Glossary

Featured term

Landfill gas (LFG)

Landfill gas is the gas produced as buried organic waste breaks down inside a landfill. Once a section of a landfill is covered and sealed off from the air, bacteria decompose the waste anaerobically, meaning without oxygen, and give off a gas that is roughly half methane and half carbon dioxide by volume. The rest is small amounts of nitrogen, oxygen, and water vapor, plus a fraction of a percent of trace gases, including hydrogen sulfide and a range of volatile organic compounds.

Landfill gas is captured by a network of wells and piping held under vacuum (the gas collection and control system) and is then burned off in a flare, used to make energy, or cleaned into pipeline-quality renewable natural gas. Its parts carry different concerns. Methane is a potent greenhouse gas and, at the right mix with air, an explosion hazard. Hydrogen sulfide and the volatile organic compounds are the main sources of the odor and health concerns raised about a landfill. When waste heats up and begins to oxidize or smolder instead of decomposing normally, the makeup of the gas changes, which is why the gas readings are watched so closely (see ETLF / SET below).

More terms

Capacity: permitted and effective: Permitted capacity is the total landfill space that a state construction permit authorizes to be filled. The state's annual capacity table reports remaining disposal capacity, which Michigan's Part 115 defines as permitted capacity minus the waste already placed in constructed areas that are not yet closed, plus the permitted capacity of cells that are permitted for construction but not yet built. It is usually stated in cubic yards or as years of use at the current fill rate. Effective capacity is not a term defined in Part 115; it is how Wayne County's July 26, 2021 memo on the Riverview Land Preserve expansion request described a broader planning figure: permitted capacity, plus space that is sited but not yet permitted, plus access to space in other counties under reciprocal import and export agreements. That memo reported that Wayne County's four landfills together had about 11.2 years of permitted capacity remaining, and "an effective capacity well in excess of 25 years."
Carbon dioxide (CO₂): One of the two main components of landfill gas, roughly half by volume. A carbon-dioxide share that rises relative to methane (a falling methane-to-carbon-dioxide ratio) can be a sign of subsurface oxidation rather than normal decomposition.
Carbon monoxide (CO): A trace gas that rises when waste is oxidizing or smoldering rather than decomposing normally. Together with hydrogen and oxygen, it is used as an indicator of an elevated-temperature event. A field reading is a screening tool; because portable field equipment is only qualitative and prone to cross-sensitivity that can give artificially high or false-positive readings, a laboratory sample is what confirms it.
Consent Judgment / Consent Order / Consent Decree: Legal settlements of environmental enforcement actions. A Consent Judgment or Consent Decree is entered through a court; a Consent Order is issued by an agency. Each can require the operator to pay a penalty and carry out specific corrective work.
Cubic yards (CY) and tons: Landfill waste is measured two ways. Cubic yards measure volume, the space waste takes up, and are the unit for landfill capacity. Tons measure weight. Michigan landfills reported the waste they received in cubic yards until March 29, 2023, and in tons since then; remaining disposal capacity is still reported, by the landfills themselves, in cubic yards. The state (EGLE) converts between the two with fixed factors, in cubic yards per ton: household and commercial waste, 3; construction and demolition debris, 2; industrial waste, contaminated soil, and most daily cover material, 1; auto-shredder residue, 3; chipped tires, 2. By the state's conversion factors, household and commercial waste accounts for a larger share of estimated landfill volume than of tonnage.
ETLF / SET: Elevated Temperature Landfill / Subsurface Temperature Event: A landfill, or an area within one, that is heating abnormally because buried waste is oxidizing or smoldering instead of decomposing biologically. Warning signs include rising temperature, falling methane, rising oxygen, hydrogen, and carbon monoxide, and a falling methane-to-carbon-dioxide ratio.
GCCS: Gas Collection and Control System: The network of extraction wells, piping, blowers or vacuum, and flares or energy devices that captures landfill gas and destroys or uses it instead of letting it escape into the air.
HOV: Higher Operating Value: A well-specific waiver the state (EGLE) can grant that allows a single gas-collection well to run hotter than the normal temperature limit.
Hydrogen sulfide (H₂S): A trace gas in landfill gas with a rotten-egg odor, detectable by smell at very low concentrations and toxic at higher ones. For the Arbor Hills Landfill, the 2022 Consent Judgment sets a fence-line action level of 30 parts per billion on a 15-minute rolling average.
Leachate: The liquid that forms as rain and moisture percolate down through buried waste, picking up dissolved and suspended contaminants along the way. It is collected and treated before it is discharged.
Methane (CH₄): The main combustible part of landfill gas, roughly half of it by volume. It is a potent greenhouse gas and is explosive when it reaches 5 to 15 percent in air. The federal surface standard for landfills is a reading of 500 parts per million of methane above background at the ground surface.
MMP: Materials Management Plan: The county-level solid-waste plan, required under Michigan's Part 115, through which Washtenaw County decides how waste is managed and whether a landfill may expand.
NESHAP / NSPS: The federal Clean Air Act standards that govern landfill emissions: the National Emission Standards for Hazardous Air Pollutants and the New Source Performance Standards. They set the gas-collection, monitoring, and surface-emission requirements a landfill must meet.
PFAS: Per- and polyfluoroalkyl substances, a large family of long-lasting synthetic chemicals often called "forever chemicals" and found in many everyday products. They enter landfills in ordinary waste and can turn up in leachate and stormwater.
RNG: Renewable Natural Gas: Landfill gas that has been cleaned and upgraded to pipeline quality so it can be injected into the natural-gas system.
SEM: Surface Emission Monitoring: Walking the landfill surface with a methane detector to find leaks. Under the federal standard, a reading of 500 parts per million of methane above background is an exceedance that starts a corrective-action clock.
Violation Notice (VN): A formal notice from a regulator, here usually the Michigan Department of Environment, Great Lakes, and Energy (EGLE), that a facility has violated a rule or permit condition. It can lead to further enforcement.
The three "Arbor Hills" facilities: Three separately regulated operations share the Arbor Hills name and site: the Arbor Hills Landfill (state ID N2688), the landfill itself; Arbor Hills Energy (N1504), the adjacent gas-to-energy power plant, which is a legally separate company; and Emerald RNG (P1488), the facility that processes landfill gas into renewable natural gas. Because they are regulated separately, a violation or a project at one is not the same as one at another.

See also the frequently asked questions and the full monitor.