The major biomass components in municipal solid waste (MSW) are food waste and paper, and thus biological (2.15.1 and 2) or thermal processes (2.15.3 and 5) are used to recover energy from the biomass fraction.

2.15.1 Methane recovery in landfills

In landfills containing organic waste, anaerobic biodegradation of biomass produces methane gas because oxygen diffused from the atmosphere is consumed near the landfill surface. Landfill gas can damage nearby plant growth and even cause neighboring buildings to explode. Therefore, gas control methods including flaring have been implemented since the 1960s. In large-scale landfills, vertical wells are installed to pump out the gas, which is used on-site for power generation or sold as fuel. Average gas recovery rates range from 120 to 150 m3/ton of dry MSW, equivalent to a heating value of 2500 MJ/ton (the methane concentration of landfill
gas is around 55%). Since the late 1990s, many American studies have investigated “bioreactor landfill,” in which moisture content is controlled to maximize biodegradation. Bioreactors also increase the rate of gas generation and consequently the rate of energy recovery.

In contrast, landfill gas is not recovered in Japan for two reasons: landfills contain little organic content due to the common practice of incineration, and landfills are aerated through the use of a semi-aerobic landfill structure, in which natural convection is allowed to form an aerobic zone around leachate collection pipes below the landfill. The Chuo-Botahei landfill in Tokyo Bay does recover methane, but its annual power generation (averaged over 20 years) is 3000 MWh, enough power for only 850 households.