Although sludge anaerobic digestion and biogas recovery could significantly
contribute to emission reduction, it was less beneficial for Oxygen Ditch than the other
two treatment technologies due to its low sludge production. The influence of choosing
“high risk” or “low risk” N2O emission factors on the carbon footprint was also investigated
in this study. Oxygen Ditch was assessed as “low risk” of N2O emissions while SBR was
“high risk”. The carbon footprint of A–A–O with sludge anaerobic digestion and energy
recovery was more resilient to changes of N2O emission factors and control of N2O
emissions, though process design parameters (i.e., effluent total nitrogen (TN) concentration,
mixed-liquor recycle (MLR) rates and solids retention time (SRT)) and operation conditions
(i.e., nitrite concentration) are critical for reducing carbon footprint of SBR. Analyses of
carbon footprints suggested that aerobic treatment of sludge not only favors the generation
of large amounts of CO2, but also the emissions of N2O, so the rationale of reducing aerobic
treatment and maximizing anaerobic treatment applies to both wastewater and sludge
treatment for reducing the carbon footprint, i.e., the annamox process for wastewater nutrient
removal and the anaerobic digestion for sludge treatment.