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Gu, D.; Liu, Y.; Zhao, W.; Qiu, S.; Cui, N.; Hu, X.; Zhao, P. Greenhouse Gases in Wastewater Collection Systems. Encyclopedia. Available online: (accessed on 18 June 2024).
Gu D, Liu Y, Zhao W, Qiu S, Cui N, Hu X, et al. Greenhouse Gases in Wastewater Collection Systems. Encyclopedia. Available at: Accessed June 18, 2024.
Gu, Dongmei, Yiwen Liu, Weigao Zhao, Shuntian Qiu, Nuo Cui, Xinyue Hu, Peng Zhao. "Greenhouse Gases in Wastewater Collection Systems" Encyclopedia, (accessed June 18, 2024).
Gu, D., Liu, Y., Zhao, W., Qiu, S., Cui, N., Hu, X., & Zhao, P. (2023, July 10). Greenhouse Gases in Wastewater Collection Systems. In Encyclopedia.
Gu, Dongmei, et al. "Greenhouse Gases in Wastewater Collection Systems." Encyclopedia. Web. 10 July, 2023.
Greenhouse Gases in Wastewater Collection Systems

Wastewater collection systems (WCSs) not only play an indispensable role in urban life but also significantly contribute to greenhouse gas (GHG) emissions. Based on extensive literature research, this study summarizes current research on the production mechanisms, influencing factors, control techniques, and quantitative estimates of GHGs emitted from WCSs and presents initial estimates of total GHG emissions from WCSs in China. A variety of factors affect GHG production, but standard methods are still lacking to quantify GHG emissions from WCSs. China’s WCSs emit approximately 3.86–15.35 Mt of CO2eq annually (equivalent to 5.1–20.2% of the GHG emissions from wastewater treatment). Thus, GHG emissions from WCSs are significant and deserve attention. Control of GHGs in WCSs can be achieved through the application of chemical agents, while the construction of a green stormwater infrastructure can further facilitate GHG reductions. This research provides valuable insights for policymakers to consider in future policy planning related to GHG reduction and the improved operation of WCSs. Future research should focus on quantifying the impacts of various factors and accumulating field data on GHGs in various regions to facilitate the development of standardized calculation methods.

wastewater collection system greenhouse gases carbon emission reduction

1. Greenhouse Gases

GHGs refer to gases that can cause global warming, including CO2, CH4, N2O, and other trace gases [1]. 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]. 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]. 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].
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]. The global average temperature has risen by more than 1 °C since pre-industrial times due to the increased concentrations of GHGs [2]. 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]. 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.

2. Wastewater Collection Systems

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]. Apart from causing inconvenience in urban life, these problems also have adverse effects on carbon emissions.

3. Greenhouse Gases in Wastewater Collection Systems

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]. 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]. 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]. Zan et al. found that biological processes in two laboratory-scale sewer reactors consumed 25% and 30% of the total COD, respectively [9]. In a laboratory-scale WCS, CH4 production accounted for ~70% of the total soluble COD (sCOD) loss [14].
Figure 1. (a) Wastewater collection pipeline cross-section; (b) CH4 and N2O generation mechanisms in biofilm (VFAs—volatile fatty acids); (c) CH4 generation mechanisms in sediment.
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]. 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]. Moreover, high CH4 concentrations also present an explosion hazard [17][18].

4. Mechanism of Greenhouse Gas Production in Wastewater Collection Pipelines

Li et al. provide a detailed summary of the mechanism of CO2, CH4, and N2O production in the WCS biofilm (Figure 1b) [6]. 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]. 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]. 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]. 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).


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