[Direct CO₂ Emission Characteristics and Fossil Source CO₂ Contributions of Municipal Wastewater Treatment Plant with...

In order to analyze the characteristics of fossil source CO₂ emissions in the AAO process of municipal wastewater treatment, in-situ monitoring in a typical AAO process of a municipal wastewater treatment plant in North China was conducted in this work. The CO₂ emission flux of each main process unit (selection tank, anaerobic tank, anoxic tank, aerobic tank, sludge return gallery, and secondary sedimentation tank) from September 2023 to August 2024 was obtained, and the 24 h day and night continuous change pattern and the emission contribution of fossil source CO₂ was analyzed. Through 12 months of continuous monitoring, the direct CO₂ emission fluxes of major process units such as selection tank, anaerobic tank, anoxic tank, aerobic tank, sludge return gallery, and secondary sedimentation tank were (30.20±2.85), (43.50±5.81), (44.41±4.69), (2 736.82±213.26), (82.68±7.21), and (11.59±1.15) g·(m²·d)^-1, respectively. In summer, the AAO process showed "double peaks" of direct CO₂ emission flux in 24 h, with peak periods at 06:00-09:00 [average 12 443.14 μg·(m²·s)^-1] and 21:00-24:00 [average 12 395.38 μg·(m²·s)^-1], both of which were 20% higher than the average value of direct CO₂ emission flux in summer for 24 h. In winter, the AAO process showed a "single peak" of direct CO₂ emission flux in 24 h, with peak periods at 09:00-12:00 [average 16 705.90 μg·(m²·s)^-1], which was 21% higher than the average value of direct CO₂ emission flux in winter for 24 h. The direct CO₂ emission flux in winter [24 h average 13 811.81 μg·(m²·s)^-1, CV=9.0%] was higher than that in summer [24 h average 10 388.41 μg·(m²·s)^-1, CV=14.4%] but with smaller fluctuations. The monthly direct CO₂ emission monitoring results showed that the average direct CO₂ emission flux of the AAO process for 12 months was (1 094.86±80.97) g·(m²·d)^-1 with large-size monthly fluctuations (CV=35.6%); the peak occurred in March 2024 [1 737.74 g·(m²·d)^-1], which was 59% higher than the average value of 12 months. The direct CO₂ emission intensity of the AAO process varied significantly with the seasons. The average direct CO₂ emission intensities in spring, winter, summer, and autumn were (3 546.76±616.24), (3 089.66±363.98), (2 738.55±120.38), and (2 267.45±229.33) kg·d^-1, respectively. The direct CO₂ emissions of different process units were obviously different, and the aerobic tank was the main source of CO₂ emissions in the AAO process, with an average annual emission flux, average annual daily emission intensity, and average annual emission factor (measured in CO₂/COD) of (2 736.82±213.26) g·(m²·d)^-1, (2 859.14±214.32) kg·d^-1, and (3.83±0.75) kg·kg^-1, respectively, which were significantly (P < 0.001) higher than those of other treatment units. The direct CO₂ emission flux was significantly positively correlated with oxidation-reduction potential (ORP; P < 0.000 1), DO (P < 0.05), NO₃^--N (P < 0.05), and NO₂^--N (P < 0.05) and was significantly negatively correlated with TP (P < 0.01), NH₄⁺-N (P < 0.01), and pH (P < 0.05). The direct CO₂ emissions from fossil sources in the municipal wastewater treatment process were estimated based on the measured values of direct CO₂ emissions, and the overall fossil source direct CO₂ emission range of the AAO process was (38.05±5.31)-(148.41±20.72) g·m^-3 (converted into fossil source CO₂ emissions per m³ of wastewater treated). Direct CO₂ emissions from fossil sources accounted for approximately 28.7%-67.1% of the greenhouse gas emissions of the whole plant calculated by the Intergovernmental Panel on Climate Change (IPCC) method. The direct CO₂ emissions from fossil sources in municipal wastewater treatment plants are underestimated by the IPCC carbon emission accounting system, and it is recommended to include direct CO₂ emissions from fossil sources in the carbon emission accounting system of wastewater treatment plants.