By Ramin Yazdani

MSW requiring treatment/disposal contains large amounts of organics, even with the intensive waste recycling/reuse efforts over the past quarter century. This situation will likely remain, even with with future attainment of the 50% national waste-reduction goal. Thus, generation of methane from MSW, both from source-separated organics and mixed MSW, is a subject of high interest. This is due to the substantial energy recoverable as methane from wastes and from ensuing reduction in waste volumes (which can be considered waste reduction). A great deal of information has accumulated for more than two decades about the kinetics and yields of waste-to-methane processes from various MSW fractions and sources and from cost/performance analyses performed at varying levels of detail.

The conversion of MSW to methane, variously termed anaerobic digestion or biomethanation, has been commercialized in Europe, the US, and elsewhere using stirred tanks, packed beds, high-solids semibatch or plug-flow processes, and other in-vessel approaches. Also, landfills–including controlled and optimized bioreactor landfills–have been exploited for methane gas recovery from MSW. Landfill methane now fuels about 1,000 MWE in the US. Comparison of alternative MSW-to-methane processes allows assessment of both their near- and long-term environmental and economic costs. Environmental "balance sheets" including both benefits and cost suggest landfills can compare surprisingly well environmentally to waste-to-methane alternatives. For example, for given MSW feeds, landfills with designs now being implemented can recover methane as well or better than in-vessel processes can. Advantages accrue for landfills due to far longer residence times and introduction of the highest possible amount of organics in mixed feeds or wastes. Controlled landfills can recover close to maximum methane potential of as-received MSW resources. By contrast, in-vessel processes take lesser fractions of the MSW organics and recover lower amounts of the renewable energy potential of organics, particularly when considering parasitic energy consumption. In-vessel digestion kinetics also dictate incomplete conversion at economic residence times. Greenhouse-gas balance sheets can also favor optimized landfill disposal with engineered methane recovery over in-vessel processes because of greater methane emissions mitigation and the long-term photosynthetically derived C-sequestration possible in landfills. Conversely, composting MSW residuals in-vessel can result in CO₂ emissions and the transfer to land of any remnant plastics, refractory organics, and other unconverted components such as heavy metals. Groundwater pollution from modern landfills can actually be lower than from MSW composting and land application atop permeable soil. Furthermore, landfill mining offers long-term options for compost recovery from landfilled wastes, in particular from controlled bioreactor landfills.

Aside from environmental considerations, landfills with methane recovery are also strongly favored economically over in-vessel biomethanation approaches. Even with favorable assumptions, in-vessel processes show costs to be a one-plus order of magnitude higher than advanced landfill designs. Perhaps most importantly, in-vessel systems can typically handle only a relatively moderate fraction of intensively preprocessed MSW and, overall, do not even significantly reduce the need for ultimate MSW disposal. Full-scale in-vessel operations are only commercially feasible using carefully segregated wastes, and where mandated, high tipping fees and other governmental subsidies encourage such ventures. The environmental and social costs of waste management policies should be reviewed in light of the comparable benefits of the technological alternatives for environmental MSW management, energy recovery, and global-warming mitigation.

Ramin Yazdani is an engineer with the Yolo County (CA) Department of Public Works and is presently engaged in an EPA study on bioreactor landfills.