Advancements in bioreactor techniques call for similar improvements in modeling skills.

By Victor O. Okereke

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The potential utilization of landfill gas for the generation of electricity and other forms of energy has been the subject of great interest during the past decade. The economic viability of a landfill gas-to-energy (LFGTE) project is mostly dependent on a sustainable level of gas production over the project life. A typical LFGTE project life is 20 years, which is long after a landfill cell has ceased operating and gas production in the cell has declined significantly It has been well documented by many studies, including Pohland (1975), that optimum moisture content and the effectiveness its distribution in the refuse matrix has the most impact on gas production and that the artificial addition of moisture to landfills through leachate recirculation increases gas production rates. Therefore, the retention and distribution of optimum moisture contents in the landfill through the duration of an LFGTE project might improve the economic viability of the project. Additional effects of increased moisture content include increases or decreases in leachate generation and accelerated refuse stabilization rates that might help extend landfill life and reduce postclosure liability costs.

Significant monetary and other resources continue to be invested in pilot and full-scale bioreactor studies. A bioreactor, when fully operational, will provide a dynamic system for optimum utilization of the beneficial effects of leachate recirculation. The Solid Waste Association of North America defines a bioreactor as “any permitted Subtitle D landfill or landfill cell where liquid or air is injected in a controlled fashion into waste mass in order to accelerate or enhance biostabilization of the waste.” This technology relies on liquids’ addition to achieve optimal moisture content of approximately 40-45% (McManus, 2002). According to a United States Department of Energy survey, LFGTE projects currently utilize about 10% of the potential LFG in the US, and it estimates that the application of the controlled bioreactor technology to 50% of the waste currently being landfilled could provide more than 270 billion ft.3 of methane gas per year to meet about 1% of the electrical energy needs in the US (McManus, 2002).

The full utilization of the benefits associated with bioreactors is currently limited due to the difficulty in determining the quantitative relationships between the key variables that influence gas and leachate production, optimum operating conditions, and the variables that control the biochemical processes in landfills. Although gas and leachate are produced simultaneously during biochemical degradation reactions and moisture redistribution in landfills and, in addition, the management of leachate account for a significant portion of landfill operating costs, the planning of LFGTE projects mostly rely on gas projections with gas-only simulation models. Opportunities for applying controlled bioreactor technology or leachate recirculation are not commonly considered during process planning and design of LFGTE projects. This model is designed to allow for the cost-effective assessment of this option.

The model discussed in this paper is an efficient tool for the concurrent simulation of leachate and gas. The model is significant as a cost-effective tool for evaluating the economic viability of LFGTE projects with the application of leachate recirculation; determining the optimum operating conditions of a leachate recirculation facility or bioreactor, which enhances the economic balance sheet of an LFGTE project; and the assessment of the range of economic benefits attainable for a broad range of leachate recirculation rates or economic conditions.

Modeling Approach

The model is a one-dimensional, quasi-two-phase continuous simulation model and is divided into two principal submodels for engineering and economic analysis. The engineering submodel is composed of four modules for simulating climatological conditions, leachate and gas generation, and leachate recirculation. The second submodel performs the economic feasibility analysis for a specified LFGTE project with the utilization of variable leachate recirculation regimes.

The growing acceptance of bioreactor landfills adds urgency to a need for better information on LFG generation. This in turn establishes the requirement for improved modeling techniques that will allow owners, operators, and designers to anticipate system requirements as well as the economic potential of these resulting increases in gas production.

References

McManus, T. and T. Hanlon. “Strategy for Municipal Solid Waste Landfills.” Proceedings of the 7th Annual SWANA’s Landfill Symposium. Louisville, KY, June 17-19, 2002.
Pohland, F.G. “Sanitary Landfill Stabilization with Leachate Recycle and Residual Treatment.” USEPA Report No. EPA/600/2-75/043. Cincinnati, OH. 1975.

Guest author Victor O. Okereke, Ph.D., P.E. is seniro engineer with King County Solid Waste Division in Seattle, WA.

MSW - July/August 2003