Hot and Cold: Potential Impacts from Biosolids Management Options on Carbon Footprints

While carbon accounting efforts for wastewater processes often focus primarily on carbon dioxide emissions related to energy use, published literature of greenhouse gas (GHG) emissions indicates that methane (CH4) and nitrous oxide (N2O) have the potential to significantly alter carbon footprints associated with biosolids management. With global warming potentials (GWP) of 21 and 310, respectively, even relatively small emissions of CH4 and N2O need to be taken into account when determining biosolids-related GHG emissions. Fugitive CH4 emissions from biosolids placed in landfills and from leaky anaerobic digesters can account for larger carbon dioxide equivalents (CO2e) debits than the emissions associated with transporting biosolids or heating digesters. Similarly, relatively small emissions of N2O gas from incineration or poorly managed composting operations can have a substantial impact on total GHG emissions.A GHG emissions calculator, referred to as the Biosolids Emissions Assessment Model, or BEAM, was developed for the Canadian Council of the Ministers of the Environment (CCME) based on an extensive review of current greenhouse gas emissions literature (download available from http://pubs.acs.org/doi/suppl/10.1021/es101210k). Use of the model allows for apples to apples comparisons of the impact of biosolids management strategies on overall GHG emissions. The model also allows decision-makers to investigate changes in biosolids management operations that could lower GHG emissions. Results from applications of the BEAM model suggest that maximizing the potential offsets inherent in biosolids, including energy capture and fertilizer and carbon sequestration value, while minimizing fugitive CH4 and N2O emissions associated with biosolids management practices, such as landfilling, low temperature combustion, or improper biosolids stockpiling, can significantly decrease the GHG emissions from biosolids management programs.