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Landfill Gases: Explosive and Toxic

The Ecology Club at Pattonville High School in Maryland Heights, Missouri, thought it would be a great idea to become the first school in the nation to use landfill gas as "fuel" to heat their school. The gas would come from the landfill located next to the school and travel through 3,600 feet of pipeline to the school's boilers, where it would be burned to heat the school's 117 classrooms and two gymnasiums. Everyone was behind the plan until someone contacted Ralph Nader, and then CHEJ, to get the other side of the story. As it turned out, no one, including the EPA who was supporting the project, had told the students or school administrators anything about the presence of toxic chemicals in the landfill gas or the possible formation of products of incomplete combustion (PICs) in the boiler.  PICs are chemicals, such as dioxins, which are not present in the landfill gas,  but are formed when the gas is burned.  These chemicals would be released in the stack emissions from the boiler. The students and school administrators were shocked to find out there were real risks to this "ideal" project. Both are rethinking the original plan and wondering why all of these issues were not disclosed up front.

Similarly, in Pottstown, Pennsylvania, residents are wondering about a plan to pipe landfill gas through their town from the local landfill to an industrial plant that wants to burn the gas in their boiler. Again, no one told the community anything about the presence of toxic chemicals in the landfill gas or about the possible formation of PICs which would be released in the stack emissions from the boiler. Armed with the truth, the community is now trying to get a true sense of the risks posed by this proposal.

The situations in Maryland Heights and Pottstown are turning out to be quite common as landfill operators attempt to meet irrational federal requirements passed in 1996 to control landfill air emissions. The regulations encourage the reuse of landfill gas as a way of controlling air emissions.  Most landfills - all those started after May 1991 and those generating more than 50,000 kilograms per year of non-methane organic compounds (NMOCs) - must install a gas collection system and gas utilization or disposal system that achieves 98 percent reduction of collected NMOC emissions.

As a result of the new regulations, many people are learning for the first time that municipal solid waste landfills not only generate a toxic leachate, but also gases that can contain toxic chemicals.

The amount of gas generated in a landfill depends on the composition, moisture content and age of the garbage, and on the landfill's  temperature and pH, or acid content. The most common gas generated in landfills is methane. Methane is generated by biological decay of organic matter found in the waste. The more organic matter, the more decay and the more methane produced. Typically, about 50% of the gas is methane with about 40% carbon dioxide, nine percent nitrogen and about one percent NMOCs. Although methane is generally not considered a toxic substance, when mixed with air at five to fifteen percent by volume, it can be highly explosive if ignited.

Methane leaves a landfill by underground migration and accumulates in basements and other structures where it can be contained. Methane levels can then build up to a point such that any spark will cause an explosion. According to an EPA report, more than 40 explosions and fires have been documented as a result of methane migration from landfill sites, some of which have caused injuries. For example, in 1969 in Winston-Salem, North Carolina, three people were killed and five seriously injured when a gas explosion occurred in an armory located next to a landfill. More recently, in West Covina, California in 1984, twenty families were temporarily evacuated because explosive levels of methane were found to be migrating from the nearby BKK landfill.

Explosions are not the only concern with landfill gases. There are also many non methane organic compounds (NMOCs). EPA has measured over 100 of these chemicals in landfill gases, with total NMOC levels ranging from 237 to 14,294 parts per million (ppm). About 85% of the NMOCs are volatile organic chemicals (VOCs) such as benzene, vinyl chloride, dichloroethane and trichloroethene. Other organic constituents include hydrogen sulfide, mercaptans and other sulfides which contribute to odor problems at landfills.

Volatile organic compounds are present in landfills primarily because of small quantity generators that legally dump toxic waste such as cleaning solvents, paints, and pesticides in with household garbage. Typical small quantity generators include businesses such as dry cleaners, photo development shops, printers and jewelers. Household hazardous waste also contributes a small portion of toxics and VOCs found in landfills.

In a study of 46 garbage landfills, EPA found that benzene, trichloroethene and vinyl chloride were found in almost all of the landfills. Table l shows common volatile organic chemicals found in landfill gases.

Volatile organic compounds are significant because of their potential toxicity. Many such as benzene and vinyl chloride are carcinogenic. Most VOCs attack the liver and central nervous system. Some, like toluene, can cause reproductive problems.

The amount of VOCs present in landfill gas depends primarily on the amount of these chemicals dumped in the landfill. In addition, the natural activity of bacteria and microorganisms can break down some larger chemicals into smaller ones. For example, vinyl chloride can be generated as a by-product of the decomposition of trichloroethene and dichloroethene.

The movement of landfill gases is similar to groundwater movement. Like groundwater, gases travel through the path of least resistance by moving through air spaces in the soil. How quickly this occurs depends largely on the type of soil and the pressure the gas is under. The more porous the soil (such as sand or uncompacted soil), the quicker the gases move. Pressure can be created by chemical reactions, weight and settlement of landfill materials, water movement and changes in atmospheric pressure.

Control of Landfill Gases

Landfill gases are collected by either active or passive methods. Active measures use vertical wells or trenches to collect gas, and blowers or compressors to move the gas to treatment devices. Extraction wells can also be used to "pull" gases out of the ground and deliver them by pipe to the treatment devices. Passive measures use natural pressure gradients to capture gas and move it by pipe to treatment devices. These are often used with plastic-lined landfills.

Once collected, landfill gases are "treated" either by burning them in a flare or passing them through some type of absorbing material. The most common treatment device is the flare, which burns the gases in an open flame. This process does produce toxic emissions, although there has been little testing of flare gases to document this problem. Flares are "expected" by EPA regulation to destroy 98% of the chemicals present in the gas. However, because it is near impossible to measure the emissions from open flares, the landfill industry is not asked to show this by testing. Thus, there is no way to verify if the 98% destruction level has, in fact, been achieved. Instead, the regulations define specific control settings (performance standards) for the flares that are intended to achieve 98% destruction.

These control settings do not address the formation of new chemicals, products of incomplete combustion (PICs) such as dioxins and furans which result when landfill gases are burned in flares. Conditions for PIC formation are ideal in open flares because there is a lot of oxygen, good mixing, many chlorinated chemicals and a short residence time. Because of this, flares may be the worst possible way to "dispose" of these gases.

Some flares can be enclosed. In this system, a cylinder surrounds the flare, extending above the top of the flame. The top remains open to the air. One advantage to this system is that emissions testing can be done by sampling the air within the cylinder. For example, tests at enclosed flares in California consistently showed 98% destruction of NMOCs. It is unclear, however, if any testing for PICs was done as part of these studies.

Another common way to treat landfill gas is to utilize the gas to make electricity, rather than burn it in a flare. This approach generally calls for first removing NMOCs by filtering the gas with carbon or other absorbing material and then using it in energy recovery. These energy recovery techniques include gas turbines, combustion engines, and boiler-to-steam turbine systems, all of which generate electricity from the combustion of landfill gas.

EPA doesn't usually go out of its way to help communities meet their standards, but they are so excited about this landfill gas-to-energy idea that they've started the "Landfill Methane Outreach Program." This program aggressively advocates the use of landfill gas to generate electricity. The agency has written a how-to handbook and brochures and makes its staff available to help convince businesses to get involved with this program.

The problem is that the presence and potential dangers of volatile organic chemicals in landfill gas have become lost in the enthusiasm to sell the program. Nowhere in the how-to handbook or brochure is there any mention of VOCs or NMOCs. Nowhere does EPA address the need to filter the landfill gas before using it. This is a problem that cannot be ignored.

In summary, landfill gases are more than methane. The real issue is the presence of volatile organic chemicals, including many toxic chemicals, which cannot be ignored. Landfill gases are a real health threat that need to be properly addressed. The disposal and use of landfill gases in energy recovery projects needs to be carefully monitored, especially since the EPA is aggressively supporting its use without disclosing the presence and potential dangers of VOCs. There may be situations where gas recovery projects at garbage landfills work, but the only way to know is to properly test for VOCs and remove them before the landfill gas is used.

Originally published in Everyone's Backyard, Vol. 16, No. 1 (Spring 1998).