by: Christian Williamson, Ph.D.

Measurements of landfill gas (LFG) production are currently (and correctly) regarded by the landfill industry as expensive and unreliable. Hydro Geo Chem, Inc. (HGC) has recently developed a proprietary procedure for measuring LFG production that is not only more accurate than others currently available, but also, in conjunction with appropriate pneumatic tests, quantifies the landfill’s gas permeability and porosity. The data developed can be used to design LFG collection systems that effectively meet regulatory requirements concerning the emission of non-methane organic compounds (NMOC), minimize LFG migration and emission, and maximize the collection of LFG for energy.

Regulations Regarding Control of NMOC Emissions

Depending on whether landfill construction activities occurred before or after May 30, 1991, the NMOC emission of a municipal solid waste landfill whose design capacity exceeds 2.5 million cubic meters is regulated by either the Emission Guidelines (Section 111d) or the New Source Performance Standards (Section 111b) of the Clean Air Act. In either case, an LFG collection system must be installed unless the landfill owner provides an annual report showing that NMOC emission is less than 50 million megagrams per year (Mg/yr). The NMOC emission rate is the product of the average NMOC concentration and yearly LFG production, both estimates obtained according to the 3-Tier EPA guidelines (§60.754). For Tier 1, the least costly alternative, the LFG production rate and NMOC concentrations are based on non-site-specific values. The conservative nature of the Tier 1 process generally results in an unrealistically high estimate for NMOC emission rate. Should it exceed 50 million Mg/yr, the owner must then install an LFG collection system or reevaluate NMOC emissions by conducting a Tier 2 estimate. For Tier 2, NMOC are directly measured,

...the landfill owner usually decides to
install the LFG collection system rather than perform
a Tier 3 estimate of LFG production that can cost
more than $100,000

but LFG production is determined as for Tier 1 and, therefore, results in a conservatively high NMOC emission estimate that may still be highly inflated. Should the estimate still exceed 50 million Mg/yr, the owner is required to install an LFG collection system or conduct a Tier 3 estimate. The Tier 3 estimate is less likely to overestimate NMOC emission because it is derived from site-specific measurements of both NMOC and LFG production. However, the landfill owner usually decides to install the LFG collection system rather than perform a Tier 3 estimate of LFG production that can cost more than $100,000.

HGC’s method is capable of measuring LFG production to meet Tier 3 requirements at much lower cost, enabling the landfill owner to select the LFG collection system on technical merit rather than the risk of financial loss. A second advantage is that the data resulting from the HGC methodology can be used to design a more efficient LFG collection system should one be required.

Optimizing LFG Collection

Engineering design of an LFG collection system ordinarily relies on experience-based “rules of thumb” regarding the number, type, placement, flow rate, and piping layout of LFG recovery wells. Little attention is paid to understanding the rate and distribution of LFG generation over time, although this understanding is critical to optimizing collection system design. The standard engineering approach can be significantly improved by constructing an engineering design model that incorporates the site-specific LFG and permeability data developed by the HGC method. The design can then be improved by interactive adjustment of placement and flow of the wells in the design model. The greatest improvement will likely be realized by reformulating the problem of LFG collection in terms of both pneumatic parameters and engineering costs. These relationships can then be incorporated into an optimization model that automatically adjusts well flow and placement to minimize costs. An advantage of the optimization-model approach is that it can easily be updated with current pneumatic data to provide operational guidance for adjusting LFG collection rates and installing or shutting down gas wells, generators, or flares as the landfill and its LFG production evolve over time.