Landfill-gas system design is a series of iterative steps often more of an art than a science.

By Daniel P. Duffy

The very nature of disposed solid waste, with its varying characteristics and inconsistencies, makes it impossible to precisely predict the amount and rate of landfill gas (LFG) production. Therefore, the design of a LFG system should have a built-in flexibility to allow for field changes and layout modifications.

Predicting Production
Waste arrives at a landfill with a typical density of 33 pounds per cubic foot or 900 pounds per cubic yard. This waste is usually compacted to an in-place density twice that of its delivered density, about 67 pounds per cubic foot or 1,800 pounds per cubic yard. The in-place waste is assumed to have typical “soil characteristics” for municipal solid waste. These characteristics are the same characteristics used to predict leachate formation and precipitation percolation rates as well as LFG production. Waste is assumed to have a typical field capacity (the amount of water in a “soil” which remains after extensive gravity drainage) of 30% by volume and a total porosity (volume of voids/total volume) of 67%. In certain areas of the landfill, liquid content may reach total porosity, leading to a saturated condition and contributing to leachate outbreaks.

In a completely saturated landfill strata, little if any LFG is produced. Therefore the moisture content of the waste can greatly affect the projected gas volumes.

These characteristics determine the LFG production potential of the in-place waste. Since these can vary significantly from landfill to landfill and even within the same landfill, these assumed characteristics can only be used as a rough average for planning purposes. All subsequent computations and predictions derived from these assumed characteristics represent a “best guess” only. The LFG management system design will almost certainly have to be modified to accommodate actual field conditions.

The single most important factor in predicting LFG production rates is the rate of decomposition of the organic components of the waste.

The available decomposable mass of refuse for any year is given by the following equation:
V₁ = V₀ + D₁
where,
V₁ = available decomposable mass for the year (cubic yards converted to lbs)
V₀ = remaining decomposable mass from the previous year (cubic yards converted to lbs)
D₁ = refuse disposed for the year (cubic yards converted to lbs)
The amount of LFG produced per year is estimated by the following:
P₁ = V₁ * (0.10 cubic feet per lbs)
where,
P₁ = annual LFG production rate (cubic feet per year)
V₁ = available decomposable mass for the year (cubic yards converted to lbs)

The value of 0.1 cubic feet of LFG per pound of in place waste is empirical and based on field observations.

But since the waste characteristics even within a landfill can vary widely, this remains a rough average for planning purposes. Further dividing P₁ by 525,600 minutes per year gives the average LFG production rate per minute.

The annual amount of waste mass decomposed by the LFG production process is given by the following:
dV₁ = P₁ / 4.49
where,
dV₁ = the amount of waste converted to LFG (lbs)
P₁ = annual LFG production rate (cubic feet per year)

The factor of 4.49 converts the cubic feet of LFG into lbs of decomposed waste. Its value is based on laboratory studies and is an average value subject to change according to local waste characteristics. The remaining decomposable waste carried over to the next year’s LFG product estimate is given by the following:

V₂ = V₁ - dV₁
where,
V₂ = decomposable waste mass available for next year (lbs)
V₁ = available decomposable mass for the year (lbs)
dV₁ = the amount of waste converted to LFG (lbs)

A hypothetical landfill with a total disposal capacity of 1,850,000 cubic yards (equivalent to about 1,150 acre-feet) might utilize an average of 92,500 cubic yards of airspace per year over a 20-year operational lifetime.

With an average in-place density of 1,800 lbs per cubic yard, the landfill would receive an average of 82,750 tons per year (or 230 tons per day, typical for a moderate-size landfill). The maximum in-place waste volume is achieved at the end of year 20, when disposal operations cease.

Initial LFG production from the first year of waste disposal would be a relatively small 32 cubic feet per minute.

LFG production also peaks at the end of year 20 with a production rate of 516 cubic feet per minute. It then declines over the course of the site’s 30-year, post-closure care period to a rate of 216 cubic feet per minute.

The annual average rate of LFG production is shown in Table 1.

Final Cap and Cover
The efficiency of the LFG extraction system is primarily dependent on the permeability of the cap. The amount of gas that can be extracted per well, and the spacing between the wells, can be greatly increased with an impermeable cap.

The greater the spacing between the wells, the fewer wells that are required to ensure complete coverage of the landfill. This results in significant capital cost savings and long-term savings in maintenance costs.

For our hypothetical landfill we will assume the following final cover system (from top to bottom):

• 6 inches of topsoil (soil type SM, silty sands with a permeability of 5.2 x 10-4 cm/sec)
• 24 inches of soil cover (soil type SM, gravelly sands with a permeability of 1.7 x 10-3 cm/sec)
• 0.24 inches thick geocomposite lateral drainage net (thick factory bonded geotextile-geonet-geotextile)
• 40 mil high-density polyethylene geomembrane (HDPE) on the top. The HDPE is factory-bonded to the underlying GCL. The edges of the HDPE geomembrane panels may not be seamed during installation, and therefore it is assumed the geomembrane’s liner leakage fraction is greatly increased.
• 0.20-inches-thick composite barrier layer. A geosynthetic clay liner (GCL) consisting of a bentonite layer (permeability of 5.0 x 10-9 cm/sec) sandwiched between a geotextile layer on the bottom and
• 12 inches of grading soil (soil type SW, well graded gravelly sands with a permeability of 5.8 x 10-3 cm/sec)

Each hydraulic conductivity results in an equivalent relative cover factor (Ms). This factor and the thickness of the layer are used to determine the final cap’s overall impermeability to gas.

Table 2 summarizes the relationship between the cover material’s vertical hydraulic conductivities and the relative cover factor, Ms. Table 3 summarizes the configuration and material characteristics of the final cover system.

Extraction Wells Depths and Zones of Control
Active landfill gas extraction wells are typically located within the limits of waste and set at depths of approximately 75% of the waste thickness at the well locations. The deeper wells are located in the center, with shallower wells around the waste perimeter. Since the well’s zone of influence (ZOI) changes with depth, and depth changes with location, several trial iterations using different well locations and depths may be required before the design can ensure complete coverage of the limits of waste. Once the well length has been determined, usually two thirds of its length is kept solid while the bottom one third of its length is slotted to allow inflow of the LFG.

Table 4 summarizes the relationship between the hypothetical depths of waste, the total lengths of the LFG extraction wells and the slotted and solid pipe segments of the wells.

Tables 5, 6, and 7 depict equivalent thickness of the geomembrane, relative cover permeability, and LFG well Zone of Influence (ZOI), respectively.

LFG well ZOIs are estimated by the following formula:
ZOI = Fs * (Sp+[Ms * Cd])
where,
ZOI = LFG extraction well zone of influence (feet)
Fs = Refuse permeability factor (dimensionless, range from 3.5 to 6.5)
Sp = Length of the solid pipe section of the LFG well (feet)
Ms = Relative cover permeability factor (dimensionless)
Cd = Cover depth (feet)

If the final cover system consists of multiple layers of soil and geosynthetics, the relative cover permeability factor for the cover system as a whole is determined by the following:
Ms = (Ms₁ * Cd₁) + (Ms₂ * Cd₂) +
… + (Ms_N * Cd_N)

The equivalent thickness of a geomembrane portion of a composite cap is determined in relation to the clay component of a composite cap, and is calculated by the following:

Cd_E = (K_C / K_H) * (Cd_H * LLF)
where,
Cd_E = Equivalent thickness of the geomembrane (feet)
K_C = Vertical hydraulic conductivity of the clay (cm/sec)
K_H = Vertical hydraulic conductivity of the geomembrane (cm/sec)
Cd_H = Thickness of the geomembrane (mils, converted to feet)
LLF = Liner leakage fraction (dimensionless, from HELP Model analyses)

(Note: for the purposes of determining ZOI, the equivalent thickness of the geomembrane assumes that the edges of the geomembrane are not welded and therefore significantly increase the geomembrane’s liner leakage fraction.)

The thickness of a geocomposite (factory bonded geotextile-geonet-geotextile) drainage blanket in the final cover system is not included in the analysis. The equivalent thickness of the geomembrane portion of the hypothetical cap is computed as follows.

For the purposes of design, a conservative ZOI of 300.0 feet will be used for each proposed LFG extraction well set in waste having a thickness of 40 feet or less. Note that if the final cover system did not use a geomembrane cap in its cover layer, its relative cover permeability factor would be reduced from 6.79 to 0.79, a reduction of 88%.

This should illustrate the importance of a geomembrane cap to an efficient LFG extraction system. A geomembrane acts to ensure the maintenance of pressure within a landfill and to prevent air infiltration into the landfill as a result of suction pressure from the wells.

Without the geomembrane, the resulting ZOI per well would be much smaller and require many more wells to provide full coverage.

Now that the wells have been properly spaced for maximum coverage, we need to determine the reduction in their areas of influence resulting form overlapping, adjacent ZOIs. Each overlap area is determined by the following formula:

Sx = [R² * COS-¹ (d/R)]
– [d * (R²– d²)_]
where,
Sx = the overlap area (square feet)
R = LFG well ZOI (feet)
d = half the distance between the adjacent gas wells (feet)
The flow reduction factor is determined by the following formula:
Rf = (Sx₁ + Sx₂ + ... + Sx_n) / (pi * R²)

Since each LFG extraction well ZOI will be overlapped by up to six adjacent ZOIs, each overlap area (Sx) cannot exceed 2.5% of the total well area of influence to ensure the maximum allowable Rf of 15% is not exceeded. If a well is located in a corner of a landfill or along its perimeter (and does not have six adjacent wells) the overlap per adjacent well can be proportionally higher, just so long as the 15% total is not exceeded. In our hypothetical example, well spacing of 530 feet for wells having ZOIs of 300 feet will provide sufficient coverage without excessive overlap.

LFG Pipe Flows, Velocities and Head Losses
After the wells have been properly spaced, the amount of gas extracted by each well can be determined.

This is done by first estimating the volume of in-place waste within the reduced ZOI of each well.

Each LFG extraction well influences a cylindrical volume of in-place waste. It is from this volume that the LFG is extracted. The volume of this cylinder is determined by the following formula:

V = pi * (ZOI² ) * Dw * (100% – Rf )
where,
V = Volume of influence (cubic feet)
ZOI = Zone of influence (feet)
Dw = Depth of waste (feet)
Rf = Flow reduction factor (percent)
The extraction rate from this volume of influence is a function of the gas generation rate of the in-place waste:
Qu = V * (dg / dt) * dw
where,
Qu = LFG extraction rate (cfm)
V = Volume of Influence (cubic feet)
dg/dt = LFG generation rate per lbs of in-place waste (cubic feet/[lbs * minute] )
dw = in-place density of waste (lbs/cubic feet)

For our hypothetical example, each LFG extraction well is estimated to have an extraction flow rate of approximately 90 cfm. Dividing the estimated peak LFG production rate (previously calculated) by this value will provide the minimum number of wells required.

An examination of the design plan showing the well locations derived from the required well spacing will determine if the proposed design has the minimum number of required wells.

In order to function properly, each LFG extraction wellhead needs a minimum pressure head of 10 inches of water column. Since the pressure is applied by a single, central blower, the pressure-head losses in the lateral pipelines connecting the wells to the blower need to be determined. These lateral pipes are usually solid, high density polyethylene (HDPE) with diameters varying from 6 inches to 24 inches.

There are two ways of installing lateral and header pipelines, either above or below the geosynthetic components of the final cover system.

Installation above the geosynthetics is a more difficult construction effort, the components of the final cover all have to be graded and trenched along the alignment of the pipelines well before they are installed.

Furthermore, the need to pre-align the pipelines severely restricts operational flexibility and field modifications to the system.

However, it makes repair and maintenance of the pipelines much easier. Installation below the geosynthetics has the reverse characteristics, ease of construction and operational flexibility, but with later difficulty (due to the need to slice through the overlaying geosynthetics) with repair and maintenance.

Of the two, the below-the-geosynthetic installation is preferred since all LFG estimates are best guesses only, and operational flexibility allowing for field changes must be retained.

Pressure drops for gaseous pipeline flows are approximated by the Spitzglass equation for relatively low pressure flows:

P = G * L * ( Q/[59.167 * K])²
where,
P = Pressure drop (inches of water column)
G = Specific gravity of the LFG (0.98, dimensionless)
L = Length of pipe segment (feet)
Q = Volumetric flow rate (cfm)
K = Spitzglass Constant (dimensionless)

When determining head losses along a header pipeline, each wellhead connected to the main header pipeline by a lateral pipe is considered a node at a point along the header. The header itself is usually circular so that if blockage occurs at a point along its length, LFG can still be extracted in the opposite direction. Though this will not occur at the same pressure head or with the same efficiencies, it is better than completely blocked gas flows. That can result in a disastrous and potentially dangerous expansion of the geomembrane cap (blowing up like a balloon) as the gas accumulates behind the blockage.

Though minor head losses from pipe bends, tees, flanges, and other fixtures is insignificant and usually ignored, there is an additional pressure-head loss in the flare stack used to flare off the extracted gas. This head loss is typically 12 inches of water column. As a safety factor, an additional 10% is added to the minimum required head. So if a maximum pipeline head loss of 4 inches occurs between the blower and the farthest extraction well, a minimum 28 inches of water column will be required for the system with the factor of safety. This is equal to 145 psf or over 1 psi. The blower should be rated for this pressure and the maximum gaseous flow rate.

The pipe diameters should be checked to ensure that the gaseous flow velocities do not exceed the maximum allowable. The maximum allowable gas flow velocity in a direction concurrent with condensate flows is 40 feet per second. For counter concurrent flows, the maximum velocity is 20 feet per second.

Condensate Collection and Control
Condensate flows are liquid flows that follow the slope gradient of the pipeline. Gas extracted by pressure follows the pressure gradient of the pipeline. Often the two are in opposing directions.

Should the pipe diameters need adjusting, the head losses in the pipeline must be recalculated. Then the velocities are checked again. This iterative process continues until an acceptable design is achieved.

Condensate is a particularly nasty and concentrated form of leachate that condenses out of the landfill gas as it cools while traveling along the pipeline. It is important to remove this accumulated liquid, as it can cause pipeline blockage. Condensate blockages are evidenced by a “sloshing” sound and indicated by rapid fluctuations in gas flow rate, temperature and increasing negative pressure.

Condensate accumulates and must be removed from low elevation points along the header pipeline.

While the condensate itself has a typical pH range of 3.5 to 7.5, it contains highly acidic compounds (at concentrations as high as 4,000 ppm). The acidic compounds that are captured by the condensate can give rise to relatively high rates of corrosion of carbon-steel pipe and fittings. Typical condensate also includes relatively high proportions of chlorides, ammonia nitrogen and phenols. Some states require the condensate to be collected and treated separately.

This would require an additional system of pipelines to convey the condensate from the low collection points in the header pipeline to an onsite pretreatment facility.

Most states however, allow the condensate to be recycled back into the landfill. Compared with the amount of leachate collected from the bottom of the landfill the amount of condensate collected from the LFG system is relatively small. The small quantities of condensate get diffused in the leachate and have there constituents mitigated by percolating through the waste.

Environmental engineer Daniel P. Duffy, PE, lives and works in Cincinnati, OH

MSW - March/April 2006