Frequently Asked Questions

Landfill gas is formed when buried solid waste decomposes. This process creates gases, the most common of which are methane (CH4) and carbon dioxide (CO2), which typically combine to make up more than 95% of the landfill gas. While these two gases are odorless, the unpleasant aromas are caused by the small percentage of other gases. The most common odors from the landfill are caused by hydrogen sulfide (H2S) and volatile organic compounds (VOCs). Hydrogen sulfide is most commonly recognized as a strong "rotten egg" smell. The smell of volatile organics can vary but typically have a somewhat sweetly pungent smell.

We currently operate 310 collection wells to capture landfill gas before it leaves the landfill, and have two mobile odor management systems located in the vicinity of the active disposal area. This is significantly above and beyond standard industry best management practices. Collected gas is routed to an on-site facility that cleans it to pipeline quality natural gas where it is sold as renewable energy.

Every day, employees spread and compact the solid waste in the active area of the landfill as it arrives. At the end of each day, it is then covered with a minimum 6-inch thick layer of clean soil, or TCEQ-approved alternate daily cover. This cover helps to control the odor, prevent blowing litter around, prevent fires, and ward off scavenging animals.

Odor patrols assess the presence of odors from 16 different locations in Melissa each day the landfill operates. The landfill surface is walked and inspected for excessive release of landfill gas by a third party on a quarterly basis.

The migration of landfill gas depends on a variety of atmospheric conditions:

• Barometric Pressure/Rainfall
  On days when the barometric pressure is low or dropping, the gas pressure inside the landfill will be higher than the atmosphere. This causes the landfill gas to shift from the landfill to balance with the atmosphere. Barometric pressure drops are often associated with rainfall which is why odors may be noticeable before and after rainfall events.
• Temperatures
  During colder months, temperature inversions may occur where warm air is trapped above cold air closer to the ground. This slows the normal rise of gas causing concentrations closer to the ground thereby creating a stronger smell.
• Wind
  Wind helps spread any escaped gas and lessen their concentration, but days with little wind mean that odors may be more noticeable.
• Moon Phases
  Certain moon phases also contribute to gas migration, similar to the influence the lunar pull has on tides. The pull of a New Moon or Full Moon can make it easier for landfill gas to migrate from the landfill. On a cold, damp, cloudy, windless day with low barometric pressure, the landfill gas and its associated odors will be hardest to control.

Even when odors are strongest, the air’s circulation significantly reduces the concentration of the gas to levels far below any that could be harmful by the time the smell is noticeable.

Email Jerry Zumwalt, NTMWD Landfill Manager or call 972-626-4456 or 972-442-5405 for odor complaints so they may be investigated. This phone number is always staffed.