Nicholas P. Cheremisinoff. Handbook of Solid Waste Management and Waste Minimization Technologies

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THE COMPOSITION OF LANDFILL GAS

Landfill gas is usually referred to as natural gas or methane; however there are

notable distinctions among these three terms. In addition, an often-used term

landfill methane is deceiving as it implies that landfill gas is simply methane.

Methane is a hydrocarbon gas whose chemical formula is CH

. It is a greenhouse

gas and it is explosive. It is generated by the decomposition of organic matter.

Natural gas is approximately 80 to 99% methane, with the remainder being

mostly other hydrocarbons (ethane, propane, butane, etc.) as well as some

nitrogen, oxygen, water, CO

, sulfur, and various contaminants.

Table 1. Composition of Landfill Gas

Constituent Gas

Methane (CH

)

Carbon dioxide (CO

)

Nitrogen (N

)

Oxygen (O

)

Hydrogen sulfide (H

Halides

Water vapor (H

Nonmethane organic

compounds (NMOCs)

Concentration in LFG

Range

35 to 60%

35 to 55%

O to 20%

O to 2.5%

1 to 1,700 ppmv

1 to 10%

237 to 14,294 ppmv

Average

50%

45%

<1%

21 ppmv

132 ppmv

2,700 ppmv

NA, Not available; ppmv, parts per million by volume.

Note: Highest values occur in perimeter wells.

Sources: GJ. Sandelli, Demonstration of Fuel Cells to Recover Energy from

Landfill Gas. Phase I Final Report: Conceptual Study, EPA-600-R-92-007,

prepared for the U.S. Environmental Protection Agency by International Fuel

Cells Corporation (Washington, D.C, January 1992); M. Doom, J. Pacey,

and D. Augenstein, Landfill Gas Energy Utilization Experience: Discussion of

Technical and Non-Technical Issues, Solutions, and Trends, EPA-600/R-95-

035,

prepared for the Air and Energy Engineering Research Laboratory,

USEPA by E.H. Pechan and Associates, Inc. (Washington, D.C, March

1995).

Table 1 provides a typical compositional breakdown of landfill gas. Landfill gas

is approximately 50% methane. The remainder of landfill gas is mostly carbon

dioxide with varying amounts of nitrogen, oxygen and assorted contaminants

known as nonmethane organic compounds or NMOCs. NMOCs usually make up

less than 1% of landfill gas. Many of these are toxic chemicals, such as benzene,

toluene, chloroform, vinyl chloride, carbon tetrachloride, and 1,1,1

trichloroethane. At least 41 of these are halogenated compounds. Many others

are nonhalogenated toxic chemicals. When halogenated chemicals (chemicals

containing halogens - typically chlorine, fluorine, or bromine) are combusted in

the presence of hydrocarbons, they can recombine into highly toxic compounds

such as dioxins and furans, which are extremely toxic and pose long-term health

risks even from low levels of exposure. Burning at high temperatures does not

eradicate or minimize the problem as dioxins are formed at low temperatures and

can be formed as the gases are cooling down after the combustion process.

Bacteria decompose landfill waste in four phases. The composition of the gas

produced changes with each of the four phases of decomposition. Landfills often

accept waste over a 20- to 30-year period, so waste in a landfill may be

undergoing several phases of decomposition at once. This means that older waste

in one area might be in a different phase of decomposition than more recently

buried waste in another area. During the first phase of decomposition, aerobic

bacteria — bacteria that live only in the presence of oxygen — consume oxygen

while breaking down the long molecular chains of complex carbohydrates,

proteins, and lipids that make up organic waste. The primary by-product of this

process is carbon dioxide. Nitrogen content is high at the beginning of this phase,

but declines as the landfill moves through the four phases. Phase I continues until

available oxygen is depleted. Phase I decomposition can last for days or months,

depending on how much oxygen is present when the waste is disposed of in the

landfill. Oxygen levels will vary according to factors such as how loose or

compressed the waste was when it was buried.

Phase II decomposition starts after the oxygen in the landfill has been used up.

Using an anaerobic process (a process that does not require oxygen), bacteria

convert compounds created by aerobic bacteria into acetic, lactic, and formic

acids and alcohols such as methanol and ethanol. The landfill becomes highly

acidic. As the acids mix with the moisture present in the landfill, they cause

certain nutrients to dissolve, making nitrogen and phosphorus available to the

increasingly diverse species of bacteria in the landfill. The gaseous byproducts of

these processes are carbon dioxide and hydrogen. If the landfill is disturbed or if

oxygen is somehow introduced into the landfill, microbial processes will return to

Phase I.

Phase III decomposition starts when certain kinds of anaerobic bacteria consume

the organic acids produced in Phase II and form acetate, an organic acid. This

process causes the landfill to become a more neutral environment in which

methane-producing bacteria begin to establish themselves. Methane-and acid-

producing bacteria have a symbiotic, or mutually beneficial, relationship. Acid-

producing bacteria create compounds for the methanogenic bacteria to consume.

Methanogenic bacteria consume the carbon dioxide and acetate, too much of

which would be toxic to the acid-producing bacteria.

Phase IV decomposition begins when both the composition and production rates

of landfill gas remain relatively constant. Phase IV landfill gas usually contains

approximately 45% to 60% methane by volume, 40% to 60% carbon dioxide,

and 2% to 9% other gases, such as sulfides. Gas is produced at a stable rate in

Phase IV, typically for about 20 years; however, gas will continue to be emitted

for 50 or more years after the waste is placed in the landfill. Gas production

might last longer, for example, if greater amounts of organics are present in the

waste, such as at a landfill receiving higher than average amounts of domestic

animal waste.

The rate and volume of landfill gas produced at a specific site depend on the

characteristics of the waste (e.g., composition and age of the refuse) and a

number of environmental factors (e.g., the presence of oxygen in the landfill,

moisture content, and temperature). The more organic waste present in a landfill,

the more landfill gas (e.g., carbon dioxide, methane, nitrogen, and hydrogen

sulfide) is produced by the bacteria during decomposition. The more chemicals

disposed of in the landfill, the more likely NMOCs and other gases will be

produced either through volatilization or chemical reactions. Generally, more

recently buried waste (i.e., waste buried less than 10 years) produces more

landfill gas through bacterial decomposition, volatilization, and chemical

reactions than does older waste (buried more than 10 years). Peak gas production

usually occurs from 5 to 7 years after the waste is buried. Methane will be

produced only when oxygen is no longer present in the landfill. The presence of

moisture (unsaturated conditions) in a landfill increases gas production because it

encourages bacterial decomposition. Moisture may also promote chemical

reactions that produce gases. As the landfill's temperature rises, bacterial activity

increases, resulting in increased gas production. Increased temperature may also

increase rates of volatilization and chemical reactions. The box on the following

page provides more detailed information about how these variables affect the rate

and volume of landfill gas production.

LANDFILL DESIGN CONSIDERATIONS

Landfill design is communicated to regulatory officials through several

documents. The landfill design package should include plans, specifications, a

design report, and an operator's manual - all of which are submitted to

regulatory agencies. A general cost estimate also should be developed, even

though it may be a rough estimate.

Plans and specifications typically include:

• A base map showing existing site conditions with contour intervals of 1

foot to 5 feet, and a scale of 1 inch equal to 50 feet to 1 inch equal to

200 feet

• A site preparation plan designing fill and stockpile areas, and site

facilities

• A development plan showing initial excavated and final completed

contours in filling areas

• Construction details illustrating site facilities

• Cross sections illustrating phased development of the landfill at several

interim points

• A completed site plan including final site landscaping and other

improvements

Additionally, a design report typically includes four major sections:

A site description, which includes existing site size, topography, slopes,

surface water, utilities, roads, structures, land use, soil, groundwater,

exploration data, bedrock and climatological information

Design criteria, which include solid waste types, volumes, and fill-area

dimensions, and all calculations

Operational procedures, which include site preparations, solid waste

unloading, handling and covering, as well as equipment and personnel

requirements

Environmental safeguards, including the control of leachate, surface

water, gas, blowing paper, odor and vectors

Waste characteristics are also critical to the design and operation of the gas

recovery system of the landfill. These will provide important design information

for determining operating procedures. Waste type affects the handling

techniques, and waste quantity determines site lifetime, daily operating

procedures, and cover requirements. A waste characterization study should

precede landfill siting work, but additional information may be needed while the

facility is being designed. For example, certain waste types may be used as daily

cover or on-site road base. When preparing a profile of the wastes that will be

received at the new landfill, pay attention to sources that may be unknowingly

mixing hazardous waste with solid waste. In suspicious cases, hazardous waste

testing procedures may be necessary. Systematic load checking during site

operation also should be planned. The types and number of vehicles that transport

solid waste to the site should be tabulated, too. Traffic information will be useful

for later analysis of roadways and access points.

The design basis is a tabulation of the general performance requirements that the

facility must satisfy to achieve project goals. It includes the facility's capacity,

waste flow rates, traffic counts and principal environmental controls. Tabulating

the design basis in this manner communicates to the project design team and

others, such as regulatory review specialists, about the nature and size of the

proposed landfill. The design basis may require revisions if unforeseen

circumstances cause significant changes in the landfill plan.

SITE LAYOUT CONSIDERATIONS

The landfill's layout will be strongly influenced by the site's geology. The

potential for gas and leachate migration and the suitability of the soil for landfill

base and cover material should be a particular concern. The site layout begins

with geotechnical information, which includes data on the surrounding site

geology, hydrology, and soils. These data usually are collected during the site

selection process, then supplemented during subsequent investigation. Soil-boring

logs and other data describing subsurface formations and groundwater conditions

are diagrammed to present an interpretation of the subsurface conditions at the

planned. Soil-boring logs help to show the extent of each formation extrapolated

between the boreholes. The depths to bedrock and the groundwater table also are

shown. Many more boring logs and additional cross sections at regular

coordinate intervals in several (minimum of two) directions typically are required

to properly locate the waste disposal area within the developing site.

The base map usually shows the landfill location in relation to surrounding

communities, roads and other features. A site map shows:

• Contour lines drawn at 2- or 5-foot intervals

• Clearly delineated property lines

• Easements and rights-of-ways

• Utility corridors, buildings, wells, roads and other features

• Drainage ways

• Neighboring property ownership and land uses

Contour maps show drainage patterns adjacent to and through possible disposal

sites.

Areas with excessive slope or direct overland flow from a site to surface

waters must be carefully evaluated. Subsurface formations and groundwater

conditions will influence the landfill's design features in the leachate collection

system and liner requirements. A formation's geotechnical characteristics will

determine its suitability as a construction material.

Site plans should describe landfill development phases in chronological order.

Landfills usually are planned to be developed, constructed, and operated in

phases of 1 to 2 years each. Dividing the project into phases minimizes the

amount of open landfill surface and reduces the potential for precipitation to

accumulate. As each phase is completed, that landfill portion can be closed and

final cover material can be placed over the waste. Another phasing advantage is

that it makes premature landfill closure, in the event of an environmental

problem, more practical and economical. In a well-planned development, the

landfill's end-use can be implemented in the completed sections while other areas

remain open for disposal.

Concurrent with plan development for liners, covers, service roads and

embankments, soil cut-and-fill balances must be calculated. The best designs

minimize soil transfer at the site. Substantial volumes of earth will be required

for cover material and possibly for liners. Some regulatory agencies mandate

screening berms or fence construction around the landfill's active areas. The

extra soil needed for berm construction must be accounted for when planning

excavation work. The berm height will depend on the sight lines into the landfill

from adjacent areas.

The site plan should chronologically illustrate the developing landfill's features.

The landfill's enduse can begin on completed sections while other areas in the

landfill still are being used for disposal. Phasing diagrams show the landfill's

evolution at different stages through the site's life. Phases should be developed

for key times in sufficient detail to ensure that the operator knows what is to be

done at any point. The engineers and management must be assured that the site is

proceeding according to plan so that contracts can be let or finances arranged for

construction. Regulatory bodies also must be assured that landfill operators are

following the plan and that the site will be completed as designed at the agreed-

upon time. The dimensions of each phase are determined by several factors.

Generally, each phase accommodates 2 to 3 years of refuse volume.

DESIGN PHASES

When practical, design phases should be laid out so that excavated soil is

immediately used. When stockpiling is necessary, the work should be organized

so that stockpiled soil is undisturbed until needed or used to surcharge completed

areas.

Stockpiled soil should be covered whenever possible to prevent wind and

precipitation erosion. After completing the phasing diagrams and earthwork

balances, a table should be prepared to summarize the waste disposal and earth

volumes that will be contained within each landfill phase.

The operating plan should detail the configuration of the landfill's working face

and helps to define terms. The "working face" is the area presently being

worked, with new refuse being deposited and compacted into it. Once the

working face has been completed and daily cover material provided, it is a

completed cell, or "daily cell." A "lift" is composed of the adjacent daily cells

that form one layer of the landfill. Lift thicknesses generally are 8 to 20 feet.

Larger landfills that accept more refuse per day have higher lift thicknesses.

"Daily cover material" is applied over the working face and can extend over the

horizontal surface at the top of each daily cell, depending on how long the cover

will be exposed to the environment. If the landfill is not expected to receive

additional wastes, closure must begin within 30 days of the final receipt of waste.

This closure requirement ensures that a proper cover is installed at the landfill.

The minimum width of the working face or daily cell should be at least wide

enough to accommodate as many trucks or vehicles as are expected to be at the

landfill. Typically, 10 to 15 feet per truck or vehicle is used in designing.

Clearly, it is not a good operating practice to have extremely wide working faces

to accommodate the peak flow of trucks that may occur once or twice a day. A

trade-off must be made between the width of the working face and the area

needed to queue vehicles entering the site during peak hours. The working face

should remain as small as possible to avoid attracting birds and creating visual

problems for passersby, and to contain blowing paper. Keeping freshly deposited

refuse in a well-defined and small working face is a good indication of a well-

operated landfill.

SANITARY LANDFILL DESIGN STEPS

The following is a summary of the steps generally applied to sanitary landfill

designs (Source: Conrad et al., 1981 with additions by the authors, Solid Waste

Landfill Design and Operation Practices, EPA Draft Report, 1981):

Determine solid waste quantities and characteristics:

a. Existing

Projected

Compile information for potential sites:

a. Performance of boundary and topographic surveys

Preparation of base maps of existing conditions on and near

sites (property boundaries, topography and slopes, surface

water, wetlands, utilities roads, structures, residences, and land

use).

c. Compile hydrogeological information and preparation of

location map: Soils (depth, texture, structure, bulk density,

porosity, permeability, moisture, ease of excavation, stability,

pH,

and CATION exchange capacity), bedrock (depth, type,

presence of fractures, and location of surface outcrops),

groundwater (average depth, seasonal fluctuations, hydraulic

gradient and direction of flow, rate of flow, quality, and uses).

d. Compile climatological data: Precipitation, evaporation,

temperature, number of freezing days, and wind direction.

e. Identify regulations (federal, state, local) and design standards:

Loading rates, frequency of cover, distances to residences,

roads,

surface water and airports, monitoring, groundwater

quality standards, roads, building codes, and contents of

application for permit.

Design of filling area

a. Select landfilling method based on site topography, site soils,

site bedrock, and site groundwater.

Specify design dimensions: Cell width, depth, length, fill depth,

liner thickness, interim cover soil thickness, and final soil cover

thickness.

c. Specify operational features: Use of cover soil, method of cover

application, need for imported soil, equipment requirements,

and personnel requirements.

Design features:

a. Leachate controls

Gas controls

c. Surface water controls

d. Access roads

e. Special working areas

f. Special waste handling

g. Structure utilities

h. Recycling dropoff

i. Fencing

Lighting

k. Washracks

Monitoring wells

m. Landscaping

Prepare design package:

a. Develop preliminary site plan of fill areas.

Develop landfill contour plans: Excavation plans (including

benches), sequential fill plans, completed fill plans, and fire,

litter, vector, odor and noise controls.

c. Compute solid waste storage volume, soil requirement volumes,

and site life.

d. Develop final site plan showing normal fill areas, special

working areas, leachate controls, gas controls, surface water

controls, access roads, structures, utilities, fencing, lighting,

washracks, monitoring wells, landscaping.

e. Prepare elevation plans with cross-sections of excavated fill,

completed fill, phase development of fill at interim points.

f. Preparation of construction details: Leachate controls, gas

controls, surface water controls, access roads, structures,

monitoring wells.

g. Prepare ultimate land use plan,

h. Prepare cost estimate.

i. Prepare design report.

Prepare environmental impact assessment.

k. Submit application and obtaining required permits.

Prepare operator's manual.

GAS CONTROL

Once gases are produced under the landfill surface, they generally move away

from the landfill. Gases tend to expand and fill the available space, so that they

move, or "migrate," through the limited pore spaces within the refuse and soils

covering the landfill. The natural tendency of landfill gases that are lighter than

air, such as methane, is to move upward, usually through the landfill surface.

Upward movement of landfill gas can be inhibited by densely compacted waste or

landfill cover material (e.g., by daily soil cover and caps). When upward

movement is inhibited, the gas tends to migrate horizontally to other areas within

the landfill or to areas outside the landfill, where it can resume its upward path.

Basically, the gases follow the path of least resistance. Some gases, such as

carbon dioxide, are denser than air and will collect in subsurface areas, such as

utility corridors. Three main factors influence the migration of landfill gases:

diffusion (concentration), pressure, and permeability. Gases can travel off-site

and into neighboring buildings, posing indoor air quality threats such as odors or

exposures to inhalation hazards, or even fire and explosion. For these reasons gas

control is needed.

Landfills install gas control measures because of regulatory requirements.

Regulations have been developed to reduce health and environmental impacts

from landfill gas emissions through the reduction of ozone precursors (volatile

organic compounds and nitrogen oxides), methane, NMOCs, and odorous

compounds. States may also have state specific landfill regulations, which must

be as strict as or stricter than the federal regulations. Odor complaints or

potential safety and health concerns may also prompt landfill gas collection.

Sulfide emissions are a common source of landfill odor complaints. At older

landfills or at smaller landfills exempt from federal and state regulations,

uncontrolled releases of landfill gases can pose potential safety and health

concerns (e.g., explosion hazards). In such cases, the landfill might implement

landfill gas control measures, even if they are not required by federal or state

regulations. Some landfills have also implemented voluntary gas collection and

control or treatment systems to recover landfill gas for energy production.

The primary objective of a landfill gas control plan is to prevent people from

being exposed to landfill gas emissions. This goal can be achieved either by

collecting and treating landfill gas at the landfill or by preventing landfill gas

from entering buildings and homes in the community. Technologies used to

control landfill gas at the landfill or in the community can be applied separately

or in combination. Landfill gas can be collected by either a passive or an active

collection system. A typical collection system, either passive or active, is

composed of a series of gas collection wells placed throughout the landfill. The

number and spacing of the wells depend on landfill-specific characteristics, such

as waste volume, density, depth, and area. As gas is generated in the landfill, the

collection wells offer preferred pathways for gas migration. Most collection

systems are designed with a degree of redundancy to ensure continued operation

and protect against system failure. Redundancy in a system may include extra gas

collection wells in case one well fails.

Passive gas collection systems use existing variations in landfill pressure and gas

concentrations to vent landfill gas into the atmosphere or a control system. Figure

1 illustrates the main design features. Passive collection systems can be installed

during active operation of a landfill or after closure. Passive systems use

collection wells, also referred to as extraction wells, to collect landfill gas. The

collection wells are typically constructed of perforated or slotted plastic and are

installed vertically throughout the landfill to depths ranging from 50% to 90% of

the waste thickness. If groundwater is encountered within the waste, wells end at

the groundwater table. Vertical wells are typically installed after the landfill, or a

portion of a landfill, has been closed. A passive collection system may also

include horizontal wells located below the ground surface to serve as conduits for

gas movement within the landfill. Horizontal wells may be appropriate for

landfills that need to recover gas promptly (e.g., landfills with subsurface gas

migration problems), for deep landfills, or for active landfills.

Figure 1. Passive gas collection system.

PERFORATED OR

SLOTTED PLASTIC

GROUND SURFACE

COLLECTION

WELL

LANDFILL

LINERS

LANDFILL WASTE