Integration of anaerobic-aerobic digestion of organic wastes: GHG emission impacts at micro and macro scale

We estimate the Greenhouse Gas (GHG) emissions from two process configurations: the aerobic composting and the integrated anaerobic-aerobic digestion with production of electricity, heat and high quality fertilizer. The reference case study is an existing composting plant, operating in aerobic conditions in the Province of Siena (Tuscany, central Italy) from 2009 to 2011. We hypothesize to install an anaerobic digester inside the existing composting plant in order to recycle the anaerobic digestates. We evaluate the climate impacts of these two different waste management practices at technological and regional scale. The integration of anaerobic-aerobic digestion of organic wastes could determine a yearly reduction of the GHG emissions equal to almost 23% of those from the existing composting plant. Moreover, the electricity production from biogas obtained by the anaerobic technologycould avoid the release of 5% of the annual GHG emissions from the waste disposal plants, operating in the integrated management system of the Province of Siena. At the same time, the provincial area could benefit from the production of electricity obtained by waste incineration and biogas (anaerobic digestion and landfills), reducing the territorial net carbon balance of almost 1.8% per year. The estimated GHG emissions, calculated according to the "2006 IPCC Guidelines for National Greenhouse Gas Inventories", can provide useful information and management suggestions to waste operators and policy makers.