Evaluating CO₂ emissions from continuous flow and batch growth systems under autotrophic mode: Implications for GHG...

The oxidation of ammonia by autotrophic bacteria is a central part of the nitrogen cycle and a fundamental aspect of biological nutrient removal (BNR) during wastewater treatment. Autotrophic ammonia oxidation produces protons and results in net-CO₂ production due to the neutralizing effect of bicarbonate alkalinity. Attention must be paid to the propensity for this produced CO₂ to be transferred to the atmosphere where it can act as a greenhouse gas (GHG). In the context of BNR systems, bicarbonate-derived CO₂ emissions should be considered distinct from the biogenic CO₂ that arises from cellular respiration, though this distinction is not made in current GHG accounting practices. The aim of this study was to evaluate the performance of two experimental systems operated under autotrophic mode and buffered with bicarbonate, to investigate the relationship between ammonia removal and gaseous CO₂ emissions. The first system consisted of continuously aerated lab-scale batch reactors, which were effective in demonstrating the important link between ammonia oxidizer activity, pH, and gaseous CO₂ production. Depletion of the buffer system always led to a rapid decline in system pH and cessation of CO₂ emissions when the pH fell below 7.0. The second system was a tubular continuous-flow biofilm reactor which permitted comparison of ammonia removal and CO₂ emission rates. A linear relationship between ammonia removal and CO₂ emissions was demonstrated and the quantified CO₂ production was relatively close to that which was predicted based on the stoichiometry of nitrification, with this CO₂ being detected in the gas phase. It was apparent that this system offered minimal resistance to the mass transfer of CO₂ from the liquid to gas, which is an important factor that determines how much of the bicarbonate-derived CO₂ may contribute to greenhouse gas emissions in engineered systems such as those used for BNR.