Evaluation of organic waste diversion alternatives for greenhouse gas reduction

Greenhouse gas (GHG) emission analysis was performed for current and proposed organic waste management practices in the City of Madison, Wisconsin, United States of America (USA). Aiming for the long-term goal of zero waste, the City of Madison has been looking into an opportunity to divert its organic waste from its landfill. Previous studies suggested that organic waste diversion could result in a GHG emission reduction: Alternative treatment of organic waste would reduce GHG emissions through avoidance of landfill methane emission, nutrient replacement, and energy recovery when anaerobic digestion technologies are employed. However, organic waste diversion requires modification of collection practices and additions to the collection fleet. This would increase the GHG emissions and there is a need to balance emission reductions and the additional costs associated with a new organic waste diversion program. Collection practices are also important for the success of such a project, as it accounts for 83% of current operational cost for managing organic waste. The GHG emissions were quantified using the Life Cycle Assessment (LCA) approach. The current practice of composting of yard waste and direct disposal of other fractions of organic waste into the landfill were estimated to emit 224kgCO2-eq./ton of GHG. In contrast with the current practice, four alternatives were assessed in this study: windrow composting, high-solids anaerobic digestion, co-digestion at a large scale industrial waste anaerobic digester facility, and co-digestion at the local wastewater treatment plant. The results show that the co-digestion of source-separated organic waste would achieve the highest GHG emission reduction among the alternatives considered. The results were as follows: 81.5kgCO2-eq./ton for windrow composting; −46.0kgCO2-eq./ton for a high solids anaerobic digester; −156kgCO2-eq./ton for co-digestion with industrial waste; and −189kgCO2-eq./ton for co-digestion with sewage sludge. Co-digestion was favored as it shows the highest GHG emission reduction capacity while saving capital cost by almost half. However, the results of the GHG emission analysis were also affected by several factors external to the technologies chosen, including public participation, collection efficiency of organic waste, and types of waste collected. Based on the findings of this work, the party in charge of the organic waste diversion project should seek opportunities to partner with regional anaerobic digestion projects and local utilities, while continuing with its community outreach efforts.