Methane and nitrous oxide emissions following anaerobic digestion of sludge in Japanese sewage treatment facilities

• CH4 and N2O emissions after anaerobic sludge digestion were investigated.
• CH4 mission factor was 509 ± 72 mg/m3-influent and especially high in winter.
• N2O emission factor was 7.1 ± 2.6 mg/m3-influent and lower than CH4 emission.
• The highest CH4 emissions were during dewatering, followed by continued digestion.
• CH4 and N2O emissions after anaerobic digestion are considered to be significant.

Methane (CH4) and nitrous oxide (N2O) are potent greenhouse gases with global warming potentials (expressed in terms of CO2-equivalents) of 28 and 265, respectively. When emitted to the atmosphere, they significantly contribute to climate change. It was previously suggested that in wastewater treatment facilities that apply anaerobic sludge digestion, CH4 continues to be emitted from digested sludge after leaving the anaerobic digester. This paper studies the CH4 and N2O emissions from anaerobically digested sludge in the subsequent sludge treatment steps. Two full-scale treatment plants were monitored over a 1-year period. Average emissions of CH4 and N2O were 509 ± 72 mg/m3-influent (wastewater) and 7.1 ± 2.6 mg/m3-influent, respectively. These values accounted for 22.4 ± 3.8% of the indirect reduction in CO2-emissions when electricity was generated using biogas. They are considered to be significant.