GHGs refer to gases that can cause global warming, including CO₂, CH₄, N₂O, and other trace gases ^[1][15]. Discussions of climate change tend to focus on CO₂, as it is the main GHG produced by burning fossil fuels, industrial production, and land use changes ^[2][16]. However, CH₄, N₂O, and other trace gases also make significant contributions to global climate change. In fact, although the amounts of CH₄ and N₂O emitted are much smaller than that of CO₂, their global warming potentials (GWPs) are much larger than that of CO₂. GWP, a metric used in policies, integrates the radiative forcing of a substance over a chosen time horizon relative to that of CO₂, which is assigned a value of 1. The GWPs of CH₄ and N₂O are 29 and 265, respectively, as assessed by the IPCC over a 100-year period ^[1][15]. In 2016, CO₂ was the largest contributor to global warming, accounting for 74.4% of total emissions, followed by CH₄ at 17.3%, N₂O at 6.2%, and other gases at 2.1% ^[2][16].

The atmospheric concentrations of CO₂, CH₄, and N₂O have all increased since 1750 due to human activity. In 2011, the concentrations of these GHGs were 391 ppm, 1803 ppb, and 324 ppb, respectively, exceeding pre-industrial levels by about 40%, 150%, and 20%, respectively ^[3][17]. The global average temperature has risen by more than 1 °C since pre-industrial times due to the increased concentrations of GHGs ^[2][16]. To reduce the risks and impacts of climate change, the Paris Agreement proposed maintaining the global average temperature rise well below 2 °C above pre-industrial levels and striving to limit it to 1.5 °C above pre-industrial levels ^[4][18]. During the 26th Conference of the Parties to the United Nations Framework Convention on Climate Change (COP26) in 2021, the United States and the European Union launched the “Global Methane Pledge” initiative, which has been signed by more than 100 countries.

Urban WCSs refer to the series of wastewater collection pipelines located underground in urban developed areas. WCSs generally receive domestic sewage from residences, as commercial and industrial sewage generally requires pretreatment to meet water quality standards before discharging. As an important component of urban infrastructure, the healthy operation of WCSs affects people’s lives. In the past few decades, China has experienced many urban environmental problems due to rapid urbanization and insufficient attention to environmental protection, in which WCSs play a role. In 2019, China’s Ministry of Housing and Urban–Rural Development, the Ministry of Ecology and Environment, and the National Development and Reform Commission jointly issued the “Three-Year Action Plan (2019–2021) for Improving the Quality and Efficiency of Urban Wastewater Treatment”, which aims to solve existing problems in WCSs. Investigations have found significant problems in WCSs in China, such as cross-connections between storm sewers and sewage pipelines, leaking pipes, and high water level operation ^[5][19]. Apart from causing inconvenience in urban life, these problems also have adverse effects on carbon emissions.

GHGs emitted from WCSs include CH₄, CO₂, and N₂O. During long-term operation, inorganic and organic particles and microorganisms in the wastewater settle at the bottom of the pipelines. The microorganisms colonize the sediments (mainly on the surface, to a depth of about 2 cm) and the pipeline walls, and grow, forming a biofilm with a thickness of several hundred to several thousand microns after several months ^{[6][7][8][9][10][11]}[20,21,22,23,24,25]. A cross-section of a wastewater collection pipeline is shown in Figure 1a. The biofilms are mainly composed of large amounts of inorganic substances (such as water and inorganic salts) and some organic substances (such as microorganisms and extracellular polymers), with bacteria playing a dominant role in the biofilm ^[12][10]. The formation of biofilms is influenced by many factors, such as the pipeline operation mode, sewage characteristics, and water flow shear force. During the transport of the sewage from the users to the WWTP in pipelines, the microorganisms in the biofilm degrade and utilize the organic substances in the sewage, decreasing the concentration of organic matter. The effect of this biological action is significant over the long term during long-distance sewage transport ^[13][26]. Zan et al. found that biological processes in two laboratory-scale sewer reactors consumed 25% and 30% of the total COD, respectively ^[9][23]. In a laboratory-scale WCS, CH₄ production accounted for ~70% of the total soluble COD (sCOD) loss ^[14][9].

The degradation of pollutants in sewage by microorganisms reduces the concentration of organic matter in the sewage, resulting in a lower organic matter concentration in the influent of the downstream WWTP ^[15][27]. To remove other pollutants such as nitrogen and phosphorus in the biological treatment process of the WWTP, additional carbon sources need to be added to the sewage, increasing sewage treatment costs. In addition, harmful gases such as H₂S, CH₄, and N₂O are released during transport in WCSs. H₂S causes pipeline corrosion, odor, and other problems, while CH₄ and N₂O are GHGs that contribute to climate change ^[16][7]. Moreover, high CH₄ concentrations also present an explosion hazard ^[17][18][28,29].

Li et al. provide a detailed summary of the mechanism of CO₂, CH₄, and N₂O production in the WCS biofilm (Figure 1b) ^[6][20]. In wastewater collection pipelines, organic substances in the wastewater are typically degraded into volatile fatty acids (VFAs), such as methanol, acetic acid, and propionic acid, as well as CH₄, CO₂, and H₂ by fermentative bacteria and hydrogen-producing acetogenic bacteria. These VFAs, CO₂, and H₂ are then utilized by methanogenic archaea (MA) to produce CH₄. N₂O is a byproduct of microbial nitrification and denitrification processes ^[6][19][20][20,30,31]. During nitrification, NH₄⁺ in the sewage is oxidized to NO₂⁻ by microorganisms. Oxidation of the intermediate product NH₂OH and reduction in NO, which is the product of NH₂OH, generate N₂O ^[21][32]. During denitrification, the intermediate product N₂O is produced during the conversion of nitrite to nitrogen. In gravity flow WCSs, some of the generated CO₂, CH₄, and N₂O evaporate into the gas phase and enter the atmosphere from exhaust ports. The production of CH₄ in the sediments in WCSs can be divided into three stages: hydrolysis, fermentation, and methane production ^[22][12]. The fermentable COD in the wastewater can be degraded into easily fermentable COD after hydrolysis, which is then utilized by microorganisms to produce H₂, acetic acid, and propionic acid. MA then primarily use acetic acid and H₂ to produce CH₄ (Figure 1c).