1. Urbanization: a majority in cities;United Nations Population Funds,2007
2. Overview of greenhouse gases: methane emissions;U.S. Environmental Protection Agency,2013
3. Landfill methane outreach program;U.S. Environmental Protection Agency,2014
4. Biomass resources in the United States;National Renewable Energy Laboratory,2008
5. Biogas opportunities roadmap;U.S Department of Energy,2014