Comprehensive Numerical Modeling of Greenhouse Gas Emissions from Water Resource Recovery Facilities.

A numerical model was developed to comprehensively predict greenhouse gas (GHG) emissions from water resource recovery facilities. An existing activated sludge model was extended to include a nitrifier-denitrification process and carbon dioxide (CO2) as a state variable. The bioreactor model was coupled to a process-based digester model and an empirical model of indirect CO2emissions. Direct emissions were approximately 90% of total GHG emissions for a plantwide simulation using the Modified Ludzack-Ettinger process. Biogenic CO2, nitrous oxide (N2O), and methane (CH4) represented 10, 43, and 34% of total emissions. Simulating a dissolved oxygen controlled closed-loop system reduced both sensitivity and uncertainty of GHG emissions. Nitrous oxide emissions were much more sensitive under different design and operating conditions compared to CH4and CO2, indicating a significant mitigation potential. An uncertainty analysis found that the uncertainty in GHGs emissions estimates could be significant. Nitrous oxide emissions dominated in both magnitude and uncertainty.