Combining art and science in projecting LFG production
One of the main issues associated with the proper construction, operation, maintenance, and closure of an MSW landfill is how to manage the landfill gas over the operational lifetime of the landfill and throughout its post-closure care period. Small, older landfills and landfills with minimal organic content—particularly those in which periodic burning was part of the operational process—can often get by with just a series of passive vents installed around the landfill’s perimeter to prevent the offsite migration of the LFG. Larger, modern landfills that contain a significantly higher portion of organic materials require active LFG management systems consisting of extraction wells connected by headers to lateral pipelines served by a blower station applying suction to remove the LFG. Once extracted, the LFG can either be destroyed in a flare apparatus, or diverted to some sort of beneficial use, typically conversion to fuel or direct use.
While the engineering of such LFG management systems may be relatively straightforward and the typical operating parameters firmly established, accurately predicting how much LFG a landfill will generate at any particular period of time has proved difficult.
During the early years of the modern (Subtitle D) landfill business, most operators and site engineers designed and laid out their gas systems based on past results and general rules of thumb for anticipating gas generation. Given that each wastestream is unique and the types and kinds of waste can vary seasonally (or even day to day), resulting in different waste compositions throughout the same landfill, this approach was often inadequate. Not being able to predict LFG generation with any certainty resulted in often-expensive refits of existing LFG management systems and repairs of damage to final cover systems caused by unexpected accumulation of LFG pockets. Conversely, operators could spend precious capital on unnecessarily large or extensive systems that operated at minimal efficiencies.
In an attempt to make LFG production estimation more of a science than an art, the EPA’s Landfill Methane Outreach Program (LMOP) created a software program call “LandGEM.” The acronym stands for Landfill Gas Emissions Model. Based on known reaction and decomposition rates, an analyst can input various operational factors (such as landfill operational lifetime or projected annual waste) to generate a projection of LFG production during and after the landfill’s operational lifetime. This article will examine the assumptions and operations of the LandGEM software package and how well it relates to real-world gas production rates.
Waste Composition and Landfill Gas Production
In the real world, according to EPA data, Americans throw away approximately 4.6 pounds of waste per capita per day. With a populating of 300 million, this is equivalent to 690,000 tons per day, or about 250 million to 255 million tons per year. Typically, more than half of all MSW is organic and therefore capable of producing landfill gas. The actual compositing of MSW can vary from location to location, whether the source is rural or urban, which season the waste is collected, and whether the landfill takes in such other wastestreams as organic sludge or inorganic construction and demolition debris. Figure 1 provides a fairly typical breakdown of MSW by weight.
Figure 1 is based on EPA data, with all figures rounded to the nearest percentage. It is the roughly 60% of the wastestream that is organic that produces landfill gas. Again, this amount can vary greatly. In some states, for example, there is a ban on the disposal of yardwaste in landfills. This can greatly reduce the amount of organics available for gas production over time.