective equipment. Horizontal collection wells can water in with leachate if the bottom of the trench is in contact with previous placed intermediate cover. To avoid these issues, geosynthetic collection strips could be used in lieu of vertical and horizontal LFG collection wells as shown in Figure 3. A structured geomembrane is available for use in this application. The membrane is a 1.5 mm HDPE geomembrane with 3.3-mm studs on one side of the membrane sheet. The studs with a covering geotextile (underlying in this application) behave similar to a geocomposite collection layer. The horizontal collector strip would be placed over a completed daily refuse cell before daily or intermediate cover soil is placed. First a geotextile would be deployed to prevent fines intrusion into the drainage matrix of the structured geomembrane. The structured geomembrane would then be deployed with studs in contact with the geotextile strip to complete the horizontal collector strip. No seaming equipment would be needed. The materials would simply be rolled out by site labor crews before soil cover was placed. Site personnel would not be in contact with exposed waste as they would walk behind the rolls as they rolled them out. This placement method would allow for the horizontal collector strip to draw LFG from the refuse lift below, out to a distance L, depending on the spacing of the strips and the refuse gas permeability. This construction technique also would reduce the potential for watering in the collector strip as percolating fluids from above lifts would be diverted to either edge of the collector strip and continue downward in contrast to fluid gathering in the gravel backfill of a horizontal collection well. No refuse excavation is required, and normal cover operations can be completed immediately after the geotextile and structured geomembrane collector strips are deployed. Laboratory performance data for this material is typically reported only for hydraulic properties. This material has a tested hydraulic transmissivity value of 5.4 x 10-3 m2/sec at an applied normal load of 250 KPa, and a flow gradient of 0.1 (applicable for a near horizontal application). The hydraulic permeability, kw, of the material is simply the transmissivity divided by the stud height of 3.3-mm, or kw = 1.6 m/sec. The gas permeability of the structured drainage geomembrane can be found by Equation 7. kg = k!

�!�!

�!�!

[7]

With µw equal to 1.01 x 10-3 Ns/m2 and γw at 9,797 N/m3, kg is found as 0.17 m/sec.

Figure 3. Conceptual Horizontal Geosynthetic Collection Strips

For flow within the refuse mass, Equation 2 is reapplied to create Table 2 for a 1.5-m wide, 90-m long horizontal structured geomembrane LFG collection strip, placed over a refuse lift 9-meters thick (t), approximately 30 meters below grade.

Table 2. Horizontal LFG Collection Strip Pressure and Collector Spacing

Collector Strip Half-Distance L (m)

Ug(x) (Pa)

5 1 10 8 15 26 20 63 25 122 30 212

The calculation in Table 2 assumed a lower intrinsic permeability in the waste mass of 13 darcies due to the much higher overburden stresses. From Equation 3, the corresponding flow in the horizontal strip collector will be 7,650 ml/sec. 3.1 Laminar Flow Conditions within the Horizontal Collector Strip Using Equation 5 and substituting the values for this application, the flow velocity within the horizontal collector strip is found to be v = (7,650 ml/sec)/(1.5 m)(.0033 m) = 1.5 m/sec. Using Equation 6, the corresponding Reynolds number is 550 which is higher than the surficial collection strip discussed above. Still, this number suggests spacing between horizontal collector strips can be increased and still satisfy laminar flow requirements within the collector itself. Additionally, the collector strip could be widened to reduce flow velocity within the system, thus reducing losses due to turbulence. This geosynthetic system requires testing in an active landfill to measure actual performance against that predicted in this paper.

3.2 Horizontal Collection Strip Replacement Horizontal collection strips will be temporary by their very nature. As succeeding lifts of waste are placed over them, additional collector strips are added in those overlying lifts reducing the importance (as well as function) of the initial collector strip. Fortunately, the strips would be very cost effective in comparison to typical horizontal well construction. Also the topmost strip is what is required for surface emissions control. Deeper strips would have decreasing importance for this function as additional waste lifts are placed.

4. LCRS LFG EXTRACTION Typical LCRS installations are either gravel collection layers above a primary geomembrane liner or geocomposite drainage layers replacing the gravel. A cross section of a collection trench in the LCRS is depicted in Figure 4.

Figure 4. LCRS Collection Trench

Base liner grades are typically designed using a herringbone pattern with multiple ridges and troughs directing leachate flow to the troughs. The base liner grades are typically 2% towards the trench as depicted in Figure 4. The collection troughs are sloped at a nominal grade between 0.5% and 1% to a LCRS sump where leachate is mechanically removed. The typical design distances from trough to ridge can be approximately 30 to 45 meters.

As leachate begins percolating down through the waste mass, the saturation of the operations soil layer will increase thereby impeding landfill gas migration into the LCRS system. The operations layer will not become fully saturated, however, thereby allowing some buildup of LFG pressure in the LCRS system. This pressure can cause surface emissions near LCRS clean out pipes and extreme venting of LFG when the LCRS pipes are opened for system inspection, exposing maintenance workers to hazardous gases and vapors. Pressure build-up in the LCRS can also increase the risk of LFG migration out of the landfill into the surrounding soil. Migration of LFG through the geomembrane is a much more likely scenario for landfill contaminant release than that risk posed by leachate leakage. Fortunately, LFG risk is also easier to control with active application of LFG system vacuum to the LCRS (Richgels 2000).

To avoid hydraulic blockage in the LCRS sump, LFG collection system vacuum can be applied to LCRS system cleanout pipes that are placed in the collection troughs away from the sump. The flow characteristics allow that system vacuum to quite effectively spread applied vacuum across the entire floor of the base liner. The authors are aware of at least one such application that produced an average LFG flow of 0.25 m3/min/ha with an average applied vacuum of 6.35 KPa across a 0.3 meter thick gravel LCRS. Back-calculation of Equation 3 indicates gas collection under this scenario may only be from a refuse thickness, t, of 2.7 meters versus the initial assumption of 15 meters. Given the applied vacuum, this confirms the operations soil layer has a significant degree of saturation.

Geosynthetics, being thinner, may not perform as well. Using the flow and refuse thickness variables from the gravel LCRS scenario above, we can estimate the applied vacuum coverage from the LCRS clean-out pipes into the field of a geocomposite LCRS using Equation 2. For a 5-mm geocomposite, typical transmissivity values are reported as 1x10-4

m2/sec with a 480 KPa normal load and a gradient of 0.1. These are appropriate conditions for a base liner application. Reducing the geocomposite thickness by a factor of 0.25 to allow for compressive loss in thickness, the resulting hydraulic conductivity of the geocomposite is 2.67 x10-2 m/sec. Using Equation 7 again to estimate gas permeability as 2.9 x 10-3 m/sec, and then substituting into Equation 2, distance from the trough to ridge for varying pressure is shown in Table 3.

Table 3. LCRS Trough-Ridge Distance, Applied Pressure, Geocomposite Thickness

Trough-Ridge Distance L (m)

Ug(x), 5-mm Geocomposite

(Pa)

Ug(x), 7.5-mm Geocomposite

(Pa) 5 7,799 584

10 62,394 4,675 15 210,579 15,780 20 499,149 37,404 25 974,901 73,055 30 1,684,628 126,238 35 2,675,127 200,462

This is too restrictive in some base liner designs with greater trough-ridge distances as noted above. If a 7.5-mm geocomposite is used as the LCRS, the gas permeability increases to 1.72 x 10-2 m/sec. This improves the vacuum “reach” across the LCRS, but is still beyond typical LFG extraction system vacuum (<12,450 Pa) for typical trough-ridge distances. By reducing the refuse thickness across which vacuum is applied (Equation 3), acceptable system vacuum levels can be eventually be found for a geocomposite LCRS. This is generally 10-percent of the vacuum influence distance for a gravel LCRS, or tenths of meters. This implies significant pressure can build within the waste mass above the LCRS. Geocomposite LCRS may be more effective at providing migration control from the landfill base then actual LFG extraction. This concept needs field evaluation from landfills with geocomposite based LCRS.

5. CONCLUSIONS In general, geosynthetic materials can be used to extract LFG from municipal waste landfills. Geocomposite collection strips placed under the synthetic final cover system have been shown to be an effective LFG extraction system. Use of structured geomembranes collection strips to control LFG movement in active areas of the landfill could provide similar LFG control in active landfill areas as horizontal collection trenches. Placement of structured geomembrane collection strips could be more easily integrated into the landfill operations than installation of excavated trenches. However, this horizontal collector strip design needs to be tested in the field to verify the design assumptions presented in this paper. Horizontal collector strips are subject to a variety of destructive forces within the active landfill. At this time, no credible estimation of a strip service life can be made without application in the field. When compared to gravel based LCRS, a geocomposite LCRS may not be effective as an active LFG extraction system. However, they should be able to provide LFG migration control from the base of the landfill. This should be confirmed with data collection from landfills with existing geocomposite LCRS.

REFERENCES

Emcon. (1980). Methane Generation and Recovery from Landfills, Ann Arbor Science Publishers, Inc., Ann Arbor, MI, USA. Kou, J. (1999), Practical Design Calculations for Groundwater and Soil Remediation, CRC Press LLC, Boca Raton, FL, USA Koerner, R.M., (2010). Advances and Concerns in Geosynthetic Materials, Geosynthetics for Success, GSE Technical Seminars, Walnut Creek, California, USA Thiel, R.S. (1998). Design Methodology for a Gas Pressure Relief Layer Below a Geomembrane Landfill Cover to Improve Slope Stability, Geosynthetics International, Vol. 5, No. 6. Richardson, (2000). Gas Transmission in Geocomposite Systems, GFR Magazine, Volume 18, Number 2, March 2000 Richgels, C. (2000) , Reasonably Foreseeable Water Quality Risks From Lined Landfills, Leachate And Landfill Gas Releases, 5th Annual Landfill Symposium, Solid Waste Association of North America, Austin, TX, USA