Greenhouse gas emissions and future forecast of the sewage treatment system in Beijing

The low-carbon operation of sewage treatment systems has received significant attention globally as an important source of greenhouse gas emissions. However, existing research shows a deficiency in the prognostic analysis of future trends in urban sewage treatment systems, and insufficient attention is devoted to greenhouse gases from the sewage pipe network and sludge treatment facilities in greenhouse gas accounting. Consequently, the calculated results fall significantly below the actual values, thus impeding the formulation of targeted emission reduction measures. Although Beijing is one of the cities in China with the highest level of wastewater reuse, the large-scale use of wastewater could significantly increase greenhouse gas emissions. Researching greenhouse gas emissions from Beijing's wastewater treatment system could bear reference significance for other cities under the strategic backdrop of China's carbon peaking by 2030 and carbon neutrality by 2060.

A greenhouse gas accounting system of the sewage treatment system was established to measure the scale of greenhouse gas emissions from 2010 to 2020 based on the sewage treatment process in Beijing. The greenhouse gas accounting system for Beijing's sewage treatment system consisted of three stages: sewage pipeline network, sewage treatment, and sludge disposal. The accounting scope comprised direct emissions of CH₄, N₂O, and fossil fuel CO₂, as well as indirect emissions of CO₂ resulting from equipment operation and chemical consumption. Greenhouse gas emissions for each stage were computed using the emission factor method. Additionally, various calculation formulas and parameters were introduced, such as CH₄ emission factors, N₂O emission factors, and chemical emission factors. Therefore, the greenhouse gas emissions of each stage were calculated and aggregated, using the sewage treatment plant as the calculation unit, to obtain the overall greenhouse gas emissions of Beijing's sewage treatment system. Additionally, the scenario analysis method was used in this study to predict the characteristics of greenhouse gas emissions under different scenarios in 2035.

The following research results are presented: (1) From 2010 to 2020, there has been a significant increase in the level of greenhouse gas emissions emitted by Beijing's sewage treatment system, rising from 1.191 3 to 2.269 1 million tons. Sewage treatment is the most significant stage, accounting for 51.27% of total emissions, followed by sludge disposal (39.92%) and sewage pipeline network (8.81%). (2) The sources of greenhouse gas emissions vary significantly across different stages. The sewage pipeline network is mainly associated with electricity consumption and CH₄ emission, whereas sewage treatment is primarily associated with electricity consumption and N₂O emission. The sludge disposal stage mainly contributes to CH₄ and N₂O emissions. Generally, it can be observed that indirect emissions, which include the consumption of electricity and chemical, constitute 62.79% of the total greenhouse gas emissions from Beijing's sewage treatment system. (3) The emission intensities of different treatment standards and processes are different: the emission intensity of the Beijing landmark exceeds the national standard, and the emission intensity of the membrane bio-reactor technology is 1.77 times that of anaerobic-anoxic-oxic technology. (4) The greenhouse gas emissions of Beijing's sewage treatment system in 2035 will increase in the baseline, high standard effluent, and low carbon emission scenarios by 59.87%, 110.69%, and 41.37% compared with 2020. Although the low carbon emission scenario has the smallest increase, its realization requires an additional investment of 4.995 billion Yuan and an area of 1.92 million m².

The research results show a significant increase in greenhouse gas emissions from Beijing's sewage treatment system and provide pertinent emission reduction recommendations.